Cyclopropylcarbene complexes

Chromium cyclopropylcarbene complexes react with alkynes to provide cyclopentenone derivatives in a formal [2c+2s+lCo] cycloaddition process (see Sect. 3.2). However, tungsten and molybdenum cyclopropylcarbene complexes... 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

Cyclopentenones. Reaction of the cyclopropylcarbene complex 1 with di-phenylacetylene in 1% aqueous dioxane results in the cyclopentenone 2 as the major product. The carbonyl group of 2 is derived from a CO ligand of 1, and C,... 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

While the first transition metal carbene complex was reported in 1964134, the first cyclopropylcarbene complex salt [(CO)5Cr=C(chromium hexacarbonyl, followed by tetramethylammonium bromide135. Subsequent reaction with trimethyloxonium fluoro-borate gave methoxycarbene complex (CO)5Cr=C(OMe)(c-Pr) (equation 62)136. 

A solution of cyclopropylcarbene complex 2 (0.276 g, 1.00 mmol) in 1,4-dioxane (100 mL) was heated to reflux while a solution of dimethyl ( )-but-2-enedioate (0.288 g, 2.00 mmol) in 1,4-dioxane (10 mL) was added to the refluxing solution over a 2 h period. When the addition was complete, the mixture was refluxed for an additional 4 h. The solvent was then removed on a rotary evaporator. The residue, after evaporation, was diluted with hexane/EtOAc (4 1, 20 mL) and filtered through Celite. This solution was subjected to flash chromatography (silica gel, hexane/EtOAc 19 1). The product in the first fraction (Rj-0.45, in hexane/EtOAc 4 1) was assigned as methyl 4-methoxy-3-methoxycarbonylhept-4-enoate (0.041 g, 18%). The product in the second fraction (Rj- 0.42, hexane/EtOAc 4 1) was concentrated and placed under vacuum (0.04 Torr) for 2 h to remove the dimethyl (Z)-but-2-enedioate impurity to give dimethyl la-methoxy-[l,l -bicyclopropyl]-2a,3) -dicarboxylate (0.142g, 62%) as colorless crystals mp 81-82°C (sublimed at 60"C/0.025 Torr). 

Cyclopropyl-substituted transition-metal carbene and carbyne complexes, especially as cyclo-propylidene- or as cyclopropylcarbene complexes, have been used as synthetic building blocks with or without ring opening. Examples of cyclopropane synthesis via decomplexation are described in Section 1.A.5.2.6. 

Cycloadditions. Pyrolysis of 0-alkynyl cyclopropylcarbene complexes generates fused cyclopentenones. This transformation has been exploited in a synthesis of a vitamin Dj synthon. ... 

Scheme 36. Stereocontrolled intramolecular cycloaddition with a cyclopropylcarbene complex.

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