Sr-X bonds

The broken bonds (boldface = BDEs (boldface = recommended data reference in parentheses) Methods (reference in  

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For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

As we have seen, reviewing catalytic S-X bond activations, some reactions complete their catalytic cycles by C-S bond-forming reductive elimination from C-M-S complexes. Hartwig et al. have reported on the mechanism of the C-S bond-forming reductive elimination from Pd(L)(R)(SR ) 122 (Eq. 7.72) [69]. 

According to our approach, it is predicted that, barring steric effects, the axial preference in pyranoses should increase in the order X = F > OR > NR2 and also in the order X = Cl > F, X = SR > OR, etc., i. e. as the Ci-X bond becomes an increasingly better instrinsic acceptor. 

Substitution of scheme a forms a well-documented class of reactions (see Costentin et al. 1999,2000, Costentin and Saveant 2000, Corsico and Rossi 2000, 2002, 2004, Adcock et al. 2001, Vanelle and Crozet 2002, Medebielle et al. 2002, Galli and Rappoport 2003, Rossi et al. 2003, Vanelle et al. 2004, and Bnden et al. 2005 and references therein). In contrast to conventional nucleophilic substitution, the nncleophile, Nu-, reacts not with the substrate, RX, to give a product but with the radical R. The latter emerges as a result of R-X bond cleavage. Snbstitnent X is very often a halogen atom, bnt other leaving groups can also be used (see section 7.8.1). In the majority of aromatic Sr I reactions, the anion-radical RX- (R=Ar) is the observable intermediate. It is depicted in scheme a. With aliphatic snbstrates, snbstitntion takes place rather than 8 2 or S l substitutions, and the concerted mechanism depicted in scheme b is feasible. 

A satisfactory explanation also cannot be made in terms of polarizability of the leaving group. The highly polarizable Si—Cl and Si—SR bonds are always displaced with inversion. However, the Si—F bond, which is far less polarizable than Si—OR and Si—H bonds, is more easily cleaved by nucleophiles with inversion of configuration. It is also impossible to explain the closely similar behavior of F and SR groups. However, the relative tendency of Si—X bonds to undergo inversion and the polarizability order run parallel in the same group of the periodic table. The order of polarizability and ability for inversion are... 

C-C or C-X bonds (X = I, NR2, SR, OR) can be cleaved anodically by a) oxidation of the substrate to a radical cation that either dissociates (Eq. (215a) ) or is hydrolytically decomposed in a subsequent CgECjq-process (Eq. (215b)) and... 

Activation parameters at coalescence temperature show that the coordinate interaction in these compounds is not a function of the electronegativity of X but is controlled by the ability of the nitrogen atom to stretch the Si—X bond. The tendency of the silicon atom to increase its valency decreases in the order X = OCOR, Br, Cl > SR F > OR, H This sequence corresponds directly to the rate of racemization of halosilanes and to the substitution of R3SiX with inversion of configuration Although no intramolecular coordination was observed in solutions of acetoxysilanes (CH3) Si(OCOR)4 by the Si NMR method the shape of the H NMR spectra of these compounds with chiral silicon atom points to Si -0 interaction... 

Solvent Effects. The nature of solvent effects is also very clear. For A1H3 (f-BuO)2AlH and (EtS)2 A1H, an increase of the basicity of the solvent leads to increased inversion at silicon (Table 23). / -Bu2 A1H is specially interesting in this respect. In hexane, it substitutes all Si-X bonds (X = Cl, SR, F, and OR) with retention (146). In contrast, the percentage of inversion at silicon increases with the basicity of the solvent (hexane < Et2 0 < THF) or the use of a complexing agent such as TMDA. 

For silicon, the preference for apical positions is not simply related to electronegativity but depends on the polarizability or the ability of the Si-X bond to be stretched. The experimental order is Br, Cl, OAc>F, SR>H, OR59 and OR>r p2>H60. (This order... 

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