Methylene story

In this talk I have focused on aspects of the methylene story that I am most familiar with. It is a remarkable story about a simple molecule the understanding of which was instrumental in establishing the credibility of computational chemistry as we know and enjoy it today. The story has many authors, too many for me to mention this evening but the paper [1] CH2 revisited has a more complete list of contributors. 

The simplest of all of the homoaromatics is the homocyclopropenium cation [2], This species was first invoked as an unusually stable intermediate by Applequist and Roberts (1956) to explain the reactions of methylene cyclobutenes with electrophiles. Since that time a wealth of studies has provided data supporting the special stability of the monohomocyclo-propenium cation (Story and Clark, 1972 Paquette, 1978). The parent cation and several simple derivatives have been examined by a variety of techniques including the convincing, direct H and 13C NMR observation... 

Controversy arose almost from the start in this story of the ground state of TME. Dowd, in 1986, prepared TME by photolysis of 4,5-bis(methylene)-3,4,5,6-tetra-hydropyridazine 35 in a methyltetrahydrofuran glass at 10 K. The ESR spectrum... 

As noted above there were three reviews of the CH2 story written in the mid-1980s. The first was by I. Shavitt and titled Geometry and Singlet-Triplet Energy Gap in Methylene A Critical Review of Experimental and Theoretical Determinations , and two excerpts follow ... 

H202 added slowly at 20-30° to a mixture of cyclohexanone and 10%-perchloric in glacial acetic acid, after stirring 15-20 min. at 40-45° more glacial acetic acid added, and the soln. warmed 0.5-1 hr. at 55-60° -> crude tricyclo-hexylidene triperoxide. Y 83%. F. e. s. J. R. Sanderson, K. Paul, and P. R. Story, Synthesis 1975, 275 mixed cyclic ketone peroxides cf. ibid. 1975, 388 J. Org. Chem. 41, 1283 (1976) cyclic ketone diperoxides in acetonitrile-methylene chloride in the presence of methanesulfonic acid s. ibid. 40, 2239 (1975) tricyclohexylidene triperoxide s. a. M. J. C. Harding and D. M. Whalen, Ind. Eng. Chem., Prod. Res. Dev. 14, 232 (1975). 

After a series of mechanical horror stories, attention was turned from transesterification to direct phosgenation. High molecular weight polymer was produced by passing phosgene into a stirred solution of bisphenol-A in a mixed methylene chloride/pyridine solvent. Excess pyridine and by-product pyridine hydrochloride was removed by water/acid washing. The polymer was recovered by addition of an anti-solvent such as alcohol or aliphatic hydrocarbon. This general process provided initial development quantities of polymer. 

Despite the fact that the a orbital on carbon in methylene is lower in energy than the n orbital, and in spite of the correlation of the electrons in the sigma orbital by the presence of the I... r ) configuration in the wave function for the lowest singlet state, the ground state of methylene is the state, which lies 9 kcal mol below the A state. The very interesting story of how calculations and experiments finally came to agree on this value for A st has been published elsewhere and will not be repeated here. 

However, dynamics calculations (see Unimolecular Reaction Dynamics) on potential energy surfaces that have been constructed to mimic the ab initio surface tell a different story. Double rotation is predicted by these calculations to be more favorable than single rotation at 400 by a factor of 3-5. The reason is that, in the reaction dynamics calculations, conservation of angular momentum favors a molecule which opens to TM in a disrotatory mode continuing to rotate its methylene groups in this mode, until it recloses to cyclopropane. 

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