Alkyl Group Substitution

When two positions are comparably activated by alkyl groups substitution usually occurs at the less hindered site Nitration of p tert butyltoluene takes place at positions ortho to the methyl group m preference to those ortho to the larger tert butyl group This IS an example of a stenc effect... 

One long alkyl group substituted onto the N atom, which makes the amine more hydrophobic without greatly increasing the steric hindrance around the nitrogen atom... 

Phenyluracils with alkyl groups substituted in the 5- and 6-positions, such as (L), are reported to have analgesic and antipyretic activity [376, 377]. The 1-position may also be alkylated. Such compounds are prepared by the treatment of a-alkyl-/3-aminocrotonates with phenyl isocyanate. When the 5-substituent is an isopropyl group, the resulting compound is a sodiuretic without increase in potassium excretion. 

The chemistry of phosphate esters is more complicated than that of sulfate esters because it is possible to have one, two, or three alkyl groups substituted for the acidic hydrogens of phosphoric acid ... 

The effect of alkyl group substitution on the tunneling splitting in A2Bj state of tropolone has been studied by Sekiya et al. [1990a] and Tsuji et al. [1991] using laser-induced fluorescence in supercooled jets. In iso-... 

Alkyl group substitutions at ring position five result in a tremendous decrease in inhibitory activity of the epithelium Na+ channel, but greatly... 

We have seen many reactions where nucleophiles attack unhindered alkyl halides and tosylates by the SN2 mechanism. An enolate ion can serve as the nucleophile, becoming alkylated in the process. Because the enolate has two nucleophilic sites (the oxygen and the a carbon), it can react at either of these sites. The reaction usually takes place primarily at the a carbon, forming a new C—C bond. In effect, this is a type of a substitution, with an alkyl group substituting for an a hydrogen. 

Field and inductive effects) Alkyl groups substituted by amine ... 

JOC247), as are 2-methylquinoline-3-carboxylic acids (78JHC687). Side reactions with SOCL have been reviewed (8IS66I). As a consequence of these chlorinations of methyl or alkyl groups, substituted hydroxy-N-... 

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

Alkyl group substituted by electronegative atoms or groups -H (aldehyde) 10... 

Carbocations stabilized by resonance with a lone pair are more stable than those stabilized only by resonance with a carbon-carbon double bond, which in turn are more stable than those stabilized only by alkyl group substitution. The effects are additive three of a lesser type of stabilization are at least as good as one of the better type. 

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