Cycloalkyl shifts 204

The last example that deserves to be highlighted in this section on spirocyclization is the DIB-induced domino reaction described by Fujioka, Kita, and co-workers [99] (Fig. 38), who utilized phenolic cyclobutanols of type 151 in aqueous HFIP with the aim of producing spiroketonic cyclohexadienones of type 153 through a 1,2-cycloalkyl shift analogous to the DIB-mediated Wagner-... 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Electron density methods such as GIAO-DFT methods require much less computational resources in terms of cpu time, memory and disk space compared to wave-function methods such as GIAO-MP2. A systematic study of a set of 16 alkyl- and cycloalkyl cation (Scheme 1) was performed to investigate the performance of GIAO-B3LYP methods for prediction of 13C NMR chemical shifts for these types of carbocations.37... 

A notable deshielding jS-effect (10 ppm) has been observed in an aminophosphonium salt on the P+NMe 13C chemical shift when the NHMe group is further alkylated to NMe237. Steric compression has been used, with some caution, to explain the relatively shielded / -carbon atoms in triphenylphosphonio cycloalkyl salts when they are compared with their methylcycloalkyl analogues36. 

Cycloalkyl(silyl)carbenes with an a-C—H bond have not yet been investigated systematically. When cyclohexyl(trimethylsilyl)carbene was generated by thermal a-elimination from cyclohexyl-bis(trimethylsilyl)methanol at 500 °C, only the (1,2) hydride shift took place, whereas cyclopentyl(trimethylsilyl)carbene, generated analogously, gave both the endocyclic alkene and the 1,3-C,H insertion product85,86 (equation 47). 

C NMR spectroscopic and theoretical studies (DFT, ab initio, IGLO) of a series of cycloacylium ions were performed by Prakash et al.577 The study showed that the cycloalkyl groups have little effect on the shift of the carbocationic carbon. Furthermore, charge calculations showed that the delocalization into the cycloalkyl group is greater than in the protonated carboxylic acid (carboxonium ion), where two oxygen atoms participate in delocalization. 

Shifts of half-wave potentials towards more positive values in the presence of bulky groups can be explained in some cases also by changes in the mechanism of the electrode process. The most thoroughly studied example is the reduction of alkyl and cycloalkyl bromides (141). Departures of the half-wave potentials from predicted values for a-branched alkyl bromides, increasing in the sequence Et < — Pr nucleophilic substitutions. Hence a similar explanation, i.e. varying participation of SnI and 5N2-like... 

The structure of the TS for the nucleophilic reaction of aniline at secondary cycloalkyl carbon centers depends on the size of the cycloalkyl group53. We note in Table 4 that although the rate is fastest with the cyclopentyl and slower with the cyclohexyl compound, the TS shifts successively to a later position along the reaction coordinate (more... 

The solution (dichloromethane) spectra of polymers X, XI, and IX can be seen in Fig. 1. Surprisingly, the absorption spectrum of the l.c. p-methoxycinnamate polymer, IX, showed a 10 nm red shift relative to the two non-l.c.-cinnamate polymers, X and XI. We suggest that, in the thermodynamically most stable conformation the p-methoxycinnamate moiety was intramolecularly perturbed by the phenyl ester group. The ester, phenyl p-methoxycinnamate also shows this perturbation, but the alkyl and cycloalkyl esters do not. CPK (Corey-Pauling-Koltun) molecular models sow that the phenyl ester group could assume an orientation that was almost coplanar with the cinnamate moiety which could easily give rise to a 10 nm red shift (see Fig. 2). 

Generation of Alkyl and Cycloalkyl Carbenes - Photolysis or thermolysis of a series of alkylchlorodiazirines (16) (Scheme 7) in the presence of alkenes, such as tetramethylethene, results in 1,2-H shifts, giving the corresponding vinyl chorides (18), in competition with additions of the carbenes (17) to the alkenes, yielding cyclopropanes (19). The mechanism of these reactions is discussed in the light of results obtained from photoacoustic calorimetry, and the ratio of vinyl chloride to cyclopropane seems to depend on the excited states of the carbene precursors and also on carbene-alkene complexes. Similar reactions of related diazirines have been investigated by flash photolysis. 

Hydrogen-Bridged Cycloalkonium Ions Tlie early studies of Prelog and Traynham " and Cope and coworkers clearly established that mediumsized cycloalkyl rings, for example, Cs-Cn, undergo ready hydride shifts in reactions involving electrophilic intermediates. ... 

Fig. 18. H and 13C NMR chemical shifts for sulfinylated alkyl(cycloalkyl)pyrenium ions (partial Aff H and A<513C values in parentheses).

The p-hydrido bridged cation [90] is equivalent to a resonance system such as exists in allyl cations (single energy minimum) and can be distinguished from a set of two degenerate cycloalkyl cations [88] undergoing a fast hydride shift (double energy minimum) as shown in (70) by comparison with suitable model cations. 

Table 6.4. chemical shifts of amines containing a cycloalkyl substituent... 

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