Vinyl cations 1-cyclopropyl

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

When two equivalents of pyridine were added to the nmr sample and the probe heated to 80° C, the enol formate 61 decreased and phenyl cyclopropyl ketone 58 appeared at a rate approximately ten times faster than in the previous buffered system. The observation of intermediate 61 and the kinetic results, together with the observed induction periods, are consistent with the idea that some and perhaps all of the rearranged product ketone in the solvolysis of this system arises via double-bond participation in 61 rather than triple-bond participation and a vinyl cation (80). 

A number of miscellaneous reactions involving diazonium ions and possible vinyl cations have been reported. Treatment of amine 138 with sodium nitrite in 20% aqueous acetic acid is reported to give methyl cyclopropyl ketone as one of four products (116). The reaction has been postulated to involve a vinyl cation, presumably by the following sequence of reactions (116) ... 

Siehl and coworkers have prepared cyclopropylcyclopropylidenemethyl cation 37, by protonation of bis(cyclopropylidene)methane with FS03H-SbF5 at liquid nitrogen temperature (equation 31 )67. The vinyl cationic center is stabilized by the a-cyclopropyl group. 

The stabilization of the vinyl cation by the a-cyclopropyl group was calculated to be significantly less than that by the phenyl group. The theoretical rotational barrier of the a-cyclopropylvinyl cation is less than that of the cyclopropylethyl cation, presumably due to the stabilization of the intermediate perpendicular conformation by the overlap of the ff-bonds with the n-electrons of the C=C bond72. 

A comparison of the 13C NMR chemical shift of the para carbon in vinyl cations 389a and 390 and the sp2 hybridized l-(p-tolyl)ethyl cations with an additional 1-cyclopropyl- (391) or 1-methyl substituent (392) (Figure 14)150151 gjVes a qualitative measure of the electron demand of various 1-p-tolyl- substituted sp2- and sp-hybridized carbocations. 

The 13 C NMR chemical shifts of the para carbon for various phenyl-substituted sp2-and sp-hybridized carbocations (Figure 15) indicate that the demand for rr-aryl delocalization of the positive charge for the -silyl-substituted vinyl cation 393 is lower compared with that in a-phenylethyl (78), a-methyl-a-phenylpropyl (397) and cumyl (292) cations150 153. The stabilizing effect of the /)-silyl group in 393 is comparable to that of the cyclopropyl substituent in 1-cyclopropylbenzyl cation (398). The para carbon shift in the /)-cr-silyl... 

On the other hand, l-(phenylethynyl)cyclopropanol 9 (R = Ph) underwent a C3 -> C4 ring expansion and subsequent decarboxylation when treated with MCPBA to yield the 2-phenylcyclobutanone 47, likely via the intermediate 2-(l-hydroxy-cyclopropyl)-2-phenyl ketene 44, formed by migration of the cyclopropyl group in the vinyl cation 43. The ketene 44 thus resulting could be attacked by a second equivalent of MCPBA and ring expanded to the (3-ketoacid 46 which would easily decarboxylate to yield 47, Eq. (15) 14>. 

Evidence for the conjugative interaction of cyclopropyl with adjacent vinyl cations comes from a number of sources. Theoretical calculations for a variety of substituted vinyl cations (109), including the case where R = c-Pr, indicated that substituent effects in 109 were similar to those of the corresponding ethyl cations (110) . The rotational barrier calculated for cyclopropylvinyl cation was half that of cyclopropylethyl cation, a result tentatively attributed to conjugation of cyclopropyl with the 7r-bond of the vinyl cation when the cyclopropyl was twisted perpendicular to the vacant p orbitaP . ... 

Rearrangement of vinyl cations to place the positive charge adjacent to a cyclopropyl group is more effective than the corresponding reaction to place the charge adjacent to phenyP. Many other aspects of the chemistry of cyclopropylvinyl cations have been summarized in a recent review ... 

The fact that hydride rearrangement of 141 to give the substituted vinyl cation (equation 24) was evidently more facile for c-Pr than for Ph was taken as evidence for the order c-Pr > Ph > Me for the stabilizing ability Cyclopropyl was also better than phenyl in accelerating protonation of alkynes, but only by a factor of 3 (Section V.F) . 

In 1969 Sherrod and Bergman and Bassler and Hanack independently reported that cyclopropyl groups strongly stabilize adjacent vinyl cations generated by solvolysis. Thus, 1-cyclopropylvinyl iodide (105) reacted with silver acetate in acetic acid at room temperature at a rate which is about 10 times faster than that of the corresponding 1-isopropylvinyl iodide (106). 

Cyclopropylidenemethyl systems, in which one carbon of the double bond is already part of the cyclopropane ring, give especially stable vinyl cations owing to favorable geometry for overlap with participating cyclopropyl orbitals. This subject has also been reviewed recently in detaiP " and thus will only be summarized here briefly. 

Because of the preferred linearity of vinyl cations, the stability of cyclic vinyl cations depends on the ring strain or the ring size. Rates of solvolysis of Inflates show that the 1-cyclohexenyl cation is 6 X 10 -fold less readily generated than the 1-methyl-1-propenyl cation. The 1-cyclopentenyl cation is not formed at all. On the other hand, 1-cyclobutenyl triflate is 3700 times more reactive than 1-cyclohexenyl triflate.The stability of the cyclobutenyl cation is due to resonance involving cyclopropyl-stabilization of the positive charge. ... 

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