Cation cyclobutanations

The unusual nature of the cyclopropyl carbinyl cation allows yet another mode of attack to form cyclobutane products. Because this mode of attack releases little strain, normally some special structural features are required to direct the reaction along this pathway. 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

A cation radical chain cycloaddition-polymerization catalysed by tris(4-bromophenyl)aminium hexachloroantimonate has been reported to afford polymers with an average molecular weight up to 150000. Both cyclobutanation and Diels-Alder polymers were obtained. " The reactivity of the phospine radical cation towards nucleophiles was studied. Tributylphosphine reacted with l,l-dimethyl-4,4-bipyridinium (methyl viologen, MV) in the presence of an alcohol or thiol (RXH X = O, S), which resulted in the gradual formation of the one-electron reduced form... 

The sodium cation chelation by the bis(enone) anion-radicals shown in Scheme 3.52 controls their further transformations although they proceed at the expense of other reaction centers (Yang et al. 2004). This kind of intramolecular cyclobutanation is characterized with the pronounced cis-stereoselectivity. However, this stereoselectivity disappears if the reaction proceeds in the presence of the tetrabutylammonium cation, when such a chelation is impossible. 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

Detailed mechanistic information concerning an intramolecular arylalkene cycloaddition yielding cyclobutane derivatives via a radical cation process gener-... 

Cleavage of a C—C bond gives a distonic radical cation as an intermediate, while concerted cleavage of two C—C bonds yields the corresponding ArO and ArO in cycloreversion of aryl-substituted cyclobutane Therefore, the cycloreversion mechanism is related to dimerization of ArO where tt- and a-dimers are detected during PR of ArO such as... 

The case of cyclobutane radical cation presents another interesting structural problem. The parent has a puckered ring with symmetry ET from one of its e orbitals leads to a IT unstable radical cation, which distorts to strucmres of Dy ... 

Figure 6.12. Possible structure types of cyclobutane radical cations.

Cyclobutanes disubstituted in the 1,2-positions should favor strucmre-type C or a related distonic structure with one broken C—C bond. Calculations [QCISD-(T)/ 6-31G //UMP2/6-31G ] suggest a trapezoidal structure for frawi-1,2-dimethyl-cyclobutane radical cation.This expectation is born out by experimental results such as the ET induced cis-trans-isomerization of 1,2-diaryloxycyclobutane (Ar = aryl), leading to IS " ", and likely involving the distonic radical cation (14 +) formed via a type C ion. ... 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

However, ab initio calculations [QCISD-(T)/6-31G //UMP2/6-31G ] on ethylene and its radical cation support an anti -7i-complex, in which the two components are joined by one long bond (190 pm), rather than the sandwich -type 71 complex. The complex is connected to two transition states leading to a (rhombic) cyclobutane radical cation (see above) or, by 1,3-H-shift, to 1-butene radical... 

Structures of protonated cyclobutanes have been studied in the same fashion (see Figure 9 B). In the corner-protonated cyclobutane, the structure corresponds essentially to a methyl cation interacting with a trimethylene diyl, and is much less favorable than that for cyclopropane. Similarly, for the edge-protonated ion, the proton must come much closer to the carbons to form a bond than for cyclopropane, and as a result, cyclobutane is much less basic. 

The energies for transferring a proton from isopropyl cation to cyclopropane and cyclobutane are ... 

The calculated energies again confirm that cyclopropane is much more easily attacked by electrophiles than is cyclobutane, and this accounts for the common observation that cyclobutanes are much less reactive toward electrophiles than are cyclopropanes, despite the similar strain energy relief for these compounds.55 The reactions of cyclopropane with other electrophiles, such as mercuric ion,65 and metal radical cations,67 have also been studied. 

The interaction with an adjacent cationic center has a similar character and, here again, cyclopropane has been found to be much more effective than cyclobutane. This may be seen in a comparison of the cyclopropyldimethylmethyl ion 11 with the corresponding cyclobutyl-methyl ion 12.6 8... 

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