Carbene complexes, of chromium and

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

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. 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

The potential of Fischer carbene complexes in the construction of complex structures from simple starting materials is nicely reflected in the next example. Thus, the reaction of alkenylcarbene complexes of chromium and tungsten with cyclopentanone and cyclohexanone enamines allows the di-astereo- and enantioselective synthesis of functionalised bicyclo[3.2.1]octane and bicyclo[3.3.1]nonane derivatives [12] (Scheme 44). The mechanism of this transformation is initiated by a 1,4-addition of the C -enamine to the alkenylcarbene complex. Further 1,2-addition of the of the newly formed enamine to the carbene carbon leads to a metalate intermediate which can... 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

The imino carbene complexes of tungsten and chromium (e.g., 81) also serve as nitrile ylide synthons (40). The mngsten complexes gave higher yields of the adducts and were strongly regioselective for product 82. For example, for 81 (M=W R = Me, Ph R = Pr, Ph R" = H), the pyrrole 82 was produced in yields of 65-75% with <1% of 83. This route to pyrroles thus has clear advantages over... 

In almost all situations the reactions of Fischer carbene complexes of chromium with alkynes lead to the formation of six-membered ring products, but on several occasions five-membered ring annulated products have been observed as minor products or as major products if the formation of six-membered rings is blocked. In an early report by Ddtz, the reaction of the 2,6-dimethylphenyl complex (209) was observed to react with diphenylacetylene to give the complexed and uncomplexed indenes (210) and... 

The reactions are stereospecific. The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), mo-lybdenum(O) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state. 

Alternatively, similar reactions were achieved with Fischer-type carbene complexes of chromium containing cyclopropane units 2. These react with alkenes, dienes and (x,/l-unsaturated esters to form dicyclopropanes 3 and 4 with small amounts of ring-opened products. 

Synthesis of Five-Membered Carbocycles An annulation of chromium arylcarbenes by alkynes without CO-insertion leads to five-membered rings. It has been observed as a side reaction along the benzannulation which may become predominant depending on the nature of the carbene heteroatom substitution pattern, [72] the metal [40] and its coligands [73] and the solvent [43] used. In this respect, indene derivatives have been obtained from arylcarbene complexes of chromium and tungsten. [73,74]... 

Table 5.6. EPR Parameters and Magnetic Properties of Carbene Complexes of Chromium(I) and Iron(I) ...

The [3+2+1] cycloaddition of an a, 3-unsaturated or aryl carbene complex of chromium, an alkyne, and carbon monoxide, that is named the Dotz benzannulation, is a useful method for the synthesis of a phenol or naphthol derivative, although this reaction requires a stoichiometric amount of the chromium carbene complex of chromium [35]. A mechanism of this reaction is shown in Scheme 21.31. When unsymmetrical alleynes are used, the regioselectivity is determined in the alkyne insertion step by the steric effect. 

Transition-metal carbene complexes have proven especially useful in the construction of six-membered rings, primarily in the synthesis of naphthalenes and higher aromatic ring systems. The major method employed is the Dbtz benzannulation reaction (depicted in Scheme 17.12), where an a,p-unsaturated or aryl carbene complex of chromium (78) couples with an alkyne to afford a phenol derivative (76). This reaction has been demonstrated for a vast array of molecular architectures and functionalities and has proven to be a very reliable and efificient process. Perhaps the major limitation on the synthetic utility is the construction of the carbene complex for systems that require structural complexity in the carbene complex. In addition to the Dotz reaction itself, several efforts to develop alternative procedures have been inspired by a detailed understanding of the mechanism of the Dotz reaction. 

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