Enders’ carbene

Most active is an in situ generated Ni complex with the Enders carbene, 1,3,4-triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene. The catalyst performance reads as follows 2.5 H2 equiv in 4h at 60°C for a dilute solution of AB in diglyme (0.14M, catalyst amount 10 mol%) 2.5 H2 equiv in 2.5 h at 60 °C for a highly concentrated solution of AB in diglyme (25 wt.%, catalyst amount 1 mol%). The obtained product mixture consists mainly of soluble polyborazylene. Besides the high yield of more than 2 equiv H2, this catalytic system is also beneficial in respect to a very low borazine... 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

Almost 20 years after the initial report of the Stetter reaction, Ciganek reported an intramolecular variant of the Stetter reaction in 1995 with thiazolium precatalyst 74 providing chromanone 73 in 86% yield (Scheme 10) [64]. This intramolecular substrate 72 has become the benchmark for testing the efficiency of new catalysts. Enders and co-workers illnstrated the first asymmetric variant of the intramolecnlar Stetter reaction in 1996 utilizing chiral triazolinylidene pre-catalyst 14 [65]. Despite moderate selectivity, the implementation of a chiral triazolinylidene carbene in the Stetter reaction laid the fonndation for future work. 

Different synthetic routes have been used to prepare these carbenes (Scheme 8.6). The most common procedure is the deprotonation of the conjugate acid. In early experiments, sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the DMSO anion were used. ° Then, Herrmann et al. showed that the deprotonation occurs much more quickly in liquid ammonia as solvent (homogeneous phase), and many carbenes of type IV have been prepared following this procedure. In 1993, Kuhn and Kratz" developed a new and versatile approach to the alkyl-substituted N-heterocyclic carbenes IV. This original synthetic strategy relied on the reduction of imidazol-2(3//)-thiones with potassium in boiling tetrahydrofuran (THF). Lastly, Enders et al." reported in 1995 that the 1,2,4-triazol-5-ylidene (Vila) could be obtained in quantitative yield from the corresponding 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole by thermal elimination (80 °C) of methanol in vacuo (0.1 mbar). 

Note that the other type of aromatic carbene isolated by Enders et al.," namely, the l,2,4-triazol-5-ylidene (Vila) is stable enough to be prepared by thermal elimination at 80 °C, and it became the first carbene to be commercially available. 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Fig. 9.3 Stable carbenes synthesized by Arduengo et al. (11) and Enders, Teles et al. (12).

Scheme 9.2 Synthesis of the stable carbene 12, as developed by Enders, Teles, and colleagues.

Enders D (1993) Enzymemimetic C-C and C-N bond formations. In Stereoselective synthesis. Springer-Verlag, Berlin Heidelberg New York, p 63 Enders D, Balensiefer T (2004) Nucleophilic carbenes in asymmetric organo-catalysis. Acc Chem Res 37 534—541... 

Enders D, Hilttl MRM (2006) Control of four stereocentres in a triple cascade organocatalytic reaction. Nature 441 861-863 Enders D, Kallfass U (2002) An efficient nucleophilic carbene catalyst for the asymmetric benzoin condensation. Angew Chem Int Ed Engl 41 1743-1745... 

Enders D, Breuer K, Raabe G, Runsink J, Teles JH, P Melder J, Ebel K, Brode S (1995b) Preparation, structure, and reactivity of l,3,4-triphenyl-4,5-dihy-dro-lH-l,2,4-triazol-5-ylidene, a new stable carbene. Angew Chem Int Ed Engl 34 1021-1023... 

Enders D, Breuer K, Raabe G, Simonet J, Ghanimi A, Stegmann HB, Teles JH (1997b) A stable carbene as ji-acceptor. Electrochemical reduction to the radical anion. Tetrahedron Lett 38 2833... 

Enders D, Niemeier O, Balensiefer T (2006c) Asymmetric intramolecular cros-sed-benzoin reactions by N-heterocyclic carbene catalysis. Angew Chem Int Ed Engl 45 1463... 

Enders D, Niemeier O, Raabe G (2006d) Asymmetric synthesis of chromanones via N-heterocyclic carbene catalyzed intramolecular crossed-benzoin reactions. Synlett 2006 2431... 

Enders D, Balensiefer T, Niemeier O, Christmann M (2007c) Nucleophilic N-heterocyclic carbenes om asymmetric organocatalysis. In Dalko PI (ed) Enantioselective organocatalysis—reactions and experimental procedures. Wiley-VCII, Weinheim, p 331... 

Teles JH, Melder JP, Ebel K, Schneider R, Gehrer E, Harder W, Brode S, Enders D, Breuer K, Raabe G (1996) The chemistry of stable carbenes. Part 2. Benzoin-type condensations of fromaldehyde catalyzed by stable carbenes. Helv Chim Acta 79 61-83... 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

Enders D, Niemeier O, Henseler A (2007b) Organocatalysis by N-heterocycbc carbenes. Chem Rev 107 5606-5655... 

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