Comparison with aliphatic series

Many common reactions of aliphatic amines, ethers and sulfides (1) involve initial attack by an electrophilic reagent at a lone pair of electrons on the heteroatom salts, quaternary salts, coordination compounds, amine oxides, sulfoxides and sulfones are formed in this way. Corresponding reactions are very rare (c/. Section 3.3.1.3) with pyrroles, furans and thiophenes. These heterocycles react with electrophilic reagents at the carbon atoms (2-3) rather than at the heteroatom. Vinyl ethers and amines (4) show intermediate behavior reacting frequently at the (3-carbon but sometimes at the heteroatom. 

Reactivity of Five-membered Rings with One Heteroatom 

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In some cases, the effect of reactant structure may outweigh the influence of catalyst nature. This is seen by comparison with the dehydration of aliphatic secondary alcohols and substituted 2-phenylethanols on four different oxide catalysts (Table 4). With aliphatic alcohols, the slope of the Taft correlation depended on the nature of the catalyst (A1203 + NaOH 1.2, Zr02 0.3, Ti02—0.8, Si02—2.8 [55]) whereas for 2-phenyl-ethanols, the slope of the corresponding Hammett correlation had practically the same value (from —2.1 to —2.4) for all catalysts of this series [95]. The resonance stabilisation of an intermediate with a positive charge on Ca clearly predominates over other influences. 

This discussion of aliphatic carbanion structures has included mainly organolithium compounds simply because the structures of most aliphatic caibanions incorporate lithium as the counterion and also because this alkali metal cation is the most widely used by synthetic organic chemists. For comparison the entire series of Group la methyl carbanion structures, i.e. MeNa, MeK, MeRb and MeCs, have been determined. Methylsodium was prepared by reaction of methyllithium with sodium r-butoxide. Depending upon the reaction conditions, the products obtained by this procedure contain variable amounts of methyllithium and methylsodium (Na Li atom ratios from 36 1 to 3 1). Hie crystal structure of these methylsodium preparations resembles the cubic tetramer (38) obtained for methyllithium with the Na— Na distances of 3.12 and 3.19 A and Na—C distances of 2.58 and 2.64 A. 

The Taft-Lewis inductive constant Oj was proposed [Taft and Lewis, 1958] to measure the inductive effect in aliphatic series on a scale for direct comparison with aromatic o values, derived from the o constant as ... 

Table 9 shows the PP MO results for this interesting series of highly strained three-membered cyclic molecules. Here a detailed comparison is possible with the best results of an all-electron study, including d functions10 (also reported in Table 9). An analysis of this table reveals how all trends in population analysis, both in charges and overlap populations, are the same in the AE + d and in the simple PP calculations, with very few and very minor exceptions. PP predicts a charge donation to the aliphatic groups, while AE predicts a withdrawal, mainly due to the availability of d orbitals on sulphur, which can allocate extra electronic charge. As outlined in the general notes on population analysis (Section III.D) comparisons should be carried out on a relative basis and,...

In the manner discussed above, a resonance energy5960 Hr = 7.8 ( 1.5) kcal mol-1 per secondary benzyl radical 30 is calculated by comparison of the axis intercept with that of the aliphatic C,— C, series 23 20). It is remarkable that in both... 

In its relative reactivity toward toluene, ethylbenzene and cumene the more highly substituted 1-methyl-2,2-diphenylcyclopropyl radicaP , derived from the decomposition of the precursor diacyl peroxide, resembles the chlorine radical more than it does the phenyl radical (Table 3). Similarly, comparison of the relative reactivities of primary, secondary and tertiary aliphatic hydrogens toward chlorine atoms (1.0 3.6 4.2) and phenyl radicals (1.0 9.3 44) with the relative reactivities of the C-H bond in the methanol/ethanol/2-propanol series toward the 1-methyl-2,2-diphenylcyclopropyl radical (1.0 2.4 15) further confirms the low selectivity of the cyclopropyl radical. Again, this radical resembles the chlorine atom in its reactivity more than it does the phenyl radical. 

In series 7 both substituents form a mesophase (MI>10) when the hydroqui-none is copolymerised with N-(4-carboxyphenyl) trimellitimide (Table 25). However, when the substituted hydroquinone in polymer 6 is replaced by aliphatic diols in polymers 8 und 9, then isotropic melt properties are obtained (MI=8). 4-Carboxy phenyletherphthalimide is a poor mesogen and relies on a predominance of ester mesogen in the PEI to achieve a mesophase. By comparison, N-4-carboxyphenyl) trimellitimide is a better mesogen. In PEIs with phe-nylhydroquinone and isophthalic acid (Table 25), it has a lower critical MI>9.5. 

A few systematic comparisons can be made for the reactions of simple aldehydes with a-heteroatom-substituted ylides. The normal Z-selectivity pattern of the PhjP CHX series decreases from X = iodine to bromine to chlorine or fluorine (Table 21, entries. 44-57). There may be a similar decrease in selectivity with decreasing atomic number from a-sulfide to a-ether substituents (compare entries 40-43 with 35-37). Unfortunately, no direct comparisons have been made, and there are very few examples involving reactions of simple aliphatic aldehydes RCH2CHO with either the a-alkoxy or the a-alkylthio ylides. It is too early for detailed generalizations, but the... 

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