EXPERIMENTAL INVESTIGATION ON EDGE INVERSION AT TRIVALENT BISMUTH ANDANTIMONY - GREAT ACCELERATION BY INTRAMOLECULAR AND INTERMOLECULAR NUCLEOPHILIC COORDINATION

The activation free energy of inversion at the bismuth atom of the tri valent organobismuth compounds {2-C6H4C(CF3)(2)O}Bi(4-CH3C6H4) (4a), { 2-C6H4C(CF3)(2)O}Bi(2-C(6)H(4)CH(2)NMe(2)) (10), and {2-C6H4C(CF3)(2)O }Bi(2-C(6)H(4)CMe(2)OMe) (11) in DMSO-d(6) was measured by use of vari able temperature F-19 NMR and found to be 21.2 (175 degrees C), 15.4 ( 55 degrees C), and 18.6 (125 degrees C) kcal mol(-1), respectively. Th e corresponding energy of 10 in pyridine decreased to 14.6 (40 degrees C), whereas that in 2,6-lutidine was 20.6 (170 degrees C) kcal mol(-1 ). These results imply that intramolecular coordination of the methoxy and the dimethylamino groups and also intermolecular coordination of the pyridine solvent greatly stabilize the transition state of inversi on at the bismuth atom. This phenomenon can only be rationalized by th e edge inversion mechanism. Inversion at the bismuth atom of -C6H4C(CF 3)(2)O}Bi{4-(R)-2,6-C6H2(CH(2)NMe(2))(2)} (21, R = t-Bu; 22, R = H) be aring two potential intramolecularly coordinating groups was so fast t hat the barrier could not even be measured at -90 degrees C. The barri er of the corresponding antimony compound {2-C6H4C(CF3)(2)O}Sb{2,6-C6H 3(CH(2)NMe(2))(2)} (23) in CD2Cl2 was found to be 9.5 kcal mol(-1) at -70 degrees C. The M-N dissociation in the NMe(2) group was determined to be 12.3 kcal mol(-1) for 22 (M = Bi, T-c = -5 degrees C in toluene -d(8)) and 12.0 kcal mol(-1) for 23 (M = Sb, T-c = -19.2 degrees C in CD2Cl2). Thus, inversion for the bismuth (22) and the antimony atom (2 3) can be regarded as taking place without dissociation of the Bi-N (o r Sb-N) bonds, that is, via a 12-M-5 (M = Sb and Bi) transition state. X-ray crystallographic analyses of 10, 22, and 23 are presented which showed the presence of intramolecular coordination of N and O to the central atom (Bi or Sb).