Regioselectivity transition-metal catalyzed allylic alkylations

In sharp contrast to the transition metal-catalyzed allylic alkylation of allylic alcohols and their derivatives (see Chapter 11.03) where 77 -allyl-transition metal complexes are key intermediates, the benzyiic alkylation of benzyiic alcohols and their derivatives catalyzed by transition metal complexes has been quite unexplored, although 7] -benzyl-transition metal complexes have often been considered to explain the regioselectivity of transition metal-catalyzed addition to vinylarenes (Scheme 21). A3,63a... 

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). 

Among the various carbon-carbon and carbon-hetero atom bond forming reactions promoted or catalyzed by transition metals, allylic substitution via electrophilic n-allyl-complexes is of utmost importance. Studies focused on the synthetic potential of alkyl or aryl substituted ( n3-allyl)Fe(CO)4 1+) complexes have shown that nucleophilic attack by soft carbon and hetero atom nucleophiles preferentially proceeds regioselectively at the less or syn-substituted allyl terminus.4 Additionally, polar effects on the regioselectivity of this reaction caused by electron-withdrawing functionalities (e.g., CO2R, CONR2) have been examined by the... 

---