Factors Controlling Bond Formation

Understanding the process of bond forming kinetics and dynamics is beyond the scope of any instrumentation and theoretical approximation alone. For example, results of numerical optimizations are subject to the assumptions made or to the 

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Dipole formation lowers the work function of the surfaces. Overdosing of adsorbates restores the work function as the dipoles serve as donor for further bond formation, which provide mechanism for surface antioxidation. Electronegativity, lattice size, dosage, and temperature are factors controlling bond and band formation and relaxation dynamics. 

The A//het(R-R ) value, whieh is derived from Arnett s empirical linear (28) and (29), might be used as an index of feasibility of salt formation. The values calculated by use of (28) or (29) for the combination of [2 ] with [1+], [24+], [26+], [28+] and [40+] are not less than zero but in the range of 10-18 kcalmoP. Tliese A//het(R-R ) values seem to be too large for salt formation. Therefore, the salt formation might be controlled not only by electronic factors, but also by steric hindrance to bond formation. 

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

The reaction of 95 with Me3SiC=CMe shows clearly that the B-CHEt bond is more reactive than the BCH2 bond, and the products (E)-96 and (E)-97 are formed in a 1 1 ratio (note that there are 2BCH2 bonds in 95). In the case of the tert-butyl derivative, steric factors control the process. Moreover, repulsion between the Et and SnMe3 substituents results in the preferential formation of (Z)-99 <2001JOM(620)51>. On the other hand, reaction with bis(triethyltin)ethyne gives only product (Z)-100 (Scheme 38) <2003JOM(687)108>. 

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