Substitution reactions allylic metals as nucleophiles

Allylic substitution using hard nucleophiles proceeds through a different mechanism. Instead of attacking the allyl group of the 71 allyl-metal complex, hard nucleophiles attack the metal first and the product is subsequently formed by reductive elimination. Nickel(O) complexes have often been used for this purpose. Reports of good enantioselectivities in this type of reaction are limited. 

Besides the transition-metal-catalyzed asymmetric addition reactions to prochiral olefins, the substitution reaction of a carbon nucleophile to allylic esters has been investigated using a variety of chiral transition-metal catalysts. Using the aforementioned sugar diphosphites 

Four reviews on allylic substitution reactions have been published. The first deals with the enantioselective allylic substitutions by carbon nucleophiles, in the presence of both palladium and non-palladium catalysts. The second reviews stere- 0 oselective allylic substitution reactions forming asymmetric C-C, C-N, and C-O bonds. The third review covers new developments in metal-catalysed asymmetric 0 allylic substitution reactions with heteroatom-centred nucleophiles. Several applications of this new methodology are included. Finally, the catalytic 5 2 and 5 2 reactions of allylic alcohols, most of which occur with a very high ee, have been reviewed.  

The transition metal-catalyzed allylation of carbon nucleophiles was a widely used method until Grieco and Pearson discovered LPDE-mediated allylic substitutions in 1992. Grieco investigated substitution reactions of cyclic allyl alcohols with silyl ketene acetals such as Si-1 by use of LPDE solution [95]. The concentration of LPDE seems to be important. For example, the use of 2.0 M LPDE resulted in formation of silyl ether 88 with 86 and 87 in the ratio 2 6.4 1. In contrast, 3.0 m LPDE afforded an excellent yield (90 %) of 86 and 87 (5.8 1), and the less hindered side of the allylic unit is alkylated regioselectively. It is of interest to note that this chemistry is also applicable to cyclopropyl carbinol 89 (Sch. 44). 

Most allylic substitution reactions catalyzed by other metals are selective for the formation of branched products. Although this had been demonstrated for a large portion of the d-block before Takeuchi s work with iridium, most of the progress in this area was restricted to stabilized enolate nucleophiles. 

A similar triarylphosphane was introduced by Leitner for the performance of transition metal catalyzed reactions in supercritical CO2 [24]. Recently, this phosphane was used for palladium catalyzed substitutions of allylic substrates in perfluorinated solvents under fluorous biphase conditions [25], Therefore, the reaction of cinnamyl methylcarbonate (36) with several nucleophiles (Nu- 

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