Isopropyl compounds

Also due to the high barrier of inversion, optically active oxaziridines are stable and were prepared repeatedly. To avoid additional centres of asymmetry in the molecule, symmetrical ketones were used as starting materials and converted to oxaziridines by optically active peroxyacids via their ketimines (69CC1086, 69JCS(C)2648). In optically active oxaziridines, made from benzophenone, cyclohexanone and adamantanone, the order of magnitude of the inversion barriers was determined by racemization experiments and was found to be identical with former results of NMR study. Inversion barriers of 128-132 kJ moF were found in the A-isopropyl compounds of the ketones mentioned inversion barriers of the A-t-butyl compounds lie markedly lower (104-110 kJ moF ). Thus, the A-t-butyloxaziridine derived from adamantanone loses half of its chirality within 2.3 days at 20 C (73JCS(P2)1575). 

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

R)3CH Alkanes and cydoalkanes with tertiary hydrogen atoms (e.g., f-butane, isopropyl compounds, decahydronaphthalenes)... 

FIGURE 6.4. Examples of bioactive hexafluoro isopropyl compounds... 

Fig. 22 The location of transition states for reactions of isopropyl compounds. The points are for solvolysis reactions. The bar shows the location for the reactions in Table 28.

Further support for this interpretation can be found by considering the data for the hydrolysis of isopropyl compounds in Table 28. The values of r are calculated from (112). It can be seen that the transition states for the isopropyl transfers are much looser than those for the methyl transfers. The solvolysis of isopropyl compounds is closer to the borderline between the SN1 and SN2 mechanisms and therefore we may expect the SN2 transition state to be looser. As discussed above there is supporting evidence from Ko and Parker s (1968) measurements of transfer activity coefficients for the transition state. 

The isopropyl compound V, (R = Me2CH) has readily been obtained from VI and isopropylmagnesium chloride in ether solution (94) but when R = cyclopentyl, THF has been found to be necessary for effecting the reaction. When R = tert-butyl the Grignard reaction yielded the dimer VII, but the desired V (R = er -butyl) has been obtained... 

We can do the same with a CH group, and in the left-hand side of Table 15.4 we take a series of isopropyl compounds, comparing the measured shifts with those for the central proton (CHMe3) or carbon (CHMej) of 2-methylpropane. We set two of the substituents as methyl groups and just vary the third. Yet again die shifts for die same substituent are broadly the same. 

The isopropyl compound, RhQ(PPr 3)3,13 is also a highly reactive and useful starting material it is made by interaction of [RhCl(cyclooctene)2]2 and excess of the phosphine. 

The kinetic acidity of trifluoromethyl cyclopropane (304) and of the corresponding isopropyl compound 305 have been determined by de Boer and coworkers As expected for the inductively operating CF3 group the cyclopropyl compound 304 exchanges with deuterium 5x10 — 5x10 times faster than 305. 

In order to examine whether the acidities observed in a-C(0)R, a-N02, a-CN and a-CF3 substituted eyelopropanes are correctly explained by a predominating mesomeric effect in the case of the a-C(0)R and a-N02 substituents, and a stronger dipole (field) effect with a-CN and a-CFa substituents Wagner and Boche " investigated this subject by means of STO-3G calculations. It was of interest to determine whether the calculations are able to reproduce the relative acidities of cyclopropyl and isopropyl compounds, as well as the configuration of the corresponding cyclopropyl anions. 

The exo-2-methyl- and ethyl-substituted 3-benzyl-3-azabicyclo[3.3.1]-nonanes were shown to adopt the CC conformation in solution (from 13C-NMR data), while for the exo-2-isopropyl compound the CB conformation is predominant (106). 

Minor Methoxypyrazines. Stable isotope-dilution mass spectrometry has also allowed quantitative analysis of 2-methoxy-3-(l-methylpropyl)pyrazine (2) sec-butylmethoxypyrazine) and 2-methoxy-3-(l-methylethyl)pyrazine (3) (isopropyl-methoxypyrazine) in grapes and wines. The corresponding trideuterated isopropyl-methoxypyrazine (6) is used in addition to the trideuterated isobutylmethoxypyrazine 5 to provide more accurate quantitative analysis of the isopropyl compound (7, 10,... 

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