Molecular-orbital calculations electrophilic addition

Michael addition, on A-2-thiazoline-5-one. with diethylazodicarboxylate, 433 with maleic anhydride, 433 4-phenyl-1,2,4-triazolin-3,5-dione, 433 Microreversibility principle, 85 Miticide, 440 Mitodepressive, 149, ISO Mn complexes, 104 Molecular orbital calculations. 17 and ambident reactivity, 61, 62 and amino-imino equilibrium, 18 and electrophilic substitution, 18 and HSAB, 1 8... 

Dienamines differ from simple enamines in that (i) there is an additional nucleophilic site at the -position and (ii) an equilibrium mixture of three isomeric dienamines is frequently formed, consisting of the linear s-trans isomer 1, the linear s-cis isomer 2 and the cross-conjugated isomer 31 (Scheme 1). A variety of factors influence the outcome of electrophilic attack on such an equilibrium system. Hiickel molecular orbital calculations indicate a significantly higher electron density at the -positions compared to the -positions of the dienamine system2 (Figure 1). 

In additions to cyanides it is not clear whether the approach is side-on or end-on. It may well be that, often, initial attack is end-on while in a subsequent step the electrophile shifts to a more side-on position. For instance, initial end-on attack seems to occur in gas-phase proton additions. Ab initio molecular orbital calculations have been performed on end-on proton additions to X—C=N of a broad variety of groups The calculated proton affinities are in good agreement with experimental values. 

The ability to promote j8 elimination and the electron donor capacity of the jS-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they promote addition and strongly control the regiochemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at the j3-carbon atom and thus directs the electrophile to the a-carbon. Molecular orbital calculations indicate a stabilization of 38 kcal/mol, which is about the same as the value calculated for an a-methyl group. The reaction is then completed by the elimination step, in which the carbon-silicon or carbon-tin bond is broken. 

Theoretical.—Molecular orbital calculations have been reported for the thiiranium ions derived from ethylene, propylene, isobutene, and butadiene. The most stable system involves a p-configuration of the valence electrons of sulphur. Differences in the extension of the lowest unoccupied MO s on the ethylene carbon atoms were said to be important in determining the regio-specificity of ring-opening addition reactions with chloride ion. Theoretical calculations on C2H4SH indicate that a bridged thiiranium structure would be more stable as an intermediate than an open structure in electrophilic additions of RSX to olefins. The barrier to pyramidal inversion at the sulphur atom was calculated to be 78.11 kcal mol . 

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989-1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the C=C, C=Si and Si=Si bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at C=C bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII). 

Ionic Reactions.— The reactivities of several fluorinated olefins in nucleophilic and electrophilic addition reactions have beat discussed in relation toelectronic structure calculations made according to the simple HQckel molecular orbital method. ... 

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