Rhodium catalysis alkylation

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Gladiali, S., Pinna, L., Delogu, G., De Martin, S., Zassinovich, G., Mestroni, G. Optically active phenanthrolines in asymmetric catalysis. III. Highly efficient enantioselective transfer hydrogenation of acetophenone by chiral rhodium/3-alkyl phenanthroline catalysts. Tetrahedron Asymmetry 990, 1, 635-648. 

In enynes, both the double and the triple bond can compete for the carbenoid that is generated from alkyl diazoacetate under copper or rhodium catalysis. The chemoselectivity is sometimes not very pronounced, but but-l-en-3-yne is selectively attacked at the triple bond [methyl diazoacetate, RhjCOAc), 70% yield], whereas in 2-methylhexa-l,5-dien-3-yne only the double bonds accept the carbenoid (Table 12, entry 2). The vinylcyclopropene obtained by addition to the triple bond may dimerize to form a 3,6-dialkoxycarbonyltricyclo-[3.1.0.0 ]hexane.2 "... 

Other functionalizations described include the direct alkylation of the title compound. For example, it was demonstrated that applying similar alkenes describe vide supra under rhodium catalysis leads to a direct hydroalkenylation, furnishing the corresponding 2-alkyl pyridine -oxides. Conversely, benzyl chlorides were effectively coupled to the title compound under similar conditions described for direct sp arylation (eq 30). ... 

Alkylation. Alkylation of the 2-position of quinoline is possible via C-H activation. Using rhodium catalysis, olefins can... 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

The Lee group originated rhodium alkenylidene-mediated catalysis by combining acetylide/alkenylidene interconversion with known metal vinylidene functionalization reactions [31], Thus, the first all-intramolecular three-component coupling between alkyl iodides, alkynes, and olefins was realized (Scheme 9.17). Prior to their work, such tandem reaction sequences required several distinct chemical operations. The optimized reaction conditions are identical to those of their original two-component cycloisomerization of enynes (see Section 9.2.2, Equation 9.1) except for the addition of an external base (Et3N). Various substituted [4.3.0]-bicyclononene derivatives were synthesized under mild conditions. Oxacycles and azacycles were also formed. The use of DMF as a solvent proved essential reactions in THF afforded only enyne cycloisomerization products, leaving the alkyl iodide moiety intact. 

The same reaction scheme can be written for (Z) -2-phenyl-2-butene, except that paths B and E would lead to erythro and threo aldehydes. In cobalt catalysis this isomerization could explain both the lack of stereospecificity and the lack of influence of the sterochemistry of the starting olefin on the distribution of aldehydes 26 and 27. This hypothesis agrees well with results with a-ethylstyrene. On the other hand, when rhodium is used, extensive isomerization occurs less readily probably because of a better stability of alkyl- and acylrhodium carbonyls, and one can thus achieve a high degree of stereospecificity. 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

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