Chromium complexes vinylidenes

In situ preparation of the vinylidene pentacarbonylchromium complexes was also reported by Fischer et al. [7]. Treatment ofthe acylchromate complexes 18, prepared by acylation of pentacarbonylchromium dianion, with trifiuoroacetic anhydride in the presence of DBU afforded very labile pentacarbonylchromium vinylidene complexes 19, which were, without isolation, reacted with several electron-rich alkynes such as ynamines, alkynyl ethers, and alkynyl thioethers to give the corresponding cyclobutenylidene chromium complexes in reasonable yield [8] (Scheme 5.5). 

In 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

A chromium(O) pentacarbonyl-methylene chloride complex 131 was formed under irradiation, which reacted with an alkyne to form a vinylidene complex 132. Complex 132 further reacted with an imine or a dialkylcarbodimide to afford /3-lactams after decomplexation of chromium (Scheme 58).233... 

Historically, vinylidene complexes of zero-valent pentacarbonyl Group 6 metals appeared as a fleeting intermediate for the preparation of Fischer-type carbene complexes. Probably the first example of the formation of such a pentacarbonyl vinylidene complex of a Group 6 metal was proposed in 1974 by Fischer et al, who examined the reaction of pentacarbonyl[hydroxy(methyl)carbene] chromium 1 with dicyclohexylcarbodiimide(DCC) [3]. Thus, treatment of 1 with DCC in CH2CI2 at —20°C rt gave a novel azetidinylidene complex 2 in 47% yield. As a possible... 

Reaction of crowded chromium alkenyl Fischer carbene (50) with bulky ketene acetals provides an interesting entry to 3-substituted pent-l-ynoate (53)45 Formation of the alkyne can be rationalized by a 1,4-nucleophilic addition of the ketene on the unsaturated carbene complex (crowded complexes will not undergo potential 1,2-addition), following by oxonium (51) formation and fragmentation to a vinylidene carbene complex (52), which undergoes a 1,3-shift to the alkynylchromium complex leading the alkyne after reductive elimination. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Also, bi- and trimetallic cumulene complexes, such as M=C=M, M=M=C, M=M=M and M=C=C=M are known. Cationic ruthenium allenylidene complexes are used as catalysts for ring closing metathesis reactions. Nonlinear optical properties have been measured for the Group 6 cumulenylidene complexes. Also, cationic chromium or iron vinylidene complexes undergo [2-1-2] cycloaddition reactions across imines to give fi-lactams. This reaction is useful for the synthesis of j8-lactam antibiotics. ... 

The reaction of the chromium vinylidene complex 27 with l-methyl-2-thiomethylacetylene at -25 °C gives rise to the formation of the four-membered ring... 

In the reaction of the chromium vinylidene complexes 29 with ynamines or ethoxypropyne, also [2+2] cycloadducts 30 are formed in moderate yields... 

Also, the chromium and tungsten vinylidene complexes 31 react with the alkynyl complexes 32 to give heterobimetallic cyclobutenylidene complexes 33, which are yellow or red crystalline compounds ". ... 

The vinylidene complexes (CO)5W=C=CPh2 react with imines and triphenylketen-imines to give [2+2] cycloadducts AUenylidene complexes of chromium 56 also undergo [2+2] cycloaddition reactions with azomethines at the center C=C bond to give the cycloadducts 57 . 

The chromium vinylidene complex, (CO)5Ci=C=CH2, reacts with dicyclohexylcarbodi-imide to give the [2+2] cycloadduct 60. ... 

CYCLOAROMATIZATION VIA CHROMIUM-, MOLYBDENUM-, AND TUNGSTEN-VINYLIDENE COMPLEXES... 

In 2002, Ohe et al. used electrocyclization in the [3,3]-sigmatropic rearrangement of cyclopropanes bearing both an ethynyl group and an acyl group 18, where chromium-vinylidene complexes were supposed to work as key reactive intermediates (Scheme 21.8) [14]. Formation of the corresponding oxepins 20 and their conversion to phenols 19 via arene oxides 21 were proposed. In 2004, Sangu et al. 

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