Scheme I

Trialkyltin hydrides are versatile dehalogenation reagents which are known to react with organic halides via a free radical chain mechanism. 1 A useful modification of this procedure has been described recently which employs stoi-chiometric quantities of sodium borohydride and catalytic amounts of trialkyl-tin chlorides. 2 Since we have shown that sodium borohydride can also perform free radical chain dehalogenations, 3 it is of interest to know whether the catalytic amount of tin hydride or the borohydride is the immediate hydrogen atom source in these reactions. Hydrogen atom transfers from tin hydrides to carbon radicals are known to be extremely fast (k = 104-lo6 M-l sec-'j4 yet there have been no comparisons of hydrogen atom transfers from tin to those from boron. This rate ratio and the stoichiometrically significant hydrogen source has been determined by ex-amination of kinetic isotope effects. Reaction of 7,7-dibromonorcarane (1) with sodium borohydride5 or tri-n-butyltin hydride6 affords cis- and tranz7-bromonorcarane (2 _ and 2) in 79 % and 02 % yields, respectively (Scheme I). With equimolar amounts of sodium boro-hydride and sodium borodeuteride (Alfa, 99.8 % D), a kinetic hydrogen isotope effect (kBH/kBD) of 4.5 is found for the formation of A and3_in ethanol. This large value contrasts with the corresponding results for a tri-n-butyltin hy-dride-tri-n-butyltin deuteride mixture for which ksnH/ksnD = 1.2. 7 When&is reduced by a catalytic system of NaBH4-NaBD4 (1:l) and 0.05 eq. of tri-n-butyltin chloride,_&is formed with an apparent isotope effect of 1.8 whereas the value forLis 1.3 (Table I)