Rhodium hydrometallation

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

The other questions raised above—about the relative roles of alkene complexation and hydrometallation, and the possible requirement for structural rearrangement before insertion—cannot be answered in this system, as the alkene binding and geometry are essentially immutable. Roe has shown for a rhodium complex with trans alkene and hydride ligands, that rearrangement does not contribute to the hydrometallation barrier the first step in equation (10) is fast, and k2 is rate determining. It is notable that for... 

Hydrometallations of alkenes and acetylenes such as hydroboration, hydrosila-tion, hydroalumination, hydrostannation and hydrozirconation are useful synthetic reactions. Rhodium-phosphine complexes catalyze hydroboration of substituted styrenes with a catecolborane (Scheme 68). An extensive survey of the phosphine... 

The RhI-catalysed rearrangement of A-allylaziridines to (Z)-A-alkenylaziridines has been investigated by computational methods and the reaction occurs via a hydrometalation//3-hydride elimination. Alkylidene cycloheptadienes have been prepared via a rhodium(I)-catalysed tandem isomerization of cyclopropylenynes through a [1,5] carbon-carbon migration via an allene intermediate with high Z E selectivities (Scheme 152). ... 

Subsequent to Brookhart s findings, Jun el al. [101] developed a rhodium-catalyzed alkylation of a,P-unsaturated carbonyl compounds in the presence of diethylamine (Scheme 19.70). The proposed mechanism involves the in silu formation of an enamine, followed by C-H activation directed by this enam-ine, olefin hydrometallation, and reductive ehmination. In situ enamine hydrolysis regenerates the carbonyl group. The authors demonstrated that the formation of product 32 originates from the isomerization of initially formed compound 31 with the rhodium catalyst. 

The reactions of organorhodium compounds or the catalytic reaction of a rhodium complex are insertions, formation reactions of carbene, hydrogenations, hydrometalations, decarbonylations, carbonylations, hydroformylations, cycliza-tions and cyclometalations, etc. 

The examples of hydrometalations are hydroboration [64,72-77], hydrosilylation [78], etc. [78,79] of unsaturated compounds such as olefins, acetylenes, carbonyls and imines. For example, styrene reacts with catecholboran in the presence of a rhodium catalyst to afford stereoselectively the R isomer of 2-phenylethanol as shown in eq. (18.34) [72]. 

Under rhodium catalysis, pyridazine iV-oxide is able to undergo hydrometalation reactions (eq 5). In this case, CsOAc was determined to be essential for reactivity and has been postulated to facilitate the insertion of Rh into the heteroaryl C-H bond. 

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