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Organic reactions with rhodium carbonyl cluster catalysts
Journal of Molecular Catalysis, 21 (1983)
ORGANIC REACTIONS CATALYSTS
HIROSHI YAMAZAKI
133 - 150
WITH RHODIUM
and PANGBU
133
CARBONYL
CLUSTER
HONG
The Institute of Physical and Chemical Research,
Wako-shi, Saitama 351 (Japan)
Summary Rhodium carbonyl clusters have been found to catalyze two types of reactions: (1) Carbon-hydrogen bond activation of aromatic compounds and (2) Cross-hydrocarbonylation of acetylenes and olefins. (1) Under pressure of carbon monoxide (20 to 30 atm) and at higher temperatures (180 to 220 “C), rhodium carbonyl clusters Rh4(C0)i2 and Rh,(CO)i6 catalyze the addition of a C-H bond of benzene to unsaturated compounds such as diphenylketene, isocyanates and acetylenes to give phenylated compounds such as diphenylacetophenone, benzamides and phenylated olefins, respectively. Five-membered heterocycles such as furan, thiophene, and N-methylpyrrole also react similarly with diphenylacetylene to give 2-furyl-, 24hienyl- and 2-(N-methylpyrrolyl)-olefins, respectively. Among these, the reactivity of furan is the highest and is about 200 times that of benzene. In the reactions of arene with olefins, aryl-substituted olefins are obtained instead of simple addition products. For example, the reaction of benzene with ethylene gives styrene directly. The hydrogen generated in these reactions is used for the hydrocarbonylation or the hydrogenation of the starting olefins, yielding ketones or saturated compounds. The reaction of furan with olefins gives fury1 olefins similarly. In the reactions of secondary amines such as indole, carbazole, and diphenylamine, the N-H bond participates in preference to the C-H bond, giving N-vinyl compounds. Regioselectivities in the reactions of substituted benzenes suggest the protophilic nature in the stage of C-H bond activation. The following mechanism is proposed: (1) initial oxidative addition of an aromatic C-H bond to the rhodium catalyst to give aryl- and hydride-rhodium species, (2) insertion of unsaturated compounds into the aryl-rhodium bond, and (3) reductive elimination with the hydride or P-hydrogen elimination affording the corresponding aryl-substituted compounds. (2) The cross-hydrocarbonylations of acetylenes and ethylene with carbon monoxide are found to give 5-ethyl-2(5H)-furanones when hydrogendonating solvents are used, and to give o, P-unsaturated ethyl ketones when molecular hydrogen is introduced into the reaction system. Since it is consistent with the regioselectivity in the reactions of unsymmetrical acetylenes, a fl-acylvinyl rhodium intermediate is proposed as a common key @ Elsevier Sequoia/Printed
in The Netherlands
134
intermediate for these two products. The convenient syntheses of 5-alkoxy2( 5H)-furanones from acetylenes, carbon monoxide and alcohols are attained by adding a basic alkali metal salt such as NaOAc or Na,CO, as a cocatalyst.
Introduction Cluster complexes have drawn much attention recently because they can be expected to provide highly selective catalysts for reactions which are thought to be inherent in heterogeneous catalysts, such as reduction of carbon monoxide and nitrogen as well as alkane isomerization [ 11; further, they can be expected to provide new examples of catalysis which are unknown with either mononuclear homogeneous catalysts or surfaces. Many examples of homogeneous reactions using rhodium carbonyl clusters as catalyst precursors are known. The hydrogenations of olefins [2] and acetylenes [ 31 have been found to proceed under low pressures of hydrogen. The hydroformylations of olefins have been examined in comparison with those occuring in the presence of the cobalt catalyst [4]. The selective reduction of the C-C double bond of o, P-unsaturated carbonyl compounds [5], reduction of aldehydes and ketones [6], reduction of nitrobenzenes [7], cooligomerization of ethylene and carbon monoxide [ 81, and hydroformylation of olefins [9] have been carried out under the water-gas shift conditions. Oxidation of carbon monoxide and cyclohexanone by oxygen [lo], and the H-D exchange reaction of tertiary alkylamines with D,O [ 111 have been also conducted under carbon monoxide pressure. It is well known that the rhodium compounds behave as active catalysts for the specific reduction of carbon monoxide to give ethylene glycol at an elevated temperature (200 - 240 “C) under a high pressure of carbon monoxide and hydrogen (1000 - 3000 atm) in the presence of bases; under these conditions, the rhodium compounds exist as high-nuclearity rhodium carbonyl cluster species, as indicated by high-pressure infrared spectroscopy [ 121. We have discovered new reactions by using rhodium carbonyl clusters as catalyst precursors at higher temperatures (180 - 220 “C) and under pressures of carbon monoxide (20 to 30 atm). This paper describes these reactions, which are divided into two topics: C-H activation of aromatic compounds and cross-hydrocarbonylation of acetylenes and olefins.
Catalytic
Activation
of Carbon-Hydrogen
Bonds of Aromatic
Compounds
Recently the activation of C-H bonds by soluble transition metal complexes has received increasing attention because of the importance of carrying out selective chemical transformation of unreactive compounds such as nonfunctionalized arenes and saturated hydrocarbons, in particular, under mild conditions [13, 141. There are numerous examples of
135
stoichiometric intra- and inter-molecular metallation on single and multimetal centers. However, little is known about the application of these reactions to catalytic synthesis of organic compounds. Addition of arenes to heterocumulenes Previously we showed that cobalt carbonyls catalyze the deoxygenation of diphenyl ketene to give 1-(diphenylmethylene)-2-(diphenylacetoxy)-3phenylindene under carbon monoxide pressure [15] ; they catalyze the decarbonylation to give tetraphenylethylene under a nitrogen atmosphere
WI. In the course of the comparative study of the catalytic behavior of rhodium carbonyls, we found a novel addition reaction of a C-H bond of benzene used as the solvent to the C-C double bond of the ketene to give diphenylacetophenone [17]. The reaction was carried out under pressures of carbon monoxide at elevated temperatures in the presence of catalytic amounts of Rh4(C0)i2: Ph,C=C=O
+ PhH -
Ph,CHCOPh
(1)
In an attempt to obtain information about the catalytically active species and the role of carbon monoxide in the aromatic C-H bond activawe carried out a series of experiments with tion catalyzed by Rh,(C0)i2, various CO partial pressures under isothermal conditions (200 “C). The results indicated that the yield of the benzene adduct depends markedly on the CO partial pressure. For examples, under partial pressures of CO of 1, 5, 10, 20 and 30 atm, diphenylacetophenone was obtained in yields of 3,11, 47, 56 and 68%, respectively. Deposits of rhodium metal were observed when the reactions were carried ,out under lower pressures. RhJCO),, gave almost the same result as that of Rh,(C0)i2, but Rh,(CO)Jl, showed lower activity. It is of interest to note that rhodium carbonyl clusters are stable at higher temperatures even under high pressures of carbon monoxide [18]. We suppose that rhodium clusters may play an important role in this catalytic reaction. Monosubstituted benzenes such as toluene, anisole, and fluorobenzene also react with diphenyl ketene to give the corresponding aryl diphenylmethyl ketones. The ortho, metu, and para ratios in the products from anisole and fluorobenzene were found to be 55/31/14 and 60/27/13, respectively. It is remarkable that the sterically hindered ortho isomers were formed predominantly in these cases. On the contrary, the isomer ratio observed in the reaction of toluene was 6/62/32. The reactivities of meta and paru positions are found to be almost equal when the statistical effect is accounted for. The relative reactivities determined in competitive reaction experiments increase as follows: &H&H3
< C,H, < C,HsOCH,
< C6H,F
Employment of aryl isocyanates in place of diphenyl the formation of N-arylbenzamide [ 171:
ketene resulted
in
136
ArN=C=O
+ PhH -
(2)
ArNHCOPh
The reductive carbonylation of nitroarenes to give aryl isocyanates has been explored with various catalysts of Group VIII transition metals 1191. This prompted us to attempt direct synthesis of N-arylbenzamides from nitroarenes, carbon monoxide and benzene with the present catalyst system. As expected, N-arylbenzamides were obtained in reasonable yields [20]. In these reactions, Rh4(CO)iz functions as the catalyst for both the reductive carbonylation of nitroarenes and the C-H bond activation of benzene: RC,H4N0,
+ PhH + 3C0 *
RC,H,NHCOPh
+ 2C0,
(3)
Addition of aromatic compounds to acetylenes The present new reaction catalyzed by a rhodium carbonyl cluster and involving C-H bond activation of arenes is not limited to the heterocumulenes. Benzene also reacts with diphenylacetylene to give triphenylethylene (la) [ 211, in which 2,3-diphenylindenone (2a) is produced as the byproduct. The formation of the indenone shows the catalytic cleavage of two C-H bonds of the reacted benzene. The reaction is remarkably affected by the partial pressure of carbon monoxide. In the absence of carbon monoxide, only trimerization of diphenylacetylene to hexaphenylbenzene occurs. As the pressure increases, the trimerization decreases rapidly, and the addition of benzene to the acetylene becomes predominant, giving la and 2a. The maximum yield of la is attained at about 25 - 30 atm:
PhCZPh
+
PhH
-
PhCli=CPh2
la
/ + ,cB;-6\
Ph C 6
Ph
(4)
2a
An unsymmetrical acetylene, l-phenylpropyne, gave (E)-methylstilbene and 2-phenyl-3-methylindenone. The formation of these products indicates that the addition of benzene proceeds regioselectively. Monosubstituted benzenes such as toluene, anisole and fluorobenzene also react with diphenylacetylene to yield isomeric mixtures of 1-aryl-1,2diphenylethylenes (1) and 2,3_diphenylindenones (2). The distribution of the isomers of 1 and the relative rates of conversion of the arenes are similar to those found in the reaction of diphenyl ketene. Five-membered heterocyclic compounds such as furan, thiophene, and N-methylpyrrole also undergo the reaction with diphenylacetylene [ 221. In a reaction of furan with diphenylacetylene, a mixture of (Z)- and (E)-1-(2furyl)-1,2diphenylethylene (78/22) was formed with a yield of 80%. The @&isomer is thermodynamically more stable than the @)-isomer, as indicated by the thermal isomerization experiment. We assume that the (E)isomer is first formed in the reaction. The reaction of a-substituted furans
137
such as 2-methylfuran, 2-(methoxycarbonyl)furan, furfural results in the formation of 2,5_disubstituted PhCXPh
2-acetylfuran, furans:
+
and
(5) C(Ph)=CHPh
R
Thus, addition of furan occurs at its a-position regioselectively; otherwise the positions are occupied. The reaction of 2,5_dimethylfuran with diphenylacetylene gives l-[ 3-(2,5_dimethyl)furyl]-1,2_diphenylethylene. Thiophene and N-methylpyrrole are also converted to 1-(2-thienyl)-1,2diphenylethylene and 1-[2-(iV-methyl)pyrrolyl]-1,2-diphenylethylene, respectively. The order of the reactivity of these heterocycles determined by a competitive experiment is as follows: furan > thiophene
> N-methylpyrrole
The rate of conversion than that of benzene.
> benzene
of furan
is about
two hundred,
times greater
Reaction of aromatic compounds with olefins In the above reactions, an aromatic C-H bond activated by the rhodium catalyst adds simply to an unsaturated C-C double or triple bond. Interestingly, the reaction of benzene with olefins affords phenyl-substituted olefins without giving addition products. For example, styrene was obtained directly from ethylene and benzene, accompanied by an almost equivalent molar amount of diethyl ketone [23 1. The ketone formation is ascribed to the hydrocarbonylation of ethylene by using the hydrogen generated in the reaction. Under the same reaction conditions (&H,: 30 atm; CO: 25 atm; reaction temperature: 220 “C), Rh,(CO)i* and Rh,(C0)i6 showed almost identical activities, giving about 100 moles of styrene per rhodium atom, but Rh,(CO)& exhibited very low activity and afforded 7 moles per rhodium atom. Rh(CO),(acac), which is readily converted into Rh,(C0)i6 under the reaction conditions, shows a relatively high activity: CH,=CH,
+ PhH
2
PhCH=CH2
+ Et&O
(6)
Many olefins having electron-withdrawing substituents such as COOH, COOR, COMe and CONRz can be used in place of ethylene, affording the corresponding phenyl-substituted olefins [ 241. The reaction of methyl acrylate and benzene gave methyl cinnamate together with the hydrocarbonylation product, dimethyl-4-oxoheptanedioate. The reaction of methyl vinyl ketone and benzene gave 4-phenyl-3-buten-a-one and 4-phenylbutan-2one, together with methyl ethyl ketone, the simple hydrogenation product of the starting olefin. The reactivities of disubstituted olefins are less than those of mono-substituted olefins. The results are summarized in Table 1. Introduction of various substituents on the benzene ring results in the formation of a mixture of three positional isomers [23]. The reaction of
138 TABLE
1
Phenylation
of mono-
and di-substituted
olefinsa
Olefin
Phenylated product (Product/Rh)
CH2=CH2COOMe CH&HCOMe CHz=CHCOOH CHz=CHCONMeZ MeCH=CHCOOMe CH2=C(Me)COOMe PhCH=CHCOOMe MeOCOCHXHCOOMe
PhCH=CHCOOMe (4700%) PhCH=CHCOMe/PhCH&H&OMe (4100%, 3/l) PhCH=CHCOOH/PhCH=CH2 (5690%, 2/l) PhCH=CHCONMez (4900%) Ph(Me)C=CHCOOMe (1690%) PhCH=C(Me)COOMe (250%) PhzC=CHCOOMe (250%) PhC( COOMe)=CHCOOMe/PhCH( COOMe)CH&OOMe (1250%, 7/l) PhC=CHCOOCO (1200%) PhC=CHCON(Ph)CO (1150%)
CH=CHCOOCO CH=CHCON(Ph)CO
aReaction of monosubstituted olefins were carried out using olefin (20 mmol), benzene (50 ml), CO (30 atm) and Rhd(CO)rz (0.025 mmol) at 220 “C! for 7 h; the reactions of disubstituted olefins were carried out using olefin (10 mmol), benzene (50 ml), CO (30 atm) and Rh4(CO)rz (0.05 mmol) at 240 “C for 7 h.
anisole with ethylene gave a mixture of ortho/metu/para-vinylanisoles in an isomer ratio of 67/23/10. Similarly, mixtures of vinylfluorobenzenes (78/17/5) and vinyltoluenes (14/57/29) were obtained from the reactions of fluorobenzene and toluene, respectively. The reaction of monosubstituted benzenes with methyl acrylate was examined in more detail, and the results are summarized in Table 2 [25]. Monosubstituted benzenes having alkyl, dimethylamino, methoxycarbonyl, alkoxy, cyano, and fluoro groups gave the corresponding cinnamates. Relative rates and the partial rate factors are also given in Table 3; these were obtained from results of competitive reaction experiments by assigning the value of 1.0 to benzene. Effects of substituents are similar to those observed in the reactions of arenes with diphenyl ketene, isocyanates and acetylenes. As shown in Tables 2 and 3, the differences in the reaction rates of the various compounds are very small in comparison with those shown in ordinary electrophilic aromatic substitution reactions. However, the results clearly demonstrate that the substituents with strong u-inductive effects favor the reaction. In particular, the partial rate factors show the higher reactivity of the ortho position of the RO- and F-groups than the ortho position of the other groups. This tendency is comparable with the fact that anisole and fluorobenzene [26] undergo facile ortho metallation upon reaction with n-butyllithium. Similar tendencies in the substituent effects were also observed in the H-D exchange reaction with strong base catalyst [ 271. The pattern may be ascribed to the enhancement of acidity of the ortho C-H bond by the inductive effect of the substituent, although the anchoring effect might also be important.
139 TABLE 2 Reaction of arenes with methyl acrylatea R in PhR
Me NMez COOMe CHzOAc H CN OAc OMe OPh F
Yield
Isomer ratio
(WRh)
ortho
meta
para
1600 600 1700 3300 4700 700 2600 3000 2500 2200
16 17 21 33
56 54 49 44 -
28 29 30 23 -
47 59 62 75 71
35 28 26 17 21
18 13 12 8 8
aOperating conditions: methyl acrylate, 0.025 mmol; 220 “C; 6 h.
10 mmol; arene, 50 ml; CO, 30 atm; RhJ(CO)rz,
TABLE 3 Relative rates and the partial rate factors of the reactions of arenes with methyl acrylate Substituent
Me NMez COOMe CHzOAc H CN OAc OMe OPh F
Relative rate
0.80 0.80 0.86 0.95 1.00 1.24 1.75 2.35 3.86 2.21
Partial rate factor
f ortho
f meta
fm
0.3 0.6 0.8 1.1 1.0 1.8 3.5 4.4 9.2 4.7
1.4 1.3 1.2 1.2 1.0 1.3 1.2 1.6 1.6 1.4
1.4 1.1 1.3 1.3 1.0 1.3 1.3 2.1 1.6 1.1
It is known that olefin-arene coupling of this type is catalyzed by Pd(I1) salts in the presence of oxygen [28]. Contrary to the electrophilic nature of the high-valent palladium catalyst, the present catalysis by the low-valent rhodium carbonyls exhibits a protophilic nature. Interestingly, when vinyl acetate was added as a hydrogen scavenger into the reaction system of substituted benzene and methyl a&ate, it was found not only to suppress the formation of the hydrocarbonylation product (dimethyL4_oxoheptanedioate), but also to result in a considerable increase in the yields of the desired aryl olefin, especially in the reactions of polysubstituted arenes [29]. It should be noted that the isomer distribution
140
is also influenced by the presence of vinyl acetate to some extent. For example, the yield of methyl-2-(o//3-naphthyl)acrylate in the reaction of naphthalene with methyl acrylate increases by a factor of about 3 in the presence of vinyl acetate, and the isomer distribution (a/P) changes from 28172 to 48152. Furan also reacts with ethylene and methyl acrylate to give 2-vinylfuran and methyl 2-(2-furyl)acrylate, respectively [30] :
CH$HR
(7)
+ CH=CHR
A competitive reaction experiment showed that the reactivity of furan is higher than that of benzene by a factor of about 75. Consistent with this fact, the reaction of benzofuran with methyl acrylate gave methyl 2-[2(methoxycarbonyl)vinyl] benzofuran exclusively. When indole was treated with olefins in the present catalytic system, N-vinylindole was obtained in a reasonable yield together with a trace amount of a-vinylindole, suggesting the N-H bond was cleaved preferentially [31]. For example, the reaction of indole and methyl acrylate in p-xylene afforded methyl 1-[2-(methoxycarbonyl)vinyl]indole (63%): CH+HCCXPle +
(8) I
H
I
CH=CHCOOMe
Carbazole, phthalimide and other secondary amines such as diethylamine, methylphenylamine and diphenylamine are also converted to Nvinylated products. On the mechanism of phenylation As described in the previous section, the present rhodium-catalyzed aromatic C-H bond activation exhibits a protophilic nature. Enhancement of acidity of the arene ring C-H bonds through complexation is well known in the case of arenetricarbonylchromium [32]. The q2-coordination of arene on a mononuclear metal complex prior to the activation of an aromatic C-H bond was suggested [33], and the chemical evidence for the intermediacy appeared recently [ 141. In the present reaction, precoordination of the arene on the rhodium catalyst (probably on a single metal center within a rhodium cluster) may also be important. Oxidative addition of the arene C-H bonds may take place favorably at the most acidic C-H bond in a second stage and change the arene ligand to o-aryl and hydride ligands. Insertion of an unsaturated molecule into the aryl-rhodium bond, and the subsequent P-hydrogen elimination or reductive coupling with the hydride gives the products, as shown in Scheme 1:
141
[ Rh] + PhH (i)
Ph--[Rh]-H
Ph-[ Rh]-H -
+ RCSR
-
PhC(R)=C(R)-[Rh]-H (ii)
Ph--[ Rh]-H PhCH,CHR-[
PhC(R)=C(R)-[Rh]-H
+ CH,=CHR Rh]-H
-
-
PhC( R)=CHR
+ [ Rh]
PhCH,CHR-[
Rh]-H
PhCH=CHR
+ H-[ Rh]-H
Scheme 1. Phenylation mechanism
In the reactions of heterocumulenes and acetylenes with arenes, an alternative pathway, insertion of the unsaturated compounds into the hydride-rhodium bond and the subsequent reductive coupling with the aryl group, is also plausible. However the exclusive formation of aryl-substituted olefin in the reactions of ethylene and methyl acrylate with arene could not be explained by this mechanism.
Cross-hydrocarbonylation
of Acetylenes
and Olefins
Cross-hydrocarbonylation of acetylene and olefin with carbon monoxide in alcohol In the previous section we have described new reactions accompanying the catalytic C-H activation of arenes by rhodium carbonyl clusters. In these reactions, carbon monoxide has appeared to be essential for realizing catalytic cycles, although it cannot be incorporated in the main product. In an attempt to discover new rhodium carbonyl-catalyzed reactions in which carbon monoxide participates as a constituent of the product, we found the formation of a five-membered unsaturated lactone, 5-ethyl-3,4diphenyl-2(5H)-furanone (3) in the reaction in which a mixture of diphenylacetylene, ethylene, and carbon monoxoide in ethanol was heated at 200 “C in the presence of Rh4(C0)12 [34]. The formation of 3 requires one hydrogen molecule, which is supplied from ethanol. The ethanol can be replaced by other hydrogen-donating solvents such as isopropanol and an acetone/ water mixture. There are several precedents for the catalytic formation of furanones by the carbonylation of acetylenes: bifurandione from acetylene and carbon monoxide [ 351, and 5-hydroxyfuranone from phenylacetylene, carbon monoxide and methyl iodide [36] by cobalt carbonyl. However the present reaction is the first example of a carbonylation reaction in which both olefin and acetylene are incorporated in a product molecule:
PhECPh
+
%H4
+
x0
+
(H2)
___t ,/‘Et
(9)
142
Several other rhodium compounds can be employed as catalysts. The results are listed in Table 4. At a higher temperature (220 “C), the catalytic activities of the rhodium compounds employed are rather close to that of (150 “C), rhodium carbonyl RhQ(CC),z. However, at a lower temperature clusters are superior to the others. TABLE 4 Activity of rhodium compounds in the carbonylation Catalyst
Rh4COh Rhs(CC)rs RhCl(CO)(PPhs)2 RhCl(PPh& RhCls*3H,O Rh& 5% Rh/C
of diphenylacetylene
Reaction temp.
Yield of 4
(“C)
(%)
220 150 150 220 150 220 220 220 150 220
60 67 59 46 30 27 41 5s 22 49
*Operating conditions: diphenylacetylene, 50 ml; CO, 30 atm; ethylene, 20 atm; 6h.
10 mmol; catalyst,
0.1 mg atom;
ethanol,
When the reaction in ethanol was carried out at higher temperatures, the dehydrogenation of ethanol occurred to give more than the necessary amount of hydrogen for formation of 3, and considerable amounts of diethyl ketone and acetal were produced. In a search for a method to avoid such byproduct formation, the influence of the reaction temperature was examined; the results are summarized in Table 5. They show that a temperature of about 150 “C is suitable for keeping reasonable yields of 3 and for minimizing the byproduct. At lower temperatures the yield of 3 decreases rapidly and new compounds, 5-ethoxy-3,4-diphenyL2(5I-&furanone (4) and 3ethoxycarbonyl-2-phenyl-indanone (5) begin to appear, although the yields are low. For the formation of these compounds, ethylene does not participate in the reaction and the ethanol is no longer a simple hydrogen donor, but is a constituent of the product molecule. The successful synthesis of alkoxyfuranones was attained by the use of alkali metal salts as cocatalysts (uide infm):
143 TABLE 5 Influence of the reaction temperature in the carbonylation presence of ethylene and ethanola 6
Temp. (“C) 220 180 150 125 100
(%) 60 73 67 31 10
-
-
3 11 10
4 13
‘Operating conditions: diphenylacetylene, atm; ethylene, 20 atm; ethanol, 50 ml; 6 h.
of diphenylacetylene
in the
Acetal (mmol)
Et&O (mmol)
49.4 25.5 4.6 0.5 -
56.6 41.7 14.4 0.2 -
10 mmol; Rh4(C0)12,
0.025
mmol; CO, 30
Diphenylacetylene can be replaced with other internal acetylenes. Thus 5ethyl-3,4dimethyl-2(5H)-furanone (47%) was obtained from 2-butyne. From 1-phenylpropyne, the regioisomers, 5-ethyl-4-methyl-3-phenyl-2( 5H)furanone (3a) and 5ethyl-3-methyl-4-phenyl-2(5H)-furanone (3b) were obtained in 48 and 4% yields, respectively:
PhCZlle
+ QH4
+ x0
+
(Hz)
-::qt
+
3a -
:$.o~
(11)
3b -
Attempts to obtain furanones from monosubstituted acetylenes such as phenylacetylene and l-hexyne were not successful. Monosubstituted olefins such as propylene and methyl acrylate can be used instead of ethylene, but the corresponding furanones are obtained in low yields. From propylene and diphenylacetylene, a mixture of 3,4diphenyl-5-(1-propyl)-2(5H)-furanone and 3,4diphenyl-5-(2-propyl)-2(5H)furanone was obtained with a ratio 1:l and a 5% total yield. From methyl acrylate and diphenylacetylene, only 5-[2-(methoxycarbonyl)-ethyl]-3,4diphenyL2(5H)-furanone was obtained in a 17% yield, accompanying a considerable amount of dimethyl 4-oxoheptandioate. Without diphenylacetylene, this system successfully gave dimethyl4-oxoheptandioate when isopropanol was used as the solvent [ 371: BCH,=CHCOOR
+ CO + i&OH
__f
0C(CH,CH,COOR)2
+ Me&O
(12)
Attempts to obtain furanones from the other olefins such as vinyl acetate, ethyl vinyl ether and styrene were not successful. It has been reported that the reaction of acylcobalt complexes, R’COCo(CO),, with acetylene gave ~3-lactonylcobalt complexes according
144
to the stepwise insertions of the acetylene and carbon monoxide into the acyl-cobalt bond and subsequent intramolecular ring closure [38]. Similar reactions of ($-CsHs)M(CO),(COCHs) (M = Mo,W) and acetylene and subsequent treatment of the resulting vinyl ketone complex with carbon monoxide have been reported to give an ~3-allylic lactone complex [ 391. Chini et al. [40] reported that the reaction of Rh,(C0)i2 with ethylene in aqueous acetone gives [Rh,(CO),,(COEt)]-, which may be derived from stepwise insertion of ethylene and carbon monoxide into a hydride complex formed by the water-gas shift reaction. On the basis of these facts, we suggest a possible mechanism outlined in Scheme 2 for the formation of the furanone from acetylene and ethylene:
R1R2C=0 H
Et H-(Rh
C2H4/C0
EtCO-[Rh]
[Rhl
x I
+0
C
'Et
Scheme 2
In the initial step, ethylene and carbon monoxide may insert. stepwise into the Rh-H bond to form an acyl complex similar to that isolated by Chini. The subsequent stepwise insertions of acetylene and carbon monoxide, followed by intramolecular cyclization, would afford an q3-allylic lactone complex which gives lactone and regenerates rhodium hydride species by reduction with alcohol. Carbonylation of acetylenes in alcohols It is known that the carbonylation of acetylenes in alcohol by soluble transition metal compounds gives mono- and di-carboxylates such as acrylate, fumarates, maleates and succinates [41]. However no report has
145
appeared on the formation of Salkoxy-2(5H)-furanones. As already described, 5ethoxy-3,4diphenyL2(5H)-furanone (4) was isolated in a low yield when the reaction of diphenylacetylene, ethylene and carbon monoxide in ethanol was carried out at a lower temperature (< 120 “C) than is appropriate to obtain 5ethyl-3,4diphenyL2(5H)-furanone (3). From an analogy of estimated intermediacy of an acylrhodium for the latter product, an ethoxycarbonylrhodium was thought to be the intermediate. It is known that an ethoxycarbonylrhodium cluster anion [Rh,(CO),,(CO,Et)]is formed by the reaction of Rh,(C0)12 or Rh,(CO)Jl, with ethanol in the presence of Na,COa [42]. The reaction of diphenylacetylene with carbon monoxide in alcohol with the exclusion of ethylene was carried out in the presence of various basic alkali metal salts as the cocatalysts, and a marked improvement in the yield of the ethoxyfuranone was attained [43]. The reactions proceed even at 50 “C, but the optimum condition is at about 125 “C! under 40 atm pressure of carbon monoxide with Rh,(CO),,/NaOAc. Rhodium compounds such as Rh,Os and RhCls may also be employed instead of Rh,( CO) i2. The present new reaction can be applied to various combinations of internal acetylenes and alcohols, and the results are summarized in Table 6.
TABLE 6 Synthesis of 5-alkoxy-2( 5H)furanoness Acetylene
diphenylacetylene
2-butyne 3-hexyne 1-phenylpropyne
Alcohol
methanol ethanol n-propanol i-propanol n-octanol ethanol
Temp. (“C)
Yield
100 125 75 125 125 125 125 125 125
86 87 76 65 60 31 63 60 93b
‘Operating conditions: diphenylacetylene, 10 mmol (other acetylenes, Rh,+(CO)r2, 0.025 mmol; NaOAc, 1.0 mmol; CO, 50 atm; 6 h. bA 84116 mixture of 5-ethoxy-4-methyl-3-phenyl-2(5H)-furanone and methyl-4-phenyl-2( 5H)-furanone.
(%)
20
mmol);
5-ethoxy-3-
The mechanism of the formation of 4 seems to be similar to that of 3. The formation of 3-ethoxycarbonyl-2-phenyl-indanone (5) may be ascribed to the cis- trans isomerization of an intermediate /3alkoxycarbonylvinylrhodium complex and the subsequent ortho-metallation to make a rhodiacycle, followed by insertion of carbon monoxide.
146
Cross-hydrocarbonylation of acetylenes and ethylene with carbon monoxide and molecular hydrogen As already described, in the rhodium-catalyzed cross-hydrocarbonylation of acetylene and olefin to give 5-alkyL2(5H)-furanone, the required hydrogen was supplied from alcohol or water used as a solvent. When molecular hydrogen was used, the reaction resulted in the formation of a$unsaturated ketone accompanied by a small amount of the lactone [44]. For example, the reaction of diphenylacetylene, ethylene, carbon monoxide and hydrogen in acetone at 150 “C afforded 1,2-diphenyl-1-penten-3-one in 75% yield on the basis of the conversion of the acetylene, together with propionaldehyde. Formation of propionaldehyde, which is an oxo-reaction product of ethylene, is unavoidable in this reaction. It is necessary to keep the hydrogen partial pressure as low as possible to minimize the byproduct formation : PhC=CPh + C2H, + CO + H, -
PhCH=C(Ph)COEt
(13)
Although other rhodium compounds such as Rh,Os, RhC1s*3H20, and Rh/C (5%) can be used as the catalyst precursors, Rh,(CO),, and Rh,(CO)16 are most active. In this reaction, various kinds of substituted acetylenes, including mono-substituted and functionally substituted ones, can be used, which is in marked contrast to the reaction system in the absence of molecular hydrogen. The results are summarized in Table 7.
TABLE
7
Synthesis of @unsaturated Acetylene
ketonesa (mmol)
Conversion
Yield
Product
(%)
(%) PhC=CPh PhCFCMe
10 20
82 99
MeCZMe PhC=CH n-BuCXH t-BuC=CH MeOCH2CX!CHzOMe MeCFCCOOMe
20 20 20 20 20 lob
100 93 100 100 75 80
PhGCCOOMe
lob
83
‘Operating conditions: Rh4(CO) 12, 0.05 30 atm; Hz, 5 atm; 150 ‘C; 6 h. bRh4(CO)lz,
0.025
mmol.
PhCH=C(Ph)COEt (E)-PhCH=C(Me)COEt (E)-MeCH=C(Ph)COEt (E)-MeCH=C(Me)COEt (E)-PhCH=CHCOEt (E)-n-BuCH=CHCOEt (E)-t-BuCH=CHCOEt MeOCH&H=C( CH20Me)COEt MeOOCCH=C(Me)COEt MeCH=C( COOMe)COEt PhCH=C( COOMe)COEt mmol;
acetone,
70 ml; ethylene,
75 49 6 25 48 42 61 76 76 4 40
25 atm; CO,
147
The reaction of terminal bon monoxide and molecular trans-o&unsaturated ketone: RCZH
+ C$H, + CO + H, -
acetylene hydrogen
and ethylene in the presence of cargave exclusively the corresponding
RCH=CHCOEt
(14)
It is interesting to note that terminal acetylenes can be employed in this reaction in spite of the failure of their use in the synthesis of 5ethyl2(5.H)-furanones. The success of the method may be ascribed to a suppressive action of molecular hydrogen in oxidative addition and polymerization of the acetylene. It has been reported that reaction of a rhodium hydride complex with terminal acetylenes gives an acetylide complex via oxidative addition of the acetylene and the subsequent elimination of hydrogen, which is converted again to the original hydride complex by treatment with hydrogen under forcing conditions [ 4 51. In the reaction of unsymmetrically substituted acetylenes, the distribution of two regioisomers is markedly influenced by the substituents. For example, (E)-2-methyl-1-phenyl-1-penten-3-one and (E)4-phenyl4-hexen3-one were obtained in the ratio of 89/11 from 1-phenylpropyne. And (E)/(Z)-3-methoxycar1-methoxycarbonyl-2-methyl-l-penten-3-one and bonyl-2-hexen-4-one were obtained in the ratio of 95/5 from methyl 3methylpropiolate. From methyl 3_phenylpropiolate, only 2-methoxycarbonyl-1-phenyl-l-penten-3-one was formed without giving the other regioisomer. These results show that the EtCO group tends to bond to a sterically less hindered acetylenic carbon atom having a smaller substituent, suggesting that a steric effect is predominant over an electronic effect in this reaction. In the formation of o$-unsaturated ketones, two paths are assumed in the stage of acetylene insertion: (a) insertion of acetylene into the Rh-COEt bond, which is the common intermediate proposed for furanone formation, and (b) insertion of acetylene into the Rh-H bond (Scheme 3). Considering the regioselectivity, in path (a) insertion of acetylene should occur in such a way that the EtCO group and the rhodium metal connect to the less and the more hindered acetylenic carbon atoms, respectively, giving a P-acylvinylrhodium complex. Similar stoichiometric insertion reactions, in which the metal atom adds to the more sterically hindered olefinic or acetylenic carbon atom, have been reported. The reaction of aryl palladium complex with olefins [46], the reaction of (CO)sMnR (R = Me, MeCO and Ph) with acetylenes [47], and the reactions of (acac)Ni(PPh,)Me with acetylenes [48] are the examples. In path (b), insertion of acetylene should occur so as to connect the rhodium metal with a sterically less hindered acetylenic carbon atom, giving a vinylrhodium intermediate. There are some examples of the complexes derived from a metal hydride complex and acetylenes [49]. However, it is not enough to compare the regioselectivity of the reaction because the number of acetylenes employed is limited. Although it is difficult to reach a definite conclusion, we tentatively assume
148
Path (a) H-[ Rh] + CH,=CH, + CO -
Et-[Rh] EtCO-[
-
Et-[ Rh]
EtCO-[
Rh] + RC=CR’ -
Rh] EtCOC( R)=C( R’)-[ Rh]
EtCOC( R)=C( R’)-[ Rh] + Hz -
EtCOC(R)=CHR’
+ H-[Rh]
Path (b) H-[Rh]
+ RC!=CR’ -
RCH=C(R’)-[Rh]
+ CO -
RCH=C(R’)CO-_[
RCH=C(R’)CO-[
Rhl + CH,=CH,
RCH=C(R’)COCH,CH,-[ RCH=C(R’)-[
RCH=C( R’)-[ Rh]
-
Rh] + Hz -
Rh] + EtCO-[
Rh]
RCH=C(R’)COCH,CH2-[ RCH=C(R’)COEt
+ H-[ Rh]
Rh] RCH=C(R’)COEt
RCH=C(R’)CO-[
Rh]
+ [ Rh] 2
Rh] + Et-[ Rh]
Scheme 3.
that path (a) is operative in the present system because the regioselectivity in the formation of the furanones and the ketones from 1-phenylpropyne can be explained by the common intermediacy of an acylrhodium complex. In the presence of molecular hydrogen, hydrogenation of the P-acylvinylrhodium complex, which is produced by insertion of acetylene into the EtCO-Rh bond, could occur to give o$-unsaturated ketone prior to the insertion of carbon monoxide, which leads to 5-ethyl-2( 5H)-furanone.
References 1 E. L. Muetterties, Science, 196 (1977) 839. 2 G. Balavoin, T. Dang, C. Eskenazi and H. B. Kagan, J. Mol. Catal., 7 (1980) 531; W. Reimann, W. Abboud, J. M. Basset, R. Mutin, G. L. Rempel and A. K. Smith, J. Mol. Catal., 9 (1980) 349; P. M. Lausarot, G. A. Vaglio and M. VaIle, J. Organometal. Chem., 240 (1982) 441. 3 P. M. Lausarot, G. A. Vaglio and M. Vaile, J, Organometal. Chem., 204 (1981) 249. 4 Y. Takegami, Y. Watanabe and H. Masada, Bull. Chem. Sot. Jpn, 40 (1967) 1459; N. S. Imyanitov and D. M. Rudkowskii, Zh. Prikl. Khim., 39 (1972) 2020, 2029; B. Heil and L. Mark& Chem. Ber., 101 (1968) 2209; ibid., 102 (1969) 2238; G. Consiglio, D. A. von Bezard, F. Morandini and P. Pino, Helv. Chim. Acta, 61 (1978) 165; D. A. von Bezard, G. Consiglio, F. Morandini and P. Pino, J. Mol. Catal., 7 (1980) 431. 5 T. Kitamura, N. Sakamoto and T. Joh, Chem. Lett., (1973) 379.
149 6 R. M. Lame, Ann. N.Y. Acad. Sci., 33 (1980) 124; W. J. Thompson and R. M. Laine, ACS Symp. Ser., 152 (1981) 133; K. Kaneda, M. Yasumura, T. Imanaka and S. Teranishi, J. Chem. Sot., Chem. Commun., (1982) 935. 7 A. F. M. Iqbai, Tetrahedron Lett., (1971) 3385; J. Org. Ckem., 37 (1972) 2791; K. Cann, T. Cole, W. Slegeir and R. Pettit, J. Am. Chem. SOC., 100 (1978) 3969; T. Cole, R. Ramage, K. Cann, R. Pettit, J. Am. Chem. Sot., 102 (1980) 6182; R. C. Ryan, G. M. Wilemon, M. P. DaIsanto and C. U. Pittman, Jr., J. Mol. Catal., 5 (1979) 319; K. Kaneda, M. Hiraki, T. Imanaka and S. Teranishi, J. Mol. Catul., 12 (1981)* 385. 8 Y. Iwashita and M. Sakuraba, Tetrahedron Lett., (1971) 2409. 9 H. Kang, C. H. Mauldin, T. Cole, W. Slegeir, K. Cann and R. Pettit, J. Am. Chem. Sot., 99 (1977) 8323; R. M. Laine, J. Am. Chem. Sot., 100 (1978) 6451. 10 G. D. Mercer, J. S. Shu, T. B. Rauchfuss and D. M. Roundhill, J Am. Chem. Sot., 97 (1975) 1967; G. D. Mercer, W. B. Beaulieu and D. M. Roundhill, J. Am. Chem. Sot., 99 (1977) 6551; M. K. Dickson, B. P. Sudha and D. M. Roundhill, J. Organometal. Chem., 190 (1980) C43; D. M. Roundhill, M. K. Dickson, N. S. Dixit and B. P. SudhaDixit, J. Am. Chem. Sot., 102 (1980) 5538; C. S. Chin, M. S. Sennett and L. Vaska, J. Mol. Catal., 4 (1978) 375. 11 R. M. Lame, D. W. Thomas, L. W. Cary and S. E. Buttrill, J. Am. Chem. Sot., 100 (1978) 6527. 12 J. L. Vidai and W. E. Walker, Znorg. Chem., 19 (1980) 896. 13 G. W. Par-shall, Act. Chem. Res., 3 (1970) 139; ibid., 8 (1975) 113. 14 W. D. Jones and F. J. Feher, J. Am. Chem. Sot., 104 (1982) 4240 and references’ cited therein. 15 P. Hong, K. Sonogashira and N. Hagihara, Tetrahedron Lett., (1971) 1107. 16 P. Hong, K. Sonogashira and N. Hagihara, Nippon Kagaku Zasski., 89 (1968) 74. 17 P. Hong, H. Yamazaki, K. Sonogashira and N. Hagihara, Chem. Lett., (1978) 535. 18 P. Chini and S. Martinengo, Znorg. Chim. Acta, 3 (1969) 21; R. Whyman, J. Chem. Sot., Dalton Trans., (1972) 1375; L. A. Hanlan and G. A. Ozine, J. Am. Chem. Sot., 96 (1974) 6324. 19 W. B. Hardy and R. P. Bennet, Tetrahedron Lett., (1967) 961; K. Unverferth, R. Hoentsch and K. Schwetlick, J. Prakt. Chem., 321 (1979) 86 and references cited therein. 20 T. Mise, P. Hong and H. Yamazaki, Chem. Lett., (1980) 439. 21 P. Hong, B-R. Cho and H. Yamazaki, Chem. Lett., (1979) 339. 22 P. Hong, B-R. Cho and H. Yamazaki, Chem. Lett., (1980) 507. 23 P. Hong and H. Yamazaki, Chem. Lett., (1979) 1335. 24 P. Hong and H. Yamazaki, unpublished work. 25 T. Mise, P. Hong and H. Yamazaki, unpublished work. 26 A. R. Lepley, W. A. Khan, A. B. Guinamini and A. G. Guinamini, J. Org. Chem., 31 (1966) 2047. 27 A. I. Schtenshteiif, in V. Gold (ed.), Advances in Physical Organic Chemistry, Vol. 1, Academic Press, London, p. 155. 28 For a review article, see I. Moritani and Y. Fujiwara, Synthesis, (1973) 524. 29 P. Hong and H. Yamazaki, unpublished work. 30 P. Hong, B-R. Cho and H. Yamazaki, unpublished work. 31 P. Hong, B-R. Cho and H. Yamazaki, unpublished work. 32 M. F. Semmelhack, J. Bisaha and M. Czarny, J. Am. Chem. Sot., 101 (1979) 768; R. J. Card and W. S. Trahanovsky, J. Org. Chem., 45 (1980) 2560. 33 G. W. Parshail, Homogeneous Catalysis, Wiley, New York, 1980, Ch. 7. 34 P. Hong, T. Mise and H. Yamazaki, Chem. Lett., (1981) 989; idem, Nippon Kagaku Kaishi, (1982) 242. 35 J. S. Sauer, R. D. Cramer, V. A. Engelhardt, T. A. Ford, H. E. Holmquist and B. W. Houk, J. Am. Chem. Sot., 81 (1959) 3677; G. A. Alvanesi and M. Tovaglieri, Chim. Znd. (Milan), 41 (1959) 189.
150 36 H. Alper, J. K. Kurrie and H. des Abbayes, J. Chem. Sot., Chem. Commun., (1978) 311. 37 P. Hong, T. Mise and H. Yamazaki, Chem. Lett., (1982) 361. 38 R. F. Heck,J. Am. Chem. Sot., 86 (1964) 2819. J. 39 M. Green, J. Z. Nyathi, C. Scott, F. G. A. Stone, A. J. Welch and P. Woodward, Chem. Sot., Dalton Trans., (1978) 1067. and G. Carlaschelli, J. Chem. Sot., Chem. Commun., (1972) 40 P. Chini, S. Martinengo 709. 41 P. Pino and G. Braca, in I. Wender and P. Pino (eds.), Organic Syntheses via Metal Carbonyls, Wiley, New York, 1977 Vol. 2, p. 419. 42 P. Chini, S. Martinengo and G. Giordano, Gazz. Chim. Ital., 102’(1972) 330. 43 T. Mise, P. Hong and H. Yamazaki, Chem. Let&, (1981) 993; idem, J. Org. Chem., 48 (1983) 238. 44 T. Mise, P. Hong and H. Yamazaki, Chem. Lett., (1982) 401. 45 C. K. Brown, D. Georgiou and G. Wilkinson, J. Chem. Sot. (A), (1971) 3120. 46 R. F. Heck, J. Am. Chem. Sot., 93 (1971) 6896. 47 B. L. Booth and R. G. Hargreaves, J. Chem. Sot. (A), (1970) 308. 48 J. M. Huggins and R. G. Bergman, J. Am. Chem. Sot., 103 (1981) 3002. D. W. Hart and J. L. Holden, J. Am. Chem. Sot., 94 (1972) 9269; 49 J. Schwartz, H. C. Clark, C. R. Jablonski and C. S. Wong, Znorg. Chem., 14 (1975) 1332; B. Longato and S. Bresadola, Znorg. Chem., 21 (1982) 168.
ORGANIC REACTIONS CATALYSTS
HIROSHI YAMAZAKI
133 - 150
WITH RHODIUM
and PANGBU
133
CARBONYL
CLUSTER
HONG
The Institute of Physical and Chemical Research,
Wako-shi, Saitama 351 (Japan)
Summary Rhodium carbonyl clusters have been found to catalyze two types of reactions: (1) Carbon-hydrogen bond activation of aromatic compounds and (2) Cross-hydrocarbonylation of acetylenes and olefins. (1) Under pressure of carbon monoxide (20 to 30 atm) and at higher temperatures (180 to 220 “C), rhodium carbonyl clusters Rh4(C0)i2 and Rh,(CO)i6 catalyze the addition of a C-H bond of benzene to unsaturated compounds such as diphenylketene, isocyanates and acetylenes to give phenylated compounds such as diphenylacetophenone, benzamides and phenylated olefins, respectively. Five-membered heterocycles such as furan, thiophene, and N-methylpyrrole also react similarly with diphenylacetylene to give 2-furyl-, 24hienyl- and 2-(N-methylpyrrolyl)-olefins, respectively. Among these, the reactivity of furan is the highest and is about 200 times that of benzene. In the reactions of arene with olefins, aryl-substituted olefins are obtained instead of simple addition products. For example, the reaction of benzene with ethylene gives styrene directly. The hydrogen generated in these reactions is used for the hydrocarbonylation or the hydrogenation of the starting olefins, yielding ketones or saturated compounds. The reaction of furan with olefins gives fury1 olefins similarly. In the reactions of secondary amines such as indole, carbazole, and diphenylamine, the N-H bond participates in preference to the C-H bond, giving N-vinyl compounds. Regioselectivities in the reactions of substituted benzenes suggest the protophilic nature in the stage of C-H bond activation. The following mechanism is proposed: (1) initial oxidative addition of an aromatic C-H bond to the rhodium catalyst to give aryl- and hydride-rhodium species, (2) insertion of unsaturated compounds into the aryl-rhodium bond, and (3) reductive elimination with the hydride or P-hydrogen elimination affording the corresponding aryl-substituted compounds. (2) The cross-hydrocarbonylations of acetylenes and ethylene with carbon monoxide are found to give 5-ethyl-2(5H)-furanones when hydrogendonating solvents are used, and to give o, P-unsaturated ethyl ketones when molecular hydrogen is introduced into the reaction system. Since it is consistent with the regioselectivity in the reactions of unsymmetrical acetylenes, a fl-acylvinyl rhodium intermediate is proposed as a common key @ Elsevier Sequoia/Printed
in The Netherlands
134
intermediate for these two products. The convenient syntheses of 5-alkoxy2( 5H)-furanones from acetylenes, carbon monoxide and alcohols are attained by adding a basic alkali metal salt such as NaOAc or Na,CO, as a cocatalyst.
Introduction Cluster complexes have drawn much attention recently because they can be expected to provide highly selective catalysts for reactions which are thought to be inherent in heterogeneous catalysts, such as reduction of carbon monoxide and nitrogen as well as alkane isomerization [ 11; further, they can be expected to provide new examples of catalysis which are unknown with either mononuclear homogeneous catalysts or surfaces. Many examples of homogeneous reactions using rhodium carbonyl clusters as catalyst precursors are known. The hydrogenations of olefins [2] and acetylenes [ 31 have been found to proceed under low pressures of hydrogen. The hydroformylations of olefins have been examined in comparison with those occuring in the presence of the cobalt catalyst [4]. The selective reduction of the C-C double bond of o, P-unsaturated carbonyl compounds [5], reduction of aldehydes and ketones [6], reduction of nitrobenzenes [7], cooligomerization of ethylene and carbon monoxide [ 81, and hydroformylation of olefins [9] have been carried out under the water-gas shift conditions. Oxidation of carbon monoxide and cyclohexanone by oxygen [lo], and the H-D exchange reaction of tertiary alkylamines with D,O [ 111 have been also conducted under carbon monoxide pressure. It is well known that the rhodium compounds behave as active catalysts for the specific reduction of carbon monoxide to give ethylene glycol at an elevated temperature (200 - 240 “C) under a high pressure of carbon monoxide and hydrogen (1000 - 3000 atm) in the presence of bases; under these conditions, the rhodium compounds exist as high-nuclearity rhodium carbonyl cluster species, as indicated by high-pressure infrared spectroscopy [ 121. We have discovered new reactions by using rhodium carbonyl clusters as catalyst precursors at higher temperatures (180 - 220 “C) and under pressures of carbon monoxide (20 to 30 atm). This paper describes these reactions, which are divided into two topics: C-H activation of aromatic compounds and cross-hydrocarbonylation of acetylenes and olefins.
Catalytic
Activation
of Carbon-Hydrogen
Bonds of Aromatic
Compounds
Recently the activation of C-H bonds by soluble transition metal complexes has received increasing attention because of the importance of carrying out selective chemical transformation of unreactive compounds such as nonfunctionalized arenes and saturated hydrocarbons, in particular, under mild conditions [13, 141. There are numerous examples of
135
stoichiometric intra- and inter-molecular metallation on single and multimetal centers. However, little is known about the application of these reactions to catalytic synthesis of organic compounds. Addition of arenes to heterocumulenes Previously we showed that cobalt carbonyls catalyze the deoxygenation of diphenyl ketene to give 1-(diphenylmethylene)-2-(diphenylacetoxy)-3phenylindene under carbon monoxide pressure [15] ; they catalyze the decarbonylation to give tetraphenylethylene under a nitrogen atmosphere
WI. In the course of the comparative study of the catalytic behavior of rhodium carbonyls, we found a novel addition reaction of a C-H bond of benzene used as the solvent to the C-C double bond of the ketene to give diphenylacetophenone [17]. The reaction was carried out under pressures of carbon monoxide at elevated temperatures in the presence of catalytic amounts of Rh4(C0)i2: Ph,C=C=O
+ PhH -
Ph,CHCOPh
(1)
In an attempt to obtain information about the catalytically active species and the role of carbon monoxide in the aromatic C-H bond activawe carried out a series of experiments with tion catalyzed by Rh,(C0)i2, various CO partial pressures under isothermal conditions (200 “C). The results indicated that the yield of the benzene adduct depends markedly on the CO partial pressure. For examples, under partial pressures of CO of 1, 5, 10, 20 and 30 atm, diphenylacetophenone was obtained in yields of 3,11, 47, 56 and 68%, respectively. Deposits of rhodium metal were observed when the reactions were carried ,out under lower pressures. RhJCO),, gave almost the same result as that of Rh,(C0)i2, but Rh,(CO)Jl, showed lower activity. It is of interest to note that rhodium carbonyl clusters are stable at higher temperatures even under high pressures of carbon monoxide [18]. We suppose that rhodium clusters may play an important role in this catalytic reaction. Monosubstituted benzenes such as toluene, anisole, and fluorobenzene also react with diphenyl ketene to give the corresponding aryl diphenylmethyl ketones. The ortho, metu, and para ratios in the products from anisole and fluorobenzene were found to be 55/31/14 and 60/27/13, respectively. It is remarkable that the sterically hindered ortho isomers were formed predominantly in these cases. On the contrary, the isomer ratio observed in the reaction of toluene was 6/62/32. The reactivities of meta and paru positions are found to be almost equal when the statistical effect is accounted for. The relative reactivities determined in competitive reaction experiments increase as follows: &H&H3
< C,H, < C,HsOCH,
< C6H,F
Employment of aryl isocyanates in place of diphenyl the formation of N-arylbenzamide [ 171:
ketene resulted
in
136
ArN=C=O
+ PhH -
(2)
ArNHCOPh
The reductive carbonylation of nitroarenes to give aryl isocyanates has been explored with various catalysts of Group VIII transition metals 1191. This prompted us to attempt direct synthesis of N-arylbenzamides from nitroarenes, carbon monoxide and benzene with the present catalyst system. As expected, N-arylbenzamides were obtained in reasonable yields [20]. In these reactions, Rh4(CO)iz functions as the catalyst for both the reductive carbonylation of nitroarenes and the C-H bond activation of benzene: RC,H4N0,
+ PhH + 3C0 *
RC,H,NHCOPh
+ 2C0,
(3)
Addition of aromatic compounds to acetylenes The present new reaction catalyzed by a rhodium carbonyl cluster and involving C-H bond activation of arenes is not limited to the heterocumulenes. Benzene also reacts with diphenylacetylene to give triphenylethylene (la) [ 211, in which 2,3-diphenylindenone (2a) is produced as the byproduct. The formation of the indenone shows the catalytic cleavage of two C-H bonds of the reacted benzene. The reaction is remarkably affected by the partial pressure of carbon monoxide. In the absence of carbon monoxide, only trimerization of diphenylacetylene to hexaphenylbenzene occurs. As the pressure increases, the trimerization decreases rapidly, and the addition of benzene to the acetylene becomes predominant, giving la and 2a. The maximum yield of la is attained at about 25 - 30 atm:
PhCZPh
+
PhH
-
PhCli=CPh2
la
/ + ,cB;-6\
Ph C 6
Ph
(4)
2a
An unsymmetrical acetylene, l-phenylpropyne, gave (E)-methylstilbene and 2-phenyl-3-methylindenone. The formation of these products indicates that the addition of benzene proceeds regioselectively. Monosubstituted benzenes such as toluene, anisole and fluorobenzene also react with diphenylacetylene to yield isomeric mixtures of 1-aryl-1,2diphenylethylenes (1) and 2,3_diphenylindenones (2). The distribution of the isomers of 1 and the relative rates of conversion of the arenes are similar to those found in the reaction of diphenyl ketene. Five-membered heterocyclic compounds such as furan, thiophene, and N-methylpyrrole also undergo the reaction with diphenylacetylene [ 221. In a reaction of furan with diphenylacetylene, a mixture of (Z)- and (E)-1-(2furyl)-1,2diphenylethylene (78/22) was formed with a yield of 80%. The @&isomer is thermodynamically more stable than the @)-isomer, as indicated by the thermal isomerization experiment. We assume that the (E)isomer is first formed in the reaction. The reaction of a-substituted furans
137
such as 2-methylfuran, 2-(methoxycarbonyl)furan, furfural results in the formation of 2,5_disubstituted PhCXPh
2-acetylfuran, furans:
+
and
(5) C(Ph)=CHPh
R
Thus, addition of furan occurs at its a-position regioselectively; otherwise the positions are occupied. The reaction of 2,5_dimethylfuran with diphenylacetylene gives l-[ 3-(2,5_dimethyl)furyl]-1,2_diphenylethylene. Thiophene and N-methylpyrrole are also converted to 1-(2-thienyl)-1,2diphenylethylene and 1-[2-(iV-methyl)pyrrolyl]-1,2-diphenylethylene, respectively. The order of the reactivity of these heterocycles determined by a competitive experiment is as follows: furan > thiophene
> N-methylpyrrole
The rate of conversion than that of benzene.
> benzene
of furan
is about
two hundred,
times greater
Reaction of aromatic compounds with olefins In the above reactions, an aromatic C-H bond activated by the rhodium catalyst adds simply to an unsaturated C-C double or triple bond. Interestingly, the reaction of benzene with olefins affords phenyl-substituted olefins without giving addition products. For example, styrene was obtained directly from ethylene and benzene, accompanied by an almost equivalent molar amount of diethyl ketone [23 1. The ketone formation is ascribed to the hydrocarbonylation of ethylene by using the hydrogen generated in the reaction. Under the same reaction conditions (&H,: 30 atm; CO: 25 atm; reaction temperature: 220 “C), Rh,(CO)i* and Rh,(C0)i6 showed almost identical activities, giving about 100 moles of styrene per rhodium atom, but Rh,(CO)& exhibited very low activity and afforded 7 moles per rhodium atom. Rh(CO),(acac), which is readily converted into Rh,(C0)i6 under the reaction conditions, shows a relatively high activity: CH,=CH,
+ PhH
2
PhCH=CH2
+ Et&O
(6)
Many olefins having electron-withdrawing substituents such as COOH, COOR, COMe and CONRz can be used in place of ethylene, affording the corresponding phenyl-substituted olefins [ 241. The reaction of methyl acrylate and benzene gave methyl cinnamate together with the hydrocarbonylation product, dimethyl-4-oxoheptanedioate. The reaction of methyl vinyl ketone and benzene gave 4-phenyl-3-buten-a-one and 4-phenylbutan-2one, together with methyl ethyl ketone, the simple hydrogenation product of the starting olefin. The reactivities of disubstituted olefins are less than those of mono-substituted olefins. The results are summarized in Table 1. Introduction of various substituents on the benzene ring results in the formation of a mixture of three positional isomers [23]. The reaction of
138 TABLE
1
Phenylation
of mono-
and di-substituted
olefinsa
Olefin
Phenylated product (Product/Rh)
CH2=CH2COOMe CH&HCOMe CHz=CHCOOH CHz=CHCONMeZ MeCH=CHCOOMe CH2=C(Me)COOMe PhCH=CHCOOMe MeOCOCHXHCOOMe
PhCH=CHCOOMe (4700%) PhCH=CHCOMe/PhCH&H&OMe (4100%, 3/l) PhCH=CHCOOH/PhCH=CH2 (5690%, 2/l) PhCH=CHCONMez (4900%) Ph(Me)C=CHCOOMe (1690%) PhCH=C(Me)COOMe (250%) PhzC=CHCOOMe (250%) PhC( COOMe)=CHCOOMe/PhCH( COOMe)CH&OOMe (1250%, 7/l) PhC=CHCOOCO (1200%) PhC=CHCON(Ph)CO (1150%)
CH=CHCOOCO CH=CHCON(Ph)CO
aReaction of monosubstituted olefins were carried out using olefin (20 mmol), benzene (50 ml), CO (30 atm) and Rhd(CO)rz (0.025 mmol) at 220 “C! for 7 h; the reactions of disubstituted olefins were carried out using olefin (10 mmol), benzene (50 ml), CO (30 atm) and Rh4(CO)rz (0.05 mmol) at 240 “C for 7 h.
anisole with ethylene gave a mixture of ortho/metu/para-vinylanisoles in an isomer ratio of 67/23/10. Similarly, mixtures of vinylfluorobenzenes (78/17/5) and vinyltoluenes (14/57/29) were obtained from the reactions of fluorobenzene and toluene, respectively. The reaction of monosubstituted benzenes with methyl acrylate was examined in more detail, and the results are summarized in Table 2 [25]. Monosubstituted benzenes having alkyl, dimethylamino, methoxycarbonyl, alkoxy, cyano, and fluoro groups gave the corresponding cinnamates. Relative rates and the partial rate factors are also given in Table 3; these were obtained from results of competitive reaction experiments by assigning the value of 1.0 to benzene. Effects of substituents are similar to those observed in the reactions of arenes with diphenyl ketene, isocyanates and acetylenes. As shown in Tables 2 and 3, the differences in the reaction rates of the various compounds are very small in comparison with those shown in ordinary electrophilic aromatic substitution reactions. However, the results clearly demonstrate that the substituents with strong u-inductive effects favor the reaction. In particular, the partial rate factors show the higher reactivity of the ortho position of the RO- and F-groups than the ortho position of the other groups. This tendency is comparable with the fact that anisole and fluorobenzene [26] undergo facile ortho metallation upon reaction with n-butyllithium. Similar tendencies in the substituent effects were also observed in the H-D exchange reaction with strong base catalyst [ 271. The pattern may be ascribed to the enhancement of acidity of the ortho C-H bond by the inductive effect of the substituent, although the anchoring effect might also be important.
139 TABLE 2 Reaction of arenes with methyl acrylatea R in PhR
Me NMez COOMe CHzOAc H CN OAc OMe OPh F
Yield
Isomer ratio
(WRh)
ortho
meta
para
1600 600 1700 3300 4700 700 2600 3000 2500 2200
16 17 21 33
56 54 49 44 -
28 29 30 23 -
47 59 62 75 71
35 28 26 17 21
18 13 12 8 8
aOperating conditions: methyl acrylate, 0.025 mmol; 220 “C; 6 h.
10 mmol; arene, 50 ml; CO, 30 atm; RhJ(CO)rz,
TABLE 3 Relative rates and the partial rate factors of the reactions of arenes with methyl acrylate Substituent
Me NMez COOMe CHzOAc H CN OAc OMe OPh F
Relative rate
0.80 0.80 0.86 0.95 1.00 1.24 1.75 2.35 3.86 2.21
Partial rate factor
f ortho
f meta
fm
0.3 0.6 0.8 1.1 1.0 1.8 3.5 4.4 9.2 4.7
1.4 1.3 1.2 1.2 1.0 1.3 1.2 1.6 1.6 1.4
1.4 1.1 1.3 1.3 1.0 1.3 1.3 2.1 1.6 1.1
It is known that olefin-arene coupling of this type is catalyzed by Pd(I1) salts in the presence of oxygen [28]. Contrary to the electrophilic nature of the high-valent palladium catalyst, the present catalysis by the low-valent rhodium carbonyls exhibits a protophilic nature. Interestingly, when vinyl acetate was added as a hydrogen scavenger into the reaction system of substituted benzene and methyl a&ate, it was found not only to suppress the formation of the hydrocarbonylation product (dimethyL4_oxoheptanedioate), but also to result in a considerable increase in the yields of the desired aryl olefin, especially in the reactions of polysubstituted arenes [29]. It should be noted that the isomer distribution
140
is also influenced by the presence of vinyl acetate to some extent. For example, the yield of methyl-2-(o//3-naphthyl)acrylate in the reaction of naphthalene with methyl acrylate increases by a factor of about 3 in the presence of vinyl acetate, and the isomer distribution (a/P) changes from 28172 to 48152. Furan also reacts with ethylene and methyl acrylate to give 2-vinylfuran and methyl 2-(2-furyl)acrylate, respectively [30] :
CH$HR
(7)
+ CH=CHR
A competitive reaction experiment showed that the reactivity of furan is higher than that of benzene by a factor of about 75. Consistent with this fact, the reaction of benzofuran with methyl acrylate gave methyl 2-[2(methoxycarbonyl)vinyl] benzofuran exclusively. When indole was treated with olefins in the present catalytic system, N-vinylindole was obtained in a reasonable yield together with a trace amount of a-vinylindole, suggesting the N-H bond was cleaved preferentially [31]. For example, the reaction of indole and methyl acrylate in p-xylene afforded methyl 1-[2-(methoxycarbonyl)vinyl]indole (63%): CH+HCCXPle +
(8) I
H
I
CH=CHCOOMe
Carbazole, phthalimide and other secondary amines such as diethylamine, methylphenylamine and diphenylamine are also converted to Nvinylated products. On the mechanism of phenylation As described in the previous section, the present rhodium-catalyzed aromatic C-H bond activation exhibits a protophilic nature. Enhancement of acidity of the arene ring C-H bonds through complexation is well known in the case of arenetricarbonylchromium [32]. The q2-coordination of arene on a mononuclear metal complex prior to the activation of an aromatic C-H bond was suggested [33], and the chemical evidence for the intermediacy appeared recently [ 141. In the present reaction, precoordination of the arene on the rhodium catalyst (probably on a single metal center within a rhodium cluster) may also be important. Oxidative addition of the arene C-H bonds may take place favorably at the most acidic C-H bond in a second stage and change the arene ligand to o-aryl and hydride ligands. Insertion of an unsaturated molecule into the aryl-rhodium bond, and the subsequent P-hydrogen elimination or reductive coupling with the hydride gives the products, as shown in Scheme 1:
141
[ Rh] + PhH (i)
Ph--[Rh]-H
Ph-[ Rh]-H -
+ RCSR
-
PhC(R)=C(R)-[Rh]-H (ii)
Ph--[ Rh]-H PhCH,CHR-[
PhC(R)=C(R)-[Rh]-H
+ CH,=CHR Rh]-H
-
-
PhC( R)=CHR
+ [ Rh]
PhCH,CHR-[
Rh]-H
PhCH=CHR
+ H-[ Rh]-H
Scheme 1. Phenylation mechanism
In the reactions of heterocumulenes and acetylenes with arenes, an alternative pathway, insertion of the unsaturated compounds into the hydride-rhodium bond and the subsequent reductive coupling with the aryl group, is also plausible. However the exclusive formation of aryl-substituted olefin in the reactions of ethylene and methyl acrylate with arene could not be explained by this mechanism.
Cross-hydrocarbonylation
of Acetylenes
and Olefins
Cross-hydrocarbonylation of acetylene and olefin with carbon monoxide in alcohol In the previous section we have described new reactions accompanying the catalytic C-H activation of arenes by rhodium carbonyl clusters. In these reactions, carbon monoxide has appeared to be essential for realizing catalytic cycles, although it cannot be incorporated in the main product. In an attempt to discover new rhodium carbonyl-catalyzed reactions in which carbon monoxide participates as a constituent of the product, we found the formation of a five-membered unsaturated lactone, 5-ethyl-3,4diphenyl-2(5H)-furanone (3) in the reaction in which a mixture of diphenylacetylene, ethylene, and carbon monoxoide in ethanol was heated at 200 “C in the presence of Rh4(C0)12 [34]. The formation of 3 requires one hydrogen molecule, which is supplied from ethanol. The ethanol can be replaced by other hydrogen-donating solvents such as isopropanol and an acetone/ water mixture. There are several precedents for the catalytic formation of furanones by the carbonylation of acetylenes: bifurandione from acetylene and carbon monoxide [ 351, and 5-hydroxyfuranone from phenylacetylene, carbon monoxide and methyl iodide [36] by cobalt carbonyl. However the present reaction is the first example of a carbonylation reaction in which both olefin and acetylene are incorporated in a product molecule:
PhECPh
+
%H4
+
x0
+
(H2)
___t ,/‘Et
(9)
142
Several other rhodium compounds can be employed as catalysts. The results are listed in Table 4. At a higher temperature (220 “C), the catalytic activities of the rhodium compounds employed are rather close to that of (150 “C), rhodium carbonyl RhQ(CC),z. However, at a lower temperature clusters are superior to the others. TABLE 4 Activity of rhodium compounds in the carbonylation Catalyst
Rh4COh Rhs(CC)rs RhCl(CO)(PPhs)2 RhCl(PPh& RhCls*3H,O Rh& 5% Rh/C
of diphenylacetylene
Reaction temp.
Yield of 4
(“C)
(%)
220 150 150 220 150 220 220 220 150 220
60 67 59 46 30 27 41 5s 22 49
*Operating conditions: diphenylacetylene, 50 ml; CO, 30 atm; ethylene, 20 atm; 6h.
10 mmol; catalyst,
0.1 mg atom;
ethanol,
When the reaction in ethanol was carried out at higher temperatures, the dehydrogenation of ethanol occurred to give more than the necessary amount of hydrogen for formation of 3, and considerable amounts of diethyl ketone and acetal were produced. In a search for a method to avoid such byproduct formation, the influence of the reaction temperature was examined; the results are summarized in Table 5. They show that a temperature of about 150 “C is suitable for keeping reasonable yields of 3 and for minimizing the byproduct. At lower temperatures the yield of 3 decreases rapidly and new compounds, 5-ethoxy-3,4-diphenyL2(5I-&furanone (4) and 3ethoxycarbonyl-2-phenyl-indanone (5) begin to appear, although the yields are low. For the formation of these compounds, ethylene does not participate in the reaction and the ethanol is no longer a simple hydrogen donor, but is a constituent of the product molecule. The successful synthesis of alkoxyfuranones was attained by the use of alkali metal salts as cocatalysts (uide infm):
143 TABLE 5 Influence of the reaction temperature in the carbonylation presence of ethylene and ethanola 6
Temp. (“C) 220 180 150 125 100
(%) 60 73 67 31 10
-
-
3 11 10
4 13
‘Operating conditions: diphenylacetylene, atm; ethylene, 20 atm; ethanol, 50 ml; 6 h.
of diphenylacetylene
in the
Acetal (mmol)
Et&O (mmol)
49.4 25.5 4.6 0.5 -
56.6 41.7 14.4 0.2 -
10 mmol; Rh4(C0)12,
0.025
mmol; CO, 30
Diphenylacetylene can be replaced with other internal acetylenes. Thus 5ethyl-3,4dimethyl-2(5H)-furanone (47%) was obtained from 2-butyne. From 1-phenylpropyne, the regioisomers, 5-ethyl-4-methyl-3-phenyl-2( 5H)furanone (3a) and 5ethyl-3-methyl-4-phenyl-2(5H)-furanone (3b) were obtained in 48 and 4% yields, respectively:
PhCZlle
+ QH4
+ x0
+
(Hz)
-::qt
+
3a -
:$.o~
(11)
3b -
Attempts to obtain furanones from monosubstituted acetylenes such as phenylacetylene and l-hexyne were not successful. Monosubstituted olefins such as propylene and methyl acrylate can be used instead of ethylene, but the corresponding furanones are obtained in low yields. From propylene and diphenylacetylene, a mixture of 3,4diphenyl-5-(1-propyl)-2(5H)-furanone and 3,4diphenyl-5-(2-propyl)-2(5H)furanone was obtained with a ratio 1:l and a 5% total yield. From methyl acrylate and diphenylacetylene, only 5-[2-(methoxycarbonyl)-ethyl]-3,4diphenyL2(5H)-furanone was obtained in a 17% yield, accompanying a considerable amount of dimethyl 4-oxoheptandioate. Without diphenylacetylene, this system successfully gave dimethyl4-oxoheptandioate when isopropanol was used as the solvent [ 371: BCH,=CHCOOR
+ CO + i&OH
__f
0C(CH,CH,COOR)2
+ Me&O
(12)
Attempts to obtain furanones from the other olefins such as vinyl acetate, ethyl vinyl ether and styrene were not successful. It has been reported that the reaction of acylcobalt complexes, R’COCo(CO),, with acetylene gave ~3-lactonylcobalt complexes according
144
to the stepwise insertions of the acetylene and carbon monoxide into the acyl-cobalt bond and subsequent intramolecular ring closure [38]. Similar reactions of ($-CsHs)M(CO),(COCHs) (M = Mo,W) and acetylene and subsequent treatment of the resulting vinyl ketone complex with carbon monoxide have been reported to give an ~3-allylic lactone complex [ 391. Chini et al. [40] reported that the reaction of Rh,(C0)i2 with ethylene in aqueous acetone gives [Rh,(CO),,(COEt)]-, which may be derived from stepwise insertion of ethylene and carbon monoxide into a hydride complex formed by the water-gas shift reaction. On the basis of these facts, we suggest a possible mechanism outlined in Scheme 2 for the formation of the furanone from acetylene and ethylene:
R1R2C=0 H
Et H-(Rh
C2H4/C0
EtCO-[Rh]
[Rhl
x I
+0
C
'Et
Scheme 2
In the initial step, ethylene and carbon monoxide may insert. stepwise into the Rh-H bond to form an acyl complex similar to that isolated by Chini. The subsequent stepwise insertions of acetylene and carbon monoxide, followed by intramolecular cyclization, would afford an q3-allylic lactone complex which gives lactone and regenerates rhodium hydride species by reduction with alcohol. Carbonylation of acetylenes in alcohols It is known that the carbonylation of acetylenes in alcohol by soluble transition metal compounds gives mono- and di-carboxylates such as acrylate, fumarates, maleates and succinates [41]. However no report has
145
appeared on the formation of Salkoxy-2(5H)-furanones. As already described, 5ethoxy-3,4diphenyL2(5H)-furanone (4) was isolated in a low yield when the reaction of diphenylacetylene, ethylene and carbon monoxide in ethanol was carried out at a lower temperature (< 120 “C) than is appropriate to obtain 5ethyl-3,4diphenyL2(5H)-furanone (3). From an analogy of estimated intermediacy of an acylrhodium for the latter product, an ethoxycarbonylrhodium was thought to be the intermediate. It is known that an ethoxycarbonylrhodium cluster anion [Rh,(CO),,(CO,Et)]is formed by the reaction of Rh,(C0)12 or Rh,(CO)Jl, with ethanol in the presence of Na,COa [42]. The reaction of diphenylacetylene with carbon monoxide in alcohol with the exclusion of ethylene was carried out in the presence of various basic alkali metal salts as the cocatalysts, and a marked improvement in the yield of the ethoxyfuranone was attained [43]. The reactions proceed even at 50 “C, but the optimum condition is at about 125 “C! under 40 atm pressure of carbon monoxide with Rh,(CO),,/NaOAc. Rhodium compounds such as Rh,Os and RhCls may also be employed instead of Rh,( CO) i2. The present new reaction can be applied to various combinations of internal acetylenes and alcohols, and the results are summarized in Table 6.
TABLE 6 Synthesis of 5-alkoxy-2( 5H)furanoness Acetylene
diphenylacetylene
2-butyne 3-hexyne 1-phenylpropyne
Alcohol
methanol ethanol n-propanol i-propanol n-octanol ethanol
Temp. (“C)
Yield
100 125 75 125 125 125 125 125 125
86 87 76 65 60 31 63 60 93b
‘Operating conditions: diphenylacetylene, 10 mmol (other acetylenes, Rh,+(CO)r2, 0.025 mmol; NaOAc, 1.0 mmol; CO, 50 atm; 6 h. bA 84116 mixture of 5-ethoxy-4-methyl-3-phenyl-2(5H)-furanone and methyl-4-phenyl-2( 5H)-furanone.
(%)
20
mmol);
5-ethoxy-3-
The mechanism of the formation of 4 seems to be similar to that of 3. The formation of 3-ethoxycarbonyl-2-phenyl-indanone (5) may be ascribed to the cis- trans isomerization of an intermediate /3alkoxycarbonylvinylrhodium complex and the subsequent ortho-metallation to make a rhodiacycle, followed by insertion of carbon monoxide.
146
Cross-hydrocarbonylation of acetylenes and ethylene with carbon monoxide and molecular hydrogen As already described, in the rhodium-catalyzed cross-hydrocarbonylation of acetylene and olefin to give 5-alkyL2(5H)-furanone, the required hydrogen was supplied from alcohol or water used as a solvent. When molecular hydrogen was used, the reaction resulted in the formation of a$unsaturated ketone accompanied by a small amount of the lactone [44]. For example, the reaction of diphenylacetylene, ethylene, carbon monoxide and hydrogen in acetone at 150 “C afforded 1,2-diphenyl-1-penten-3-one in 75% yield on the basis of the conversion of the acetylene, together with propionaldehyde. Formation of propionaldehyde, which is an oxo-reaction product of ethylene, is unavoidable in this reaction. It is necessary to keep the hydrogen partial pressure as low as possible to minimize the byproduct formation : PhC=CPh + C2H, + CO + H, -
PhCH=C(Ph)COEt
(13)
Although other rhodium compounds such as Rh,Os, RhC1s*3H20, and Rh/C (5%) can be used as the catalyst precursors, Rh,(CO),, and Rh,(CO)16 are most active. In this reaction, various kinds of substituted acetylenes, including mono-substituted and functionally substituted ones, can be used, which is in marked contrast to the reaction system in the absence of molecular hydrogen. The results are summarized in Table 7.
TABLE
7
Synthesis of @unsaturated Acetylene
ketonesa (mmol)
Conversion
Yield
Product
(%)
(%) PhC=CPh PhCFCMe
10 20
82 99
MeCZMe PhC=CH n-BuCXH t-BuC=CH MeOCH2CX!CHzOMe MeCFCCOOMe
20 20 20 20 20 lob
100 93 100 100 75 80
PhGCCOOMe
lob
83
‘Operating conditions: Rh4(CO) 12, 0.05 30 atm; Hz, 5 atm; 150 ‘C; 6 h. bRh4(CO)lz,
0.025
mmol.
PhCH=C(Ph)COEt (E)-PhCH=C(Me)COEt (E)-MeCH=C(Ph)COEt (E)-MeCH=C(Me)COEt (E)-PhCH=CHCOEt (E)-n-BuCH=CHCOEt (E)-t-BuCH=CHCOEt MeOCH&H=C( CH20Me)COEt MeOOCCH=C(Me)COEt MeCH=C( COOMe)COEt PhCH=C( COOMe)COEt mmol;
acetone,
70 ml; ethylene,
75 49 6 25 48 42 61 76 76 4 40
25 atm; CO,
147
The reaction of terminal bon monoxide and molecular trans-o&unsaturated ketone: RCZH
+ C$H, + CO + H, -
acetylene hydrogen
and ethylene in the presence of cargave exclusively the corresponding
RCH=CHCOEt
(14)
It is interesting to note that terminal acetylenes can be employed in this reaction in spite of the failure of their use in the synthesis of 5ethyl2(5.H)-furanones. The success of the method may be ascribed to a suppressive action of molecular hydrogen in oxidative addition and polymerization of the acetylene. It has been reported that reaction of a rhodium hydride complex with terminal acetylenes gives an acetylide complex via oxidative addition of the acetylene and the subsequent elimination of hydrogen, which is converted again to the original hydride complex by treatment with hydrogen under forcing conditions [ 4 51. In the reaction of unsymmetrically substituted acetylenes, the distribution of two regioisomers is markedly influenced by the substituents. For example, (E)-2-methyl-1-phenyl-1-penten-3-one and (E)4-phenyl4-hexen3-one were obtained in the ratio of 89/11 from 1-phenylpropyne. And (E)/(Z)-3-methoxycar1-methoxycarbonyl-2-methyl-l-penten-3-one and bonyl-2-hexen-4-one were obtained in the ratio of 95/5 from methyl 3methylpropiolate. From methyl 3_phenylpropiolate, only 2-methoxycarbonyl-1-phenyl-l-penten-3-one was formed without giving the other regioisomer. These results show that the EtCO group tends to bond to a sterically less hindered acetylenic carbon atom having a smaller substituent, suggesting that a steric effect is predominant over an electronic effect in this reaction. In the formation of o$-unsaturated ketones, two paths are assumed in the stage of acetylene insertion: (a) insertion of acetylene into the Rh-COEt bond, which is the common intermediate proposed for furanone formation, and (b) insertion of acetylene into the Rh-H bond (Scheme 3). Considering the regioselectivity, in path (a) insertion of acetylene should occur in such a way that the EtCO group and the rhodium metal connect to the less and the more hindered acetylenic carbon atoms, respectively, giving a P-acylvinylrhodium complex. Similar stoichiometric insertion reactions, in which the metal atom adds to the more sterically hindered olefinic or acetylenic carbon atom, have been reported. The reaction of aryl palladium complex with olefins [46], the reaction of (CO)sMnR (R = Me, MeCO and Ph) with acetylenes [47], and the reactions of (acac)Ni(PPh,)Me with acetylenes [48] are the examples. In path (b), insertion of acetylene should occur so as to connect the rhodium metal with a sterically less hindered acetylenic carbon atom, giving a vinylrhodium intermediate. There are some examples of the complexes derived from a metal hydride complex and acetylenes [49]. However, it is not enough to compare the regioselectivity of the reaction because the number of acetylenes employed is limited. Although it is difficult to reach a definite conclusion, we tentatively assume
148
Path (a) H-[ Rh] + CH,=CH, + CO -
Et-[Rh] EtCO-[
-
Et-[ Rh]
EtCO-[
Rh] + RC=CR’ -
Rh] EtCOC( R)=C( R’)-[ Rh]
EtCOC( R)=C( R’)-[ Rh] + Hz -
EtCOC(R)=CHR’
+ H-[Rh]
Path (b) H-[Rh]
+ RC!=CR’ -
RCH=C(R’)-[Rh]
+ CO -
RCH=C(R’)CO-_[
RCH=C(R’)CO-[
Rhl + CH,=CH,
RCH=C(R’)COCH,CH,-[ RCH=C(R’)-[
RCH=C( R’)-[ Rh]
-
Rh] + Hz -
Rh] + EtCO-[
Rh]
RCH=C(R’)COCH,CH2-[ RCH=C(R’)COEt
+ H-[ Rh]
Rh] RCH=C(R’)COEt
RCH=C(R’)CO-[
Rh]
+ [ Rh] 2
Rh] + Et-[ Rh]
Scheme 3.
that path (a) is operative in the present system because the regioselectivity in the formation of the furanones and the ketones from 1-phenylpropyne can be explained by the common intermediacy of an acylrhodium complex. In the presence of molecular hydrogen, hydrogenation of the P-acylvinylrhodium complex, which is produced by insertion of acetylene into the EtCO-Rh bond, could occur to give o$-unsaturated ketone prior to the insertion of carbon monoxide, which leads to 5-ethyl-2( 5H)-furanone.
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