The selective generation of 1,2- and 1,4-addition products (3 and 4, respectively) in the addition of TiIV enolates 1 to α,β-unsaturated carbonyl compounds 2 can be almost completely controlled by the choice of Lewis acid. The high selectivity for 1,4 addition when the enones are activated by TiCl4 or SnCl4 is probably a result of the bridging chlorine atoms.
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Deng et al. (2002) studied this question.