Conformational studies on heteroleptic trifluoromethyl-substituted phenylboranes

Abstract

Organoboranes carrying electron-withdrawing substituents are commonly used as Lewis acidic catalysts or cocatalysts in a variety of organic processes. These Lewis acids also became popular through their application in `frustrated Lewis pairs', i.e. combinations of Lewis acids and bases that are unable to fully neutralize each other due to steric or electronic effects. We have determined the crystal and molecular structures of four heteroleptic arylboranes carrying 2-(trifluoromethyl)phenyl, 2,6-bis(trifluoromethyl)phenyl, 3,5-bis(trifluoromethyl)phenyl or mesityl substituents. [3,5-Bis(trifluoromethyl)phenyl]bis[2-(trifluoromethyl)phenyl]borane, C22H11BF12, (I), crystallizes with two molecules in the asymmetric unit which show very similar geometric parameters. In one of the two molecules, both trifluoromethyl groups of the 3,5-bis(trifluoromethyl)phenyl substituent are disordered over two positions. In [3,5-bis(trifluoromethyl)phenyl]bis[2,6-bis(trifluoromethyl)phenyl]borane, C24H9BF18, (II), only one of the two meta-trifluoromethyl groups is disordered. In [2,6-bis(trifluoromethyl)phenyl]bis[3,5-bis(trifluoromethyl)phenyl]borane, C24H9BF18, (III), both meta-trifluoromethyl groups of only one 3,5-bis(trifluoromethyl)phenyl ring are disordered. [3,5-Bis(trifluoromethyl)phenyl]dimesitylborane, C26H25BF6, (IV), carries only one meta-trifluoromethyl-substituted phenyl ring, with one of the two trifluoromethyl groups disordered over two positions. In addition to compounds (I)-(IV), the structure of bis[2,6-bis(trifluoromethyl)phenyl]fluoroborane, C16H6BF13, (V), is presented. None of the ortho-trifluoromethyl groups is disordered in any of the five compounds. In all the structures, the boron centre is in a trigonal planar coordination. Nevertheless, the bond angles around this atom vary according to the bulkiness and mutual repulsion of the substituents of the phenyl rings. Also, the ortho-trifluoromethyl-substituted phenyl rings usually show longer B-C bonds and tend to be tilted out of the BC3 plane by a higher degree than the phenyl rings carrying ortho H atoms. A comparison with related structures corroborates the conclusions regarding the geometric parameters of the boron centre drawn from the five structures in this paper. On the other hand, CF3 groups in meta positions do not seem to have a marked effect on the geometry involving the boron centre. Furthermore, it has been observed for the structures reported here and those reported previously that for CF3 groups in ortho positions of the aromatic ring, disorder of the F atoms is less probable than for CF3 groups in meta or para positions of the ring.

Facile Route to Quadruply Annulated Borepins

“…Step 2 : At a first glance, the formal nucleophilic substitution reaction between a negatively charged trifluoroborate ( 2 ) and a naphthyl anion may seem counterintuitive, and related examples are indeed rare. ,− However, we reproducibly obtained high yields of 3a (88%), 3b (57%), and 3c (67%), while none of our attempts to replace 2 by Mes–BR 2 (R = OMe, Br) was met with success. In addition, the reaction between Mes–BBr 2 and 1-bromo-8-(trimethylstannyl)­naphthalene did not provide useful quantities of 3a .…”

Rapid, iterative syntheses of unsymmetrical di- and triarylboranes from crystalline aryldifluoroboranes

“…The B-F bonds are signicantly longer than in 3, more comparable to symmetric Ar 2 BF (1.312(3)-1.354(2) Å). 15,29,[42][43][44] The B-C bonds are, however, insignicantly different from their counterparts in 3. The trigonal plane at boron is coplanar with the less sterically hindered aryl group and the increased steric shielding prevents any notable solid-state contacts to boron.…”

A computational tool to accurately and quickly predict 19F NMR chemical shifts of molecules with fluorine-carbon and fluorine-boron bonds

“…[133][134][135][136][137][138] These seven molecules are shown in Figure 11 and the chemical shift values in Table 12. 142 -57.0, -136.9 -58.0,-12.9…”