Titanium Lewis Acids in Radical Reactions

The use of Lewis acids to impart chemoselectivity and stereoselectivity to free-radical polymerization and copolymerization is well documented [670]. Recent progress in radical reactions in organic synthesis has revealed the importance of Lewis acids in selective transformations [671,672]. Lewis acids have also been found to enhance the reactivity both of radical acceptors [673-675] and of radicals themselves [676], thus increasing the efficiency of radical reactions. 

Titanium compounds are frequently investigated as Lewis acids in radical reactions [677-680]. When addition of an alkyl radical to a chiral vinylsulfoxide was conducted in the absence or presence of Ti(0-/-Pr)2Cl2, the stereochemistry of the product was reversed, very high diastereoselectivity being observed in the presence of the titanium salt (Eq. 302) [681,682]. The stereochemistry and high selectivity in the presence of the titanium salt were readily rationalized on the basis of a chelation intermediate between the titanium metal and the carbonyl and sulfoxide oxygens, as shown in Eq. (302). 

In contrast with the above discussion, a radical itself (rather than a radical acceptor) can be activated by complexation with a Lewis acid. The aminyl radical cycliza-tion shown in Eq. (304) is a slow process and yields of the cyclic product are often low. It is, however, known that a Brpnsted acid promotes this cyclization [676], and by analogy the reaction proved to be promoted by the addition of a Lewis acid, which should coordinate with the nitrogen atom to increase the reactivity of the nitrogen radical. The effect of a series of titanium salts, Ti(0-i-Pr) CL , at a concentration of 0.025 M, is shown in Eq. (304). It is apparent that use of Ti(0-i-Pr)Cl3 resulted in significantly improved yield. 

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Titanium(IV) Lewis Acids 779 15.12 Titanium Lewis Acids in Radical Reactions... 

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