Royal Society of Chemistry Alicyclic Chemistry, Vol 4: Volume 4 (Specialist Periodical Reports - Alicyclic Chemistry)

Excerpt. © Reprinted by permission. All rights reserved. Alicyclic Chemistry Volume 4A Review of the Literature Published During 1974By W. ParkerThe Royal Society of ChemistryCopyright © 1976 The Chemical SocietyAll rights reserved.ISBN: 978-0-85186-582-9ContentsChapter 1 Three- and Four-membered Rings By S. A. Math, 1, Chapter 2 Five- and Six-membered Rings and Related Fused Systems By D. G. Morris, 196, Chapter 3 Medium- and Large-ring Compounds By E. J. Thomas, 278, Chapter 4 Bridged Carbocyclics By J. Carnduff, 345, Author Index, 489, CHAPTER 1Three- and Four-membered RingsBY S. A. MATLIN1 Introduction, Theory, and StructureSince the last Report, the number of publications relevant to this chapter has increased by 35 %. There have been numerous reviews and articles concerning aspects of the chemistry of three- and four-membered rings.Amongst theoretical calculations for these systems is included a quantum chemical study of the π-electron delocalization in triphenylphosphonium ylides, leading to an evaluation of the criteria for reactivity and aromaticity in the Wittig reaction. It was predicted that the as yet unknown ylide (1; X = PPh3) will not take part in the Wittig reaction.Geometry-optimized INDO calculations on substituted cyclopropenyl cations indicate that F and NH2 groups conjugate strongly with the ring whereas resonance interaction of a phenyl substituent is weak. Stronger conjugation of all three substitu-ents is seen with the cyclobutadienyl dication. In spite of the strong polarization of the carbonyl group in cyclopropenone (1: X = O) there was very little indication of charge build-up in the ring or of aromatic character, but protonated cyclopropenone does resemble a 2π aromatic system. Comparisons of cyclopropenone with thiiren 1,1-dioxide have also been made.Using graph-theoretical technique, Hearndon and Ellzey have identified, within the Hückel MO formalism, a new class of π-structures containing even numbers of π-orbitals of which 2-cyclopropenylallyl is the smallest member. This has a closed shell of electrons according to HMO theory, but is required to have a biradical valence bond structure, and dicationic species should be obtainable.From a graphical study of positional isomers containing bivalent sulphur, it was predicted that (2a) should be more stable than (2b). The former has been synthesized, whereas the latter is not a known compound.Following the development of the MINDO/3 method, which is considerably more successful than MINDO/2 in taking account of strain factors, the possibility of collapse of bisdehydrobenzenes (3) to bicyclic structures has been examined. The most striking conclusion was that (3a) should have a stability comparable to that of o-benzyne, and this suggests that m-benzynes may be viable reaction intermediates.MINDO/3 calculations of the cycloreversion of vinylcyclobutane to ethylene and butadiene show a biradical transition state to be involved, and the cycloaddition of cyclobutadiene and acetylene, affording Dewar benzene (4), was also examined. The equilibrium geometry of the latter, predicted by semi-empirical INDO calculations, was in good agreement with experimental results and the calculated dipole moment of <0.04 D suggests that the likelihood of obtaining a microwave spectrum is marginal at best.The calculated potential curves for the (CH)3CH2+ system show that the cyclo-propenylcarbinyl, cyclobutenyl, and bicyclobutyl cations are surrounded by relatively low-energy barriers and that they would readily collapse to the most stable conformer, the puckered homocyclopropenyl cation (5).The iterative maximum overlap approximation (IMOA) method is useful for the semiquantitative prediction of the geometry of hydrocarbons, and has been applied to several cyclopropyl and cyclobutyl derivatives. An intramolecular force field for amides has been derived and used to calculate the crystal structure of cyclopro-panecarboxamide.Several X-ray structure determinations of th