Heterocyclische 12‐π‐ und 14‐π‐Systeme, XIX: Synthesen und Reaktionsverhalten von Thia‐pseudophenafulvenen und Thia‐pentapseudophenafulvalenen

Neidlein, R.; Cepera, K. F.

doi:10.1002/cber.19781110516

Cited by 16 publications

References 9 publications

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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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Ketene Cycloadditions

1994

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[2+2]-Cycloreversionen

Schaumann

Ketcham

2006

Angewandte Chemie

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[2 + 2]‐Cycloreversions

Schaumann¹,

Ketcham²

1982

Angew. Chem. Int. Ed. Engl.

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Four-membered rings can be cleaved thermally, photochemically, or catalytically into two n: bonded fragments. Theoretical calculations, kinetic studies, and investigations of stereoand regioselectivity have been undertaken to clarify the question of whether the reaction involves one or two steps and to permit predictions on its course. [2 + 2]-Cycloreversions have been used to clarify the structure of four-membered rings, to prepare highly reactive n: electron-systems and-in combination with a [2 + 2]-cycloaddition-t0 protect double bonds.The combination of a cycloaddition and -reversion can be used to convert a carbonyl group into an olefin. Starting with compounds containing annelated four-membered rings, compounds with two functional groups or large ring systems can be prepared. (2 + 2]-Cycloreversions have also been discussed in connection with storage of solar energy. 3 5 We will classify a reaction as a [2+2]-cycloreversion only when two opposing o bonds of a carbo-or heterocyclic four-membered ring 3 are cleaved to give 112 or 4/5.Other terms used in the literature, "retrograde [2 + 21-cycloaddition", and "[4+2 + 2]-cycloelimination" are synonymous with [2 + 2]-cycloreversion. On the other hand, the concept "metathesis" has come to be used for the disproportionation of alkenes catalyzed by transition-metal compounds (cf. Section 4.1).If in 1-5, a/d and b/c are identical then the educts and products of the overall reaction of 112 to give 4/5 belong to the same chemical type, and only an exchange of substituents occurs. This specific type of cycloaddition and cycloreversion has been called an "exchange reaction" by Ul-The complete reaction often takes place in situ Angew. Chem. Inr. Ed. Engf. 21 (1982) 225-247 0 Verlag Chemie GrnbH, 6940 Weinheirn. 1982 0570-0833/82/0404-0225 S 02.50/0 Mechanism of 12 + 21-CycloreversionsAccording to the principle of microscopic reversibility the same considerations for the reaction mechanism must apply to the [2 + 2]-cycloreversion as to the [2 + 21-cycloaddition. If the two o-bonds are broken simultaneously, then according to the Woodward-Hoffmann rules['] for a thermally induced cleavage this corresponds to a [,2, + 02J process having transition state 11 in which the fragments separate from each other in a "corkscrew"[51 motion. A geometrically more favorable [,2, + ,2,]-reaction path via transition state l2[I4] is thermally "forbidden" i. e. would be associated with a much higher activation energy. In the case of photochemical [2 + 2]-cycloreversions the reverse relationships are expected, i. e. 12 would be favored over ll"].

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Heterocyclische 12‐π‐ und 14‐π‐Systeme, XIX: Synthesen und Reaktionsverhalten von Thia‐pseudophenafulvenen und Thia‐pentapseudophenafulvalenen

Cited by 16 publications

References 9 publications

Ketene Cycloadditions

Ketene Cycloadditions

[2+2]-Cycloreversionen

[2 + 2]‐Cycloreversions

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