Protonated cyclobutane

Proton transfer from H3 + and CH5+ to cyclopropane yields a C3H7 + ion, which at atmospheric pressures is largely stabilized by collision (9). This ion reacts as a sec-propyl ion with an added interceptor molecule (9). Hence, the protonated cyclopropane ion undergoes ring opening to acquire the sec-propyl ion structure. Similarly, it has been shown that protonated cyclobutane rearranges to the sec-C4H9 + structure. 

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. 

In terms of hydroisomerization selectivity, it was found that Pt/ZSM-22 exhibits highest selectivity for the conversion of n-alkanes to 2-methyl-branched isomers 308,309). For example, the dibranched isomers from n-decane are particularly rich in 2,7-dimethyloctane. On the other hand, 3-, 4-, and 5-methylnonane isomers are significant even at a low conversion level on Pt/H-USY zeolite. On Pt/H-USY, the composition of the methyl-nonane product fraction approaches thermodynamic equilibrium at medium levels of conversion through methyl shifts. In addition, ethyloctanes are observed as primary products on Pt/H-USY via substituted protonated cyclobutane, but they are absent from the reaction products with Pt/ZSM-5... 

The most probable way of forming benzene is from a Cg oligomer, obtained from C3 condensation, but not through primary carbocations. The critical step would be the formation of the protonated cyclobutane ring, however, the four member ring cycloalkane has been claimed to occur as an intermediate during the isomerization of alkanes on bifunctional zeolite catalysts... 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

A key step in the synthesis in Scheme 13.11 was a cycloaddition between an electron-rich ynamine and the electron-poor enone. The cyclobutane ring was then opened in a process that corresponds to retrosynthetic step 10-IIa 10-IIIa in Scheme 13.10. The crucial step for stereochemical control occurs in Step B. The stereoselectivity of this step results from preferential protonation of the enamine from the less hindered side of the bicyclic intermediate. 

Scheme 2 Mechanism of repair of cyclobutane pyrimidine dimers (CPD) by a CPD photolyase. 8-HDF 8-hydroxy-5-deazaflavin, ET electron transfer. FADH reduced and de-protonated flavin-coenzyme...

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]. 

ET-induced cycloadditions of polycyclic olefins and cycloreversions of cyclobutane species have been studied by ESR spectroscopy [266]. Upon chemical and electrochemical reduction, 2,2 -distyrylbiphenyl rearranges by intramolecular coupling into a bis-benzylic dihydrophenanthrene dianion (Scheme 1), which can be either protonated to a 9,10 -dibenzyl-9,10-dihydrophenanthrene or oxidatively coupled to a cyclobutane species. It is interesting to note that the intramolecular bond... 

The formation of butanes by reduction of arylethenes may arise by radical-radical coupling of two radical-anions giving a dianion, which is then protonated. An alternative route is by nucleophilic addition onto one neutral molecule of the radical-anion, followed by further reduction and protonation. In support of this alternative, cyclobutanes have been isolated from electrochemical reduction of phenylvinylsulphones [21] and vinylpyridines [22], A mechanism for the latter process is illustrated for the case of 2-vinylpyridine 7. Nucleophilic attack of a radical-anion on the substrate gives an intermediate and this disproportionates to form the cyclobutane and a 1,4-diary Ibutane. Cyclobutanes are themselves reduced with ring opening to form the 1,4-diarylbulane. 

Dibromobutane affords 29 % cyclobutane along with butane and butene at a vitreous carbon cathode. Addition of l,l,l,3,3,3-hexafiuoropropan-2-ol as a proton... 

Studies of the stereochemical dependence of 1H chemical shifts in cyclobutanes, benzocyclobute-nes and /J-lactams have been collected463. In a number of cases the protons or proton groups are shielded if they are cis to vicinal halogen, hydroxy or phenyl substituents, due to a ring-current effect. Carbonyl groups, however, can give rise to deshielding effects. 

Figure 15.4. Calculated structures of protonated cyclopropane and cyclobutane.

In contrast to cyclopropane, the cyclobutane C C bonds are only slightly bent, so that in order for the proton to form a bond, it must come close to the positively charged carbon nuclei, leading to increased Coulombic repulsion. Similarly, an attempt to bond one of the carbons does not lead to an ion with any apparent stabilization. This attempt to bond to one of the carbons leads to a relatively unstable... 

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

Figure 9. Protonated cyclopropane and cyclobutane structures calculated using the 6-31G basis set.

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

The exhaustive controlled-potential reduction of 6-chioro-l-phenylhex-l-yne at — 1.57 V in dimethylformamide containing tetrabutylammonium perchlorate gave a mixture of products. among which was ( >(2-phcnylvinyl)cyclobutane (9).11 It is probable that the mechanism involves initial isomerization of the acetylene to an allene 8 which is reduced at — 1.57 V to the radical anion. Protonation and further onc-clectron reduction then yield the allylic anion. An intramolecular nucleophilic substitution eventually gives the cyclobutane.11... 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

The checkers obtained the cyclobutane 1 as a colorless crystalline solid, m.p. 45-47° (a mixture of major and minor isomers), that is relatively free of l,2-dichloro-l,2-dicyanocyclobutane. The product had the following spectral properties proton magnetic resonance (chloroform-d) 8 (multiplicity) 2.35-3.15 (multiplet), 3.40-4.20 (multiplet) l3C magnetic resonance (chloroform-d) 8 major isomer 21.58 (triplet), 38.41 (doublet), 37.24 (triplet), minor isomer 22.48 (triplet), 36.72 (doublet), 36.91 (triplet). 

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