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Vinylketenes dimerization reactions

Reaction of the same cyclopropene with octacarbonyldicobalt resulted in a 14.6% yield of an inseparable mixture of the dimeric species (102) in which the two vinylketene moieties may adopt a cis (102.a auf Deckung ) or trans (102.b auf Liicke ) configuration. [Pg.301]

Alvarez-Toledano has also investigated150 the reaction of vinylketene complexes 221.d and 221.g with Davy s reagent, a dimeric species that generates a phosphorus-sulfur yield in situ. This then attacks the ketene carbonyl in a nucleophilic fashion, generating the intermediate 244, which collapses to yield the product (245). [Pg.341]

The reaction of (trialkylsilyl)vinylketenes with nucleophilic carbenoid reagents, such as sulfur ylides and diazo compounds, has been used for synthesis of substituted cyclopentenones by stereoselective 4 + 1-annulation (Scheme 12). The strategy relies on the remarkable ability of silyl substituents to stabilize ketenes and suppress their tendency to undergo dimerization and 2 - - 2-cycloaddition. [Pg.381]

The intermediacy of a metallacyclobutene is proposed upon reaction of the diphenylcy-clopropenone dimer spirolactone with CpCo(CO)2, ultimately yielding a >j4-vinylketene complex (equation 23 l)295a. Unlike the analogous iron complex (Section IV.B.2.a), no vinyl carbene complex was observed, and hence formation of the metallacyclobutene seems to be more likely. [Pg.587]

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) . ... [Pg.585]

Upon generation, vinylketene 49 can be dimerized via a [4 + 2] cycloaddition reaction to afford 50. The structure of 50 was elucidated... [Pg.204]

Lewis acid promoted reactions of silicon enolates, /.e., silyl enol ethers and ketene silyl acetals with various electrophiles have yielded a wealth of novel and selective synthetic methods. This combination of reagents has been used in the past to perform such reactions as aldol-condensations with aldehydes and acetals, imine-condensations, conjugate additions to a,P-enones, alkylations, electrophilic aminations, and Diels-Alder/cyclocondensations. Our own interest in this field has involved the use of titanium tetrachloride to promote the reaction of ketene silyl acetals with non-activated imines as an efficient route to P-lactams. This reaction has been applied to the asymmetric synthesis of P-lactams via a chiral imine-titanium tetrachloride template. We have also found that both ketene silyl acetals and vinylketene silyl acetals oxidativelly dimerize or cross-couple, in the presence of titanium tetrachloride to conveniently yield various diesters . Our present study concerns reactions of vinylketene silyl acetals with non-activated imines and vinylimines promoted by titanium and zirconium tetrachlorides. [Pg.37]


See other pages where Vinylketenes dimerization reactions is mentioned: [Pg.374]    [Pg.210]    [Pg.585]    [Pg.374]    [Pg.374]    [Pg.202]   
See also in sourсe #XX -- [ Pg.247 ]




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