Butterfly-type transition

The Simmons-Smith cyclopropanation is a concerted process, and it proceeds via a three-centered "butterfly-type" transition state. This is in agreement with the result of theoretical studies as well as the stereochemical outcome of a large number of reactions. 

After generation of the active species, the reaction proceeds via concerted addition of the methylene group to the olefin substrate with retention of configuration. This process was postulated to proceed via a three-centre butterfly-type transition state 11 on the basis of experimental observations, and numerous theoretical calculations are in agreement with this postulate. However this transition state may not be the favoured reaction pathway under all relevant experimental conditions. For example,... 

In very special cases, the square-pyramidal ligand arrangements in these butterfly-type sub-units are distorted towards trigonal-bipyramidal coordination sites. Within the picture of the bent bond, this transition would disrupt the metal-metal bond and should be directly attributable to electronic variations within the ligand spheres of the metal atoms. 

A large fraction of the binuclear mixed chalcogen/carbonyl transition metal complexes of iron and manganese contain the M2E (CO)6 core with the butterfly-type structure (n = 2) or substructure (n — 3, see Section 1.10.3). As an example of a complex with thiolate ligands, the structure of [Fe2(SC3H7)2(CO)6] is shown in... 

Concerted mechanisms have been proposed on the basis of work carried out with soluble Movl, Wvl and TiIV peroxo compounds. The experimental evidence is consistent with the hypothesis that these compounds act as oxidants in stoichiometric epoxidations and that the reactions involve electrophilic attack of the peroxo compound on the organic molecule or, what is equivalent, a nucleophilic attack of the organic molecule on the peroxidic oxygen, in a butterfly transition state. The reaction product is formed and, after desorption, the peroxo compound is regenerated by reaction of TiIV with H202 this accounts for the catalytic nature of the reaction (Amato et al, 1986). The same type of mechanism... 

The whole phenomenology of phase behavior and emulsion inversion was interpreted wifli a butterfly catastrophe model with amazing quahtative matching between theory and experiment. The phase behavior model used the Maxwell convention which allows the system to split into several states, i.e., phases at equilibrium. On the other hand, the emulsion-type model allows for only one state (emulsion type) at the time, with eventually catastrophic transition and hysteresis, according to the perfect delay convention. The fact that the same model potential permits the interpretation of the phase behavior and of the emulsion inver sion (204, 206) is a symptomatic hint that both phe-nomenologies are linked, probably through formulation and water/oil composition which are two of the four manipula-ble parameters in the butterfly catastrophe potential. 

The butterfly catastrophe model explains why the transitional inversion is not really an inversion but a surfactant transfer from one phase to the other, while the catastrophic inversion is a nonreversible hysteresis type instability. This approach, which is out of the scope of this chapter, is well documented elsewhere (197). 

Peracid oxidation is an electrophilic process in which the driving force of the reaction is provided by the electron donating nature of the alkene double bond and the electron accepting nature of the peracid group. The transition state of this type of reaction was originally beheved to be planar or butterfly [5]. However, recent calculations favour the spiro transition state (Scheme 1.3) [6]. The transfer of oxygen is concerted and hence the reactions are stereospecilic. 

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