595. Chemical and spectroscopic evidence for the occurrence of π-character in carbon–boron bonds

Abstract

Many organoboron compounds possess properties which suggest theThis is discussed, presence in these compounds of carbon-boron x-bonding. and additional spectroscopic evidence is presented.x-BONDING between carbon and boron has frequently been postulated to explain the properties of certain organoboron compounds. a@-Unsaturated boronic esters and acids, for example, possess chemical and spectral properties strongly suggesting such bonding, a hypothesis in this instance supported by simple molecular orbital ca1culations.l We now supply additional experimental evidence for carbon-boron x-bonding, and discuss some earlier postulates of this bonding.Evidence from Chemical Bekaviowr.-Carbon tetrachloride adds to dibutyl vinylboronate to form dibutyl 1,3,3,3-tetrachloropropylboronate; the low value for the first transfer constant (3.3 & 0.7 x is as expected if the intermediate radical is stabilized by electron delocalization involving the carbon-boron T;-bonding, viz.,The relative stability towards disproportionation of vinyl-and propenyl-alkylboron compounds provides further chemical evidence for carbon-boron x-bonding2 Unless steric effects are a determining factor, as in large and branched-chain alkyl groups, unsymmetrical triorganoboron compounds (e.g., BMe,Et) are believed to have only a transient existence at ambient temperatures, disproportionating into the symmetrical triorganoboron compounds. These disproportionations undoubtedly proceed via a fourcentre mechanism involving the p,-orbital of boron. Clearly, if this +,-orbital is involved in internal dative bonding, as is believed to occur in alkoxy-and amino-boranes and might occur in alkenylalkylboron compounds,2 the tendency of the boron compound to disproportionate will be greatly reduced. Ritter and his co-workers found that dimethylvinylborane decomposes only to the extent of about 33% after storage at room temperature for about 5 days. Similarly, methyldivinylborane can be heated to 100" for short periods without significant decomposition. The stability of these unsymmetrical boron compounds contrasts with the transient existence of such species as BMeEt,, supporting the suggestion that mesomerism in alkenylboron derivatives restricts the boron atoms to a planar configuration and therefore inhibits disproportionation.The stability of the perfluorovinylboron compounds is also intere~ting.~ These are the first covalent organoboron compounds to contain a fully fluorinated organic group bonded to boron. Organoboron compounds having halogen atoms in the organic groups tend to decompose to form boron trihalides. Thus fluoromethylboron difluoride readily yields boron trifluoride,* and although 2-chloroethylboron compounds have not yet been obtained pure there is clear evidence that they are ~n s t a b l e . ~ It has long been held that a boron compound in which a highly fluorinated organic group is attached to boron would tend to decompose with formation of substances having boron-fluorine bonds. Thus, reaction between bispentafluoroethylmercury and boron trichl...

The preparation of HCF2CdX and HCF2ZnX via direct insertion into the carbon halogen bond of CF2HY (Y=Br, I)

“…GC-MS, m/z (relative intensity) of the mixture: 102 (1.5), 101 (4.4), 83 (66.1), 82 (7.4), 64 (36.9), 63 (11.9), 51 (100.0), 45 (39.9), 44 (29.6). The chemical shift and coupling constants reported by Stone for cis-CFH CFH are consistent with those observed in our spectrum [53], and the 19 F NMR spectrum reported by Ellerman et al match our observed spectral data for CF 2 HCF 2 H [54].…”

13C NMR SPECTRA OF ALKENYLBORANES. EVIDENCE FOR THE PRESENCE OF Π-Character IN THE B–C BOND

“…Consequently, the present 13C spectral study supports that the resonance structure (4) contributes to the ground state of alkenylboranes, the conclusion being in agreement with other experimental results.…”

Synthesis and Characterization of Allylic Dihaloboranes

“…2-Propenyldifluoroborane (2a). Yield: 98%. τ 1/2 (5% in CDCl 3 ) ≈ 18 h. 1 H NMR (CDCl 3 , RT): δ 1.98 (br s, 2H), 5.07 (d, 1H, 3 J HHcis = 9.9 Hz), 5.11 (d, 1H, 3 J HHtrans = 17.1 Hz), 5.82 (ddt, 1H, 3 J HHtrans = 17.1 Hz, 3 J HHcis = 9.9 Hz, 3 J HH = 7.3 Hz).…”

“…In 1961, Coyle et al prepared by reaction of BF 3 with tetraallylstannane the first allylic dihaloborane, CH 2 CHCH 2 BF 2 , which was characterized by 19 F NMR spectroscopy . Fourteen years later, on the basis of the 1 H and 11 B NMR spectra of the crude mixture, Mikhailov et al reported CH 2 CHCH(Me)BCl 2 as the product formed by redistribution of tricrotylborane with BCl 3…”