Systematic Investigation of the Ring‐Expansion Reaction of N‐Heterocyclic Carbenes with an Iminoborane Dihydride

Abstract

Conversions of iminoboranes with an N-heterocyclic carbene (NHC) result in borane dihydride formation (BDF) concomitant with dihydrogenated NHC. The iminoborane dihydrides are prone to a hydride-mediated ring-expansion reaction (RER) at elevated temperature, that is, the insertion of the boron atom into the adjacent CN bond of the NHC to yield boracycles. Upon conversion of a saturated-backbone NHC with respective iminoborane precursors RER yet occurs at ambient temperature to yield the ring-expanded products. When a less bulky iminoborane is brought in contact with sterically unhindered NHC neither the iminoborane dihydride is stable at room temperature nor is the RER observed to take place upon heating. The conversions of iminoboranes with very bulky NHC do not show BDF at ambient temperature and only in the case of the less hindered borane precursor the RER is found to proceed in a controlled fashion upon heating.

Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

“…4), where ring insertion requires migration of at least one methyl group to the carbenic carbon, it is found that the favoured pathway involves initial hydride migration (T1) followed by ring expansion (T2) and a final methyl migration (T3). 5,6 Trends in the T1 barriers (H migration) for HBMe 2 are similar to that of Ph 2 SiH 2 and H 2 BMe, being lowest for 2 (74 kJ mol −1 ) and 5 (105 kJ mol −1 ), with the other three NHCs having higher barriers of 132 to 150 kJ mol −1 . The preference for hydride migration has been consistently predicted by theoretical studies, [9][10][11][12][13] and also observed experimentally.…”

“…1 Despite NHCs being considered to be robust spectator ligands in catalysis, the activation of C-N bonds has significant generality, being thus far observed for the elements Be, 2 B, [3][4][5] Si, 6 Ni, 7 and Zn. 1 Despite NHCs being considered to be robust spectator ligands in catalysis, the activation of C-N bonds has significant generality, being thus far observed for the elements Be, 2 B, [3][4][5] Si, 6 Ni, 7 and Zn.…”

“…5,6 Thus far, theoretical studies with NHCs have almost exclusively focused on unsaturated NHCs with the standard structure 1 (Fig. 5,6 Thus far, theoretical studies with NHCs have almost exclusively focused on unsaturated NHCs with the standard structure 1 (Fig.…”

A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes

“…During this dehydrogenative redox process, the tellurium atom is further reduced to the −2 oxidation state with concomitant formation of dihydrogenated NHC (L Et (H 2 ), hydrogenated at the formerly carbenoid atom). The abstraction of hydrogen from iminoboranes using NHC to produce NHC(H 2 ) had been previously investigated25.…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…303 The ring expansion of іmіdazol-2-ylіdenes is also effected by other Lewis acids, particularly boranes 304 and amіnoboranes. 305,306 These result in the formation of borazines of types 67D,E, along with boroxoles 68B (Scheme 68). 307 A beryllium analogue 308 of the silazines 67C, compound 67F (Scheme 67), is also known.…”