om9710327_si_002.pdf (426 kB)
Synthesis and Solution and Solid-State Structures of Tris(pentafluorophenyl)borane Adducts of PhC(O)X (X = H, Me, OEt, NPri2)
journal contribution
posted on 1998-03-06, 00:00 authored by Daniel J. Parks, Warren E. Piers, Masood Parvez, Reinaldo Atencio, Michael J. ZaworotkoReaction of the highly electrophilic borane
B(C6F5)3 with the carbonyl Lewis
bases
benzaldehyde, acetophenone, ethyl benzoate, and
N,N-diisopropylbenzamide 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(C6F5)3 follows the
order
N,N-diisopropylbenzamide > benzaldehyde >
acetophenone > ethyl benzoate. The solution
and solid-state structures were probed to rationalize these
observations. In solution, the
borane coordinates to the carbonyl lone pair syn to H and Me in the
aldehyde and ketone
adducts, as indicated by 1H/19F 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 associated with the ketone adduct
(C−O−B =133.6(3)° vs 126.7(5)°
for the benzaldehyde complex) accounts for the observed order of
basicity with these two
bases. For ethyl benzoate and
N,N-diisopropylbenzamide, the borane coordinates
syn to
the phenyl group in both solution and the solid state. In addition
to the carbonyl oxygen−boron interaction, the two complexes engage in a π-stacking
interaction between one of the
borane C6F5 rings and the syn phenyl group.
In addition to the structural proof of this
interaction in the solid state, variable-temperature 19F
NMR experiments suggest it is
important in the solution structures of these adducts as well.