Cyclobutyl cation

Extensive experimental and theoretical work has been devoted to study the nature of cationic intermediates involved in cyclopropylmethyl, cyclobutyl and homoallylic 

Ion 38 (Fig. 16) could be generated both from cyclobutyl and cyclopropylmethyl precursors. At lowest temperatures studied (= 140 °), ion 38 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 169 and the bicyclo-butonium ion 170. 

From the comparison of calculated NMR shifts, the low lying species is considered to be bicyclobutonium ion 1709s However, in the case of CSH9+ 39, the low lying species are the methylbicyclobutonium ions 171 with no contribution from either 

The first cyclopropylmethyl cation to be directly observed was the tricyclo-propylmethyl cation (100) and the subsequent study of a variety of cyclopropylmethyl cations led to the conclusion that the tertiary cations are static and, in the absence of constraining skeletal rigidity, adopt a bisected geometry rather than an eclipsed one (making the ex substituents on the carbenium ion 

It was suggested that the temperature dependence of the chemical shifts in the C NMR spectrum of ion 101 was due to an equilibrium between two or more energetically similar structural isomers of C4H7+ that interconvert rapidly, even at -155°C. Using chemical shift arguments, the major contributing isomer was assigned by Roberts and coworkers to the nonclassical bicyclobutonium structure 104. 2° 

Saunders and SiehP subsequently reported a small, temperature-dependent equilibrium isotope effect in deuterated derivatives of 101 and this lend 

C4H7+ ions were generated by collisionally activated dissociation (CAD) in the gas phase from various precursors. Mass spectrometric analysis showed that homoallyl chloride and cyclopropylmethyl chloride generated primarily cation 103, whereas cyclobutyl chloride gave a substantial amount of bicy-clobutonium ion 104. 

The correspondence in the spectroscopic properties of 102 with those of 101 suggests that the parent ion (101) can also be best presented in the same way. The 1-ethyl and 1-propyl analogs of 102 are similarly nonclassical but rearrange irreversibly upon warming to cycloalkyl cations.  

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This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropyl-methyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 58 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

Bisoxo substitution, 44, 48, 58 Bridged cyclobutyl cations, 146, see also Bicyclobutonium ions... 

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

These results have been interpreted in terms of HOMO-LUMO interactions. As a result of the orbital perturbation, the interaction of the HOMO of the cyclohexene double bond with the LUMO of the developing cation may become effective. At the first stage of this interaction, an overlap of the LUMO of the cyclobutyl cation with the p lobe of the double bond located close to the cation center is probably important. However, when the reaction progresses, the interaction with the p lobe on the remote carbon atom has been assumed to increase significantly. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

The theoretical conclusion that the cyclopropyl-methyl and cyclobutyl cations have similar energies and a low barrier to interconversion has been confirmed by low temperature NMR studies of these ions ... 

Whenever the intermediate cyclobutyl cation has the same degree of substitution as the corresponding cyclopropylmethyl and/or but-3-enyl cation, ring opening with concomitant formation of but-3-enyl compounds can occur. Examples are the formation of mixtures of trans-2-methylcyclobutan-1-ol (6), (E -pent-S-en-l-ol [( )-7], (Z)-pent-3-en-l-ol f(Z)-71 and pent-4-en-2-ol (8) from both 1-cyclopropylmethanol (4)17 and /ra/i,v-2-methylcyclopropancmethanol... 

However, the ring expansion of 1-substituted tertiary cyclopropanemethanols to tertiary fluo-rocyclobutanes is efficiently brought about with pyridinium poly(hydrogenfluoride) in the presence of diisopropylamine and potassium fluoride-hydrogen fluoride.19 2(1 Under the strictly anhydrous conditions employed, the intermediate cyclobutyl cation is efficiently trapped. Illustrative is the rearrangement of methanesulfonate 9, the key step in a synthesis of ( )-2-fluorograndisol.21 Other examples are collected in Table 3. 

The planar cyclobutyl cation and the perpendicular cyclopropylcarbinyl cation are of nearly equal energy and much less stable than the bisected cyclopropylcarbinyl or bicyclobutonium ion (ca 36 kcalmol"1) while the latter two structures have very similar energies. Inclusion of correlation at the MP4SDQ/6-31G //MP2-6-31G levels showed that bicyclobutonium ion is more stable than the bisected cyclopropylcarbinyl cation only by 0.7 kcalmol"1. Selected structural parameters for the bisected and perpendicular conformers (6 and 7) of the cyclopropylcarbinyl cation as well as the bicyclobutonium ion 2, as calculated at the MP2/6-31G level, are as shown26. 

From solvolytic studies of iso topically labeled substrates it was shown that cyclopropyl-carbinyl-cyclobutyl interconversion is stereospecific51 52. The stereospecific interconversion of cyclobutyl cations to the corresponding cyclopropylcarbinyl cation was also cleanly observed in superacid medium, and was used to prepare otherwise unstable cis-(a-methylcyclopropyl)carbinyl cation 1753. Thus ionization of d.s-2-chloro- or cw-3-chloro-l-methylcyclobutane in SbF5-S02ClF at -135 °C yielded the ris-isomer which rapidly rearranged irreversibly into the trans-isomer 18 at about -100 °C. The trows-isomer 18 is the only cation obtained when the preparation was carried out at -80 °C, or when prepared from the cyclopropylmethyl carbinol20b 38 50ac (equation 24). 

No reactions of t with protic solvents have been reported however, its cyclic analogue 1,2-diphenylcyclobutene (7) reacts with the protic solvents methanol, acetic acid, and water, to yield adducts 85 and 86 (eq. 28). The proposed mechanism for the formation of 85 and 86 involves the formation of singlet exciplex followed by proton transfer to yield a cyclobutyl cation 87. Stereoselective nucleophilic capture of 87 by solvent from its less hindered side yields 85, while skeletal rearrangement of 87 yields the cyclopropylmethyl cation 88, which reacts with solvent to yield 86 ... 

On the other hand, there is now a good deal of evidence that the solvolysis of most cyclobutyl derivatives does lead directly to the cyclopropylcarbinyl cation. For example, orbital symmetry considerations (Section 11.3) indicate that the conversion of cyclobutyl cations into cyclopropylcarbinyl cations should occur by disrotatory ring opening as shown in Figure 6.11 but any steric factors that would hinder such a process decelerate most cyclobutyl solvolyses. Thus 86 See note 84(b). 

Cyclobutyl cations certainly do exist if they are especially stabilized. For example, 1-phenylcyclobutyl cation shows no tendency to rearrange in superacid solution.89... 

Figure 6.11 Orbital symmetry allowed (disrotatory) opening of a cyclobutyl cation. Note that the orbitals of the C—C bond being broken overlap with the back side of the orbital used for bonding to the departing group.

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... 

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