Chromium Fischer alkoxy carbene complex

Conjugated chromium Fischer alkoxy carbene complex 1 is quantitatively transformed into a mixture of Z-vinyl ether 2 and ii-allyl ether 3 by NaBH4 reduction in EtOH (Scheme 7.1). 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

The reaction of alkoxy(alkyl)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). 

Cr(CO)3-coordinated hydroquinone from vinylic alkoxy pentacarbonyl chromium carbene (Fischer carbene) complex and alkynes. 

The Dotz benzannulation reaction, based on the alkyne cycloaddition to chromium carbene complexes, is the most important application of Fischer carbene complexes. Among the various Fischer carbene complexes, alkoxy and aminocarbene complexes of chromium undergo a novel inter- and intramolecular tandem alkyne insertion/ carbene annotation sequence to give 9H-carbazoles and nf/-benzo[fl]carbazoles. 

The results of a series of reactions of Fischer carbene complexes with enynes are summarized in Tables 1 and 2. Cyclopropane synthesis is accomplished in the alkoxy series (Y = OMe) by the generation of a mixture of geometric isomers of enol ethers, whereas in the dialkyl-amino series, ketones are directly obtained after hydrolysis of the enamines. Higher yields have been obtained using the amino analog pentacarbonyl(l-pyrrolidinoethylidene)chromium [Y = N(CH2)J. - ... 

A number of Fischer carbene complexes of chromium and tungsten have been prepared by conventional and microwave heating (YIF = 0.79-1.1). Mono- and dimethylureas were reacted with alkynyl alkoxy Cr or W carbenes using THF as the solvent (Scheme 7.22). The resultant complexes contained a uracil-moiety. A sol-vent-free approach to the reaction was also attanpted, but this gave somewhat lower yields of the desired products. 

Metalla-Diels-Alder Reaction via Metal Exchange A significant metal effect on the reactivity of a,(3-unsaturated Fischer carbene complexes is found in the reaction with alkenes and alkynes. Thus, the reaction of electron-poor alkynes with chromium alkenyl(alkoxy)car-benes is usually slow [26], whereas rhodium-catalyzed reactions of chromium alkenyl(methoxy)carbenes 75 with electron-poor alkynes 76 readily afford polysubstituted cyclopentenones 78, 79, and 80 (Scheme 5.17) [27]. Although the reactions proceed with high regioselectivity, the regioselectivity depends on the substituent (R ) of... 

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