SYNTHESIS OF S-ALKYL AND S-ACYL DERIVATIVES OF MERCAPTOUNDECAHYDRODODECABORATE, A POSSIBLE BORON CARRIER FOR NEUTRON-CAPTURE THERAPY

The synthesis of S-alkylated and S-acylated derivatives of mercaptoundecahydrododecaborate (B12H11SH2-) (1) is described. Under conventional alkylation conditions, 1 reacts with primary alkyl halides to form S,S-bis-substituted sulfonium salts. With secondary halides, monoalkylation to thioethers is observed. The sulfonium salts act only as poor alkylating agents. The cyanoethyl-substituted sulfonium salt 3 was found to lose one substituent upon treatment with tetramethylammonium hydroxide, to yield the thioether 8. Unsymmetrically substituted sulfonium salts were obtained through realkylation. Selective removal of the remaining cyanoethyl group yielded thioethers. Reaction of 1 with acid halides resulted in thioesters, which showed surprising stability toward hydrolysis.