A theoretical study focusing on the ring expansion that occurs in the reaction of N-heterocyclic carbenes with silanes has been carried out. A detailed reaction pathway was determined which indicates that formation of C-H bonds is the crucial factor in the transformation.

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A computational investigation has been carried out on the mechanism and energetics of the experimentally observed insertion/ring expansion of N-heterocyclic carbenes (NHCs) by boranes (H 2 BNHR, BH 3 ; R = Me, Ph) and beryllium hydrides (BeH 2 ) in comparison with silanes (SiH 2 R 2 ; R = Me, Ph). The results suggest that the ring insertion mechanisms are similar for boranes, beryllium hydrides, and silanes. The principal mechanism components are (1) hydrogen atom migration to the carbene carbon, (2) C−N bond expansion of the NHC with insertion of the main-group hydride into the ring, and (3) migration of a second hydrogen atom to the carbene carbon. The synthetically important NHC•BH 3 adduct is also predicted to be thermodynamically unstable with respect to this transformation but is kinetically stabilized with a high barrier to the first hydrogen atom migration. The BeH 2 insertion product provides a rare example of a Be−N π interaction.

We report a novel C-H to C-N bond metathesis at the 3-position of 1,2-diphenyltellurophene via oxidation of the Te(II) center to Te(IV) using the I(III) oxidant [PhI(4-DMAP)2](2+). Spontaneous reduction of a transient Te(IV) coordination compound to Te(II) generates an electrophilic equivalent of 4-DMAP that substitutes at a C-H bond at the 3-position of the tellurophene. Theoretical and synthetic reaction pathway studies confirm that a Te(IV) coordination complex with 4-DMAP is an intermediate. In the course of these pathway studies, it was also found that the identity of the I(III) oxidant generated from PhI(OAc)2 and 2 TMS-OTf is PhI(OAc)(OTf) and not PhI(OTf)2, as had been previously thought.

The β-diketiminato aluminum dihydrides, [HC­{(Me)­CNAr}2AlH2] [4: Ar = 2,6-di-isopropylphenyl (Dipp), 5: 2,4,6-trimethyl­phenyl (Mes)] react directly with N-aryl-substituted N-heterocyclic carbenes (NHCs) by C–N bond activation to afford aluminum amido-alkyl derivatives of the form [HC­{(Me)­CNAr}2AlCH2(N­(Ar′)­CH)2]. The more sterically congested alane (4), bearing N-Dipp substitution, does not react with either 1,3-bis­(2,6-di-isopropylphenyl)­imidazol-2-ylidene (IPr) or 1,3-bis­(2,4,6-trimethyl­phenyl)­imidazol-2-ylidene (IMes), even under forcing conditions. In contrast, in situ generation of 1,3-bis­(phenyl)­imidazol-2-ylidene through deprotonation of the corresponding imidazolium tetrafluoroborate by KN­(SiMe3)2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield [HC­{(Me)­CNDipp}2AlCH2(N­(Ph)­CH)2]. Although compound 5 also does not react with IPr, relaxation of the steric demands of the supporting β-diketiminate ligand to N-mesityl substitution enables analogous ring opening of IMes, with the formation of [HC­{(Me)­CNMes}2AlCH2(N­(Mes)­CH)2] (7), when the reaction is heated to 80 °C. DFT calculations performed on model systems suggest that in comparison to the parent alane (AlH3) the enhanced propensity of these systems to induce NHC ring cleavage is a consequence of the relative stability of the initially formed five- and four-coordinate adducts as well as the augmented hydridic character of the Al–H bonds within the β-diketiminate-supported molecules.