N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading.

We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and ß-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/ß-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.

Breslow Intermediates from Aromatic N-Heterocyclic Carbenes (Benzimidazolin-2-ylidenes, Thiazolin-2-ylidenes).

We report the first generation and characterization of elusive Breslow intermediates derived from aromatic N-heterocyclic carbenes (NHCs), namely benzimidazolin-2-ylidenes (NMR, X-ray analysis) and thiazolin-2-ylidenes (NMR). In the former case, the diamino enols were generated by reaction of the free N,N-bis(2,6-diisopropylphenyl)- and N,N-bis(mesityl)-substituted benzimidazolin-2-ylidenes with aldehydes while the dimer of 3,4,5-trimethylthiazolin-2-ylidene served as the starting material in the latter case. The unambiguous NMR identification of the first thiazolin-2-ylidene-based Breslow intermediate rests on double 13 C labeling of both the NHC and the aldehyde component. The acyl anion reactivity was confirmed by benzoin formation with excess aldehyde.

O-Propyl N-phenyl-thio-carbamate.

Two independent mol-ecules comprise the asymmetric unit in the title thio-carbamide derivative, C(10)H(13)NOS. These differ in the relative orientations of terminal ethyl groups [C-C-C-O torsion angles = -66.95 (13) and 55.92 (13)°, respectively]. The phenyl ring is twisted out of the plane of the central residue [C(q)-N-C(ph)-C(ph) = -146.20 (12) and -144.15 (12)°, respectively; q = quaternary and ph = phen-yl]. The independent mol-ecules are linked into a dimeric aggregate by N-H⋯S hydrogen bonds and an eight-membered thio-amide {⋯H-N-C=S}(2) synthon.