Ketenes, carbenes from preparation

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

Cycloaddition of a ketene complex with unsaturated bonds other than alkenes and alkynes is also possible. The ketene 312, formed from 311, adds to imine 313 to give the /1-lactam 314 under sunlight photolysis. The optically active /1-lactam 314 was prepared from the optically active carbene complex 311 with 99% ee, and converted to 315 [95]. Irradiation of carbene complex 316 generates ketene 317, which cyclizes to the o-hydroquinone derivative 318 [96],... 

In light of the above results it is interesting to note that the reaction of diphenylcyclo-propenone dimer spirolactone with ironenneacarbonyl yields a mixture of ring-opened vinyl carbene and -vinylketene complexes, and these interconvert under addition (or removal) of CO (equation 225) . A possible pathwav to vinylketene Fe-complexes, prepared earlier from cyclopropenes and ironcarbonyls " , may thus involve initial f -coordination, followed by ring cleavage to vinyl carbene and finally carbonylation to the ketene iron // -complexes. An analogous // -manganese complex is prepared similarly by the reaction of CpMn(CO),THF with 3,3-dimethylcyclopropene complex (equation 226) . ... 

Transition metal ketene complexes have been derived from various metals through the study of CO reduction relating to the Fischer-Tropsch synthesis. The old preparative method for the ketene-metal complex is carbonylation of carbene complex, in which gem-dibromide is used as a common source of carbene. The reaction is carried out in the presence of Zn powder as a reducing agent. Among the examined metals such as Pd, Ni, Pt, and Co, Pd catalyst is shown to be inferior to the other metals, especially Co complex. Some typical examples using Pd catalyst are summarized in Table... 

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. ... 

A new thiophen synthesis is based on deprotonation and cyclization of the resultant carbanion (65) of a keten dimethylthioacetal. Other applications of cr-(phenylthio)- or cr-(alkylthio)-alkyl-lithiums include preparations of disparlure (the sex pheromone of the gypsy moth) and other chiral epoxides,of an anti-peptic-ulcer diterpene from Croton sublyratus, of carbene-thiometal chelates through reaction with fCr(CO)gl or fW(CO)jl, of olefins via )ff-hydroxy-sulphides, and of Michael adducts of enones. In other work, the chiral solvent l,4-dimethylamino-2,3-dimethoxybutane is employed in enantioselective addition reactions of cr-thio-carbanions with ketones and Michael acceptors. a,P -Elimination of cr-thio-carbanions such as (66) provides a useful synthesis of olefins, and a 2-(lithiomethylthio)-A -oxazoline (67) has been used to prepare a C-labelled thiiran for a microwave study of the valence tautomerism of allene episulphide. ... 

The Staudinger synthesis is catalysed by NHCs (A -heterocyclic carbenes), via Ye s possible ketene-first or imine-first mechanisms. To test these alternatives, four zwitterionic NHC adducts have been prepared two using A -tosyl benzaldimine and two using diphenylketene. All four adducts had 1 1 stoichiometry and have been extensively characterized by H- and C-NMR, X-ray crystallography and catalytic tests. The imine-derived zwitterions proved poor catalysts, whereas those derived from diphenylketene replicated the free carbene catalysts, strongly supporting the ketene-first route. 

Ketene Ph3Si(EtO)C=C=0 prepared in situ from the carbene complexes (CO)3M=C(OEt)SiPh3 (M=Cr, Mo, W) with 50bar carbon monoxide was found to react at 80-100 °C, among others, vdth N-methylbenzimine to give the corresponding P-lactam in 90% isolated yield (reaction 8.21) [39]. 

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