Fischer carbene complex photolysis

In attempts to understand the photochemical reactions of Fischer carbene complexes, several matrix isolation and flash photolysis studies have been conducted using both Cr and W (but not Mo) complexes [5-11]. Although the complexes studied and conditions used varied, several general conclusions were drawn ... 

One of the earliest reported thermal reactions of Fischer carbene complexes was the reaction with olefins to give cyclopropanes [127]. More recently it has been shown that photolysis accelerates inter molecular cydopropanation of electron-poor alkenes [128]. Photolysis of Group 6 imine carbenes with alkenes... 

B. Transition Metal Organometallic Methods Photolysis of Fischer Carbene Complexes... 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

These reactions have been extensively investigated,and photolysis of the bis (chromium(0) Fischer carbene complex) 90 in the presence of the vinylaziridine 91 proceeds through the formation of a nominal bis (ketene metal complex) 92 leading to the bis(azepinone) product 93 (Eqn (4.49))." " Whether formation of the ketene metal complexes are... 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

Fischer type carbene complexes Cp Mn(CO)2 =C(OMe)Ph are unreactive towards 1-hexyne under photochemical conditions. However 1-hexyne reacts with Cp Mn(CX))2 =C(OTiClCp2)Ar (Ar = Ph, Tol) under photochemical conditions or upon heating to form naphdioquinone derivatives. It appears that the mechanism does not involve a prinuiry photolytic dissociation of CO away from manganese. l photolysis of various... 

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