Transition metal catalyzed carbonylations of vinylsilanes, allyl sulfides and related systems.

Abstract

It has been demonstrated that the zwitterionic rhodium complex (Rh zw), [Rh(COD)+]BPh4-, is an effective catalyst for the hydroformylation of vinylsilanes. With vinyltrimethylsilane, the branched aldehyde could be obtained as the major aldehyde isomer if the CO:H2 ratio was 1:2 (total pressure = 200 psi). Under these conditions, 20-40% isolated yield was obtained and the low Yield of these reactions was confirmed by experiments performed in deuterated solvents. The hydroformylation of vinyltriethylsilane, on the other hand, gave 66% of the desired aldehydes after 4h. The branched isomer was, however, the minor isomer with B:L selectivities of ca. 35:65. A cationic rhodium complex [Rh(COD)2+]BF 4-, abbreviated as (Rh+), was also found to catalyze the hydroformylation of vinylsilanes, but only in aromatic solvents. With the addition of 1 equivalent of triphenylphosphine, the reaction did proceed in non-aromatic solvents. The addition of PPh3 to the Rhzw catalyzed reactions dramatically improved the yield and selectivity for the linear isomer such that after the addition of only 2 equivalents, the products were formed in 97% yield with a B:L ratio of 7:93. This system is therefore much more sensitive to the effect of phosphines than the neutral complexes reported previously by Takeuchi et al.66 where the addition of 50--100 equivalents of phosphine was required in order to obtain >90% selectivity for the linear isomer. The unique solvent effects observed in the reactions of [Rh(COD) 2+]BF4- were explained by postulating that the aromatic solvent was acting as a ligand for the low-ligated rhodium complex. Furthermore, the activity of the zwitterionic complex in non-aromatic solvents was explained by co-ordination of one of the phenyl rings of the counterion. To gain support for this proposal, the stability of Rhzw under the reaction conditions was examined using high pressure and simple NMR techniques. These experiments led us to conclude that under severe conditions (140°C, 16h), 40% of the counterion is decomposed to benzaldehyde, benzyl alcohol and benzene. The mixture that resulted from this exhaustive treatment was shown to catalyze the hydroformylation of vinyltriethylsilane. However, when the rhodium complex was exposed to the conditions normally employed for carbonylation reactions, (namely 75°C, 3h, 200 psi), less than 15% decomposition to benzaldehyde was observed, and no benzene or benzyl alcohol could be detected. Finally, we prepared a number of arene-rhodium complexes and showed that they catalyzed the hydroformylation of vinyltriethylsilane in non-aromatic solvents. Chiral arene complexes were also prepared in order to test the enantioselectivity of the hydroformylation, but in these reactions, the branched isomer was completely isomerized to the enol silyl ether which did not permit assessment of the enantioselectivity. Other olefins were poor substrates. Styrene and vinyl naphthalene were polymerized under the reaction conditions, and olefin isomerization was problematic in the attempted hydroformylation of aliphatic substrates. The insertion of carbon monoxide into 6-membered ring heterocycles was examined using a variety of 1-alkyl-1,2,3,4-tetrahydroquinoline derivatives. It was found that under forcing conditions using Pd/Cu or Ru/Co complexes, up to 15% of products resulting from insertion of carbon monoxide could be observed. The isolation of these compounds and the determination of their structure was complicated due to the large amounts of metal salts employed, the decomposition of the remaining starting material and the low yields. MS data, however, confirmed that insertion of carbon monoxide had occurred. Finally, we examined the carbonylation of allyl aryl and allyl alkyl sulfides and found that the combination of Pd(OAc)2 and DPPP was effective only if a 1:1 ratio of metal:ligand was employed. Under these conditions, the alpha,beta-unsaturated thioester resulting from carbonylation and isomerization was formed in fair to good yields. Analysis of this compound by 1H NMR indicated that it was 100% trans. When Ru3(CO)12 was employed, the carbonylation could also be effected in lower yields after longer reaction times, and no isomerization of the product was observed. Thus the beta, gamma-unsaturated thioester was obtained from this reaction. The palladium catalyzed carbonylation was shown to proceed through a pi-allyl intermediate since 2-butenyl and 1-(3-butenyl) phenyl sulfide gave the same product. Furthermore, this experiment suggests that the rate determining step was carbonylation and not oxidative addition.