Tritiated alkylboranes

Two tritiated alkylboranes have been reported to date. They have been synthesized at very high specific activities via Li H in one-pot procedures and can be used in situ. 

3-Dithian-2-ylmyrtanyl[ H2]borane (MDBT2) (272) is the only chiral borane so far reported in radiolabeled form. It is prepared by treatment of Li H with AICI3 followed by reaction of the resulting Al Hjin situ with [2-(l,3-dithianyl)]myrtanylcatecholborane.  

Methyl[ H2]borane, prepared in the same way as MDBT2 except using methylcate-cholborane, was tested in a hydroamination reaction with 1-phenylcyclopentene. Cleavage of the B-C bond of the resultant alkyl borane with H2NOSO3H afforded trons-2-phenyl[2- H]cyclopentylamine in 20-40% yield.  

Because of its lack of selectivity and undependable commercial availability LiAPl-14 has usually been used only in cases where reductions with NaB H4 alone or in the presence of 

The ability of LLA1 H4 to reduce carbamates to labeled methyl groups, such as in the synthesis of [iV-methyl- H]nortriptylin (274) , provides an alternative to Af-[ H]methyl-ation with [ HJmethyl iodide for the preparation of high specific activity tracers for invitm binding studies (see also Chapter 4.4.3). An example of a uniquely usefiil carbamate reduction is that of O-benzyl carbamate to give [ HJmethylamine at a specific activity of 70 Ci/mmol. The latter was further reacted with 3-azido-4-chlorophenylisothiocyanate to furnish the thiourea 275. an efficient photoaffinity probe for the urea carrier . 

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Hydroboration followed by protonolysis of the resulting alkylborane can be used as an alternative method for hydrogenation of alkenes, although catalytic hydrogenation (Section 7.12) is the more common procedure. Reaction of alkylboranes with deuterated or tritiated acetic acid also provides a very useful way to introduce these isotopes into a compound in a specific way. 

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