Cyclization energy

Thus, the cyclization energy is given by a sum of alternating terms which increase in magnitude as they approach the HOMO.10 Therefore, the outcome of the reaction can be predicted by just looking at the HOMO contribution. 

According to ab initio calculations by Schleyer et al. 1,4-Dilithiobutane 28 has a similar doubly bridged structure as 1,3-dilithiopropane 16 although somewhat twisted (Cj symmetry), 28 is 34.6 kcal/mol (144.8 kJ/mol) more stable than the extended zick-zack form of 1,4-dilithiobutane, the cyclization energy being 10 kcal/mol (41.8 kJ/mol) higher than in the case of 16. 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Compounds of the type HC=C—CH=CHXR are not involved in a primary reaction with weak nucleophiles such as CH acids meanwhile, a final (secondary) cyclization with participation of active methylene groups happens to be feasible. Evidently, in most cases the energy gain in the heteroaromatic system realization is the decisive factor (81UK1252). 

The free energy of activation at the QCISD(T)/6-31 H-- -G(d,p) level amounts to 21.1 kcal/mol. According to the authors, the large electron density redistribution arising upon cyclization makes it necessary to use extended basis sets and high-order electron correlation methods to describe the gas-phase thermodynamics, which indicates clearly the gas-phase preference of the azido species. However, the equilibrium is shifted toward the tetrazole as the polarity of a solvent is increased. For instance, SCRF calculations (e = 78.4) yield a relative free energy of solvation with respect to the cw-azido isomer of —2.4 kcal/mol for the tmns-zziAo compound and of —6.8 kcal/mol for the tetrazole isomer. At a much lower level, the... 

Earlier studies [14,15] clearly reveal that there is a reaction between two polymers and that the extent of reaction depends on the blend ratio. As 50 50 ratio has been found to the optimum (from rheological and infrared studies) ratio for interchain crosslinking, the higher heat of reaction for the NBR-rich blend may be attributed to the cyclization of NBR at higher temperatures. There is an inflection point at 50 50 ratio where maximum interchain crosslinking is expected. Higher viscosity, relaxation time, and stored elastic energy are observed in the preheated blends. A maximum 50-60% of Hypalon in NBR is supposed to be an optimum ratio so far as processibility is concerned. 

On the basis of model studies, it appears that a preexisting ring(s) in the cyclization substrate (e. g. 39) is necessary for the success of this reaction type. Moreover, irradiation at elevated temperatures leads to cleaner reactions and higher yields, suggesting that thermal energy may be necessary to achieve the proper conformation for coupling to occur.22... 

Table 8 shows results obtained from the application of various bulk and surface analysis methods to lithium metal at rest or after cyclization experiments, as well as at inert and carbon electrodes after cathodic polarization. The analytical methods include elemental analysis, X-ray photoelectron spectroscopy (XPS or ESCA), energy-dispersive analysis of X-rays (X-ray mi-... 

The activation energy of the reaction of thermal cyclization of PAN containing 5% of thioamide groups amounts to 20 kcal/mol (83,7 kJ/mol). 

Emulsifiers (see also Surfactants) 27, 46 End-blockers 12, 18, 19, 76 End-capping 157 End-linking processes 163 End-stoppers (see also End-blockers) 10 End-to-end cyclization 159,160 Energy, cavitation 188,189 —, electrostatic interactions 188, 189... 

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