The Chemistry of Stable Carbenes. Part 2. Benzoin‐type condensations of formaldehyde catalyzed by stable carbenes

Abstract

Stable carbenes derived from thiazole, 1H‐imidazole, and 4H‐1,2,4‐triazole are efficient catalysts for benzointype condensations of formaldehyde. Catalysts derived from N‐substituted thiazolium salts trimerize formaldehyde to dihydroxyacetone (II). Catalysts based on 1,4‐disubstituted 4H‐1,2,4‐triazol‐1‐ium salts give glycolaldehyde (I) as the main product and no II, whereas N,N′‐disubstituted 1H‐imidazol‐3‐ium salts yield mixtures of both products. The isolation of several intermediates in the catalytic cycle provide a better insight into the reaction mechanism.

1,2-migration inN-phosphano functionalizedN-heterocyclic carbenes

“…It has been suggested previously that the phenyl substitution in position 3 stabilizes triazol‐ylidenes towards the 1,2‐rearrangement in the experiment . One could expect increasing ΔE ST value in carbenes 3 compared with unsubstituted species 2 .…”

“…A significant part of total negative charge in anion 7 (−0.219) is delocalized on the phenyl moiety, while hydrogen atom in the unsubstituted anion 6 is positively charged (+0.163). Unfortunately, no experimental data for verification of this hypothesis is available: all known to date stable crystalline triazol‐5‐ylidenes are substituted at C3, and few representatives of 3‐H‐triazol‐5‐ylidenes were identified only in solution . Apparently, instability of carbenes 2 is determined by pronounced acidity of proton in position 3 …”

Imidazolium Catalysts Formed by an Iterative Synthetic Process as a Model System for Chemical Evolution

“…N,N-diphenylimidazoliums were chosen for two major reasons: (i) they are capable of catalyzing sugar synthesis from formaldehyde (Teles et al 1996) and (ii) they are more stable towards hydrolysis than thiazoliums (Duclos and Haake 1974). The formation of imidazoles and imidazoliums in reactions of sugars with ammonia or amino acids has been documented, though there remains disagreement on an exact reaction mechanism (Kort 1971;Velisek et al 1989).…”

“…Three additions of formaldehyde yield the product ketose DHA, which is theoretically unreactive. However, sugar rearrangements forming aldose isomers can continue to add equivalents of formaldehyde, and other sugars have been detected using similar reaction conditions (Castells et al 1980;Teles et al 1996) The middle trace (gray) displays reaction of the purified imidazolium ions with 0.75 mmol paraformaldehyde for 30 min at 80°C. New peaks appeared in the gray trace, between 3 and 5 ppm, that were indicative of sugar formation.…”

“…Through a similar mechanism, both thiazolium and imidazolium ions (Castells et al 1980;Teles et al 1996) have been shown to catalyze the self-condensation of formaldehyde to short chain sugars-a highly valued reaction in the prebiotic community. The work presented here is based on the following hypothesis.…”