Enhanced acidity of cyclopenta-2,4-dienylborane and its Al and Ga analogues. The role of aromatization.
The intrinsic acidity of cyclopenta-2,4-dienylborane and its Al and Ga analogues has been compared to that of cyclopentadiene by means of B3LYP/6-311+G(3df,2p)//CCSD/6-311+G(d,p) calculations. Substitution of one of the H atoms of the C(sp(3))H(2) group of cyclopentadiene by an XH(2) (X = B, Al, Ga) leads to an acidity enhancement which is significantly large for the boron derivative (95 kJ mol(-1)); but much smaller for the Al and Ga containing analogues. This acidity enhancement reflects the stabilization of the anion, in the substituted derivatives, due to a significant reinforcement of the C-X bond. This enhancement is however smaller than expected because, although XH(2) (X = B, Al, Ga) substitution leads to a significant aromatization of the neutral compounds, the aromaticity significantly decreases upon deprotonation, whereas for the unsubstituted parent compound is the other way around. Cyclopenta-2,4-dienylborane and its Al and Ga analogues behave as highly fluxional systems.
Hurtado
Marcela
M
Departamento de Química, C-9. Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain.
Yáñez
Manuel
M
Guillemin
Jean-Claude
JC
D000143:Acids / Q000737:chemistry*; D000535:Aluminum / Q000737:chemistry*; D003198:Computer Simulation; D003517:Cyclopentanes / Q000737:chemistry*; D005708:Gallium / Q000737:chemistry*; D008956:Models, Chemical
D000143:Acids; D003517:Cyclopentanes; C551203:cyclopenta-2,4-dienylborane; D005708:Gallium; D000535:Aluminum