Experimental and Theoretical Evidence of an S_N2-Type Mechanism for Dissociation of B−N Coordination Bonds in 2,6-Bis((dimethylamino)methyl)phenylborane Derivatives¹

Abstract

To distinguish between dissociation of a B-N coordination bond by S N 1-and S N 2-type mechanisms, two series of 1,3,2-dioxaborolanes (boronates) and BEt 2 (borane) complexes carrying a 2,6-bis((dimethylamino)methyl)phenyl group as a third substituent were synthesized by the reaction of the corresponding organolithium compound with an appropriate boron reagent. In the solid state, the boronate complex exhibits a structure in which only one NMe 2 group is coordinated to a tetracoordinated boron atom according to the X-ray analysis and the solid-state NMR. In solution there is a rapid exchange between the coordinated and uncoordinated amine ligands. The barriers to B-N dissociation in the boronate and borane complexes are lower by >3.4 and 6.6 kcal/mol than in the corresponding monoamino complexes, respectively, which is due to electronic assistance in an S N 2-type mechanism. This observation is supported by ab initio calculations for the system of NH 3 and BH 3 . The dynamic process observed in the boronate complex with 4,4-diphenyl substituents is also discussed.

Synthesis and structure of potential Lewis acid–Lewis base bifunctional catalysts: 2-N,N-Diisopropylaminophenylboronate derivatives

“…Since it is known that ortho-N,N-dimethylbenzylamine boronic acid shows strong N-B chelation both in the solution and solid states [6], we decided to prepare the N,N-diisopropylbenzylamine analogue, using a Snieckus-type directed ortho-metallation [7]-boronation route, followed by an amide reduction. Diisopropylamide 1 was therefore treated with n-butyllithium-TMEDA (sec-butyllithium-TMEDA can also be used for directed metallation, as on the more common diethylamide system [6], though n-butyllithium works just as well), followed by addition of a borate electrophile, with the expectation of deriving the required boronate derivative.…”

Facile dissociation of BS coordination bonds in [2,6‐bis(ethylthiomethyl)phenyl]diethylborane by an SN2‐type mechanism

“…We consider that this conclusion can be extended to the observed dynamic behavior of 3 in solutions [11]. The energy gap between the two mechanisms (8.4 kcal/mol) is smaller than that for the corresponding NH 3 complexes (20.8 kcal/mol) calculated at the same level [1].…”

“…The facile dissociation in 3 is explained by the S N 2-type mechanism via a trigonal bipyramidal transition state (7 * in Scheme 1, L = SEt), as proposed for the corresponding amine analog 1 previously [1].…”

“…Compounds 3 and 4 were prepared by the reaction of the corresponding organolithium compounds with B(OMe)Et 2 in a similar manner as the preparation of 1 and 2 [1]. These compounds are stable toward water and air because of the stabilization by the intramolecular coordination.…”

“…10 5 than that in the corresponding mono dimethylaminomethyl compound 2 ( Fig. 1) [1]. The facile dissociation in 1 is explained by a contribution of the S N 2-type mechanism [2], in which the uncoordinated ligand assists the dissociation of the coordinated ligand through a trigonal-bipyramidal (TBP) transition state, on the basis of experimental and theoretical data (Scheme 1).…”