Bishomocyclopropenyl cation

There are numerous reports of the direct NMR observation of the bishomocyclopropenyl cation [32 n = 2] under stable ion conditions. The first reports of the H NMR spectrum of [32 n = 2] appeared simultaneously (Brookhart et al., 1966 Richey and Lustgarten, 1966), and subsequently various accounts of the 13C NMR spectrum appeared, culminating in an extensive study by Olah and Liang (1975). The 13C data were taken as clear evidence for the bishomoaromatic nature of [32 n = 2] and to preclude the equilibrating classical ions [34] and [35] (Olah and Liang, 1975). 

Other potential bishomocyclopropenyl cations include the various birdcage cations [37] and [38] (Winstein and Hansen, 1960 Bruck et al., 1960) and the related bicyclic cation [39] (Leal and Pettit, 1959). However, these species are less well characterized. 

Bicyclo[3.1.0]hex-2-ene, 291 sigmatropic rearrangement, 290-291 Bicyclo[2.1.0]pent-2-ene, 289 Bishomocyclopropenyl cation, 277 Bond, definition of, 50 Bond angles... 

Insertion of a methylene group into the four-membered ring of the pyramidal (CH)5+ cation 615 would give rise to the homo derivative 628. As discussed earlier,41 1037-1041 hexamethylbicyclo[2.1.1]hexenyl cation is best represented as a bishomocyclopropenyl cation 586 and not pyramidal-type ion 629. 

Bishomocyclopropenyl cation, 277 Bond, definition of, 50 Bond angles... 

Despite considerable effort to prepare the parent bishomocyclopropenium ion (23), it remains unknown experimentally. 23 is calculated to be puckered (pucker angle 90°) and 6 kcal mol (MP4SDQ/ 6-31G //MP2/6-31G ) more stable than the nonhomoaromatic, but less strained, planar conformation 24. Since Roberts et al. first proposed the enhanced stability of the bishomocyclopropenyl cation 5, numerous 3,5-bridged derivatives (25) were prepared and fully characterized as homoaromatic both by experiment and theory. ... 

Recently Laube et al. and Evans et al. reported X-ray structures for the three different 3,5-bridged bishomocyclopropenyl cations 26—28. In each case, the formal double bond is elongated (Cl—C2 1.41 A) compared with a normal double bond and C4 leans toward this former double bond (average Cl—C4 and C2—C4 distances 1.75—1.88 A). These results are in excellent agreement with calculated structures for related bishomocyclopropenyl cations with both experiment and theory strongly supporting the homoaromaticity of these systems. 

Just as Applequist and Roberts (1956) were the first to classify the cyclobutenyl cation as the homocyclopropenyl ion, the Roberts group was also the first to designate the system [32 n = 2] as bishomocyclopropenyl (Woods et al., 1956). Winstein et al. (1955) initially attributed the exceptional solvolytic lability of systems [30 n = 2] to homoallylic stabilization of the cation. The early studies on cation [32 n = 2] have been extensively reviewed by Winstein (1967, 1969), and by Story and Clark (1972). 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

An example of a bishomocyclopropenyl system is the 7-norbornenyl cation, 71 (see Table 15) 212-214) ( whose chemical shifts and coupling constants are not entirely incompatible with the formulation in which C(7) interacts equally with C(2) and C(3). However, the high chemical shift of the proton on the bridging carbon atom (H(7), t 6.76), although explained in terms of the tendency of the bridging carbon atom to rehybridize from sp3 to sp3 212,213 suggests that ring current effects are not operative in this system. 

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