New active and selective catalysts for homogeneous metathesis of disubstituted alkynes

Journnlo~KoiecrclarCatalysis,15 (1982)93

- 10E

NEW AfXTVE METATHESIS

AND SELECTIVE OF DISUES~

A. BENCHEIC~

M. PETIT, A. MORTREUX*

Luborafoire de Cizimie Ogcnique 69652 Villeneuue d’Ascq CPdex

93

CATALYSTS ED ALKYNES

App6queh. (France)

FOR

HOMOGEhiOUS

and r’. PRTIT’ ERA

C.N.R.S.

158. B-P-

108,

Homogeneous metathesis of disubstituted alkynes is performed catalytically on new 0&o(acac)2-ALEt3-PhOH combinations. These systems are much more efficient than the L,Mo(CO),-LhOH catalysts which require thermal activation; activities as high as 16 800 h-l are reached at 110 “C, with entire selectivities in acetylenic hydrocarbons without isomerization. A mechanism is proposed, in which the role of phenol is to interact with the triple bond and to enhance alkyd scrambling in metallacyclobutadiene intermediates for metathesis compound production.

Introduction Homogeneous metathesis of disubstituted alkynes has already been performed on molybdenum type catalysts, in which the metal is initially in a low oxidation state, e.g. Mo(CO)s and (Mes)Mo(CO)s, in the presence of phenolic reagents as cocatalysts [I - 53 : 2R,--C=C--R,

['Mo']IPhOH (

R+Z=C:R,

f R,-C=C-R2

Since it is well known that active species for metathesis of olefinic hydrocarbons are generated from both low and high oxidation state transition metal precursors [6], it seemed to us of interest, during our investigations of the mechanism of the above reaction, to look for new catalytic systems arising from other low oxidation state complexes of molybdenum L,Mo(CO), and especially from higher oxidation state Group VI B salts, reduced by aluminium alkyls. ..A_lthough early attempts in this direction with WCls and MoCls [7] failed before our discovery of the ‘Mo(CO)s-PhOK’ combination, this paper will show that the most active systems obtained so far are produced by this reduction method; we wiB try therefore to define the mechanistic implications of this catalytic reaction. *Authortowhom

corre.spondenceshou:dbeaddressed. @E~evierScqucia/Frinted

inThe Pt’efheriands

94

Experimental

Preparation 0 f catalysts

PPh,Mo(CO), [8], (NBD)Mo(CO), [IO] and (i,3-cyclohexadiene).@o(CO)s according to the literature.

191, (cycloheptatriene) Mo(CO)s [II] were prepared and purified

Reactants The typical hydrocarbon used in this series of experiments is 4-nonyne, purchased from Farchan _ This hydrocarbon, as well as toluene and nonane, was freed from peroxides and water by distillation from CaPIsunder nitrogen_ Catalytic reactions with L,Mo(CO), The catalytic tests were conducted under nitrogen in a 20 ml roundbottomed flask equipped with a reflex condenser. After introduction of phenol (94 mg, 1 mmol), toluene (8.75 ml) and 4-nonyne (0.16 ml, 1 mmol) by means of a hypodermic syringe, the reactor was heated to 110 “C in 2 thermostatted oil bath. The catalyst was then introduced in solution (1 ml. IO-’ mmol) and the products collected at regular intervals for analysis. Catalytic reactions with reduced systems The catalyst was prepared in toluene during 5 min at ambient temperature by interaction between the transition metal salt and the organoaluminium compound in order to obtain a 10m2 M solution. One ml of this solution was injected ai; the desired temperature into the reaction flask containing 9 ml of a toluene solution of 4-nonyne (1 mmol) and phenol (1 mmolj. Ahquots were collected at regular intervals and poured immediately into a 20% NaOH solution in order to stop the reaction and eliminate the phenolic reagent into the aqueous phase. Analysis Conversion (I) and selectivities were determined by gas phase chromatography using ;L3 m X 0.32 mm 10% SE 30 column, with nonane as internal standard. The activities were obtined from the initial slope in the curves T = f(t), and expressed as turnover rates, moles of substrate transformed per mole of catalyst per hour.

L,Mo(CO), catalysts As shown in Table 1, several L,Mo(COj, complexes can be used as catalytic precursors for metathesis of 4-nonyne, some of them exhibiting activities higher than Mo(CO),.

95

TAELE

1

Activity of different L,Mo(CO),-FhOH Catdytic precursor

Activity (h-l)”

Mo(C0)6

120

catalysts.

(PPh&~%o(C0)5

48

(Norbornadiene)-

(CycIoheptatriene)-

Mo(COIc

Mo(C0)3

adiene)zMo(C0)2

312

216

240

aAII selectivities in d-octyne and S-deeyne > 95%. Solvent = toluene (IO ml); [s] = [PhOH] = 0.1 M; [B]/[Mo]

h4oiybdenw-n oxyphenoxide

and molybdenyl

precursors En the presence of AlEt,, UMo(OPh), catiytic mixtures at ambient temperature TABLE

(1,3cycIohex-

= 100; T = 110 “C.

acetylacetomte

as aztaly

tic

and O,Mo(acac), give efficient or lower (Table 2).

2

Preliminary resuits on reduced systems at ambient temperature for 4-nonyne metathesis. ck31yst

OMo(OPh)+

Activity (h-l)

0.3

Solvent = toluene (10 ml); [W]

= 1.

2.6

= [PhOH]

= 0.1 M; [B]/[Mo]

= 10; [AlEt3]/[Mo]

Effect

of fhe Ai/rMo rafio on the catalytic activity Since it is known that for olefin metathesis an excess of orgarroalumtiium compound generally enhances catalytic activity, a series of experiments has been performed on the 0&u(acac)2 -AL&-PhOH combination by varying the AI/MO ratio. These results are summarized in Table 3 and show that maximum activity is practically reached for an Al/hforatio of 6, and is already better at 30 "C than the most active I&Ma(CO), type catzdyst at 110 “C. TABLE 3 Effect of the Al/&so ratio on the catalytic activity during 4-nonyne metathesis. Al/M0

1

3

4

5

6

IO

Activity

3.2

107

204

272

333

3%

U--=1 Solvent

= toluene (10 ml);

[a]

= [PhOH]

= 0.1 M; [a]/[Mo]

= 100; T = 30 “C.

96

Tempemture dependence; optimization of the catalytic system Most homogeneous catalytic systems for olefm metathesis are efficient at ar.?bient temperature so that only 2 few of them have been the subject of a detailed study zt elevated temperatures; increasing activity has been observed, but to the detriment of selectivity at high temperature [X2]. Within the range 0 - 110 %. selectivity in 4-nonyne and Sdecyne metathesis is not affected at all by this new O,Mo(acac),-AlEt,-PhOK (l/6/100) system, so that an activation energy of 8.2 + 1 kcaI/mol can be cakxlated from an Arrhenius plot. A system has been optimized at 110 “C and gave initial rates of 3480 h-l with a [CS] /[MO] ratio cf 100 (_41/Mo = 3) and 16 800 h-l with a [m] /[MO] ratio of 1000 (AI/MO = 6), the equilibrium being reached in each case within 5 min. blffect of phenol on tF,z reaction From the former study on the Mo(CO),-PhOH catalytic combination [ 21, a first-order rate in [PhOH] has been observed. Here again, the absence of phenol completely inhibits metathesis, the starting material remaining unchanged. The results in Table 4 indica’ti that a first-order rate law can be considered again with this system. TABLE Effect

4 of phenol

concentration

on the metathesis

[ThOI-I]

0

0.025

Activity

c

100

M

rate.

0.05 M

0.1 M

0.2 M

196

435

855

(h-l) Solvent = toluene 100; T = 60 ‘C.

(10

ml);

;m

] = [PhOH]

= 0.1 M.

[Al]/[Mo]

= 6; [m]/[Mo]

=

Nature of the reducing agent At 60 “C under the Same conditions with O,Mo(acac)s, trimethyl aluminium, triisobutylaluminium and bis(isobutyl)aluminiumhydride, all of low Lewis acidity, give catalytic mixtures cf good efficiency (Table 5). Nevertheless, AlEt,Cl, AlEtClz, ZnEt,, SnMe,, NaF3H4 and LiAIHe are totally inefficient. TABLE Effect

5 of the natcre

of reducing

agents

on rate of metathesis

Reducer Activity

of 4-nonyne. Xl(isobutyl)3

1980

435

(h-l) =T = 110 “C. Conditions: as in Table

4.

696

830

97

Other catalytic

precursors

These results encouraged us to look at new systems arisiig &om reduction of Group VIB transition metal s&s by AlEt,. Some successful attempts are given in Table 6. TABLE

6

Activity

of reduced

Group

VI B transition

metal

salts for I-nonyne

metzthesis.

Catalytic precursor

Cr(acac)$

[Mo(C0)5L]-[NBu4]+

Mo(acac)3

Mo(NO)zCl#y)zb

W(CO)&$

Activity

0.3

13

750

1433

0.2

(h-l) ‘Selectivities < 70%. b[C=C]/[MO] = 100. Solvent = totuene (10 6; ‘I‘ = 110 “C.

ml);

C-1

= [PhOH]

= 0.1 M; [5]/[Mo]

= 10;

[Al]/[Mo]

=

Discussion

L,Mo(CO),

cafulytic precursors

As expected from our first experiments in this field, and those conducted later on by Devajaran and coworkers [5], all molybdenum carbonyl complexes mentioned in this study are active for alkyne metathesis. Thermal or photochemical 131 labilization of carbon monoxide to give unsaturated Moo species is certainly one of the initial processes necessary to provide the catalytic moieties_ Such an assumption can be further evidenced by other experiments made on a Chat&type complex, obtained by reduction of molybdenyl acetylacetonate with AlEt in the presence of dppe under nitrogen [13] ; one obf&ns a yellow-orange complex, exhibiting an infrared rN=N band at 1985 cm- ‘. This dinitrogen complex is slightly less active than the preceding ones, as .a turnover rate of 78 h-’ is observed at 110 “C, but promotes metathesis at lower temperature (90 “C)_ These results show.that (i) carbonyl groups are not necessary to obtain metathesis catalysts; (ii) vacant coordination sites can be-provided fiorn dinitrogen complexes of molybdenum, probably by easy removal of this labile ligend by the acetylenic substrate_ Catalysts of this type have already been used for metathesis of olefins in the presence of AlBr3 [14]. From these observations, an in sifrr preparation of a new system was investigated, starting directly from catalytic amounts of O&o(2cac), (0.1 mmol), AlEt (0.2 mmol) and diphenyiphosphinoethane (0.2 mmol) which converted 4-nonyne (I mmol) with equimolar amounts of phenol in toluene (10 ml) at 80 “C to an equilibrium mixture of 4-octyne and 5decyne within 30 min. High

oxi&fiorc state catalytic precursors Reduction of molybdenum s&s by an excess of AlEts

must lead to

98

low oxidation levels of metal (Mo(acac)a rs reduced to Moo during the pr+ paration of Chatt’s complex [ 13]), but there is also the possibility of praducing carbene moieties

of the L,Mo=C

AH3

type, arising from a-hydride ‘H elimination in alkylmolybdenum complexes 1151. If such a carbene is formed during reduction of the molybdenum salt, a mechanism involving metallacyclobutenes as intermediates, already proposed during the initiation step of polymerization of alkynes on tungsten carbene complexes [I(?] would be particularly attractive:

PbOH

Scheme

1. Metallocarbenetype

R

mechanism

for alkyr.e

metathesis

In this scheme, one can suggest that the acidic phenol reagent might !Iave several roles: (ij Through intermolecular hydrogen bonding 131, the triple bond character is weakened, so that the hydrocarbon is rendered much more olefinic in nature; this function should be particularly efficient at avoiding side reactions such as polymerisation ]lS] or cyclotrimerisation. (ii) A destabilising effect on the transient metallacyclobutenes is a consequence of the above assumption. (iii) Scrambling of the alkyl units in the metallacyclobutenes can also be achieved by the acidic character of this essential ‘cocatalyst’. The intervention of such intermediates is supported by recent investigations on titanacyclobutene compounds, which are now easily prepared by interaction of the potential metallocarbene ‘Tebbe’ reagent Cp2TiCH2A1Me2Cl and disubstituted acetylenes [17] _ However, an alternative mechanism could be envisaged if one considers that metallocarbynes are formed during the initiation process [18]:

Ma:athesis

Campaeads R---Ill Scheme

2. Metallocerbyne-type

XH

mechtim

for dkyne

metaathti

99

Here again, the phenolic co-catalyst wouId be necessary to induce isomerisation of the metallacyclobutadiene intermediates; proton addition to the metaIIacyclobutad.iene 1 would be abIe to give a cationic ?r-allyE complex intermediate which further rearranges to 2. Unless ahcyhnolybdenum carbene and carbyne complexes are prepared and shown to be CatalyticalIy active or inert under these conditions, we cannot decide whether the process is initiated by metallocarbene or carbyne moities. In this context, attempts to catalyse metathesis of Pnonyne with phenol a~ co-catiyst, by the phenyltungsten carbyne complex C1(CO)4WZ-CGHs (prepared in sr’fu by interaction of the carbene complex (CO},with CJ&AlCl, Cl91 ) have failed. However, this experWC(OCHs)C&” iment cannot exclude the carbyne mechanism, as only littie activity was found with W(CO),CI, as catalytic precursor (Table 6). Carbyne compIexes might be present in the Cr(acac)a-MEt, catalyst system, since mixtures of phenyl and p-tolyl chromium cartyne complexes have been shown to decompose to give the cmetathesis’ compounds toIan, phenyl-p-tolyIacetyIene and di-p-tolylacetyIene 1203. This ‘coupling’ reaction did not ‘de pIace with tungsten carbyne compIexes under the same conditions. Coming back to the ‘carbene’ type mechanism depicted in Scheme 1, one can assume that if the yield of initial carbene is reasonabIy good, this should induce the production of initial by-products; if R is a methyl radical, one should observe at the early stage of the reaction at least 2-pentyne and 2-hexyne, if catalysis is performed with 4nonyne as the substrate. Unfortunately in our hands, we have not been able to detect any of these compounds at low conversion, even with large amounts of catalysts (C=--%/Mo = 5). So, whatever is the mechanism, a question arises concerning the initiation step of this reaction, which could aIso be applied to molybdenum carbonyl complexes, where no reducing agents are required. The answer is not clear, but the absence of initial acetylenic by-products favours somewhat the carbyne-type mechanism. These carbyne species could be made directly by interaction of the molybdenum unsatruated moieties with the aIkyne in the presence of phenol, so that the alkylcarbyne groups immediately undergo productive metathesis. FinalIy, one must also keep in mind that if such mechanisms are consistent with labelling experiments [al, these metahocyclic pathways cannot exclude other reaction schemes in which cyclobutadienic complexes might intervene: flash vacuum py-ro?ysis of substituted q4-cyclobutadiene n5-cyclopentadienyl cobalt complexes produces metathesis products [2X], whereas an asymmetrically tetrasubstituted cyclobutadiene molybdenum complex requires Fhenol to thermally decompose into metathesis products at equihbrium [22].

*We thar& Dr. Y. aauviin for a generous gift of this complex.

100

conclusion The above results show that efficient catalytic systems for alkyne metathesis can be provided by reduction of molybdenum salts by ahrminium alkyls, giving rise to activities very much higher than with molybdenum carbonyls L,Mo(CO),. In any case, it is found that a phenolic co-catalyst is essential to induce productive met&thesis, so that this co-catalyst must be taken in account in the proposed mechanism(s)_ From the present data, and incared spectroscopic results [3], one can suggest that the primary role of phenol is to remove some acetylenic character of the substrate, through hydrogen bonding with the triple bond. A second determining role is assigned to this particular reagent, which is to make possible the scrambling of the alkyl units in the suggested metallacyclobutadiene intermediates. This hypothesis is presently the subject of further work. Electrochemical investigations 1231 of these new systems are also in progress in order to elucidate the initiation step in this reaction and confirm the critical role of phenoi in these highly active catalytic combinations.

Acknowledgements We are deeply

indebted

to Mrs. F. Hlawka

for technical

assistance.

References 1 A. Mortreux and M. Elanchard, J. Chem. Sot.. Chem. Commun., (1974) X37. 2 A. Mortreux, N. Dy and M. Elanchard, J. Mol. Catal.. I (297W76) 101. 3 A. Mortreux, J. C. Delgrange, M. Elanchard and B. Lubochinsky, J. hid. Cataf.. 2 (1977) 73. 4 A. Mortreux, F. Petit end M. Elanchard, Terrahedron. Lett.. 2 (1978). 4967. 5 S. Devajaran. D. R.-M. Walton and G. J. Leigh, J. Organomet. Chem.. 281 (1379) 59. 6 For recent re-ziews on ole6n metathesis, see (a) 5. C. Mol and J. A. Moulijn, AduCatal.. 24 (1975) 131; (b) R. J. Haines and G. J. Leigh, Chem. Sot. Reu.. 4 (1975) 1, (c) T. J. Katz. cldv. Organomet. Chem.. 16 (1977) 283; (d) J. J. Rooney and A. Stewart, Catal.. I (1977) 277; (e) R. H. Grubhs. Frog. Inorg. Chem.. 24 (1978) 1; (f) N. Calderon. J. P. Laurence and E. A. Ofstead. Adu. Organomet. Chem.. Z 7 (1979) 449. i R. Greco, F. Pirinohi and G. Ddl’Asta, J. Organomet. Chem., 60 (2973) 115. 8 T. A Magee, C. N. Matthews, T. S. Wang and J. H. Wotiz; J. Am. Chem. Sot.. 83 (1961) 3206 9 M. A. Bennett and G. Wilkinson, Chem. fnd.. (1959) 1516. 10 E. W. Abel, M. A Bennett, R. Burton and G. Wilkinson, J. Chem. Sot.. (1958) 4559. 11 E. 0. Fischer and W. Froblich. Z. Nafurfoorsch.. 156 (1960) 266. 12 J. L. Wang and Ii. R. Menapace, J. Catat.. 28 (1973) 303. 13 M. Hid+ K. Tominari, Y. Uchida 2nd A. Misono, J. Chem. Sot.. C:hem. Commrrn. (1969) 814. 14 M. Hidai, T. Tatsumi end Y. Uchida, Bull. Chem. So;. Jpn. 47, 12 (1974) 3177. 15 R. H. Grubbs and C. R. Hoppin, J. Chem. Sac.. Chem. Commun. (1977) 634. 16 T. J Katz and S. J. Lee, J. Am. Chem. Sot.. ZC2 (1980) 422.

101 F_ N. Tebbe and R_ L. Harlovz. J_ Am_ Cfzem. Sac.. Z02 (1980) 6151_ 18 T. d. Katz and 5. McGinnis, J. Am. Chem. Sot., 97 (1975) 1592. 19 E. 0. Fischer, S. WaIz and W. R. Wagner, J_ Qrgcrnomet. Cfzem.. I34 (1977) C37. 20 E. 0. Fischer. A. Rubs and D. Plahst, Z_ NuCurforscf~. Teil B. 32 B. i (1977) 802. 21 J. R. Fritch and K P. C. VolIhardt, Angew. Cheer;.. 18. 5 (1979) 409. 22 M_ Blznckard anti A_ Mortreux, RoL Koodinckti V Kafaiire (1976) 137. cf. Cherm 17

Abstr23

86 188853

h.

M. Gil-et, A. Mortreux, J. Nicole and F. Petit, J. Ckem. Sot. Chem. Common_ (1979) 522.