Ethers shift increments

Using the increment formalism, this is rationalized as a p effect of the O-alkyl carbon to the a E nd a y effect to the /Tcarbon transmitted through the ether oxygen [210, 275-277]. Corresponding changes are observed for methoxy relative to hydroxy shift increments for a and fi carbons of cyclohexane [263-265]. 

Fig. 1 Charge-transfer absorption spectra of enol silyl ethers complexes with re-acceptors. (a) Spectral changes accompanying the incremental additions of cyclohexanone enol silyl ether [2] to chloranil in dichloromethane. Inset Benesi-Hildebrand plot, (b) Charge-transfer absorption spectra of chloranil complexes showing the red shift in the absorption maxima with decreasing IP of the enol silyl ethers, (c) Comparative charge-transfer spectra of EDA complexes of a-tetralone enol silyl ether [6] showing the red shift in the absorption maxima with increasing EAs of the acceptors tetracyanoben-zene (TCNB), 2,6-dichlorobenzoquinone (DCBQ), chloranil (CA), and tetracyanoqui-nodimethane (TCNQ). Reproduced with permission from Ref. 37.

Carbon-13 shift values of parent heterocycloalkanes [408] collected in Table 4.61 are essentally determined by the heteroatom electronegativity, in analogy to the behavior of open-chain ethers, acetals, thioethers, thioacetals, secondary and tertiary amines. Similarly to cyclopropanes, three-membered heterocycloalkanes (oxirane, thiirane, and azirane derivatives) display outstandingly small carbon-13 shift values due to their particular bonding state. Empirical increment systems based on eq. (4.1) permit shift predictions of alkyl- and phenyl-substituted oxiranes [409] and of methyl-substituted tetrahydropyrans, tetrahydrothiapyrans, piperidines, 1,3-dithianes, and 1,3-oxathianes [408], respectively. Methyl increments of these heterocycloalkanes are closely related to those derived for cyclohexane (Table 4.7) due to common structural features of six-membered rings. 

Figure 8 shows the characterization of these poly (propylene ether) diols by gel-permeation chromatography. There is a shift in the peak position to lower elution volumes, in accord with an increase in molecular weight with each monomer increment polymerized. The molecular-weight distributions of the three diols are similar and remain narrow after the addition of monomer increments. Since all of the molecules apparently continued to grow, this polymerization must proceed with very little chain termination under these conditions. 

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