Biomimetic Organocatalytic C–C-Bond Formations
89
In 1995 we synthesized the triazol-5-ylidene 94 starting from the triazole precursor 92 (Enders et al. 1995b). The crystalline carbene 94 was
obtained from 92 by addition of methanolate to give the adduct 93 followed by α-elimination of methanol, either at 80°C under vacuum or in
refluxing toluene. 94 proved to be stable at temperatures up to 150°C in
absence of air and moisture (Enders et al. 1996a; Enders et al. 1997a;
Enders et al. 1997b; Enders et al. 2003). Ab initio calculations on the
parent heterocyclic system suggested that the π-donation from the adjacent nitrogen atoms lead to a significant transfer of electron density into
the formally vacant p-orbital at the carbon atom (Raabe et al. 1996). 94
showed the typical behavior of a nucleophilic Wanzlick-type carbene
and was the first carbene to be commercially available (Scheme 22).
On an industrial scale, methane, the main component of natural gas,
can easily be converted to formaldehyde. An efficient catalytic condensation of formaldehyde to dihydroxyactone or glycolaldehyde would
thus provide a route to C 2 - and C 3 -chemicals from methane. The triazolin-5-ylidene 94 turned out to be a powerful catalyst for the conversion of formaldehyde (95) to glycolaldehyde (96) in the ‘formoin reaction’ (Teles et al. 1996). This reactivity is a useful complement to
the catalytic properties of thiazolium salts which mainly afford 1,3dihydroxy acetone as product (Scheme 23) (Castells et al. 1980; Matsumoto and Inoue 1983; Matsumoto et al. 1984). As triazolium ylides
are much more stable than thiazolium ylids, the elimination of glycolaldehyde occurs faster than the addition of the third formaldehyde
molecule.
3.2.3 Asymmetric Intermolecular Benzoin Reactions
The promising results of triazolium salt catalysis inspired our research
group to synthesize a variety of chiral triazolium salts for the asymmetric benzoin condensation (Enders et al. 1996b; Enders and Breuer
1999; Teles et al. 1999). Extensive investigations have shown that the
enantiomeric excesses and catalytic activities are highly dependent on
the substitution pattern of the triazolium system. The most active catalyst (S, S)-97, which is readily available from an intermediate of the
industrial chloramphenicol synthesis, provided benzoin (85
) in its (R)configuration with 75% ee and a good yield of 66%. Remarkably, only
89
In 1995 we synthesized the triazol-5-ylidene 94 starting from the triazole precursor 92 (Enders et al. 1995b). The crystalline carbene 94 was
obtained from 92 by addition of methanolate to give the adduct 93 followed by α-elimination of methanol, either at 80°C under vacuum or in
refluxing toluene. 94 proved to be stable at temperatures up to 150°C in
absence of air and moisture (Enders et al. 1996a; Enders et al. 1997a;
Enders et al. 1997b; Enders et al. 2003). Ab initio calculations on the
parent heterocyclic system suggested that the π-donation from the adjacent nitrogen atoms lead to a significant transfer of electron density into
the formally vacant p-orbital at the carbon atom (Raabe et al. 1996). 94
showed the typical behavior of a nucleophilic Wanzlick-type carbene
and was the first carbene to be commercially available (Scheme 22).
On an industrial scale, methane, the main component of natural gas,
can easily be converted to formaldehyde. An efficient catalytic condensation of formaldehyde to dihydroxyactone or glycolaldehyde would
thus provide a route to C 2 - and C 3 -chemicals from methane. The triazolin-5-ylidene 94 turned out to be a powerful catalyst for the conversion of formaldehyde (95) to glycolaldehyde (96) in the ‘formoin reaction’ (Teles et al. 1996). This reactivity is a useful complement to
the catalytic properties of thiazolium salts which mainly afford 1,3dihydroxy acetone as product (Scheme 23) (Castells et al. 1980; Matsumoto and Inoue 1983; Matsumoto et al. 1984). As triazolium ylides
are much more stable than thiazolium ylids, the elimination of glycolaldehyde occurs faster than the addition of the third formaldehyde
molecule.
3.2.3 Asymmetric Intermolecular Benzoin Reactions
The promising results of triazolium salt catalysis inspired our research
group to synthesize a variety of chiral triazolium salts for the asymmetric benzoin condensation (Enders et al. 1996b; Enders and Breuer
1999; Teles et al. 1999). Extensive investigations have shown that the
enantiomeric excesses and catalytic activities are highly dependent on
the substitution pattern of the triazolium system. The most active catalyst (S, S)-97, which is readily available from an intermediate of the
industrial chloramphenicol synthesis, provided benzoin (85
) in its (R)configuration with 75% ee and a good yield of 66%. Remarkably, only