Metal enol carbene intermediates

The direct formal Rh(II)-catalysed 3 + 3-cycloaddition reaction of enol diazoacetates with A(-acyliminopyridinium ylides yielded highly substituted bicyclic and tricyclic 1,2,3,6-tetrahydropyridazines in high yields (88%) and up to 98% ee. The key intermediates in this reaction are metal enol carbenes. " Again, the Rh2(OAc)4-catalysed 0... 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

In related chemistry, Jones postulated metal-laphenol intermediates in the thermal conversion of metallaindenes to indenyl—metal complexes. As shown in Scheme 25, a likely pathway for this conversion involves CO insertion into the metallaindene ring in A to generate B, followed by enolization to produce metallaphenol C. Carbene migratory insertion woirld then lead to D and ultimately to the observed... 

Metal carbenes generated from the reaction of paUadium or ruthenium catalysts with a-diazo esters have also been found to be electrophilic enough to undergo electrophilic aromatic substitution with pyrrole. The Ru-catalyzed reaction leads to a 2-alkylated pyrrole upon rearomatization. In contrast, the Pd car-bene, in combination with a phosphoric acid cocatalysL generates an intermediate enolate, which subsequently combines with an imine electrophile to furnish a three-component adduct with good enantioselectivity and moderate S37i/a -selectivity (eq 6). 

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