Molecular-orbital calculations elimination reactions

Tsuda and Oikawa (1989) investigated the photolysis of the 1,2-isomer of 10.89 (1,2-benzoquinone diazide) by means of MINDO/3 molecular orbital calculations with configurational interaction. These authors came to the conclusion that no ketocarbene of the type of 10.90 is formed, but that the rearrangement into the cyclopentadienyl ketene 10.94 is a concerted reaction in which the elimination of nitrogen and the rearrangement take place simultaneously. In the opinion of the present author the theoretical result for 1,2-quinone diazide is not necessarily in contradiction to the experimental investigations of Sander, Yankelevich et al., and Nakamura et al., as the reagents used were not exactly the same. 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

Molecular orbital calculations of the w-electron distribution in pyridine predict that more 4- than 2-aminopyridine should be formed in the Tschitschibabin reaction.4 The fact that no 4-aminopyridine can be detected when the two positions are allowed to compete for a deficiency of sodamide (see, e.g., Abramovitch et al 268) has led to the suggestion that the observed orientation in this reaction depends on the relative ease of elimination of a hydride ion from C-2 and C-4 and not upon the initial mode of addition (which, by implication, must take place predominantly at C-4 as predicted by the molecular orbital calculations).4 This hypothesis necessitates that the addition step be rapidly reversible and that the second stage, the elimination of hydride ion, be the rate-determining one (Scheme VII). Although it seems reasonable to assume that the hydride ion eliminations are the slow steps in this reaction, the fact that no deuterium isotope effect was observed in the reaction of 3-picoline-2d and of pyridine-2d with sodamide implies that the first stage must be virtually irreversible,268 as was found also in the case of the addition of phenyllithium to pyridine.229 The addition stage must, therefore, be the product-... 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

Puddephatt etal. [41] have studied the C-H or C-C bond activation in the alkane complexes [PtMe(CH4)L2] or [PtMe(CHjCH3)L2] (L = NH3 or PH3) as well as the reductive elimination of methane or ethane from the five-coordinate model complexes [PtHMe2L2] or [PtMesLi], respectively, by carrying out extended Hiickel molecular orbital calculations and density functional theory. The oxidative addition and reductive elimination reactions occur by a concerted mechanism, probably with a pinched trigonal-bipyramidal complex on the... 

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. 

Reactions of methyl(vinyl)iodonium ion and its jS-substituted derivatives with chloride ion have been treated by ab initio molecular-orbital calculations (MP2, double-zeta -I- d level). Transition states for 5n2, ligand-coupling substitution (LC), and -elimination ifE) were found. In the gas phase, the barrier to LC is usually the lowest, but the relative barriers for S 2 and change with the substituents. Solvent effects were treated in terms of a dielectric continuum model and found to be large on 5 n2 but small on LC. 

The photo-NOCAS reaction with 2,6-dimethyl-l,6-heptadiene gave two major cyclic aryl-methoxy adducts (a cyclohexane and a cycloheptane) as well as an acyclic heptene adduct Variation in concentration of the nucleophile, methanol, and co-donor, biphenyl, has been shown to affect the product ratios. Further applications of the photo-NOCAS S 2Ar reactions with the aUeyl-4-enols, a-terpineol, limonene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, (3-myrcene, and 1,4-bis (methylene) cyclohexene have been reported.The aryl-methoxy adduct product ratios have been investigated and discussed in terms of the stability of radical intermediates and the factors controlling the regiochemistry of reaction with the nucleophiles alcohols, cyanide, and fluoride attempts to justify the results by ab initio molecular orbital calculations have been made. The photo-NOCAS reaction with 2-methylpropene in the absence of methanol and a donor molecule has shown that solvent acetonitrile can act as a nucleophile. Under these conditions a tetrahydroisoquinoline product is formed, prior to HCN elimination, in high yield as illustrated in Scheme 8. The adduct product formation was rationalized on the basis of the relatively high oxidation potential of the olefin. 

Note that only the same coefficient derivatives are required as in the SCF second derivative case in particular, the exact derivatives of the canonical molecular orbitals are not required. The formulation of Gaw et al. (1984) does apparently use the latter. As discussed in Section III.C, this may lead to numerical difficulties in the case of degeneracies or quasi-degeneracies. Recently Gaw and Handy (1986) eliminated the canonical orbital derivatives from their program, in agreement with the results above. An extension of the closed-shell third derivative program to open-shell and MCSCF wavefunc-tions would be highly desirable for calculating reaction surfaces. 

This chapter is devoted to pseudopotential calculations of molecular excited states, however, some words should be said about spin-orbit effects on the groimd state when dealing with the reactivity of open-shell systems. Even though, the reactivity of transition metal compounds has been extensively studied (see for example Ref [115]), spin-orbit effects are quite rarely taken into accoimt. This can be imderstood from the fact that in all the considered reactions, such as for example inert bond activation through the classical oxidative addition-reductive elimination mechanism, both the reactants and the products are closed-shell systems. In this case, spin-orbit contributions on the... 

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