The chemistry of N-heterocyclic carbenes has long been limited to metal coordination compounds derived from azolium precursors, a development that was started by Ofele and Wanzlick in 1968. Since free carbenes are now available through the work of Arduengo (1991), a renaissance in this little-recognized area of chemistry has occurred. A leading motive is the advantage of Nheterocyclic carbenes as ligands in organometallic catalysts, where they extend the scope of application reached by phosphanes (functionalized, chiral, water-soluble, and immobilized derivatives). The present review summarizes the state of the art with regard to synthesis, structure, bonding theory, metal coordination chemistry, and catalysis. Chelating, functionalized, chiral, and immobilized ligands can be generated and attached to metal centers in straightforward procedures under mild conditions. A wealth of new chemistry is thus opened. It is also shown how carbenes derived from imidazoles and triazoles behave as ligands in catalysis. It is reasonable to assume that N-heterocyclic carbenes surpass the ubiquitous phosphanes as ligands in a number of organometallic catalytic reactions.

A plethora of methods have been developed over the years so that carbon dioxide can be used as a reactant in organic synthesis. Given the abundance of this compound, its utilization in synthetic chemistry, particularly on an industrial scale, is still at a rather low level. In the last 35 years, considerable research has been performed to find catalytic routes to transform CO(2) into carboxylic acids, esters, lactones, and polymers in an economic way. This Review presents an overview of the available homogeneous catalytic routes that use carbon dioxide as a C(1) carbon source for the synthesis of industrial products as well as fine chemicals.

Stabilized by “carbene” donor ligands, the Pd complex 1 catalyzes the Heck olefination of aryl halides unexpectedly efficiently and yet has long‐term stability at elevated temperatures. The active Pd0 species can be generated during the Heck reaction or deliberately prepared by reduction of 1 with, for instance, hydrazine or sodium formate. Another similar catalyst can be synthesized in situ from Pd0 complexes and 1,3‐dimethyldihydroimidazoline‐2‐ylidene.

269. 21949 330% [h] a1 ( i . M. Clorc. A M Gronenhorn. .I .M~i,!yi. R c i i i i 1982. 48. 402 -417: h) G. M. Chi-e. A. M GI-onenhorn. ihid 1983. 53. 423 ~442: c ) F. Xi, .I , M q y Rcwrio~i. 1992. !Zi. 651 656 [7] €5. Ernst, N. Cooke. P M. ,&her?. unpublished. The qrithetic tetrasaccharide corresponds to thc formula shown in the text.181 The cctndnm;iins of h u m a n E-selectiii were combined b! means of the poly-mera\echnin iretictimi (FCR) uith the Fc region of liuman I g G l , and subcloned iii the expression vector pcDNAl iieo (Invitrogen). Following trmsfection of CHO-KI cells (ATCC CCL-61) and selection with '3418 (Gibco). :i stahle cell line w s obtained which secreted c' r 25 pgni1. ' recombinant E-selectin IiIgG. For thc productimi of larger aniouiit~ t h i s cell line was cultivated in 3 hollo~r fiber hioreactor (1.1 in2. 50 mL, Cellco) in Opli. MEM culture medium (Ciihco) rupplementcd with 2 % fetal calf \ei-um nnd 200 m L -' gentamyciii. The supernatant o l~o n d i t i o n~d cclh was lir5t purified by d h t y chromatography on proteiii A agarme (Sigma). The proteiii fraction obtained hy elution with 20mM glqcine-HCl. pH 3.0. w a s iicutralired and chi-oin;itographed direct11 on an .inti-human-E-selectiii affinity column ( 3 mg monoclonal antibody 7A9 per mL affigel). Specifically horiiid E-selectin l g G was eluted nitli 5 v urea in buffered s a l i solution and finally dialyzed against Dubeccos PBS solution (PBS = phosphate buffered \aline). The protein thus obtained showed. i n SDS (sodium dccyl sull'atc) polyacry1;rmidc pel electrophoresis tinder reducing coiiditions. : i single band :it ca 140 kD. For NMR triialyris ca 6 me E-selectin hJgG wab diiilyxd against 50mM pcrdeutei-ated imidazol (Signs). I mh4 Ca-CI,. pH 7.4. iii D1O and concentrated by-means of Centricon YM-50 (Amicon) t o il Sinal volume ofO.5 mL. [O] a ) All NMR experiment.; w r e performed i n the Jnstitut fur Binpliqsikalisclie . . C;hemic (lei-Uniccrsitiit Fi-ankfiirt on :i Rruker DMXhOO specti-ometei-. We ~i s h to tliaiik Prof. Dr. H. Ruterjans for the the opportunilq to make the measurements. b) For the N MR eyperiments on the complex an E-selectin(1gG c1iiiner:i)'retrasacchnride ratio of 1: 15 M R S chosen. The concentration ofsialyl Letcis" \\>is 0.X1 inhi. that of E-selectin(lgG chimera) 5 4~~. [D,]Imidarolc (30iiihr) mas used as buffer. In addition the tolution contained NaCl (50mxi) and CaCI, (1 nib<). The pH w a s 7.4. Spinlock filtered NOESY spectra [lo] with mixing times T,,, of 25. 50. 75. 100. 125. 150. 175. and 200 ms were recorded at 600 MMr and 310 K. The relaxation lime way 1 X5 s. The spectral \bid111 was i000 H r (5 ppiii) in all casea. 512 Increments in T , Rere recorded. For each iiicrcment 32 transients were xcqnired each with 2K data points. After Lero-filling and multiplicirti~~n with squared coc functions in r l and T?. ii 2K x 1 K data matrix bas obtaincd by 2D Fourier transformation. T. Scherf. J Anglister. Biophr.~. .I 1993. 64. 7.54-761 NOESY specti-a o f lkcc F ciin(JgC~ chimera) and sialyl Lewis" tetr...

N‐Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N‐heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C−H activation, C−C, C−H, C−O, and C−N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N‐heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.