Bicyclobutonium ions rearrangement

Scheme 3 Rearrangement of [C4H6R] + cations, for R = H a threefold degenerate interconversion of bicyclobutonium ion (54) and cyclopropylmethyl cations (55).

The 1-methylbicyclobutonium ion (56) and the l-(trimethylsilyl)bicyclobutonium ion (57) also undergo fast threefold methylene rearrangements (Scheme 3, R = CH3 or Si(CH3)3). 

Unusual cationic species, e.g. nonclassical bicyclobutonium ions, are discernible in the solvolytic rearrangements, depending on the structure of the substrate.3,4 Computational studies using ab initio at the STO 4-31G level of theory indicate that the molecular symmetry of the unsubstituted parent cation C4H, is a bisected form of the cyclopropylmethyl system.5... 

They have postulated that the solvolysis of 3-substituted cyclobutyl tosylates 12 (X = Alkyl, Ar, Cl, OEt, SiR 3) proceeds through the initial formation of bicyclobutonium ion in the rate determining step, which rearranges stereospecifically to the cyclopropyl-... 

Gas-phase protonation of spiropentane 19 using 3HeT+ or D3+ as protonating agents provided initially the comer protonated spiropentane, which rearranged into 1 -methylcy-clobutyl cation (i.e. bicyclobutonium ion 9)45. Under mild protonating conditions, cis- and trows-2-methylcyclopropylcarbinyl cations 17 and 18 were observed. These cations were presumably formed through the isomerization of the 1-methylcyclobutyl cation 9 into the... 

The parent secondary cyclobutyl cation 47 undergoes threefold degenerate rearrangement viao-bond delocalization involving nonclassical bicyclobutonium ion-like system171,172 (see Section 3.5.2.5). Similar behavior is also observed for the 1-methylcyclobutyl cation 48a.173 176 The 1-phenylcyclobutyl cation 48b, on the other... 

Siehl and co-workers949 950 have used the matrix co-condensation technique to generate the l-(trimethylsilyl)bicyclobutonium ion 528 (Scheme 3.23). The 1H and 13 C NMR spectra of ion 528 show averaged methylene signals, which is in accord with a fast threefold degenerate rearrangement and a puckered hypercoordinate structure. 

No cyclobutanol derivative was formed. It was proposed that the degenerate cyclopropylcarbinyl rearrangement proceeded via the bicyclobutonium ion [127]. 

The parent secondary cyclobutyl cation 38 undergoes immediate rearrangement via o-bond delocalization into the equilibrating non-classical bicyclobutonium ion like system94,95 (see subsequent discussion of non-classical ions). Similar behaviour is also observed for the 1-methylcyclobutyl cation94, 94 98) 39. The 1-phenylcyclo-butyl cation 40 on the other hand is a classical tertiary carbocation94). 

In fact, a cyclopropylcarbinyl ion is formed as the result of the attack by the cationic initiator. This carbo cation may rearrange, through intermediate bicyclobutonium ions, to give cyclobutylium or allylcar-binyl ion. The basic units obtained from such rearrangements have a structure that is either cyclobutenic or allylic. The predominant structures of the polymers are P17a and Pi7b of Equations 28 and 29. 

Summary The l-(trimethylsilyl)bicyclobutonium ion and the 3-e c fe>-(/er/-butyldi-methylsilyl)bicyclobutonium ion were investigated by NMR spectroscopy in superacid solution and by quantum chemical ab initio calculations. The l-(trimethyl-silyl)bicyclobutonium ion undergoes a threefold degenerate methylene rearrangement. The 3-e /o-(/ert-butyldimethylsilyl)bicyclobutonium ion is the first static bicyclo-butonium ion. The NMR spectra of this carbocation are a direct proof for the hypercoordinated and puckered structure of bicyclobutonium ions. 

The averaged H- and C-NMR methylene signals observed for the parent bicyclobutonium ion 1 (Fig. 2,1, R = H) are in accord with a fast methylene rearrangement. 

Fig. 2. Threefold degenerate rearrangement of bicyclobutonium ions via cyclopropylmethyl cation structures.

Fig. 8. C -NMR spectra rearrangement of the l-(tert-butyldimethylsilyl)bicyclobutonium ion 8 to the 3-endo-(tert-...

At -115 °C the first set of signals disappears within 10 minutes and only the peaks for cation 9 remain (Fig. 8 upper spectrum). Structural assignment for cation 9 was confirmed by HC-COSY-and COSY45-NMR spectra shown in Figs. 9 and 10, respectively. Experiments with (3-CD2-labeled progenitors and quantum chemical model calculations of transition states for 1,3-hydride shifts indicate that the rearrangement of the 1-silyl-substituted bicyclobutonium ion 8 to the 3-silyl-sub-stituted bicyclobutonium ion 9 occurs most probably by a 1,3-hydride shift from Cy to C across the bridging bond. 

The rearrangement of the cyclopropylcarbinyl chloride in solution is well known in the literature (//). In polar solvents three products, arisen from the nucleophilic substitution of the solvent to the chloride, are usually detected, which are formed via nucleophilic substitution of chloride by solvent. This chemistry can be explained by the formation of the bicyclobutonium cation (C4H7+), which acts as a tridentated ion, generating the three products shown in scheme 3. 

Rearrangement of the cyclopropylcarbinyl chloride takes place over NaY zeolite, indicative of the formation of the bicyclobutonium cation. Theoretical calculations show that the bicyclobutonium is an intermediate on the zeolite surface and might be in equilibrium with the alkyl-aluminumsilyl oxonium ion. 

The results of cyclopropylcarbinyl chloride rearrangement over NaY impregnated with NaBr suggest that there is an equilibrium between the bicyclobutonium cation and the alkyl-aluminumsilyl oxonium ion, explaining the preferred formation of the allylcarbinyl bromide in the rearranged products. It also suggests that zeolites may act as solid solvents, providing unsymmetrical solvation for the ions inside the cavities. 

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