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A novel hydrogenation of conjugated dienes catalyzed by halogenotris(triphenylphosphine)cobalt(I) activated with boron trifluoride etherate
Journal of Molecular Catalysis, 5 (1979) 175 @ Elsevier Sequoia S-A., Lausanne -Printed
175
- 187 in the Netherlands
A NOVEL HYDROGENATION OF CONJUGATED DIENES BY HAI.OGENOTRIS(TRIPHENYLPHOSPHINE)COBALT(I) WPI‘H BORON TRIFLUORIDE ETHERATE
KIYOSHI
KAWAKAMI,
TSUTOMU
MIZOROKI
and ATSUMU
Research Laborafory of Resources UfiZtiafion, Tokyo Nagafsufa-cho, Mirodri-ku, Yokohama, 227 (Japan) (Received
February
Insfifufe
CATALYZED ACTIVATED
OZAKI of Technology,
20.1978)
Summary CoX(PPh3)a (X = Cl, Br. I) exhibits a fairly high catalytic selectivity in the presence of BF, - OEta for the hydrogenation of conjugated dienes such as 1,3-butadiene, 2-methyl-l,3-butadiene, and 1,3-pentadiene at ambient temperature and pressure to give 1-butene, 3-methyl-1-butene, and 1pentene, respectively, as the maiu product, where the substituted double bond is preferentially hydrogenated. The rate of hydrogenation is independent of the diene concentration and proportional to the hydrogen pressure and to the catalyst concentration_ Addition of AgClO, instead of BFa - OEta has essentially the same effect on the activity and selectivity for the hydrogenation of dienes when an analogous cobalt(I) complex, CoBr(PPhsMe)s, is used as the catalyst. It is proposed that the rate of hydrogenation is determined by a reaction of molecular hydrogen with a cationic tris(tertiaryphosphine) (diene)cobalt(I) species_
Introduction A large number of transition metal complexes are known to catalyze selectively the hydrogenation of conjugated diene to monoene [l] . Among cobalt catalysts, anionic pentacyanocobalt(I1) complex has been most extensively studied to determine why the anionic complex catalyst gives a product rich in 1-butene at CN-/Co(H) > 5, and rich in 2-butene at CN/Co(H) < 5, in the hydrogenation of 1,3-butadiene at ambient temperature and pressure [2] _ 1,3-Butadiene is also hydrogenated to butenes with the HCo(dpe)s-AlEtaCl or CoC&(dpe)-LiARI* (dpe = 1,2-bis(diphenylphosphino)ethane) catalyst system at 80 - 120 “C under 30 - 50 atm of hydrogen pressure [3] _ Another cobalt complex to be noted is Cos(CO)s(PPh,),, which hydrogenates polyunsaturated hydrocarbons, such as cyclododeca-
176
triene, cyclooctadiene, and cyclohexadiene, selectively to monoenes, although enforced reaction conditions (110 - 180 OC, 20 - 30 atm hydrogen) are required [4] _ On the other hand, halogenotris(tertiaryphosphite)cobalt(I) reportedly absorbs molecular hydrogen under mild conditions, and catalyzed the hydrogenation of alkyne as well as vinyl acetate and vinyl ether in the presence of catalytic amounts of triethylamine [ 5] , but there are no reports of hydrogenation with halogenotris(tertiaryphosphine)cobalt(I) complexes_ During the course of our study on the dimerization of ethylene with bromotris(triphenylphosphine)cobalt(I), we found that in the presence of BFs - OEts this complex selectively catalyzed the hydrogenation of conjugated dienes to give terminal olefins as the main products [6]. The present paper reports, in detail, the characteristics of this novel hydrogenation_
Experimental Materials Halogenotris(triphenylphosphine)cobalt(I) complexes were the same as those used in the previous paper dealing with the selective dimerization of ethylene [7] _ 1,3-Butadiene, 2-methyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and cyclooctadienes, were dried by anhydrous copper(I1) snlfate and distilled under vacuum_ Solvents, BFa - OE&, and hydrogen gas were purified by the same methods described in the previous paper 1’7J _ Commercial silver perchlorate was dried under vacuum at 40 “C for 1 h before use_ Procedure To a two-necked flask (100 ml) equipped with a sampling tube sealed by a silicone rubber bung were added CoX(PPh&, bromob&zene, BF, -OEt,, and one of dienes in that order under nitrogen atmosphere_ Nitrogen gas was evacuated at liqnid nitrogen temperature, and then hydrogen gas was introduced to the homogeneous solution at 0 “C with vigorous stirring. The hydrogenation was conducted at 0 “C under a constant pressure of hydrogen using a gas burette- The course of the hydrogenation with time was followed by intermittent sampling of the reaction mixture with a micro syringe (50 ~1). The samples were immediately quenched by being exposed to the air and quantitatively analyzed by gas chromatography, as described in a preceding paper [83 - The percent product composition was determined from peak areas. Products were identified either by gas chromatographic retention or by IH-n.m.r. after separation with a preparative gas chromatographIsotopic exchanges of Hz-Dz and Dz-olefin were performed by an analogous procedure, and kptopic mixtures were analyzed by mass spectrometry. The deuteration products of 1,3-butadiene and 1,3-pentadiene were, after separation with a gas chromatograph, identified by %I- and ?D-[H] -n.m.r. spectra on a JEOL PS-100 spectrometer in FT mode.
177
ResuIts Hydrogen scrambling reactions H-D exchange between Da and Ha (Ha/D, = 48/52,35.5 ml STP) took place readily at 0 “C in the bromobenzene solution (10 ml) of CoBr(PPhs)s (0.2 mmol) in the presence of BF, -0Etz (O-96 mmol); the gas composition after 2 h was HD 37%, Dz 33% and Hz 30%_ No exchange was observed in the absence of BF, - OEta within 4 h. The hydrogen scrambling between Dz and monoolefin such as ethylene, propylene, or 1-butene also proceeded under simiIar reaction conditions (TabIe l), aIthough no hydrogenation product was detected. The same treatment of 1,3-butadiene with Da, however, gave mainly the Da-adduct, C4HsD2, as shown in Tabie 1 (run 4), the hydrogen exchange between D2 and diene being significantly suppressed_ It is also shown that the isotopic scrambling between Hz and Da is effectively suppressed during the hydrogenation, as indicated by the run with an Ha-D, mixture (run 5), where the products are in conformity with a paired addition of Hz or Dz [9] to 1,3-butadiene. TABLE 1 H-D exchange reaction between Dz and olefin or 1,3-butadiene CoBr(PPhg)s, 0.2 mmol; BFa-OEta, O-96 mmoI; PhRr. 10 ml_ Composition of starting deuterium (do = 3_1%, dl = 11.6%, dz = 85_3%). Run
Deuterium
Starting material (ml s.t.p_)
.
distribution
of products
(W)
J52
HD
D2
do
dl
d2
d3
de
c2H4
(21-g),
D2 (21-4)
14.1
37-7
48.2
30-3
23-7
21-7
15-9
C,%
(22.21,
D2 (21.0)
39.3
23.8
37-O
35.5
37-l
19.5
6.8
1.1
D2 (22.0)
13-7
40.8
45-5
62.5
25.9
S-7
2.9
-
54
55
3-O
11-l
85.9
64.5
-
45.2
49-3
l-C4Ha
(22.2),
C4H6
(22-2),
D2 (24-S)
C4Htj
(30-41,
Hz (185)
51-5
3-2
3.8
56-
57
S-4
58Wle)
1.9
5.1
28.5
23.5
3.8
19.5
D2 (11.5)
Hydrogenation
of conjugated
dienes
Hydrogenation of dienes (3-O mmol) was performed in the bromobenzene solution (20 ml) of CoBr(PPh,), (O-2 mmol) in the presence of BFs-OEtz (0.44 mmol) at 0 “C under a constant pressure of hydrogen (76 cmHg). The results are summarized in Table 2.1,3-Butadiene, 2-methyl1,3-butadiene, 1,3_pentadiene, 1,4-pentadiene, 1,3-hexadiene, and 1,3-cyclooctadiene are selectively hydrogenated to the corresponding monoenes, in which the double bond is terminal, only trace amounts of aIkanes being detected_ It is noteworthy that 3-methyl-1-butene, 1-pentene, and 1-hexene are mainly formed from 2-methyl-1,3-butadiene, 1,3_pentadiene, and 1,3-
83 37 53 38 27
60 260 60
1110
840
1,3aPontadienc
1,49ontadiono
1,3Jiexadieno
*Otherproductsare 1,3cyclooctadiene(40%)and 1,4-cyclooctadiene (46%),
1,3-Cyclooctadiono
96
30
S.Mothyi1,3wbutadione
t-2aButone -0 2.Mothyllebuteno 4.6 t-2.Pcntenc 21 t-2wPonteno 12 t-2.Hcxcnc 18
1.Butone 09 3.Mothyll-butono 86 l-Pentone 56 l-Pontono 65 l-Hexenc 61 Cyclooctene 100 Cycloocteno 14”
28
Productdistribution (%)
Conv, (%)
90
Timo (min)
1,3mButadiono
Dieno
c-2.Butene 11 2mMothyl. 2ebutone 9,l cs2ePontene 23 c*2dcntone 23 c-2.Hoxene 22
TABLE2 Hydrogenationof dienescatalyzedby CoBr(PPha)a Diene,3,Omm& CoBr(PPh&,0,2 mmol;BFa*OEtz,0,44 mmol;PhBr,20 ml; PHZ,76 cmHg;temp. o Y_!,
0.46
0,ll
0*97
1.83
0,19
Initialabsorption rate of hydrogen (ml/min)
179
hexadiene, respectively, the substituted doubIe bonds being preferentially hydrogenated_ The initial absorption rates of hydrogen, and the conversion of dienes (Table 2) show that the rate of hydrogenation decreases in the order, 2-methyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,3-butadiene, 1,4-pentadiene. 1.3-cyclooctadiene, l,Scyclooctadiene_ During the hydrogenation of less reactive 1,5_cyclooctadiene, 1,3- and 1,4_cyclooctadiene were found as isomerization products, while no isomerization product of 1,4-pentadiene was detected_ It is clear that the catalyst system is less active for the isomerization of unconjugated dienes_
”
60
120
180
240
300
Time (mm]
Fig_ 1. Time
course of the hydrogenation
BFs-OEt2,0.66
of 1,3-butadiene_
CoBr(PPh,),,
0.6 mmol;
mmol; PhBr, 30 ml; 1.3-butadiene, 9.0 mmol;PHq, 76 cmHg; temp., O”C_
100
80
Q T;W g =
s
040
20
0
m
60
4.0
lii
Fig_ 2_ Time course of mmol; BF3-OEt2.0.66
cmHg; temp., 0 “C.
60
100
(mm1
the hydrogenation of 2-methyl-1,3-butadiene- CoBr(PPh3)3, 0.6 mmol; PhBr, 30 ml; 2-methyl-1,3-butadiene, 9.0 mmol; pH2, 76
180 The time courses of the hydrogenation of 1,3-butadiene, 2-methyl-1,3butadiene, and 1,3-pentadiene under the same conditions are shown in Figs_ 1, 2, and 3, respectively- 1,3-Butadiene gives 1-butene, which is rapidly iso-
b Fig_ 3_ Time course of the hydrogenation of 1,3-pentadieneCoBr(PPh&, 0.6 mmol; BF,-OEt2,0_66 mrrol; PhBr. 30 ml; l-3-pentadiene, 9-O mmol; PH*, 76 cmHg; temp_, 0 ‘TX C& = pentadiene, C$, = pentene_
merized to 2-butenes when 1,3-butadiene is mostly consumed. 2-Methyl-1,3butadiene is consumed more rapidly than 1,3-butadiene to give $-methyl-lbutene exclusively, the double bonds at 3 position being hardly hydrogenated, whereas the enhanced isomerization of produced monoene, as observed in the hydrogenation of 1,3-butadiene, is not observed even after ah the 2-methyl-1,3-butadiene is consumed. In the hydrogenation of a mixture of ci’s- and trans-1,3-pentadiene (trans/cis = 64/37), the trans-isomer is consumed faster than the cis-isomer, and a preferential hydrogenation of the internal double bond is also found, although the isomerization of 1-pentene produced proceeds more extensively than that of 3-methyl-1-butene during the hydrogenation of 2-methyl-1,3-butadiene. To investigate the coordination and reactivity sequences of 1,3&enes, the competitive hydrogenation of 1,3-butadiene with 2-methyl-1,3-butadiene, or 1,3-pentadiene, was carried out at a lower catalyst concentration (CoBr(PPhs)s 0.3 mmol, BFs - OEta 0.44 mmol) following the time course in detail_ The results are shown in Figs. 4 and 5. In both cases, 1,3-butadiene significantly retards the hydrogenation of the other dienes, which are rapidly hydrogenated in the absence of 1,3-butadiene, as shown in Figs, 2 and 3_ In particular, 1,3-pentadienes are hardly hydrogenated before 1,3-butadiene is almost consumed_ Thus, the coordination sequence of 1,3dienes on the
181
Fig. 4. Time course of the competitive hydrogenation of 1,3-butadiene and 2-methyl-1,3butadiene. CoBr(PPha)s, O-3 mmol; BFQ-OEt 2, O-44 mmol; 1,3-butadiene, 2.5 mmol; 2methyl-1,3-butadiene, 2.5 mmol; PhBr, 30 ml; PH*, 76 cmHg; temp., 0 “C. =Based on 1,3-butadiene and 2-methyl-1,2-butadiene, respectively_
0
30
60
90
120
150
Time (mm1
Fig. 5. Time course of the competitive hydrogenation of 1,3-butadiene and 1,3-pentadiene_ CoBr(PPb&, 0.3 mmol; BF3-OEt 2.0.44 mmol; 1,3-butadiene, 2.5 mmol; 1,3pentadiene, 2.5 mmol; PhBr, 30 ml; PHI, 76 cmHg; temp-, 0 “C- aBased on 1,3-butadiene and 1,3-pentadiene, respectively. ck = butene, Ca = butadiene, C& = pentene, Cs = pentadiene. active cobalt(I) species is assessed to be 1,3-butadiene > 2-methyl-1,3-b&adiene > 1,3-pentadiene, which is in contrast with the reactivity sequence of dienes obtained in individual runs: 2-methyl-1,3-butadiene > 1,3-pentadiene > 1,3-hexadiene > 1,3-butadiene (Table 2).
182
Effect of solvent When BF, -0Etz was added to the benzene solution of CoBr(PPh, )a in the absence of diene, the colour of the solution turned from pale green to blue giving a black precipitate in a few minutes at 0 “C. The foliowing disproportionation probably takes place in the absence of diene [ 71, 2CoBr(PPhz)s
CoBrz(PPhs)z
-
+ 4PPhs + Co
whiIe the formation of the black precipitate was much less extensive in bromobenzene after 30 min at room temperature_ The color of the benzene or bromobenzene solution containing both CoBr(PPh3), and one of the dienes, however, immediately turned to deep green on the addition of BFz - 0Et2 and remained almost unchanged without forming any precipitate. The catalyst solutions thus prepared absorbed molecular hydrogen smoothly without an induction time- These observations indicate that the diene, as well as bromobenzene, stabilizes the cobalt complex activated with BFa - OEtz, aTthough no direct evidence for complex formation could be obtained by n.m.r. measurement because of the paramagnetic solutions. Mechanistic study of the hydrogenation The kinetics of the hydrogenation
was investigated with 1,3-pentadiene by fohowing the hydrogen uptake as a function of time. The reaction was inSated by introducing hydrogen gas. As shown in Fig_ 6, the rate of uptake
Tie
(mm)
Fig. 6. Effect of hydrogen pressure on the rate of hydrogenation_ CoBr(PPh&, BF3-OEt2,1.32 mmol; 1,3-butadiene, 6.0 mmol; PhBr, 20 mI; temp., 0 “C;PH~, 50 (a), 25 (0) cmHg.
0.5 mmol; 75 (0).
183
is proportional to hydrogen pressure, no induction period being observed, while the rate is independent of the diene concentration, as suggested by the results in Fig_ 6 and confirmed by a run at a lower diene concentration. Time courses of the hydrogenation of 1,3-butadiene (Fig- 1) and a-methyl1,3-butadiene (Fig_ 2) also indicate that the rates of hydrogenation are almost independent of these dienes’ concentration over 50% conversion. The rate was also proportional to the concentration of CoBr(PPhS)S in the presence of an excess amount of BF, -OEtz (BF,-OEt,/CoBr(PPhB)s = 2.6). No appreciable difference was observed in the rate of hydrogenation when the halogen ligand in CoX(PPh,), was changed_ The addition of triphenylphosphine decreased the rate of hydrogenation_ The absorption of hydrogen was still observed at PPh,/CoBr(PPhB)3 = 3, at which ratio the ethylene dimerization was completely retarded, as reported in the previous paper [7] _ An isotope effect was observed in the hydrogenation of 1,3-butadiene (~/rn= 1.6). Deuteration products of 1,3-butadiene and 1,3-pentadiene, C4H6D2 and C5HsD2, were identified by n.m.r_ to be CH,=CH-CHDCH2D and CH*=CH-CHD-CHD-CH,, respectively, as shown in Fig. 7, demonstrating 1,2- and 3,4-addition to 1,3-butadiene and 1,3-pencadiene, respectively. The addition of BFB-OEt2 seems to be essential for the activation of the cobalt complex, since the rate of hydrogenation increases almost linearly with increase in BFB - OEtP concentration up to BF, -OEt,/CoBr(PPhs)B = 1, where the rate attains a plateau value, as is seen in Fig_ 8_ The same effect of the BF3 - OEtP concentration was found with the same cobalt(I) complex catalyst in the dimerization of ethylene [7] _ Silver perchlorate, however, also activates an analogous cobalt(I) complex, CoBr(PPhzMe),, for the hydrogenation, as noted in Table 3, although the effect of AgClO, concentration was difficult to examine because of its low solubility. The solution of CoBr(PPha)3 catalyst and 1,3-diene absorbs hydrogen much less readily on the addition of AgC104 instead of BF, - 0Et2. It is known that AgClO, reacts with triphenylphosphine to give [Ag(PPh,),] f [ClO,] - (n d 4) [lo] _ When -CoBr(PPh;?Me), was used instead of CoBr(PPh,), in the hydrogenation of 2-methyl-1,3-butadiene and 1,3_pentadiene, AgClO, showed essentially the same effect on the catalytic activity and selectivity. In the absence of BF,-OEtz or AgC104, a very slow uptake of hydrogen was observed with the bromobenzene solution of CoBr(PPh,), and one of the 1,3-dienes, while the monoenes formed were completely different from those described in Tables 2 and 3; cis-2-butene, 2-methyl-2-butene, and cis-2-pentene resulted from 1,3-butadiene, 3-methyl-1,3-butadiene, and 1,3-pentadiene, respectively, suggesting that the reaction path is different_ Discussion On the active cobalt species The present results demonstrate that the addition of BFB - OEta remarkably activates halogenotris(tertiaTphosphine)cobalt(I) complexes as catalysts
184 a) Cl-i: = CH-CHD-CHID
bl CH: = CH CHD-CHD-CH.
528
‘D-IHI
Fig. 7. ru’.m.r. spectra
of the deuteration
products_
589
C4HsDz
(a), and CsHsDz
(b)_
for the partial hydrogenation of dienes_ Similar activation by BFs - OEta has been shown for a haIogeno(a-aryl)bis(triphenylphosphine)nickel(II) complex [ 111) in addition to that noted for the cobalt(I) complexes [ 7 3 in olefin dimerization. Such an effect of BFs - OEts might be caused by abstraction of the PPh, Zigand from the cobaIt(I) or the Ni(II) complex. No interaction of BFs -0Eta with PPh,, however, was detected by 3’P-n.m.r_ in a well dried bromobenzene soIvent_ The colour change of CoBx(PPhs)s solution on addition of BFs -0Etz and the 111 interaction (Fig. 8) suggest the formation of a complex which is stabilized by the presence of diene_ The folIowing reaction is suggested I CoX(PPh,)s -
+ BF,-OEts + diene [Co(PPh,)3 (diene)]+
[BFs-XI-
+ EtzO-
185
Fig_ 8. Rate of hydrogen uptake us. BF, -OEt&oBr(PPh&_ PhBr, 20 ml; 1,3-pentadiene, 3-O mmol. temp_. 0 “C; Paz,
CoBr(PPh&, 76 cmHg_
0.2
mmol;
Another reason to justify the above type of interaction is that AgC104, which is frequently utilized for the preparation of cationic transition metal complexes [12], activates the CoBr(PPhsMe)a complex for the partial hydrogenation of dienes. It should be noted, however, that the extent of acti3 ) 3_ Indeed, it has been rqorted vation by AgClO, is not great for CoBr(PPh that 31P-n.m.r. does not show any interaction of AgClO, with PPhsMe or PPhMes in bromobenzene [ 133, while complexation of AgC104 is detected with PPhs. It is reasonable to assume that AgC104 removes not only the bromide ligand, but also PPh, from CoBr(PPh,), to give an unstable species, which is possibly transformed into an inactive form_ On the other hand, -BFs - OEts does not interact even with PPhs as described above, so that the etherate can activate the triphenylphosphine-Co(I) complex_ The role of BFs - OEt, can thus be understood in terms of the cationic cobalt(I) complex_ Mechanism of diene hydrogenation Recently Schrock and Osbom reported that a cationic rhodium(I) complex [ Rh(NBD) (diphos)] + [ Cl0.J - (NBD = norbomadiene, diphos = bidentate phosphine ligand), efficiently catalyzes the partial hydrogenation of 1,3dienes and that the rate determining step of the hydrogenation is the reaction of molecular hydrogen with [ Rh(l,3diene) (diphos)] + species [14] In this hydrogenation, 1,4-addition was predominant, giving 2-methyl-2butene from 2-methyl-1,3-butadiene with 70 - 90% selectivity, whereas the present cationic Co(I) catalyst undergoes mainly 1,2addition, giving 3methyl-1-butene as the main product (>80%) from 2-methyl-1,3-butadiene, and also 3,4-addition, giving I-pentene from 1,3-pentadieneThus, there seems to be a difference in the mechanism of hydrogenation-
1,BSentadiene
BF3a0Et2 (0,44) ApJJl04 (OJ)
CoBr(PPhzMe)s CoBr(PPhzMo)a
87 17 66 82 67 22
60 60
Conv, (%)
30 100 16 16
Timo (min)
*3.Methyl-1.buteno, 2.Mcthybl-butono, and 2eMethyI-2.butone,respectively, **lqentene, fransd-pontono, and G-2.pentone, rospcctivcly.
S-Methyl1,3-butadiena
BF3+OEt2 (0,44) AgCI04 (0,l) BFS+OEt2 (0,44) AgC104 (0,l) !
CoBr(PPh& CoBr(PPh& CoBr(PPhzMo)a CoBr(PPhzMe)s
1,3diene
Activator (mmol)
Complex
Effect of A@104es tha activator Co(I) complex, 0,2 mmol; 1,3-dlcnc,3,Ommol; PhBr, 20 ml; temp,, 0 “0, &I,, 76 cmHg,
TABLE 3
86 61 44 83 1.P”” 65 68
3.Me-l-b*
26 26
1.;;**
4,4 2,3 39
9,l 36 17 11 c-2-p** 10 690
2.Mo.l.b” 2.Me-2-b*
Product distribution (%)
187
The rate determining step of hydrogenation, as suggested by the first order kinetics in hydrogen pressure and in the cobalt(I) catalyst concentration, as well as by the zero order kinetics in diene concentration, is also the addition of molecular hydrogen to [Co(PPh,), (diene)] + species. Accordingly, it is suggested that hydrogen attack may be different from that on the rhodium(I) catalyst_ Indeed, the rhodium(I) species has two coordination sites available for oxidative addition of molecular hydrogen, while the cobalt(I) species has only one site available, so that molecular hydrogen would have to be heterolytically dissociated to leave a hydride on Co(I) species, that is
H-C
H-C
(P=tertiaryphosphine)
In view of the cationic nature of the Co(I) complex, the attacking hydrogen molecule would be polarized, with the negative end being closer to the Co(I) ion, and the other positive end attacking on the coordinated diene molecule. In fact, it is commonly observed in the present study that the addition of hydrogen takes place more readily at the double bond with more substituents, or of higher electron density_ This is a unique feature of the present hydrogenation which seems reasonable on the grounds of the electrophilic nature of attacking hydrogen_
References 1 B_ R. James, Homogeneous Hydrogenation, Wiley, New York, 1973_ 2 J_ Kwiatek, I. L. Mador and J_ K_ Seyler, J_ Am. Chem. Sot., 84 (1962) 304. 3 Y_ Tajima and E_ Kunioka. J_ Catal.. ll(1968) 83; M_ Iwamoto, J. Chem. Sot_ Jpn_ (Ind_ Chem. Sect.). 71 (1968) 1510_ 4 A. Misono and I_ Ogata. Bull_ Chem_ Sot_ Jpn., 40 (1967) 2 718_ 5 M_ E. Volpin and I. S. Kolomnikov, Russ. Chem. Rev., 38 (1969) 273. 6 K. Kawakami. T_ Mizoroki and A_ Ozaki_ Chem- Lett__ 11976) 847_ 7 K. Kawakamii ‘I?_ Mizoroki and A. Ozaki; Bull. Chem_ _&x. J&X_, 51 (1978) 21_ 8 N. Kawata, T. Mizoroki and A_ Ozaki, J. Mel_ CataL, 1 (1975/76) 275_ 9 A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Kodansha and Academic Press, Tokyo and New York. 1977, p_ 78. 10 F. A. Cotton and D_ M. L. Goodgame, J_ Chem. Sot_, (1960) 5 276. 11 K. Maruya, T_ Mizoroki and A. Ozaki, BuU. Chem. Sot. Jpn., 45 (1972) 2 255; N. Kawata, K. Maruya, T. Mizoroki and A. Ozaki, Bull. Chem. Sot_ Jpn., 47 (1974) 413. 12 H. C. Clark and K_ K. Dixon, J. Am. Chem. Sot., 91(1969) 596; P_ M. Druce, M. F. Lappert and P_ N_ Riley. Chem_ Commun_, (1967) 486; R_ R_ Schrock and J_ A_ Osborn, J. Am. Chem. So+ 93 (1971) 2 397_ 13 T_ Koike, K. Kawakami, K. Maruya, T. Mizoroki and A. Ozaki, Chem. Lett., (1977) 551. 14 R_ R_ Schrock and J. A_ Oshorn, J_ Am_ Chem. Sot., 98 (1976) 4 450.
175
- 187 in the Netherlands
A NOVEL HYDROGENATION OF CONJUGATED DIENES BY HAI.OGENOTRIS(TRIPHENYLPHOSPHINE)COBALT(I) WPI‘H BORON TRIFLUORIDE ETHERATE
KIYOSHI
KAWAKAMI,
TSUTOMU
MIZOROKI
and ATSUMU
Research Laborafory of Resources UfiZtiafion, Tokyo Nagafsufa-cho, Mirodri-ku, Yokohama, 227 (Japan) (Received
February
Insfifufe
CATALYZED ACTIVATED
OZAKI of Technology,
20.1978)
Summary CoX(PPh3)a (X = Cl, Br. I) exhibits a fairly high catalytic selectivity in the presence of BF, - OEta for the hydrogenation of conjugated dienes such as 1,3-butadiene, 2-methyl-l,3-butadiene, and 1,3-pentadiene at ambient temperature and pressure to give 1-butene, 3-methyl-1-butene, and 1pentene, respectively, as the maiu product, where the substituted double bond is preferentially hydrogenated. The rate of hydrogenation is independent of the diene concentration and proportional to the hydrogen pressure and to the catalyst concentration_ Addition of AgClO, instead of BFa - OEta has essentially the same effect on the activity and selectivity for the hydrogenation of dienes when an analogous cobalt(I) complex, CoBr(PPhsMe)s, is used as the catalyst. It is proposed that the rate of hydrogenation is determined by a reaction of molecular hydrogen with a cationic tris(tertiaryphosphine) (diene)cobalt(I) species_
Introduction A large number of transition metal complexes are known to catalyze selectively the hydrogenation of conjugated diene to monoene [l] . Among cobalt catalysts, anionic pentacyanocobalt(I1) complex has been most extensively studied to determine why the anionic complex catalyst gives a product rich in 1-butene at CN-/Co(H) > 5, and rich in 2-butene at CN/Co(H) < 5, in the hydrogenation of 1,3-butadiene at ambient temperature and pressure [2] _ 1,3-Butadiene is also hydrogenated to butenes with the HCo(dpe)s-AlEtaCl or CoC&(dpe)-LiARI* (dpe = 1,2-bis(diphenylphosphino)ethane) catalyst system at 80 - 120 “C under 30 - 50 atm of hydrogen pressure [3] _ Another cobalt complex to be noted is Cos(CO)s(PPh,),, which hydrogenates polyunsaturated hydrocarbons, such as cyclododeca-
176
triene, cyclooctadiene, and cyclohexadiene, selectively to monoenes, although enforced reaction conditions (110 - 180 OC, 20 - 30 atm hydrogen) are required [4] _ On the other hand, halogenotris(tertiaryphosphite)cobalt(I) reportedly absorbs molecular hydrogen under mild conditions, and catalyzed the hydrogenation of alkyne as well as vinyl acetate and vinyl ether in the presence of catalytic amounts of triethylamine [ 5] , but there are no reports of hydrogenation with halogenotris(tertiaryphosphine)cobalt(I) complexes_ During the course of our study on the dimerization of ethylene with bromotris(triphenylphosphine)cobalt(I), we found that in the presence of BFs - OEts this complex selectively catalyzed the hydrogenation of conjugated dienes to give terminal olefins as the main products [6]. The present paper reports, in detail, the characteristics of this novel hydrogenation_
Experimental Materials Halogenotris(triphenylphosphine)cobalt(I) complexes were the same as those used in the previous paper dealing with the selective dimerization of ethylene [7] _ 1,3-Butadiene, 2-methyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and cyclooctadienes, were dried by anhydrous copper(I1) snlfate and distilled under vacuum_ Solvents, BFa - OE&, and hydrogen gas were purified by the same methods described in the previous paper 1’7J _ Commercial silver perchlorate was dried under vacuum at 40 “C for 1 h before use_ Procedure To a two-necked flask (100 ml) equipped with a sampling tube sealed by a silicone rubber bung were added CoX(PPh&, bromob&zene, BF, -OEt,, and one of dienes in that order under nitrogen atmosphere_ Nitrogen gas was evacuated at liqnid nitrogen temperature, and then hydrogen gas was introduced to the homogeneous solution at 0 “C with vigorous stirring. The hydrogenation was conducted at 0 “C under a constant pressure of hydrogen using a gas burette- The course of the hydrogenation with time was followed by intermittent sampling of the reaction mixture with a micro syringe (50 ~1). The samples were immediately quenched by being exposed to the air and quantitatively analyzed by gas chromatography, as described in a preceding paper [83 - The percent product composition was determined from peak areas. Products were identified either by gas chromatographic retention or by IH-n.m.r. after separation with a preparative gas chromatographIsotopic exchanges of Hz-Dz and Dz-olefin were performed by an analogous procedure, and kptopic mixtures were analyzed by mass spectrometry. The deuteration products of 1,3-butadiene and 1,3-pentadiene were, after separation with a gas chromatograph, identified by %I- and ?D-[H] -n.m.r. spectra on a JEOL PS-100 spectrometer in FT mode.
177
ResuIts Hydrogen scrambling reactions H-D exchange between Da and Ha (Ha/D, = 48/52,35.5 ml STP) took place readily at 0 “C in the bromobenzene solution (10 ml) of CoBr(PPhs)s (0.2 mmol) in the presence of BF, -0Etz (O-96 mmol); the gas composition after 2 h was HD 37%, Dz 33% and Hz 30%_ No exchange was observed in the absence of BF, - OEta within 4 h. The hydrogen scrambling between Dz and monoolefin such as ethylene, propylene, or 1-butene also proceeded under simiIar reaction conditions (TabIe l), aIthough no hydrogenation product was detected. The same treatment of 1,3-butadiene with Da, however, gave mainly the Da-adduct, C4HsD2, as shown in Tabie 1 (run 4), the hydrogen exchange between D2 and diene being significantly suppressed_ It is also shown that the isotopic scrambling between Hz and Da is effectively suppressed during the hydrogenation, as indicated by the run with an Ha-D, mixture (run 5), where the products are in conformity with a paired addition of Hz or Dz [9] to 1,3-butadiene. TABLE 1 H-D exchange reaction between Dz and olefin or 1,3-butadiene CoBr(PPhg)s, 0.2 mmol; BFa-OEta, O-96 mmoI; PhRr. 10 ml_ Composition of starting deuterium (do = 3_1%, dl = 11.6%, dz = 85_3%). Run
Deuterium
Starting material (ml s.t.p_)
.
distribution
of products
(W)
J52
HD
D2
do
dl
d2
d3
de
c2H4
(21-g),
D2 (21-4)
14.1
37-7
48.2
30-3
23-7
21-7
15-9
C,%
(22.21,
D2 (21.0)
39.3
23.8
37-O
35.5
37-l
19.5
6.8
1.1
D2 (22.0)
13-7
40.8
45-5
62.5
25.9
S-7
2.9
-
54
55
3-O
11-l
85.9
64.5
-
45.2
49-3
l-C4Ha
(22.2),
C4H6
(22-2),
D2 (24-S)
C4Htj
(30-41,
Hz (185)
51-5
3-2
3.8
56-
57
S-4
58Wle)
1.9
5.1
28.5
23.5
3.8
19.5
D2 (11.5)
Hydrogenation
of conjugated
dienes
Hydrogenation of dienes (3-O mmol) was performed in the bromobenzene solution (20 ml) of CoBr(PPh,), (O-2 mmol) in the presence of BFs-OEtz (0.44 mmol) at 0 “C under a constant pressure of hydrogen (76 cmHg). The results are summarized in Table 2.1,3-Butadiene, 2-methyl1,3-butadiene, 1,3_pentadiene, 1,4-pentadiene, 1,3-hexadiene, and 1,3-cyclooctadiene are selectively hydrogenated to the corresponding monoenes, in which the double bond is terminal, only trace amounts of aIkanes being detected_ It is noteworthy that 3-methyl-1-butene, 1-pentene, and 1-hexene are mainly formed from 2-methyl-1,3-butadiene, 1,3_pentadiene, and 1,3-
83 37 53 38 27
60 260 60
1110
840
1,3aPontadienc
1,49ontadiono
1,3Jiexadieno
*Otherproductsare 1,3cyclooctadiene(40%)and 1,4-cyclooctadiene (46%),
1,3-Cyclooctadiono
96
30
S.Mothyi1,3wbutadione
t-2aButone -0 2.Mothyllebuteno 4.6 t-2.Pcntenc 21 t-2wPonteno 12 t-2.Hcxcnc 18
1.Butone 09 3.Mothyll-butono 86 l-Pentone 56 l-Pontono 65 l-Hexenc 61 Cyclooctene 100 Cycloocteno 14”
28
Productdistribution (%)
Conv, (%)
90
Timo (min)
1,3mButadiono
Dieno
c-2.Butene 11 2mMothyl. 2ebutone 9,l cs2ePontene 23 c*2dcntone 23 c-2.Hoxene 22
TABLE2 Hydrogenationof dienescatalyzedby CoBr(PPha)a Diene,3,Omm& CoBr(PPh&,0,2 mmol;BFa*OEtz,0,44 mmol;PhBr,20 ml; PHZ,76 cmHg;temp. o Y_!,
0.46
0,ll
0*97
1.83
0,19
Initialabsorption rate of hydrogen (ml/min)
179
hexadiene, respectively, the substituted doubIe bonds being preferentially hydrogenated_ The initial absorption rates of hydrogen, and the conversion of dienes (Table 2) show that the rate of hydrogenation decreases in the order, 2-methyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,3-butadiene, 1,4-pentadiene. 1.3-cyclooctadiene, l,Scyclooctadiene_ During the hydrogenation of less reactive 1,5_cyclooctadiene, 1,3- and 1,4_cyclooctadiene were found as isomerization products, while no isomerization product of 1,4-pentadiene was detected_ It is clear that the catalyst system is less active for the isomerization of unconjugated dienes_
”
60
120
180
240
300
Time (mm]
Fig_ 1. Time
course of the hydrogenation
BFs-OEt2,0.66
of 1,3-butadiene_
CoBr(PPh,),,
0.6 mmol;
mmol; PhBr, 30 ml; 1.3-butadiene, 9.0 mmol;PHq, 76 cmHg; temp., O”C_
100
80
Q T;W g =
s
040
20
0
m
60
4.0
lii
Fig_ 2_ Time course of mmol; BF3-OEt2.0.66
cmHg; temp., 0 “C.
60
100
(mm1
the hydrogenation of 2-methyl-1,3-butadiene- CoBr(PPh3)3, 0.6 mmol; PhBr, 30 ml; 2-methyl-1,3-butadiene, 9.0 mmol; pH2, 76
180 The time courses of the hydrogenation of 1,3-butadiene, 2-methyl-1,3butadiene, and 1,3-pentadiene under the same conditions are shown in Figs_ 1, 2, and 3, respectively- 1,3-Butadiene gives 1-butene, which is rapidly iso-
b Fig_ 3_ Time course of the hydrogenation of 1,3-pentadieneCoBr(PPh&, 0.6 mmol; BF,-OEt2,0_66 mrrol; PhBr. 30 ml; l-3-pentadiene, 9-O mmol; PH*, 76 cmHg; temp_, 0 ‘TX C& = pentadiene, C$, = pentene_
merized to 2-butenes when 1,3-butadiene is mostly consumed. 2-Methyl-1,3butadiene is consumed more rapidly than 1,3-butadiene to give $-methyl-lbutene exclusively, the double bonds at 3 position being hardly hydrogenated, whereas the enhanced isomerization of produced monoene, as observed in the hydrogenation of 1,3-butadiene, is not observed even after ah the 2-methyl-1,3-butadiene is consumed. In the hydrogenation of a mixture of ci’s- and trans-1,3-pentadiene (trans/cis = 64/37), the trans-isomer is consumed faster than the cis-isomer, and a preferential hydrogenation of the internal double bond is also found, although the isomerization of 1-pentene produced proceeds more extensively than that of 3-methyl-1-butene during the hydrogenation of 2-methyl-1,3-butadiene. To investigate the coordination and reactivity sequences of 1,3&enes, the competitive hydrogenation of 1,3-butadiene with 2-methyl-1,3-butadiene, or 1,3-pentadiene, was carried out at a lower catalyst concentration (CoBr(PPhs)s 0.3 mmol, BFs - OEta 0.44 mmol) following the time course in detail_ The results are shown in Figs. 4 and 5. In both cases, 1,3-butadiene significantly retards the hydrogenation of the other dienes, which are rapidly hydrogenated in the absence of 1,3-butadiene, as shown in Figs, 2 and 3_ In particular, 1,3-pentadienes are hardly hydrogenated before 1,3-butadiene is almost consumed_ Thus, the coordination sequence of 1,3dienes on the
181
Fig. 4. Time course of the competitive hydrogenation of 1,3-butadiene and 2-methyl-1,3butadiene. CoBr(PPha)s, O-3 mmol; BFQ-OEt 2, O-44 mmol; 1,3-butadiene, 2.5 mmol; 2methyl-1,3-butadiene, 2.5 mmol; PhBr, 30 ml; PH*, 76 cmHg; temp., 0 “C. =Based on 1,3-butadiene and 2-methyl-1,2-butadiene, respectively_
0
30
60
90
120
150
Time (mm1
Fig. 5. Time course of the competitive hydrogenation of 1,3-butadiene and 1,3-pentadiene_ CoBr(PPb&, 0.3 mmol; BF3-OEt 2.0.44 mmol; 1,3-butadiene, 2.5 mmol; 1,3pentadiene, 2.5 mmol; PhBr, 30 ml; PHI, 76 cmHg; temp-, 0 “C- aBased on 1,3-butadiene and 1,3-pentadiene, respectively. ck = butene, Ca = butadiene, C& = pentene, Cs = pentadiene. active cobalt(I) species is assessed to be 1,3-butadiene > 2-methyl-1,3-b&adiene > 1,3-pentadiene, which is in contrast with the reactivity sequence of dienes obtained in individual runs: 2-methyl-1,3-butadiene > 1,3-pentadiene > 1,3-hexadiene > 1,3-butadiene (Table 2).
182
Effect of solvent When BF, -0Etz was added to the benzene solution of CoBr(PPh, )a in the absence of diene, the colour of the solution turned from pale green to blue giving a black precipitate in a few minutes at 0 “C. The foliowing disproportionation probably takes place in the absence of diene [ 71, 2CoBr(PPhz)s
CoBrz(PPhs)z
-
+ 4PPhs + Co
whiIe the formation of the black precipitate was much less extensive in bromobenzene after 30 min at room temperature_ The color of the benzene or bromobenzene solution containing both CoBr(PPh3), and one of the dienes, however, immediately turned to deep green on the addition of BFz - 0Et2 and remained almost unchanged without forming any precipitate. The catalyst solutions thus prepared absorbed molecular hydrogen smoothly without an induction time- These observations indicate that the diene, as well as bromobenzene, stabilizes the cobalt complex activated with BFa - OEtz, aTthough no direct evidence for complex formation could be obtained by n.m.r. measurement because of the paramagnetic solutions. Mechanistic study of the hydrogenation The kinetics of the hydrogenation
was investigated with 1,3-pentadiene by fohowing the hydrogen uptake as a function of time. The reaction was inSated by introducing hydrogen gas. As shown in Fig_ 6, the rate of uptake
Tie
(mm)
Fig. 6. Effect of hydrogen pressure on the rate of hydrogenation_ CoBr(PPh&, BF3-OEt2,1.32 mmol; 1,3-butadiene, 6.0 mmol; PhBr, 20 mI; temp., 0 “C;PH~, 50 (a), 25 (0) cmHg.
0.5 mmol; 75 (0).
183
is proportional to hydrogen pressure, no induction period being observed, while the rate is independent of the diene concentration, as suggested by the results in Fig_ 6 and confirmed by a run at a lower diene concentration. Time courses of the hydrogenation of 1,3-butadiene (Fig- 1) and a-methyl1,3-butadiene (Fig_ 2) also indicate that the rates of hydrogenation are almost independent of these dienes’ concentration over 50% conversion. The rate was also proportional to the concentration of CoBr(PPhS)S in the presence of an excess amount of BF, -OEtz (BF,-OEt,/CoBr(PPhB)s = 2.6). No appreciable difference was observed in the rate of hydrogenation when the halogen ligand in CoX(PPh,), was changed_ The addition of triphenylphosphine decreased the rate of hydrogenation_ The absorption of hydrogen was still observed at PPh,/CoBr(PPhB)3 = 3, at which ratio the ethylene dimerization was completely retarded, as reported in the previous paper [7] _ An isotope effect was observed in the hydrogenation of 1,3-butadiene (~/rn= 1.6). Deuteration products of 1,3-butadiene and 1,3-pentadiene, C4H6D2 and C5HsD2, were identified by n.m.r_ to be CH,=CH-CHDCH2D and CH*=CH-CHD-CHD-CH,, respectively, as shown in Fig. 7, demonstrating 1,2- and 3,4-addition to 1,3-butadiene and 1,3-pencadiene, respectively. The addition of BFB-OEt2 seems to be essential for the activation of the cobalt complex, since the rate of hydrogenation increases almost linearly with increase in BFB - OEtP concentration up to BF, -OEt,/CoBr(PPhs)B = 1, where the rate attains a plateau value, as is seen in Fig_ 8_ The same effect of the BF3 - OEtP concentration was found with the same cobalt(I) complex catalyst in the dimerization of ethylene [7] _ Silver perchlorate, however, also activates an analogous cobalt(I) complex, CoBr(PPhzMe),, for the hydrogenation, as noted in Table 3, although the effect of AgClO, concentration was difficult to examine because of its low solubility. The solution of CoBr(PPha)3 catalyst and 1,3-diene absorbs hydrogen much less readily on the addition of AgC104 instead of BF, - 0Et2. It is known that AgClO, reacts with triphenylphosphine to give [Ag(PPh,),] f [ClO,] - (n d 4) [lo] _ When -CoBr(PPh;?Me), was used instead of CoBr(PPh,), in the hydrogenation of 2-methyl-1,3-butadiene and 1,3_pentadiene, AgClO, showed essentially the same effect on the catalytic activity and selectivity. In the absence of BF,-OEtz or AgC104, a very slow uptake of hydrogen was observed with the bromobenzene solution of CoBr(PPh,), and one of the 1,3-dienes, while the monoenes formed were completely different from those described in Tables 2 and 3; cis-2-butene, 2-methyl-2-butene, and cis-2-pentene resulted from 1,3-butadiene, 3-methyl-1,3-butadiene, and 1,3-pentadiene, respectively, suggesting that the reaction path is different_ Discussion On the active cobalt species The present results demonstrate that the addition of BFB - OEta remarkably activates halogenotris(tertiaTphosphine)cobalt(I) complexes as catalysts
184 a) Cl-i: = CH-CHD-CHID
bl CH: = CH CHD-CHD-CH.
528
‘D-IHI
Fig. 7. ru’.m.r. spectra
of the deuteration
products_
589
C4HsDz
(a), and CsHsDz
(b)_
for the partial hydrogenation of dienes_ Similar activation by BFs - OEta has been shown for a haIogeno(a-aryl)bis(triphenylphosphine)nickel(II) complex [ 111) in addition to that noted for the cobalt(I) complexes [ 7 3 in olefin dimerization. Such an effect of BFs - OEts might be caused by abstraction of the PPh, Zigand from the cobaIt(I) or the Ni(II) complex. No interaction of BFs -0Eta with PPh,, however, was detected by 3’P-n.m.r_ in a well dried bromobenzene soIvent_ The colour change of CoBx(PPhs)s solution on addition of BFs -0Etz and the 111 interaction (Fig. 8) suggest the formation of a complex which is stabilized by the presence of diene_ The folIowing reaction is suggested I CoX(PPh,)s -
+ BF,-OEts + diene [Co(PPh,)3 (diene)]+
[BFs-XI-
+ EtzO-
185
Fig_ 8. Rate of hydrogen uptake us. BF, -OEt&oBr(PPh&_ PhBr, 20 ml; 1,3-pentadiene, 3-O mmol. temp_. 0 “C; Paz,
CoBr(PPh&, 76 cmHg_
0.2
mmol;
Another reason to justify the above type of interaction is that AgC104, which is frequently utilized for the preparation of cationic transition metal complexes [12], activates the CoBr(PPhsMe)a complex for the partial hydrogenation of dienes. It should be noted, however, that the extent of acti3 ) 3_ Indeed, it has been rqorted vation by AgClO, is not great for CoBr(PPh that 31P-n.m.r. does not show any interaction of AgClO, with PPhsMe or PPhMes in bromobenzene [ 133, while complexation of AgC104 is detected with PPhs. It is reasonable to assume that AgC104 removes not only the bromide ligand, but also PPh, from CoBr(PPh,), to give an unstable species, which is possibly transformed into an inactive form_ On the other hand, -BFs - OEts does not interact even with PPhs as described above, so that the etherate can activate the triphenylphosphine-Co(I) complex_ The role of BFs - OEt, can thus be understood in terms of the cationic cobalt(I) complex_ Mechanism of diene hydrogenation Recently Schrock and Osbom reported that a cationic rhodium(I) complex [ Rh(NBD) (diphos)] + [ Cl0.J - (NBD = norbomadiene, diphos = bidentate phosphine ligand), efficiently catalyzes the partial hydrogenation of 1,3dienes and that the rate determining step of the hydrogenation is the reaction of molecular hydrogen with [ Rh(l,3diene) (diphos)] + species [14] In this hydrogenation, 1,4-addition was predominant, giving 2-methyl-2butene from 2-methyl-1,3-butadiene with 70 - 90% selectivity, whereas the present cationic Co(I) catalyst undergoes mainly 1,2addition, giving 3methyl-1-butene as the main product (>80%) from 2-methyl-1,3-butadiene, and also 3,4-addition, giving I-pentene from 1,3-pentadieneThus, there seems to be a difference in the mechanism of hydrogenation-
1,BSentadiene
BF3a0Et2 (0,44) ApJJl04 (OJ)
CoBr(PPhzMe)s CoBr(PPhzMo)a
87 17 66 82 67 22
60 60
Conv, (%)
30 100 16 16
Timo (min)
*3.Methyl-1.buteno, 2.Mcthybl-butono, and 2eMethyI-2.butone,respectively, **lqentene, fransd-pontono, and G-2.pentone, rospcctivcly.
S-Methyl1,3-butadiena
BF3+OEt2 (0,44) AgCI04 (0,l) BFS+OEt2 (0,44) AgC104 (0,l) !
CoBr(PPh& CoBr(PPh& CoBr(PPhzMo)a CoBr(PPhzMe)s
1,3diene
Activator (mmol)
Complex
Effect of A@104es tha activator Co(I) complex, 0,2 mmol; 1,3-dlcnc,3,Ommol; PhBr, 20 ml; temp,, 0 “0, &I,, 76 cmHg,
TABLE 3
86 61 44 83 1.P”” 65 68
3.Me-l-b*
26 26
1.;;**
4,4 2,3 39
9,l 36 17 11 c-2-p** 10 690
2.Mo.l.b” 2.Me-2-b*
Product distribution (%)
187
The rate determining step of hydrogenation, as suggested by the first order kinetics in hydrogen pressure and in the cobalt(I) catalyst concentration, as well as by the zero order kinetics in diene concentration, is also the addition of molecular hydrogen to [Co(PPh,), (diene)] + species. Accordingly, it is suggested that hydrogen attack may be different from that on the rhodium(I) catalyst_ Indeed, the rhodium(I) species has two coordination sites available for oxidative addition of molecular hydrogen, while the cobalt(I) species has only one site available, so that molecular hydrogen would have to be heterolytically dissociated to leave a hydride on Co(I) species, that is
H-C
H-C
(P=tertiaryphosphine)
In view of the cationic nature of the Co(I) complex, the attacking hydrogen molecule would be polarized, with the negative end being closer to the Co(I) ion, and the other positive end attacking on the coordinated diene molecule. In fact, it is commonly observed in the present study that the addition of hydrogen takes place more readily at the double bond with more substituents, or of higher electron density_ This is a unique feature of the present hydrogenation which seems reasonable on the grounds of the electrophilic nature of attacking hydrogen_
References 1 B_ R. James, Homogeneous Hydrogenation, Wiley, New York, 1973_ 2 J_ Kwiatek, I. L. Mador and J_ K_ Seyler, J_ Am. Chem. Sot., 84 (1962) 304. 3 Y_ Tajima and E_ Kunioka. J_ Catal.. ll(1968) 83; M_ Iwamoto, J. Chem. Sot_ Jpn_ (Ind_ Chem. Sect.). 71 (1968) 1510_ 4 A. Misono and I_ Ogata. Bull_ Chem_ Sot_ Jpn., 40 (1967) 2 718_ 5 M_ E. Volpin and I. S. Kolomnikov, Russ. Chem. Rev., 38 (1969) 273. 6 K. Kawakami. T_ Mizoroki and A_ Ozaki_ Chem- Lett__ 11976) 847_ 7 K. Kawakamii ‘I?_ Mizoroki and A. Ozaki; Bull. Chem_ _&x. J&X_, 51 (1978) 21_ 8 N. Kawata, T. Mizoroki and A_ Ozaki, J. Mel_ CataL, 1 (1975/76) 275_ 9 A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Kodansha and Academic Press, Tokyo and New York. 1977, p_ 78. 10 F. A. Cotton and D_ M. L. Goodgame, J_ Chem. Sot_, (1960) 5 276. 11 K. Maruya, T_ Mizoroki and A. Ozaki, BuU. Chem. Sot. Jpn., 45 (1972) 2 255; N. Kawata, K. Maruya, T. Mizoroki and A. Ozaki, Bull. Chem. Sot_ Jpn., 47 (1974) 413. 12 H. C. Clark and K_ K. Dixon, J. Am. Chem. Sot., 91(1969) 596; P_ M. Druce, M. F. Lappert and P_ N_ Riley. Chem_ Commun_, (1967) 486; R_ R_ Schrock and J_ A_ Osborn, J. Am. Chem. So+ 93 (1971) 2 397_ 13 T_ Koike, K. Kawakami, K. Maruya, T. Mizoroki and A. Ozaki, Chem. Lett., (1977) 551. 14 R_ R_ Schrock and J. A_ Oshorn, J_ Am_ Chem. Sot., 98 (1976) 4 450.