Rhodium catalysts enantioselective allylic substitutions

Much effort has been devoted to developing catalysts that control the enantioselectiv-ity of these substitution reactions, as well as the regioselectivity of reactions that proceed through unsymmetrical allylic intermediates. A majority of this effort has been spent on developing palladium complexes as catalysts. Increasingly, however, complexes of molybdenum, tungsten, ruthenium, rhodium, and iridium have been studied as catalysts for enantioselective and regioselective processes. In parallel with these studies of allylic substitution catalyzed by complexes of transition metals, studies on allylic substitution catalyzed by complexes of copper have been conducted. These reactions often occur to form products of Sj 2 substitution. As catalylic allylic substitution has been developed, this process has been applied in many different ways to the synthesis of natural products. ... 

In the asymmetric intramolecular cyclopropanation of 40 (Scheme 32), comparative studies of chiral copper, rhodium, and ruthenium catalysts showed none of the catalysts is omnipotent in providing high enantiocontrol for all substrates (117). Complementary factors between these catalysts were observed. For instance, for the copper/7 catalyzed reaction, whereas high enantioselection was achieved with substituted allylic diazoacetates, any substitution at and R resulted in low percentage of ee. Interestingly, the opposite observation was observed with the rhodium catalyst 37. In the case of ruthenium catalyst 25, while high enantioselectivities were obtained for substrates with substitutions... 

An interesting two-catalyst system has been developed for the highly enantioselective aUylation of a-cyanoesters. The allyl carbonate 143 reacts with the nucleophile 144 to give the substitution product 145 with up to a remarkable 99% ee (Scheme 31). The nucleophile 144 is activated to deprotonation by the rhodium catalyst, which also seems to be responsible for the control of enantioselectivity. However, in the absence of a palladium catalyst, the allyl carbonate 143 is inert. 

Intriguingly, rhodium catalysts display with the same substrates a complementary reactivity profile (Scheme 19) [55]. Moreover, the pathway after the retro-allylation step is heavily dependent on the conditions employed and substrate substitution. For instance, enantioselective retro-allylative C-C bond cleavage... 

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrroli-done-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene transformations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubsti-tuted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro-gen insertion reactions of diazoacetate esters that result in substituted y-butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabilization of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

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