Alkenylcarbene-metal complex

Preparation of alkenylcarbene metal complexes was reported by Le Bozec and Dixneuf et al. in 1991 by activation of propargylic alcohols in the presence of methanol [13[. Thus, photolysis of M(CO)6 (M = Cr, W) in the presence of 2-propyn-l-ol derivatives 30 in the presence of MeOH gave the corresponding... 

Coupling of alkenylcarbene complexes and siloxy-substituted 1,3-dienes affords vinylcyclopentene derivatives through a formal [3C+2S] cycloaddition process. This unusual reaction is explained by an initial [4C+2S] cycloaddition of the electron-poor chromadiene system as the 471 component and the terminal double bond of the siloxydiene as the dienophile. The chromacyclohexene intermediate evolves by a reductive elimination of the metal fragment to generate the [3C+2S] cyclopentene derivatives [73] (Scheme 26). 

The mechanism for aldehyde-derived enamines involves a Michael-type 1,4-addition of the enamine to the alkenylcarbene complex to generate a zwit-terionic intermediate which evolves to the final product by cyclisation. On the other hand, ketone-derived enamines react through an initial 1,2-addition to the carbene carbon to generate a different zwitterionic intermediate. Then, a [l,2]-W(CO)5 shift-promoted ring closure produces a new intermediate which, after elimination of the metal moiety, furnishes the corresponding cyclopen-tene derivatives (Scheme 30). 

The insertion reaction between alkenylcarbene complexes and electron-rich alkynes such as 1-alkynylamines (ynamines) leads to mixtures of two regioi-someric cyclopentyl derivatives [78]. Thus, if the insertion occurs on the carbon-metal bond a new aminocarbene complex is produced which evolves to a cyclopentenylmetal derivative. On the other hand, if the insertion reaction occurs on the carbon=carbon double bond of the alkenyl complex, the reaction gives a l-metala-4-amino-l,3,5-triene complex which finally generates a different regioisomer of the cyclopentenylmetal derivative (Scheme 31). 

The reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and... 

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... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

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). 

Bicyclic 3-imidazolines form from primary isocyanides and alkenylcarbene complexes by a metal-induced anomalous insertion of a C=N into the a-C—bond of the isocyanide (Scheme 167) <87CB1287>. 

In alkenyl- and alkynylcarbene complexes the addition of nucleophiles to the carbene carbon competes with the addition to the 3-carbon of the conjugated C-C multiple bond. [17] The regioselectivity of the addition of amines to alkynylcarbene complexes is temperature dependent 1,2-addition is favoured by lower temperatures. [17c] Enolates turned out to be efficient C-nucleophiles for Michael addition reactions to unsaturated metal carbenes. The product distribution may depend on steric factors as shown in Scheme 7 for the addition of different enolates to alkenylcarbene complex 10. The less bulky acetone enolate 11 adds to the carbene carbon protonation of the primary addition product results in demetalation and in the formation of a mixture of isomeric enones 12. In contrast, the more bulky cyclopentanone enolate 13 adds to the less shielded vinylic position. 

Transition Metals. - Carbene complexes of the chromium triad have proven to be attractive reagents in modern organic chemistry. Since the advent of the catalyst RuCl2(CHCH=CPh2)(PR3)2 much interest has been dedicated to the synthesis of the alkenylcarbenes of the Pe triad. Bemad et are the first to describe the aUcenylaminocarbenes of Ru. The ellipticities of the Ru-Ca and C -N bonds are reported to be low. 

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