Alkylrhodium complexes

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Some examples of this method are given in Table 11. Most of the entries proceed in about 60-80% yield, with the exception of secondary and tertiary alkyllithiums. In these cases, the facility of -hydride elimination in the intermediate alkylrhodium complex predominates over oxidative addition to the acid halide. One example of formation of an optically active ketone proceeded without racemization at the a-center. 

Table 11 Acylation of Alkylrhodium Complexes with Acid Chlorides...

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

In the precatalytic process the rhodium hydride precursor undergoes insertion into the butenyl carbonate to form an alkylrhodium complex. (3-Elimina-tion yields 1-butene and phenylcarbonatorhodium complex. Upon decarboxylation a phenoxorhodium complex is produced that undergoes the SN2 type reaction with 2-butenyl phenyl carbonate to liberate the branched allylic ether, 1-... 

To account for these results, the hydrosilylation reaction in the presence of Wilkinson s catalyst was proposed to occur via a cis addition process (according to the Chalk and Harrod mechanism, cf. Sect. IV-A-l-a) with concomitant isomerization taking place, the catalyst retaining the olefinic product in its coordination sphere. Conversion of the more stable trans to cis product is proposed to be directed by steric interactions in an intermediate o-alkylrhodium complex. 

In addition, many different combinations of steps can control the n i ratio. The regiochemistry of aldehyde might be controlled by irreversible addition of the Rli-H unit across the alkene, irreversible insertion of CO into an equilibrating mixture of n-alkyl- and i-alkylrhodium complexes, or by irreversible hydrogenolysis of an equilibrating mixture... 

In contrast, deuterioformylation of 1-hexene with catalysts containing dppe and other bisphosphines under these mild conditions formed products containing deuterium only in the p-positions of the aldehdydes. This observation shows that formation of both n- and f-alkylrhodium complexes was irreversible with these less selective catalysts. 

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