Chromium cyclopropyl carbene complexes

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Alkoxy(alkyl)carbene complexes of chromium, upon photolysis in the presence of 1,3-dienes, give cyclobutanones as [2 +1 + 1] or formal [2-1-2] cycloadducts. Thus, cyclopropylcar-bene complex 5 gave the corresponding cyclopropyl-cyclobutanone adduct 6 with cyclohexa-1,3-diene. 

Several examples have been reported, where cyclopropyl containing carbenes were used in [4 -H 2], [2 -f 2] and [3 -t- 2] cycloadditions giving monocyclic and polycyclic ring systems. Thus, the chromium cyclopropylethynyl(ethoxy)carbene complex 19 formed a [2-1-2] cycloadduct with silyl enol ethers 20 underwent further conversion with alkynes to form the bicyclo[4.2.0]oc-tadienone skeleton 21. ... 

Pentacarbonyl[cyclopropylethynyl(ethoxy)carbene]chromium complex 19 reacted with cyclo-penta-1,3-diene to give a cyclopropyl-substituted carbene complex of norbornadiene 22, which was converted to the corresponding carboxylic acid 23 upon oxidation or to a 1,4-quinone-type derivative 24 after Dotz-type cyclization with phenylethyne. ... 

Similarly, the pentacarbonyl[(2-cyclopropyl-2-dimethylaminovinyl)ethoxycarbene]chromium complex 27, prepared from pentacarbonyl[cyclopropylethynyl(ethoxy)carbene]chromium 19 and dimethylamine (or other secondary amines), reacted with pent-l-yne or phenylethyne to give 1-propyl- or 1-phenyl-substituted 5-cyclopropyl-5-dimethylamino-3-ethoxycyclopenta-1,3-diene in 62-77% yield.With substituents other than cyclopropyl yields were drastically lower unless the reaction of the carbene complex was performed in pyridine or acetonitrile. 

Simple alkenes, norbomene and styrene do not undergo cyclopropanation or insertion reactions with cyclopropyl(methoxy)carbene chromium pentacarbonyl complex. However, the conjugated 1-vinylcyclopentene is cyclopropanated under the reaction conditions at the terminal double bond, affording an isomeric mixture (trans cis = 40 60), in 66% yield (equation 72). 

The reaction of the chromium complex of cyclopropyl(methoxy)carbene with an alkene results in the formation of bicyclopropyl derivatives 4 in good yield, accompanied occasionally by minor amounts of ring-opened products. 

Pentacarbonyi[cyclopropyl(methoxy)carbene]chromium (Complex 2/ ... 

Similarly, the [cyclopropylmethyl(methyl)amino]carbene system reacted to give a cyclopropyl-methyl-substituted pyrrolinone. This cyclopropane-containing aminocarbene complex of chromium underwent alkyne insertion and formation of a heterocyclic ylide. Further rearrangements gave dihydro pyrrolones 17 and 18 with cyclopropylmethyl substituents. ... 

More interestingly, acyloxycarbene- chromium complexes, which are generated in situ by acylation of the corresponding tetraalkylammonium metal acylates, transfer their carbene ligand to silyl enol ethers under very mild conditions and with excellent simple diastereoselectivity to provide 2-(/m-butyldimethylsilyloxy)cyclopropyl acetates. When R2 is hydrogen the cu-iso-mers predominate, whereas formation of the trans-product is favored when R2 is phenyl18. 

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