Four-membered NHC

The outstanding performances of five-membered NHC ligands in organometallic chemistry and catalysis prompted Grubbs and co-workers to develop a novel stable four-membered NHC [64]. Following their interest in developing new ruthenium olefin metathesis catalysts, they synthesised and fully characterised complex 51 to study the impact of the architecturally unique NHC ligand on the activity of the Ru-based catalyst [65] (Fig. 3.20). In the RCM of 1 at 40°C in CH Cl with 51 (5 mol% catalyst), the reaction reached completion within 20 min, whereas less than 10 min are required for standard catalysts 14 and 16. It should be noted that catalysts 14 and 16 are able to complete the RCM of 1 with only 1 mol% catalyst at 30°C. 

The study was also extended toward the synthesis of four-membered NH(Zs. In 2005, Despagnet-Ayoub and Grubbs reported the synthesis of a ruthenium metathesis complex 119 featuring a four-membered NHC and compared its activity to that of complexes with five-membered NHCs (Figure 11.26) [102,103]. In all substrates tested, the reactions were found to be slower than those catalyzed by 10 or 11. 

A number of studies have been carried out on various frameworks of the NHC ligand. The effects of the substituents on the heterocycle (on the nitrogen atoms or on the C - C backbone) as well as the ring size of the NHC (six- and four-membered ring NHC) have been investigated. 

Fig. 9 Ruthenium catalysts with six- and four-membered ring NHCs 6...

An amino-pendant NHC-stabilized boronium cation was prepared from the reaction of a LiBr adduct of an NHC with borane in THF, giving a bromide salt. Subsequent reaction with silver triflate and lithium borohydride gave the amino-pendant NHC adduct with two equivalents of borane, one at the carbeneic carbon and one at the pendant nitrogen (Scheme 15.9). A four-membered metallacycle intermediate with a B-H-B motif has been postulated [108]. 

In addition to the range of asymmetric routes toward 13-lactone and (3-lactam scaffolds, Lewis bases have been used to access a variety of other important four-membered heterocyclic classes via formal [2+2] cycloadditions. In this section, the use of tertiary amines or NHCs to generate ammonium or azolium enolates, respectively from ketenes and their subsequent application toward the formation of other synthetically interesting heterocycles, will be discussed. A series of reports concerning the use of sulfenes, the sulfonyl equivalents of ketenes, in Lewis base-catalyzed asymmetric [2+2] cycloadditions will also be detailed. 

We also note that another possibility of introducing chirality within the heterocycle is the use of a seven-membered N-heterocyclic carbene. Most of the structures with four, five or six-membered rings possess a nearly planar heterocycle with the exception of seven-membered ring NHCs [63]. 

The coordination chemistry of heterocycles featuring the Group 13 element in the +1 oxidation state and supported by an -chelating substituent has been developed in the last decade. Monomeric heterocycles of types VI-VIII have been isolated and used as ligands the neutral four and six-membered heterocycles are isolobtil to NHCs in that they bear a singlet lone pair and a formaUy vacant p-orbital at the metal center, and the anionic five-membered heterocycles are valence isoelectronic to NHCs. 

Most of the known N-heterocyclic carbenes are derived from five-membered heterocyeles with additional oxygen, sulfur or phosphorus heteroatoms. Stable NHCs with up to four heteroatoms and saturated or unsaturated five-membered heteroeyeles are discussed in the following Section. 

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