Cyclobutenyl cation

The cyclobutenyl cation is the homoaromatic analog of the very stable cycloprope-nium cation. This ion can be prepared from 3-acetoxycyclobutene with the use of superacid conditions ... 

Solutions of aiyl-substituted vinyl cations have been reported to be stable for at least a short time at low temperatures. The NMR spectra have been obtained Abram, T.S. Watts, W.E. J. Chem. Soc., Chem. Commun., 1974, 857 Siehl, H. Carnahan Jr., J.C. Eckes, L. Hdnack, M. Angew. Chem. Int. Ed. Engl., 1974, 13, 675. The 1-cyclobutenyl cation has been reported to be stable in the gas phase Franke, W. Schwarz, H. Stahl, D. J. Org. Chem., 1980, 45, 3493. See also Siehl, H. Koch, E. J. Org. Chem., 1984, 49, 575. 

Extended Hiickel calculations have been carried out on the 1-cyclopropylvinyl cation 156 (122). These results show that the most favorable conformation for this ion is the linear bisected structure 156a. However, Hanack et al. (166b), by means of a modified CNDO technique, calculate the most stable geometry of the intermediate ion resulting from homopropargyl participation to be a bridged cyclobutenyl cation rather than 156a. 

The cyclobutenyl cation (92) and the homotropylium cation, CgHcf 93 are both prototypes of homoaromatic systems. 

The cyclobutenyl cation 92 is one of the first examples demonstrating that electron correlation is required both for geometry optimization and NMR chemical shift calculations.14 The IGLO/6-31G(d,p) calculated 13C NMR chemical shifts of the planar form of a homotropylium cation 94 clearly deviate from the experimental values (mean deviation A = 45.6 ppm) alternating in the seven-membered ring between 122 and 194 ppm, whereas those of the non-planar structure for the homotropylium cation 93 are in good agreement with experiment (mean deviation A = 6.2 ppm).108... 

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). 

We reiterate that both homoaromaticity and aromaticity are more pronounced in ions than in related neutrals. In the tour-de-force of computational theory, S. Sieber, P. v. R. Schleyer, A. H. Otto, J. Gauss, F. Reichel and D. Cremer [7. Phys. Org. Chem., 6,445 (1993)] document considerable homoaromatic stabilization of the cyclobutenyl cation. Yet the difference of the enthalpy of formation they calculate from their quantum chemical cations, 1021 kj mol-1, is only 54 kJmol-1 lower than that archivally recommended for the cyclopropenium ion [S. G. Lias,... 

J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 17 (1988), Supplement 1], This last number allows us to conclude that the aromatic stabilization of cyclopropenium ion exceeds the homoaromatic stabilization of cyclobutenyl cation by at least (160 - 54) = 106 kJ mol-1. Regrettably, inadequate time and space does not allow us to discuss the energetics of ions containing three-membered rings in the current chapter. 

Shubin and co-workers325 have generated long-lived cyclobutenyl cations and studied their varied rearrangements, which involve cyclialkylation steps. For example, cation 94 gives the isomeric pentacycle 95 under superacid conditions [Eq. (5.125)]. 

The dichloride (347) is converted to (348) on heating, apparently again through an intermediate cyclobutenyl cation. Hydrolysis of either compound with water leads to the ketone (349), which is reconverted to the cyclobutene by treatment with phosphorus pentachloride 274). 

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>. 

Cremer and coworkers investigated a number of potentially homoconjugated cyclopropyl compounds such as the monohomotropylium cation the 1,4- and 1,3-bishomotropylium cation the trishomotropylium cation , the barbaralyl cation and the cyclobutenyl cation. All these cations have the choice between a closed cyclopropyl structure (la), an open cyclopolyenyl structure (Ic) and an intermediate structure (Ib) as demonstrated in the case of the monohomotropylium cation. 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

Bishomo-square pyramidal cation, 2-bicyclo[3.2.1]octa-3,6-dienyl cation and related cations 302 Cyclobutenyl cations 308 Cyclopentenyl cations 310 Cyclohexenyl cations 311 Benzenium ions and related cations 313 9,l0-Dihydro-10-phenanthrenium ions and related cations 323 Acenaphthenium ions 332 Vinyl cations 334... 

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 dichloride 1 rearranged under a variety of conditions to 2, a reaction most effectively carried out by heating in an inert atmosphere for 5 hours at 150°C. The cyclobutene 2 was obtained in 64% yieldT The ring expansion apparently proceeds by loss of chloride ion and ring expansion of the cyclopropenylmethyl cation to the corresponding cyclobutenyl cation. 

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. ... 

Since the first preparation of cyclobutenyl cations in 1964 (97), the importance of 1,3 overlap in connection with the question of the homoaromatic nature of these cations has attracted considerable attention (98). In the case of the pentamethyl-substituted cation 67-l,3-(CD3)2 labeled by trideuteriomethyl... 

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. ... 

Reaction of the vinyl cation is with another molecule of alkyne, giving the cyclobutenyl cation (47). 

The convincing corroboration of Winstein s correct concept as to the monohomoaromatic structure of this type of ions are the data of the NMR spectra, in particular for the unsubstituted cyclobutenyl cation 454 - ) in the latter case the signals of C and resonate in a higher field than that of C does contrary to what is observed in the spectrum of cycloalkenylic ions, but in full accord with the concept of 1,3-orbital interaction. The PMR spectra show l a reversible temperatiu e dependence of the methyl proton signal shape which was atrributed to the nonplanar ion inversion with the intermediate formation of a planar cation. The barrier value of this inversion determined experimentally (AG = 8.4 kcal/mole) agrees... 

These differences in the rates are due to essential differences in the structure of the MCRs of these cations. For example, the hindrances to 1,2-shifts in cyclobutenyl cations relative to those in benzenonium ones are likely to be connected with these two types of ions being topologically opposite the latter are antihomoaromatic systems in the ground state (true, the overlap between the ends of the pentadienyl system and, hence, the extent of antihomoaromaticity seems to be very small. 

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