Mechanism of diphenylketene cycloaddition to a carbodiimide

Brady, William T.; Dorsey, Edwin D.

doi:10.1021/jo00833a055

Cited by 17 publications

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“…The thermal dimerisation of cis,trans-cyclo-octa-1,3diene gives (1) as the major product, suggesting that a predominant portion of the reaction occurs by the symmetry-allowed process.I2 Pyrolysis of bicyclo[2,2,0]hexanedicarboxylicesters(2) also gives products compatible with orbital symmetry theory, but a biradical process is preferred since both the cis,endo-and cis'exoisomers give the same product ration and the concerted transition state would be extreinely congested in the former case. Pyrolytic decomposition of bicyclo- [4,2,0]octane may be, in part, ~0ncerted.I~ Thermolysis of 1,1,2,2-tetramethylcyclobutane yields isobutylene ; the recovered deuteriated molecule (3) was not , -,. or +, - The mechanisms of the formation of cyclobutanes in the sensitized photolysis of isomeric diethyl deca-2,8-diene-l ,lO-oates,' the photochemical reaction of styrene with tetramethylethylene,20 and the butadiene to bicyclobutane phote isomerisation2' have also been studied.…”

Section: B G Ode11mentioning

confidence: 99%

“…Schollkopf has reviewed the mechanism of [ 1,2] sigmatropic rearrangements in anionic ~ysterns.~'~" He notes that the anionic [1,4] shift"3b is a symmetryallowed process. A systematic study of the thermal rearrangements of 2-alkoxypyridine N-oxides to N-alkoxypyridones (for which it had previously been shown that migrating ally1 groups were not invertedL0&) has given conclusive proof that it is a concerted [ls,4s] process (except for the case of benzhydryl migration where a radical pair process intervene^).…”

Section: I57mentioning

confidence: 99%

“…Cubane (14) is isomerised by Rh' catalysts to syn-tricycluoctadiene (15) while interaction with Ag' leads to cuneane (16), a wedge-shaped itself rearranges in the presence of Rh' to semibullvalene. 43The above type of [,2, + ,2,] isomerisations of compounds with strained o-bonds is general for homo-and bishomo-cubane derivative^.^^ Some further evidence for metal-containing intermediates in rhodium-catalysed processes has been presen ted.43*44The role of secondary orbital interactions' in stabilising the endo-transition states of[4 + 21 cycloadditions in the absence of great steric demands has received much support. Cyclopentadiene gives more than 97 o/, em-adduct (17) with 2,5-dimethyl-3,4-diphenylcyclopentadienone while cyclopentene, which has similar steric demands but lacks the second double bond, gives approximately equal amounts of em-and e n d o -a d d u c t ~.…”

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Chapter 3. Reaction mechanisms. Part (iii)

Odell¹

1970

Annu. Rep. Prog. Chem., Sect. B

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SOPHISTICATED stereochemical and kinetic arguments continue to be used to show whether reactions are concerted or not, and hence whether the principles of orbital symmetry conservation' are applicable.Orbital Symmetry Correlations.-The tendency to maintain bonding governs the complex motions of molecules in the course of reactions.2 This will direct the nuclear motions which may or may not be the least-motion ones. The nonleast-motion paths will often, however, be discriminated against by 'extrasymmetric factor^',^ and thus will often have higher activation energies than nonconcerted processes, but not always. * * = epimerised or racemised." These results were discussed in terms of rotations in 1,4-butanediyl intermediates. Bartlett's Centenary Lecture16" on the mechanisms of cycloadditions presents much recent work. If a diradical or dipolar intermediate in a two-step [2 + 21 cycloaddition is formed in an extended conformation (4) it is likely to have a sufficient lifetime for rotation about the terminal C-C bonds, with resulting configurational loss, to occur before closure to a four-membered ring. The addition of cis-anethole (5) to tetracyanoethylene (TCNE) proceeds with greater loss of configuration in acetonitrile, which favours formation of an extended dipole, than in benzene.16" The diradical intermediate formed in the addition of a diene to an olefin can close to a cyclobutane or a cyclohexene. It has been found, however, that [2 + 21 addition of difluorodichloroethylenes to cyclopentadiene is a two-step process, while the competing [4 + 21 reaction is

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Section: B G Ode11mentioning

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Chapter 3. Reaction mechanisms. Part (iii)

Odell¹

1970

Annu. Rep. Prog. Chem., Sect. B

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Ketene Cycloadditions

1994

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The reaction of ketene with itself was described almost simultaneously in 1908 by Chick and Wilsmore in England, and by Staudinger and Klever in Germany; priority for the discovery was attributed to Wilsmore by the German group. Ketene cycloaddition was an early example of a peculiar process, one that formed carbon–carbon bonds with ease, often without the need for solvent, catalyst, or high heat. Subsequent work by others was done in the shadow of Staudinger's exhaustive and rigorous study of all phases of ketene reactivity. Three factors led to a resurgence of interest in this reaction beginning in the 1960s. Haloketenes, which had previously eluded study, were found to have high reactivity, and the halogens could easily be removed after the reaction. The increasing sophistication of powerful analytical methods, particularly nuclear magnetic resonance spectroscopy, led to discovery of the interesting stereochemical aspects of this reaction. Finally, the new theory of orbital symmetry conservation provided a conceptual framework to rationalize these puzzling “no mechanism” reactions. This chapter is a review of cycloaddition reactions of ketenes. Here, a cycloaddition is defined as a reaction of a ketene with an unsaturated organic compound to give a cyclic product by a mechanism that, in principle, involves the almost simultaneous formation of two bonds between two reactants. While there is no concern here as to whether bond formation is concerted or stepwise, no other chemical process can take place between the formation of the first and second bond. Definition here involves both structural and mechanistic factors, and it is difficult to avoid a certain amount of arbitrariness. Products, such as dehydroacetic acid, which seem to us to arise by concatenation of ketene molecules followed by cyclization are excluded. These reactions, are more properly considered to arise from a series of ionic reactions, and the prediction of the eventual product does not take advantage of the special mechanistic features commonly associated with true cycloadditions. Additions to imines to give β‐lactams are numerous and will be covered in a separate review. The literature has been searched to the end of 1988. Many reviews devoted partially or exclusively to ketene cycloadditions have been published. Specific topics that have been reviewed include haloketenes, fluoroketenes, cyanoketenes, intramolecular cycloadditions, conjugated ketenes, and β‐lactam antibiotics. Ancillary topics pertinent to ketene cycloadditions that have been reviewed include cycloreversion reactions, ketene equivalents, which provide ketene functionality with olefin‐like reactivity especially in [4 + 2] reactions, application of frontier molecular orbital theory to cycloadditions, and a critical discussion of cycloadditions with polar intermediates. Applications of cyclobutanones in synthesis have also been reviewed.

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General Introduction

2009

Cumulenes in Click Reactions

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Mechanism of diphenylketene cycloaddition to a carbodiimide

Cited by 17 publications

References 9 publications

Chapter 3. Reaction mechanisms. Part (iii)

Chapter 3. Reaction mechanisms. Part (iii)

Ketene Cycloadditions

General Introduction

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