Cyclobutanation rates

A further improvement in platinum catalysis is claimed from use of tin(Il) haUde and phosphine ligands which are rigid bidentates, eg, l,2-bis(diphenylphosphinomethyl)cyclobutane (27). High rates for a product containing 99% linear aldehyde have been obtained. However, a pressure of 10 MPa (1450 psi) H2 CO is requited. 

The spontaneous polymerization of styrene was studied in the presence of various acid catalysts (123) to see if the postulated reactive intermediate DH could be intentionally aromatized to form inactive DA. The results showed that the rate of polymerization of styrene is significantly retarded by acids, eg, camphorsulfonic acid, accompanied by increases in the formation of DA. This finding gave further confirmation of the intermediacy of DH because acids would have Httie effect on the cyclobutane dimer intermediate in the Flory mechanism. 

Fluorinaied dienophiles. Although ethylene reacts with butadiene to give a 99 98% yield of a Diels-Alder adduct [63], tetrattuoroethylene and 1,1-dichloro-2,2-difluoroethylene prefer to react with 1,3-butadiene via a [2+2] pathway to form almost exclusively cyclobutane adducts [61, 64] (equation 61). This obvious difference in the behavior of hydrocarbon ethylenes and fluorocarbon ethylenes is believed to result not from a lack of reactivity of the latter species toward [2+4] cycloadditions but rather from the fact that the rate of nonconcerted cyclobutane formation is greatly enhanced [65]... 

Thermal cycloadditions of butadiene to 3-bromo- 133 and 3-methoxy-5-methylene-2(5//)-furanones 220 were studied (95TL749). These systems contain substituents at C3 capable of stabilizing also a possible radical intermediate, influencing hereby the rate and/or the course of the reaction. Thus, the reaction of 133 and 220, respectively, with butadiene at 155°C afforded mixtures of the expected 1,4-cycloadducts 221 and 222, respectively, and of the cyclobutane derivatives... 

From the observed drop in yield, one can calculate the rate of Reaction 10 relative to hydride transfer with cyclobutane (on the rate scale used in Table I) as 1.55 for the ethyl ion and 0.83 for the propyl ion. 

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 strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

C15-0028. Estimate the rate constant for the decomposition of cyclobutane at 25 °C (see Extra Practice Exercise). 

It is noteworthy to point out that the rate of retro-aldol reaction is enhanced by ring strain. Thus, suitably substituted 3-oxygenated carbonyl cyclobutanes would... 

The thermal decomposition of cyclobutane to yield ethylene has been very extensively investigated (Genaux and Walters, 1951 Kem and Walters, 1952, 1953). The reaction is homogeneous and kinetically first order. Addition of inhibitors to the reactant does not affect the rate, and... 

The relative percentage yields of products at 135-7°C are (A) ll-l, (B) 1-3, (C) 45-7, (D) 32-1, (E) 9-8. The yield of isoprene has been scaled down by a factor of 2 since the decomposition of one molecule of the cyclobutane yields two molecules of isoprene. The products and rates of reaction of this cyclobutane and the previous one discussed are both consistent with the reaction paths already mentioned. 

The large difference in reactivity between cyclopropanes and cyclobutanes toward most electrophiles arises because there is little strain relief in the rate-determining step. The cleavage of C-C bonds by transition metal species leads to metallocy-cloalkanes, and if cleavage proceeds to a significant extent in the rate-determining step, the reactivities of cyclopropanes and cyclobutanes should become more comparable. 

It has been shown that the cyclization step of 3-halopropylmalonic esters to give cyclobutane-1,1-dicarboxylatcs, e.g. 3. is rate-determining.4 A convenient synthesis of cyclobutyl derivatives via hydroboration has been reported.5 In this case, 4-tosyloxybut-1-yne was bishydroborated with 9-borabicyclo[3.3.1]nonane (9-BBN), and this was followed by treatment with methyllithi-um to give 9-cyclobutyl-9-BBN 4 in 65% yield.5 24... 

H6]dimethylcyclobulane (4) was pyrolyzed at 338 401 °C under initial pressures of 10 and 30 Torr. The results arc inconclusive, showing that there is less than 4% racemization and no geometric isomerization in the recovered cyclobutane after its 11.7% cleavage to 2-methyl-propene.74 However, it appears that the ratio of the rate constants for the thermal cracking to geometric isomerization is a function of substituent size, as evidenced by the fact that dimethyl... 

Finally, the relative rates of cleavage and rotation of 1,4-diradicals have been directly studied by thermal decomposition of the tetrahydropyridazines 9 and 10 (R = Me or D) at 415 "C.85-87 Judging from the rate constants and product distributions obtained for various processes, it is likely that the fate of the diradicals 11 and 12 is identical to those generated by thermolysis of cyclobutanes.85 - 87 Obviously, a choice in favor of the nonconcerted diradical pathway can, therefore, be made on the basis of the aforementioned theoretical as well as experimental endeavors. 

For example, acetolysis of r/n /-6-tosyloxytricyclo[5.2.0.02 5]nona-3.8-diene proceeded smoothly at 35 C with stereospeeific rearrangement to c.Y0..mj-9-acetoxylricyclo[4.2.1,02 5]nona-3,7-diene (l).30 Interestingly, the rate of acetolysis of the substrate was considerably enhanced kre[25 C = 6.8 x 104) as compared with that of its bicyclic counterpart 2.30 An important conclusion from these studies is that the anchinteric assistances by cyclobutene in the form of homoallylic participation is effective in the stabilization of the carbocation intermediate. It was found in another study that cyclobutene is better than cyclobutane in terms of anchimeric assistance.31... 

In the cyclobutane series67 syn and anti eliminations from cis- and rans-l//,2//-hexafluorocy-clobutanes 8 again proceed at rates much more comparable than syn and anti eliminations from cyclohexane systems to give products 9 and 10. 

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