Cyclopropyl carbocation

In the first step, Ag+ promotes the departure of Cl- to give a cyclopropyl carbocation. This undergoes two-electron disrotatory electrocyclic ring opening to give the chloroallylic cation, in which the empty orbital is localized on Cl and C3. Then 09 can add to C3 desilylation then gives the product. 

The cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

Cyclopropyl carbocations react rapidly to form allyl carbocations. Explain how this process is allowed. Explain why the allyl carbocation is more stable than the cyclopropyl carbocation. Predict the stereochemistry of the allyl carbocation that is formed from the following c/s-dimethylcyclopropyl carbocation ... 

A number of reviews dealing with various aspects of the solvolytic chemistry of systems leading to cyclopropyl carbocations have appeared The most recent of these reviews is... 

Initial interest in the solvolyses of cyclopropyl derivatives stemmed from the observation that they underwent solvolysis with concerted ring-opening , and that the reaction was strongly dependent on the nature and stereochemistry of substituents on the ring. This was explained by Woodward and Hoffmann who predicted from orbital symmetry considerations that the electrocyclic transformation in which a cyclopropyl carbocation is converted to an allyl cation should occur in a disrotatory fashion. Also, the particular disrotatory path a given system will take should be dependent on the stereochemistry of the leaving group. This is illustrated as follows. 

Carbalumination of 271b with tri-isopropylaluminium affords acyclic products, the formation of which can be attributed to rearrangement of the initially formed cyclopropyl carbocation (equation 84). Ring-opened products also result from reactions with trialkylboranes, but only cyclopropanes, from addition to the n bond, are seen in reactions with organomagnesium reagents . 

This is a remarkably stable ion, already observed in 1955. Its first NMR spectrum (in H2SO4 at room temperature ) was reported in 1962 and so were a little later, the spectra of a variety of cyclopropyl carbocations in superacid media. Its spectrum has been carefully analyzed. 

The opening of a cyclopropyl carbocation to form an allyl carbocation is an electrocyclic reaction involving two electrons. Because it involves an odd number of electron pairs, disrotatory opening is thermally allowed. The allyl carbocation is much more stable than the cyclopropyl carbocation because of resonance stabilization and relief of ring strain, so the allyl carbocation is favored. The thermal ring opening of the c/s-dimethylcyclopropyl carbocation occurs with a disrotatory motion. 

The ester Kulinkovich reaction was used to prepare a key allylic bromide 5333 in a synthesis of neolauh-malide and isolaulimalide (Scheme 5.88). The cyclopropanol 5.332 produced by the Kulinkovich reaction of ester 5.331, as its mesylate, was subjected to ring opening with magnesium bromide. This is likely to involve electrocyclic ring opening of the cyclopropyl carbocation, followed by bromide trapping. 

Besides the carbocations mentioned above, numerous highly stable carboca-tions have been isolated as the salts of inorganic anions. Azulene analogues of triphenylmethylium ion [ll ]-[20 ], [21 j, [22 " ] and [23 j trisubsti-tuted cyclopropenylium ions [l" ] and [24 ]-p6 ] cyclopropyl-substituted tropylium ions [27 ]-[30 ] tropylium ions annelated with bicyclic frameworks [31 ]-[36 ] and [37 ]-[39 ] fulvene-substituted cyclopropenylium [40 ] and tropylium [41 ] ions a tropylium ion condensed with acenaphthylene [42 ] and a cation containing a 147t periphery [43 ] have... 

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

Fig. 5 Schematic representation of the linear combination of the filled antisymmetric Walsh orbital of a a-cyclopropyl ring and the vacant p2-orbital of the C + -carbon of a carbocation leading to delocalization of the positive charge in a 4c-2e MO.

The experimental 3J(HH) spin-spin coupling constants for Ha, Hp, and Hp for the -isomer 22 were quite satisfactorily reproduced (A = 0.1 - 1 Hz) by calculations, using a finite perturbation method (FPT level (26), Perdew/IGLO-III at a MP2/6-31G(d)) geometry for the model structure (E)-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation. The calculations confirm the /rexperimentally observed carbocation 22 (20, 27, 28). The (E)- 1-cyclopropy 1-2-... 

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