Bora-aromatic systems. Part 8. The physical and chemical consequences of cyclic conjugation in boracyclopolyenes. The antiaromatic character of pentaarylboroles.

TLDR: From such considerations, it is evident that the conjugation between the tricoordinate boron's 2p,-orbital and the four-carbon butadienylidene array is destabilizing and is the source of the high reactivity of boroles.

Abstract: Pentasubstituted boroles or boracyclopentadienes have been prepared by two routes: (1) the interaction of an (E&)-( 1,2,3,4-tetraaryl-1,3-butadien-l,4-ylidene)dilithium in ether solution with an aryl(diha1o)borane to form the borole etherate; and (2) the exchange reaction between a 1,1-dialky1-2,3,4,5-tetraarylstannole in nondonor media to produce the unsolvated borole. The boroles are unexpectedly strong Lewis acids, complexing with amines and even ethers and nitriles, and very prone to oxidation, solvolytic cleavage, and Diels-Alder reactions. The foregoing chemical behavior, taken together with their unusual nuclear magnetic and electronic spectral properties, can be straightforwardly interpreted in terms of the Hiickel antiaromatic character of the 4-*-electron boracyclopentadiene nucleus. By treating such nuclei as perturbed cyclopentadienyl 7r-systems, a qualitative understanding of both the spectral and chemical properties can be attained. From such considerations, it is evident that the conjugation between the tricoordinate boron's 2p,-orbital and the four-carbon butadienylidene array is destabilizing and is the source of the high reactivity of boroles. An $-hybridized boron atom might be expected to form cyclic conjugated systems with arrays of sp2-hybridized carbons, because both the covalent radius (0.80 A) and the Allred-Rochow electronegativity (2.01) of boron are similar to those of carbon (0.77 A and 2.51).4 In such conjugated rings the boron center might exert a stabilizing influence if the total number of x electrons would conform to the Hiickel 4n + 2 criterion for a r o m a t i ~ i t y . ~ ~ In fact, heterocyclic organoboranes bearing the isoelectronic J3=N unit in place of one or more C=C linkages of a benzenoid ring have been shown to possess chemical and physical properties similar to classic aromatic compounds.5b Boracyclopolyenes not possessing the appropriate number of ?r electrons might be destabilized by conjugation and accordingly display unusual reactivity or antiaromaticityS6 An example of the destabilization of a carbocyclic ring by such antiaromaticity is the instability of the pentaphenylcyclopentadienyl cation (1). This deep-blue ion is highly reactive and has a thermally populated triplet (compound 2). This behavior indicates that the gain in conjugated stabilization attained by placing the four A electrons as pairs in bonding molecular orbitals is not large.