The Reactivities of Iminoboranes with Carbenes: BN Isosteres of Carbene–Alkyne Adducts

Abstract

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N-heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (-45 °C). The adducts show short BN bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di-tert-butyliminoborane was reacted with 1,3-bis(isopropyl)imidazol-2-ylidene (IPr), the initially formed Lewis acid-base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4-position. Warming the iminoborane-CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2-azaborilidine compound.

From an Electron‐Rich Bis(boraketenimine) to an Electron‐Poor Diborene

“…It is undoubtedly the combined electron-withdrawing capacity of the cyanide and CAAC substituents [21] on the diborene that delocalize the p-electrons ( Figure S4), resulting in the high stability of the frontier orbitals and comparative electron deficiency between the boron atoms. Taken together, CV indicates an electron-rich compound with a B-B single bond (3), yet an electron-poor diborene (7).…”

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

“…Note that the BN distance in tert-butyl(tert-butylimino)borane is 1.258(4) Å and ranges between 1.330–1.340 Å in some of its N-heterocyclic carbene adducts. [9a, 16] With the ambiphilic CO 2 molecule both the electron deficient boron and the electron rich nitrogen centers are involved in binding with an overall binding energy of 29 kcal mol –1 at the CCSD(T)/TZ2P + ZPVE(B3LYP/6-311+G**) level of theory.…”

Isolation of a Reactive Cyclopropane Intermediate via a Unique Catalyst‐Free Spontaneous Cyclopropanation Step at 0 °C

“…In 2015, we published the reaction of the cyclic (alkyl)(amino)carbene ( CAAC , Figure ) with iminoborane t BuBN t Bu ( IBa ) at −45 °C, which led to the isolation of the iminoborane–carbene adduct 1 a . This adduct was unstable above this temperature, leading to complete conversion to the room‐temperature‐stable bicyclic compound 3 a , in which the BN unit and an additional carbon atom had fused to the CAAC unit.…”