Deoxygenations of (silox)(3)WNO (12) and R(3)PO (R = Me, Ph, (t)Bu) by M(silox)(3) (1-M; M = V, NbL (L = PMe(3), 4-picoline), Ta; silox = (t)Bu(3)SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd) = -47 kcal/mol), Ph) but not (t)Bu(3)PO (85 degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaG degrees (rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)(3)V-OPR(3) (3-OPR(3)) with OPR(3). 1-Nb(4-picoline) (S = 0) and 1-NbPMe(3) (S = 1) deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaG degrees (rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (t)Bu(3)PO. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of approximately 17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier approximately 2 kcal/mol) are competent. Once binding occurs, significant mixing with an (1)A(1) excited state derived from population of a sigma-orbital is needed to ensure a low-energy intersystem crossing of the (3)A(2) (reactant) and (1)A(1) (product) states. Correlation of a reactant sigma-orbital with a product sigma-orbital is required, and the greater the degree of bending in the (silox)(3)M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and (t)Bu(3)PO to attain a bent 90 degree angle M-O-E due to sterics explains their slow or negligible deoxygenations. Syntheses of relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe(3) (90 degree angle V-O-P = 157.61(9) degrees), 3-OP(t)Bu(3) ( 90 degree angle V-O-P = 180 degrees ), 1-NbPMe(3), and (silox)(3)ClWO (9).

Hold me close: Highly enantioselective catalysis of tandem acetalization/cycloisomerization reactions of o‐alkynylbenzaldehydes has been achieved using gold complexes of chiral acyclic diaminocarbene ligands that have electron‐deficient aryl substituents. X‐ray crystallography and DFT calculations implicate weak gold‐arene interactions—absent in the case of simple phenyl substituents—that define the chirality of the substrate binding site.

Acyclic aminocarbenes have received much less attention as ancillary ligands in homogeneous catalysis compared with their cyclic relatives (i.e., N-heterocyclic carbenes, NHCs), despite having a longer history and greater structural variety. Although these species are generally more fragile than NHCs, recent advances in the synthesis and catalytic application of metal complexes of acyclic carbenes have brought increased attention to these underutilized ligands. It is increasingly clear that acyclic carbenes possess unique properties that distinguish them from other ligand classes and make them potentially valuable for catalysis. These include exceptional donor ability, conformational flexibility, and wide N−C(carbene)−N angles that can place bulky or chiral substituents near catalytic substrate binding sites. This purpose of this Perspective is to review and critically assess recent progress in this forefront area of catalyst design. Syntheses and ligand properties of acyclic diaminocarbenes, aminooxycarbenes, and other aminocarbenes are reviewed with a view toward catalytic relevance. A special focus is to highlight catalytic reactions in which acyclic carbene ligands confer unusual selectivity or activity on metal catalysts compared with conventional ligand types. Particularly promising catalytic results have been obtained with acyclic carbene complexes of gold, including some highly enantioselective catalytic transformations.