Bartlett mechanism

The oxidation of amines, acylhydrazines, and alkoxy amines described in this section involves the formation of nitrenes or other intermediates, depending on the nature of the nitrogen substituent and the oxidant, although lead tetraacetate is commonly employed. For example, a nitrenium ion or an amino lead derivative was proposed as the intermediate in the oxidation of alkoxyamines with lead tetraacetate 2. However, evidence has been provided that the jV-acetoxy species is the intermediate in the aziridination of alkenes with V-aminophthal-imide and /V-aminoquinazolinone, where a mechanism analogous to the Bartlett mechanism for the peracid epoxidation of alkenes should be operating3,4. 

Alkenes can be epoxidized with a variety of peroxy acids benzoic acid (MCPBA) is the most commonly used. Woods and Beak have provided experimental evidence for the butterfly transition state (A) in Scheme 61 (Bartlett mechanism <50RCP47 involved in peroxy acid epoxidations <9iJA628i>. 

A. Szabo and N. S. Ostlund, Modern Quantum Chemistry, McGraw-Hill, 1982 R. McWeeny, Methods of Molecular Quantum Mechanics, Academic Press, 1992 W. J. Hehre, L. Radom, J. A. Pople and P. v. R. Schleyer, Ab Initio Molecular Orbital Theory, Wiley, 1986 J. Simons, J. Phys. Chem., 95 (1991), 1017 R. J. Bartlett, J. F. Stanton, Rev. Comput. Chem., 5 (1994), 65. 

I dedicate this book to Paul D. Bartlett, one of the grand pioneers of physical organic chemistry, who investigated perceptively several basic problems in organic chemistry at a time when the chemical community did not yet see their crucial aspects, for example, in the mechanism of the diazo coupling reaction (Wistar and Bartlett, 1941). I learned much from him when he invited me to attend his Friday seminars at Harvard University in 1951-1952. 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

The conversion of [49] into [50] involves a Claisen rearrangement. Once this was realized it was less surprising that no specific catalytic groups on the enzyme are involved. Support for the Claisen-type mechanism comes from the inhibition shown by the bicyclic dicarboxylate [51], prepared by Bartlett and Johnson (1985) as an analogue of the presumed transition state [52], This same structure [51], coupled through the hydroxyl group to a small protein, was used as a hapten to induce antibodies, one (out of eight) of which mimics the behaviour of chorismate mutase, albeit less efficiently (Table 7). 

The accepted mechanism for this reaction was proposed by Bartlett, and it is shown in the following equation ... 

This is a catalytic-chain mechanism because the agent which adds to the olefins is regenerated in the last step.The addition reaction of the anion to the olefin is similar to the noncatalytic reaction of alkyllithium compounds with ethylene as reported by Ziegler and Gellert 37) and by Bartlett et al. 38). In this reaction (5), the less stable secondary and tertiary alkyl lithium compounds add most readily. 

For more than forty years the Bartlett butterfly TS was the generally accepted mechanism for peracid epoxidation and numerous experimental studies supported this transition structure" " . During these formative years theoretical calculations did not play a major role due to limitations of available methods that could adequately treat the peroxide functional group. Theoretical contributions in 1978 were at the Hartree-Fock (HF) level since... 

M. Urban, J. Noga, S. J. Cole, and R. J. Bartlett, J. Chem. Phys. 83, 4041 (1985). T. J. Lee, G. E. Scuseria, Achieving Chemical Accuracy with Coupled-Cluster Theory. In S. R. Langhoff (Ed.) Quantum Mechanical Electronic Structure Calculations with Chemical Accuracy (Kluwer, Dordrecht, 1995), pp. 47-108. 

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