Sterically assisted electronic effect

Functional groups proximal to the alkene may direct the addition by steric and electronic effects. Strong complexation of a borane molecule, e.g., by an amino group, renders the complexed molecule inactive and increases the steric bulk of the group this directs the attack of an uncomplexed borane from the opposite side22. On the other hand, groups that complex weakly, e.g.. methoxy, may assist the addition from the same side (see Table l)23. 

Until now, hydrogen sources other than formates have been rarely reported in microwave-assisted transfer hydrogenations of carbon-carbon multiple bonds. An exception is a transfer hydrogenation of electron-deficient alkenes where a series of 1,4-dihydropyridines supported on silica gel were used as the hydrogen source (Scheme 4.6). The influences of electronic effects of the alkene, steric effects of the dihydropyridine and type and power of the microwave irradiation were studied24. 

While 2/f-heptachlorotoluene hydrolyses (to 1,2,3,4-tetrachlorobenzoic acid), 2//,2 //-dodecachlorobibenzyl, under the same conditions, does not (Ballester et al., 1959b). This result can be accounted for as in (21a). The adverse electronic effect of the trichloromethyl group on the stability of the carbenium-ion intermediate in the hydrolysis of perchloro-p-xylene to tetrachloroterephthalic acid in concentrated H2SO4 becomes insignificant compared with the steric assistance (Ballester et al., 1959b). 

The effect of weak forces on the equilibrium constant for the diaza-Cope rearrangement suggests that the anion effect is the strongest followed the resonance-assisted hydrogen-bond, steric, conjugation, and electronic effects. These weak forces are said to be additive." An intramolecular Fischer indole synthesis with a double bond in the tether allows a tandem [3,3]-sigmatropic rearrangement access to tricyclic benzo[cindole systems (Scheme 1). ... 

Scheme 52 explains the [(Cp )Rh(MeCN)3]2+-assisted regioselective hydrogenation of pyridines, benzoquinolines, acridines as well as indoles and benzothiophene.258 The relative hydrogenation rates were attributed to both electronic and steric effects, the rate generally decreasing with increasing basicity and steric hindrance at the nitrogen atom. 

For steric reasons, the preferred orientation of the addition to a monosubstituted alkene is to the unsubstituted end of the C = C bond however, the polarity of the C = C bond can influence the magnitude of the regiosectivity and this effect is dependent on the electronegativity of the substituents on the alkene. Polarity can also have a major effect on the rate of the condensation polyhaloalkyl radicals behave generally as electrophiles whose addition is retarded by electron-withdrawing and assisted by electron-donating substituents. 

The Yukawa-Tsuno equation continues to find considerable application. 1-Arylethyl bromides react with pyridine in acetonitrile by unimolecular and bimolecular processes.These processes are distinct there is no intermediate mechanism. The SnI rate constants, k, for meta or j ara-substituted 1-arylethyl bromides conform well to the Yukawa-Tsuno equation, with p = — 5.0 and r = 1.15, but the correlation analysis of the 5 n2 rate constants k2 is more complicated. This is attributed to a change in the balance between bond formation and cleavage in the 5 n2 transition state as the substituent is varied. The rate constants of solvolysis in 1 1 (v/v) aqueous ethanol of a-t-butyl-a-neopentylbenzyl and a-t-butyl-a-isopropylbenzyl p-nitrobenzoates at 75 °C follow the Yukawa-Tsuno equation well, with p = —3.37, r = 0.78 and p = —3.09, r — 0.68, respectively. The considerable reduction in r from the value of 1.00 in the defining system for the scale is ascribed to steric inhibition of coplanarity in the transition state. Rates of solvolysis (80% aqueous ethanol, 25 °C) have been measured for 1-(substituted phenyl)-l-phenyl-2,2,2-trifluoroethyl and l,l-bis(substi-tuted phenyl)-2,2,2-trifluoroethyl tosylates. The former substrate shows a bilinear Yukawa-Tsuno plot the latter shows excellent conformity to the Yukawa-Tsuno equation over the whole range of substituents, with p =—8.3/2 and r— 1.19. Substituent effects on solvolysis of 2-aryl-2-(trifluoromethyl)ethyl m-nitrobenzene-sulfonates in acetic acid or in 80% aqueous TFE have been analyzed by the Yukawa-Tsuno equation to give p =—3.12, r = 0.77 (130 °C) and p = —4.22, r — 0.63 (100 °C), respectively. The r values are considered to indicate an enhanced resonance effect, compared with the standard aryl-assisted solvolysis, and this is attributed to the destabilization of the transition state by the electron-withdrawing CF3 group. 

The pathway chosen depends on the geometry of the disilametallacycle intermediate (%), the electronic properties (e.g., hardness) of the central metal and the steric effects of the substituents on the 1,3-diene. The disilacyclopentene products (e.g., (93)) generally are favored by the use of dienes with at least one end unsubstituted (to allow 1,1-addition), steric crowding in the coordination sphere around the metal in the metallacycle intermediate (preventing 1,4-addition) and use of a soft metal (such as Fe) to assist in the migration of the softer base, H (rather than the harder F ). 

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