Isomers Isopropyl cation

More recent work93,127-129 has shown that /1-hydrogen transfer takes place only if the cation corresponding to the N—R substituent is stable. Thus, CH2=N+(CH3)CH(CD3)2 and related species expel C3HD5, via specific D-transfer128 129, because isopropyl cation is stable with respect to a 1,2-H shift. In contrast, if the associated cation readily rearranges to a more stable isomer, there is actually a discrimination against... 

The Ce isomer distribution—2-methylpentane (28%), 3-methylpentane (14%), and w-hexane (32%)—is very far from thermodynamic equilibrium, and the presence of these isomers indicates that both isopropyl cation 31 [Eq. (5.62)] and n-propyl cation 34 [Eq. (5.63)] are involved as intermediates [as shown by 13C(2)—13C(1) scrambling in the stable ion147]. 

The relative rates of hydrogen and carbon interchange have been measured by Saunders et al. (1973c) using a mixture of (1,1,1- H3)- and (2- C)-labelled isopropyl cations at —88°C. The changes in the relative areas of different peaks as well as C-satellites were observed, and the time dependence of the concentrations of different labelled isomers were simulated by computer [using mechanisms (34)-(36)]. A combination of mechanisms (34) and (35) or mechanisms (34) and (36) could match the measurements. The rate for (34)... 

Protonated cyclopropane has been reported in the gas phase" "" to be ca 8 kcal mol" in energy above the isopropyl cation. The bent bonds of the cyclopropane ring are susceptible to electrophilic attack leading to the expectation that cyclopropane will be more basic than saturated alkanes and that protonation will occur on the C—C bond, i.e. the edge-protonated isomer will have the lowest energy. There is, however, considerable evidence from solution chemistry that corner-protonated cyclopropanes exist as intermediates in 1,2-alkyl shifts in carbocations. There have been several reviews of protonated cyclopropanes " and, in the current work, only the very recent theoretical work will be reviewed. 

Reduction of the cation (M = W) with NaBH4 affords a yellow oil identified as the isopropyl complex (C5H6)W(CO)3[CH(CH3)2] with none of the w-propyl isomer present. A hydride ion addition corresponding to that with the analogous iron complex (255) is postulated. 

It has been proposed 23) that the CH2 group of the isomer (Z)-2J rotates in such a way that the cation migrates through the anti-face of the allyl plane, away from the isopropyl group (Scheme 1). On the other hand, the (E)-l-isopropyl substituent in (E)-26 permits the jy/2-location of the cation in the transition state (Scheme 1). Calculations show a significant preference for jy -transition states in such rearrangements 30). 

The cationic dihydride [Cp(CO)2(PMe3)W(H)2] OTr can be isolated, and has been fully characterized, including a crystal structure [41]. It reacts readily at room temperature with acetone, giving cis- and trans-isomers of the isopropyl alcohol complex (3.17) an analogous reaction occurs with propionaldehyde. 

The relationship of the nitrous acid and nitrosoamide methods of deamination is more tenuous in water since only one example of the i nitrosoamide decomposition in aqueous media has been reported. The decomposition of A -nitroso-A -n-propylbenzamide in dimethyl-formamide-water (60/40 by vol) yielded a propyl benzoate fraction containing 9% of the isopropyl isomer and a propanol fraction con- taining 33% of isopropyl alcohoP . The nitrous acid deamination of n-propylamine under the same conditions yielded an alcohol fraction containing 31% of isopropyl alcohoP. The similarity in the isomer contents of the alcohol portions suggests that common intermediates were involved. The less extensive isomerization of the benzoates may be a reflection of the shorter lifetime of the propyl cations trapped by negatively charged coimter ions compared to those trapped by neutral solvent molecules on the other hand, the role of diazopropane must be determined before definitive conclusions are possible. 

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