Thermodynamics, Lewis base

Despite the favorable thermodynamics associated with the cyclization of unsaturated organolithiums, the isomerization is often sluggish when the ring closure involves generation of a quaternary center or formation of a strained framework. In such cases it has been found that addition of lithiophilic Lewis bases such as THF or TMEDA facilitate the reaction.7 9 The preparation of cuparene, a sterically congested sesquiterpene possessing two adjacent quaternary centers, illustrates the methodology.11... 

The acid-base interaction in group 13-stibine and -bismuthine adducts seems to be very weak as is indicated by mass spectroscopic studies, which never showed the molecular ion peak but only the respective Lewis acid and Lewis base fragments. The extreme lability in the gas phase may also account for the fact that there are only very few reports on thermodynamic data of group 13-stibine or bismuthine adducts in the literature. Therefore, multinuclear NMR spectroscopy and single crystal X-ray diffraction are the most important analytical tools for the characterization of such adducts. 

There is a large amount of data available concerning the thermodynamic effects of ligands on other coordination sites (i. e., the thermodynamic cis- and iraws-effects). However, very little is known about the effects of ligands on the kinetic lability of other coordination sites. In fact, very little work has been carried out, directly with Bi2-derivatives, or with models of B12, on the kinetics of ligand substitution at the cobalt center. Of particular biochemical interest would be studies on the rate of displacement of coordinated benzimidazole by various ligands. Such work has not been reported at present. If the benzimidazole is replaced during enzymatic catalysis so that the lower axial position is occupied by some other Lewis base, one would expect this displacement, and the reverse step, to be very facile. This appears to be qualitatively true in that when water displaces benzimidazole as the benzimidazole is pro-... 

Our analysis of literature data will focus on two closely related questions about the influence of changes in the relative thermodynamic driving force and Marcus intrinsic barrier for the reaction of simple carbocations with Bronsted bases (alkene formation) and Lewis bases (nucleophile addition) on the values of ks/kp determined by experiment. 

Optically active selenoxides are known to be unstable toward racemization. An optically active selenoxide having a steroidal frame was obtained for the first time by Jones and co-workers in 1970.7 Enantiomeric selenoxides were prepared by Davis et al. in 1983,8 and an enantiomerically pure selenoxide was isolated for the first time by us in 1989.9 Many optically active selenoxides, which are kinetically stabilized by bulky substituents, were synthesized over the last two decades, and their stereochemistry and stability toward racemization were studied.3,5,10 Recently, some optically active selenoxides, which were thermodynamically stabilized by the intramolecular coordination of a Lewis base to the selenium atom, have been isolated. Optically active selenoxides 1 and 2 were obtained by optical resolution on chiral columns, and their stereochemistry and stability toward racemization under various conditions were clarified (Scheme 1).11,12... 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

The thermodynamic tendency of a substance to act as a Lewis base. The strength of a Lewis base depends on the nature of the acid with which the Lewis base forms a Lewis adduct. Hence, comparative measures of Lewis basicities are given by equilibrium constants for the formation of the adducts by a common reference acid. See Lewis Base Nucleophilicity Hard Bases Soft Bases Donor Number... 

Kinetic and thermodynamic studies on the nickel(II)/Hacac system have been carried out in water solution1553-1555 and NMR studies have been carried out on the adducts of Ni(acac)2 with a variety of Lewis bases.1556-1559... 

The trihalides (except the fluorides), and other R3M compounds such as the trialkyls, triaryls, mixed R2MX compounds, and A1H3, all function as Lewis acids, forming 1 1 adducts with a great variety of Lewis bases. This is one of the most important aspects of the chemistry of the Group 13 elements. The Lewis acidity of the A1X3 groups (where X = Cl, CH3, etc.) has been extensively studied thermodynamically, and basicity sequences for a variety of donors have been established. 

The analysis of E j2 according to Eq. (21) follows from a more general proposal of Krygowski and Fawcett [80, 81] to analyse solvent effects on thermodynamic parameters, taking into account both Lewis base and acid properties of the solvents. 

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