Lone pairs alkylation

With anionic allylic sources, highly polar aprotic solvents increase the amount of lone pair alkylation because poor solvent stabilization of the anion leaves the heteroatom end less hindered by solvent. Conversely, groups bound to the heteroatom increase the steric hindrance about it, and therefore decrease the tendency toward heteroatom alkylation. For example, enamines (R2N-C=C) tend to alkylate on carbon rather than nitrogen (as shown in Section 8.4.5). 

Amines are good nucleophiles (the nitrogen has a lone pair). Alkyl bromides are good electrophiles, since bromine is electronegative and bromide is a good leaving group. 

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

A similar situation obtains in the aminoboranes where one or more of the substituents on B is an R2N group (R = alkyl, aryl, H), e.g. Me2N-BMc2. Reference to Fig. 6.22 indicates the possibility of some p, interaction between the lone pair on N and the vacant orbital on trigonal B. This is frequently indicated as... 

The 4,5-dihydro compounds (34) might be expected to show different properties. Here, as with the pyrazolines discussed in Section IV, A, lone pairs of electrons should be available on both nitrogen atoms for reaction to give salts of type 35 and/or 36. No salts of type 35 have been reported. Indeed, the reaction between the alkyl halide... 

An exception to the lone pair or donor electron requirement of organic inhibitors is provided by the quaternary ammonium compounds. Meakins reports the effectiveness of tetra-alkyl ammonium bromides with the alkyl group having C 10. Comparative laboratory tests of commercial inhibitors of this type have been described . The inhibiting action of tetra-butyl ammonium sulphate for iron in H S-saturated sulphuric acid has been described, better results being achieved than with mono-, di- or tri-butylamines . 

Q The nucleophile OH uses its lone-pair electrons to attack the alkyl halide carbon 180° away from the departing halogen. This leads to a transition state with a partially formed C-OH bond and a partially broken C-Br bond. 

To control the first factor, one of the two lone pairs of the sulfide must be blocked such that a single diastereomer is produced upon alkylation. For C2 symmetric sulfides this is not an issue, as a single diastereomer is necessarily fonned upon alkylation. To control the second factor, steric interactions can be used to favor one of the two possible conformations of the ylide (these are generally accepted to be the two conformers in which the electron lone pairs on sulfur and carbon are orthogonal) [14], The third factor can be controlled by sterically hinder-... 

In the case of sulfide 7 the bulky camphoryl moiety blocks one of the lone pairs on the sulfide, resulting in a single diastereomer upon alkylation. One of the conformations (29b) is rendered less favorable by non-bonded interactions such that conformation 29a is favored, resulting in the observed major isomer (Scheme 1.11). The face selectivity is also controlled by the camphoryl group, which blocks the Re face of the ylide. 

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

In 5-alkyl-substituted 1,3-dioxanes, the 5 substituent has a much smaller preference for the equatorial position than in cyclohexane derivatives the A values are much lower. This indicates that the lone pairs on the oxygens have a smaller steric requirement than the C—H bonds in the corresponding... 

The argument of the directing effect of lone pairs on the substiment [92] easily extends to the alkyl cases. The orbital interaction (Scheme 20) [103] in the pere-poxide quasi-intermediate suggests the stabilization occurs by the simultaneous interaction of O with two allylic hydrogens on the same side of the alkene. Photooxygenation of trisubstituted olefins revealed a strong preference for H-abstraction from disubstituted side of the double bond [104, 105],... 

In a related strategy, ethynylphosphines (81) have been prepared with a view to synthesising polyphosphacyclopolyyne materials such as (82) (Scheme 24) [70]. Interaction between the phosphorus lone pairs and the organic 71-systems is supported by the fact that the phosphorus atoms of mono- and di-silyl-capped analogues of bis(arylphosphine) derivative (81) exhibit an unusually low inversion barrier (65 kj mol versus 130-140 kj mol for classical alkyl-or aryl-phosphines). Indeed, the rings (82) exhibit values of at ca. 300 nm, consistent with this assumption. 

Ru—C(carbene) bond distances are shorter than Ru—P bond lengths, but this can simply be explained by the difference in covalent radii between P and The variation of Ru—C(carbene) bond distances among ruthenium carbene complexes illustrates that nucleophilic carbene ligands are better donors when alkyl, instead of aryl, groups are present, with the exception of 6. This anomaly can be explained on the basis of large steric demands of the adamantyl groups on the imidazole framework which hinder the carbene lone pair overlap with metal orbitals. Comparison of the Ru—C(carbene) bond distances among the aryl-substituted carbenes show... 

Push-spectator carbenes of the type 31 (R, R = alkyl) were synthesized and reacted with various Lewis Acids to compare the reactivity of the phosphorus and carbene centers. Two such reactions are shown in Scheme 7.11. From an X-ray structural analysis, the phosphorus substituent was shown to act as a spectator, leaving its lone pair available to react in a Lewis basic manner. When carbene 31 was reacted with BF3, only the carbene adduct 32 was formed. By contrast, when 31 was reacted with the softer Lewis Acid BH3, it was the phosphorus that reacted to yield adduct 33. These types of carbenes exhibited C-NMR shifts in the range of 320-348 ppm, a P-C-N angle of 116.5° a short C-N distance of 1.296 A, and a long C-P distance of 1.856 A. The latter is very similar to that of a typical C-P single bond. 

From a molecular structure point of view, for diCN-HBO, the lone pair electrons of the benzo-nitrogen atom are intrinsically involved in the Ji-electron resonance to establish the aromaticity, such that its electron donating strength, compared with those of alkyl and aryl amines, is negligibly weak. Thus, upon Franck-Condon... 

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