Bicyclobutyl cation

The question of two rings going-on one is also seminal to the study of homoaromaticity. For n odd, the set of [(CH) CH2] ions demonstrate a delicate balance between mono and bicyclic structures (see Ref 133 and numerous references cited therein to both the experimental and theoretical literature). In the case of n = 3, one can imagine a planar cyclobutenyl cation (63) and a markedly non-planar, highly puckered bicyclobutyl cation (64). Both calculational theory on the parent and experiment on derivatives show the latter geometry to be preferred. However, as in the case of the other purported bicyclobutane derivatives characterized by the 2,4-carbons trigonally coordinated that were discussed earlier in this section, formal theory shows there is no 1,3-bond. The ion is not homoaromatic and there is no cyclopropane ring. In the case of n = 5,... 

The preferred conformation of the cyclopropylmethyl cations is one in which the p orbital at the cationic center is oriented for maximum overlap with the cyclopropane C-C bonds (bisected conformation). It has essentially the same energy as the cyclobutyl cation, and the barrier to interconversion is very low ( 2 kcal/mol). The low energy of cyclobutyl cation is due to a strong 1,3-interaction, forming a bicyclobutyl cation. The extreme of this sort of interaction is found with the bicyclo[l.l.l]pent-l-yl cation, in which the structure is found to have a 1.54 A cross-ring distance. The ion appears to correspond to a proton attached to the nonbonded charge density near the bridgehead carbons of [l.l.ljpropellane. 

The calculated potential curves for the (CH)3CH2 system show that the cyclo-propenylcarbinyl, cyclobutenyl, and bicyclobutyl cations are surrounded by relatively low-energy barriers and that they would readily collapse to the most stable conformer, the puckered homocyclopropenyl cation (5). ... 

The thermolysis of (358) also leads to aromatisation, in this case in a process believed to involve an intermediate nitrile ylid. Evidence for this is obtained by thermolysis of a series of cyclopropenyl-substituted oxazolinones such as (359) for which cycloreversion with elimination of C02 is known to lead to a nitrile ylid. In some cases the ylid could be trapped by addition to methyl propiolate. Substituent effects suggest that the nitrile ylids undergo stepwise addition to produce a bicyclobutyl zwitterion which can either collapse to an azabenzvalene or rearrange to a cyclobutenyl cation 286>. 

The ylides were also generated by thermolysis of oxazol-5(4//)-ones 2 by 1,3-dipolar reversion and loss of carbon dioxide. In the case of 4, three pyridines were formed,apparently by stepwise addition of an intermediate nitrile ylide to produce a bicyclobutyl zwitterion which can cyclize to an azabenzvalene or via a cyclobutenyl cation which can rearrange to two different aza-Dewar benzenes. ... 

Drawn as puckered cyclobutyl cation but called bicyclobutyl. 

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