SYNTHESIS AND SOLUTION AND SOLID-STATE STRUCTURES OF TRIS(PENTAFLUOROPHENYL)BORANE ADDUCTS OF PHC(O)X (X = H, ME, OET, NPR2I)

Reaction of the highly electrophilic borane B(C6F5)(3) with the carbon yl Lewis bases benzaldehyde,acetophenone, ethyl benzoate, and N,N-diis opropylbenzamide led to isolation of the crystalline adducts 1-H, 1-Me , 1-OEt, and 1-NPr, respectively, in good to excellent yields(63-89%). Equilibrium measurements and exchange experiments indicated that the order of basicity (from highest to lowest) of these bases toward B(C6F 5)(3) follows the order N,N-diisopropylbenzamide > benzaldehyde > acet ophenone > ethylbenzoate. The solution and solid-state structures were probed to rationalize these observations. In solution, the borane coo rdinates to the carbonyl lone pair syn to H and Me in the aldehyde and ketone adducts, as indicated by H-1/F-19 NOE difference experiments. The same coordination geometry was observed in the solid state upon X- ray diffraction analysis of the two adducts. The added front strain as sociated with the ketone adduct (C-O-B = 133.6(3)degrees vs 126.7(5)de grees for the benzaldehyde complex) accounts for the observed order of basicity with these two bases. For ethyl benzoate and N,N-diisopropyl benzamide, the borane coordinates syn to the phenyl group in both solu tion and the solid state. In addition to the carbonyl oxygen-boron int eraction, the two complexes engage in a pi-stacking interaction betwee n one of the borane C6F5 rings and the syn phenyl group. In addition t o the structural proof of this interaction in the solid state, variabl e-temperature F-19 NMR experiments suggest it is important in the solu tion structures of these adducts as well.