Synthesis, structure, and thermochemistry of adduct formation between N-heterocyclic carbenes and isocyanates or mesityl
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ORIGINAL RESEARCH
Synthesis, structure, and thermochemistry of adduct formation between N-heterocyclic carbenes and isocyanates or mesitylnitrile oxide Manuel Temprado • Subhojit Majumdar • Xiaochen Cai • Burjor Captain • Carl D. Hoff
Received: 20 June 2013 / Accepted: 27 June 2013 Ó Springer Science+Business Media New York 2013
Abstract Reaction of N-heterocyclic carbenes (NHCs) with isocyanates yields stable zwitterionic imidates/amidates in toluene solution. These compounds were fully characterized and the crystal structures of several species were determined by X-ray crystallography. The thermochemistry of binding of these and related species was studied by solution calorimetry. Comparison is made of the enthalpies of binding of NHC to isocyanates (RNCO) and isomeric nitrile oxides (RCNO) as well as CO2. DFT calculations were performed to additionally assess the nature of bonding in these compounds. Keywords Carbenes NHC Zwitterions Betaines Thermochemistry
Introduction The isolation and characterization of the first stable crystalline carbene was performed by Arduengo et al. [1]. Since then, these species have generated great interest in several different
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Electronic supplementary material The online version of this article (doi:10.1007/s11224-013-0305-2) contains supplementary material, which is available to authorized users. M. Temprado (&) Department of Physical Chemistry, Universidad de Alcala´, Ctra. Madrid-Barcelona Km. 33,600, 28871 Madrid, Spain e-mail: [email protected] S. Majumdar X. Cai B. Captain C. D. Hoff (&) Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146, USA e-mail: [email protected]
fields of chemistry due to their important applications as ligands in organometallic complexes [2–6], and as organic catalysts [7–11]. By far, the most used carbenes nowadays are N-heterocyclic carbenes (NHCs) derived from imidazole. The stability of NHCs arises from a combination of electronic effects, p-donation by the lone pairs of the N atoms into the empty pp orbital of the carbene, and steric effects incorporating bulky substituents in the nitrogens. NHCs are strong r-donors and their application as powerful nucleophilic reagents has been documented in several important organic catalyzed transformations [7–15]. It is well known that addition of NHCs to electrophilic reagents leads to formation of zwitterionic adducts [16–18]. In this regard, Louie and co-workers have synthesized and structurally characterized a series of zwitterionic imidazolium carboxylates by reaction of NHCs with CO2 [19, 20]. Analogously, the synthesis and X-ray structures of a series of imidazol(in)ium dithiocarboxylates [21] and thiocarboxylates [22] have also been reported by Delaude et al. We have recently reported [23] that addition of solid MesCNO (Mes = mesityl) to a toluene solution of NHC resulted in the immediate formation of the corresponding NHC/MesCNO adduct. Moreover, Severin and co-workers [24] have also r
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