Catalysis of Radical Cyclizations

Treatment of Cp (CO)3CrD with a large excess of MMA dimer allows the measurement of fejeinit by monitoring the increase in the hydride resonance due to H/D exchange [80]. This method gives a value of 3 x 10 M s, which is much smaller than the for MMA itself (1.74 x 10 M s ) [51]. 

Methods involving carbon-centered radicals are used extensively in synthesis [81-83]. The cyclization of such radicals forms the rings common in natural products, and tolerates functional groups-for example, unprotected hydroxyls-that cannot be present during some heterolytic transformations. 

Generally, cyclizations begin with the photo- or initiator-stimulated formation of a tin radical, which abstracts X from R-X (1.29) where X is Br, I, occasionally Cl, or PhSe or PhS [84]. The resulting R then undergoes cyclization or rearrangement to R before abstracting H from a tin hydride (typically BusSnH), which regenerates the tin radical (1.30) and continues the chain. Such methods are necessarily stoichiometric both in tin and in another heavy element (Br, I, Se, S). 

Related methods begin with the addition of a radical Rad to the C=S double bond in a xanthate (1.31) or thiohydroxamate (1.32) ester [84, 85]. (Rad is often RsSn but in initial investigations t-BuS and CCb were also used [86, 87].) The resulting intermediates give R by P scission (31 and 32) and decarboxylation (1.32). 

Special care is required to handle trialkyltin hydrides and the waste they generate, and standard laboratory purification techniques often leave toxic levels of tin compounds in the product [88]. The industrial application of these methods has been hindered by the need to remove these tin-containing contaminants. Methods catalytic in tin have been developed [89-91], and tin hydride reagents modified to make their removal easier [92-98], but the need for alternatives to tin 

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