Cycloaddition reactions ketenes, carbene catalysis

An interesting alternative improvement of this catalysis concept was recently discovered by the Ye group, who applied iV-heterocychc carbenes in the [4+2] cycloaddition of ketenes with A-benzoyldiazenes (Scheme 11.50) [131]. Reactions carried out in THF at 0°C employing 20 mol% of pre-catalyst 63, and 20 mol% of CS2CO3 afforded the corresponding l,3,4-oxadiazin-6-ones in good yields and enan-tioselectivities from high to excellent. 

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

Catalytic enantioselective cycloaddition reactions of ketenes have been intensively explored over the past several decades. Various chiral catalysts have been developed for these cycloadditions, such as cinchona alkaloids, N-heterocyclic carbene (NHC) catalysis (2013SL1614), planar-chiral 4-(JV,JV-dimethylamino)pyridine (DMAP)-derived catalysts, and Lewis acid-based catalysts (2009T6771, 2004ACR542). 

Phenylketene generated from phenylacetyl chloride with triethylamine gives [2 + 2] imine cycloaddition with catalysis by the N-heterocyclic carbene prepared by partial electrochemical reduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) as solvent gives iraws-p-lactam formation (Eqn (4.92)). The zwitterionic intermediate formed in the reaction is stabiHzed by the highly ionic solvent, which also allows isomerization to form the trans product. The use of such N-heterocyclic carbenes in the catalysis of [2 + 2] cycloadditions of ketenes has been reviewed. ... 

Arylalkylketenes react with P,y-unsaturated a-ketophosphonates with chiral catalysis by N-heterocyclic carbenes giving enantioselective formation of products of formal [4 + 2] cycloaddition (Eqn (4.103)). ° These ketenes react similarly with acylcarboxylates forming the corresponding esters and amides with high enantioselectivity (Eqn (4.104)). ° A similar lactone synthesis by [4 + 2] cycloaddition is shown in Eqn (4.99), and such ketene reactions catalyzed by N-heterocycHc carbenes have been recently reviewed. 

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