Cyclopropyl inductive effect

The decreased acceleration due to the cyclDpropyl group was attributed to the requirement that the cyclopropyl was forced by the two adjacent methyl groups to assume the perpendicular conformation 16 which prevented optimum conjugation. The inductive effect of cyclopropyl in this conformation is an item of some interest, and is considered further in Section V.D. 

Thus the low reactivity of 87 certainly shows the absence of the strong conjugative stabilization found for the bisected cyclopropylcarbinyl arrangement, but does not provide a quantitative assessment as to the respective roles of ring strain and the inductive effect of cyclopropyl in giving the observed rate of deceleration relative to dimethyl substituents. Accurate molecular mechanics calculations for 87 and the derived cation would provide an assessment of the former, and a measure of o-j for c-Pr free of resonance effects is much to be desired. 

Ree and Martin" used the data of Brown and Cleveland to calculate a value of — 0.21 for a cyclopropyl group twisted out of conjugation with the aryl ring by two ortho-methyl groups. The cr value of — 0.03 for cyclopropyl also indicates a cation-stabilizing ability intermediate between H and CH3 for a non-conjugated cyclopropyl group. The inductive effect of cyclopropyl is discussed further in Section VI.D but is yet to be fully resolved. 

The unique substituent properties of cyclopropyl make it a useful probe for organic reaction mechanisms. The strong 7r-donor ability of cyclopropyl is manifested by the (Tp and <7 constants of — 0.48 and — 1.33 (Table 3), and there is evidence that cyclopropyl can act as a 7r-acceptor as well (Section III.A). However, when direct conjugation is not possible the donor ability of cyclopropyl is greatly reduced, as evidenced by the constant of —0.04 (Table 3). As discussed in Section V.D.2 the inductive effect of cyclopropyl is not well understood, but cyclopropyl is clearly a poorer or-donor than saturated alkyl groups, while the relative position of cyclopropyl and hydrogen is ambiguous. 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

The He-I photoelectron spectra of vicinal tricarbonyl molecules, including (28) and its bicyclo[3,2,l] analogue, have been studied. The first two bands for (28) are interpreted in terms of the ionization of an electron from lone pair combinations of the 2p-oxygen orbitals. The dispirocyclopropyl derivatives (29) and (30) exhibit pronounced interaction between the antisymmetric Walsh orbitals the monospiro-cyclopropyl compounds were studied for purposes of comparison. " Inductive effects... 

Because of its unique structure both the 7c-conjugative and or inductive electronic properties of cyclopropyl are quite different from alkyl and unstrained cycloalkyl groups. These electronic propoerties are manifested in a variety of ways, including the effect of cyclopropyl on rates and equilibria compared to other substituents, and also in the way that cyclopropyl affects the sensitivity of reactions to other substituents. 

In consideration of these results" de Meijere concluded" the cyclopropyl group has an inductive electron-attracting effect in this case and destabilizes the intermediate carbenium ion . However, the destabilization referred to is for spiro cyclopropyl relative to a tert-butyl... 

The effects of initially added Me Hr. to 1)1 1 can be dramatic 98. figure 7.38 shows that 0.18 M MgBry in DHK eliminates Ihe induction period lor the Grignard reaction of cyclopropyl bromide. Other salts, such as 1-eCl.i [ 169. can also promote initiation (Section 7.3.4). 

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