Cyclopentenes from pyrolysis

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

Concern for the stereochemistry of the 1,3-shift portion of the reaction, i.e. the pathway for formation of cyclopentene, was addressed by examination of numerous substituted materials, but ultimate resolution of the problem was provided by pyrolysis of 5 y/2- -2,3,2 -trideuteriovinylcyclopropane at 300°C to roughly only 1 % conversion, which resulted in a 16 60 24 ratio of Z,Z-, E,Z, and -3,4,5-trideuteriocyclopentene, respectively. Pyrolysis of 5 y/2-Z-2,3,2 -trideuteriovinyl-cyclopropane to obtain roughly 1% of cyclopentene product resulted in a 23 41 36 ratio of Z,Z-, E,Z-, and , -3,4,5-trideuteriocyclopentene, respectively (Scheme 6.15). The Z,Z- and E,E- products from syn-E are formed by the ar and ai pathways, respectively, and these same products from syn-Z are formed by the sr and si pathways, respectively, thus providing data on the relative utilization of the four pathways. 

In an effort to determine the stereochemistry of the 1,3-shift of what at the time was thought to be a minimally perturbed vinylcyclopropane system, which also would not suffer from extensive geometric isomerization via an s-trans alkylallylic species, Olson prepared and pyrolyzed c 5-2,3-dideuterio-fran5 -l-(Z-2-deuterio-l- r -buty-lethenyl)cyclopropane." The H NMR of the epoxide of the major cyclopentene product from short-term pyrolysis revealed greater than 90% suprafacial inversion stereochemistry in the rearrangement despite the fact that the stereochemistry of the starting material was also compromised in a faster reaction, apparently via reversible formation of an s-cis alkylallylic species (Scheme 6.19). 

Next, in order to learn more about the rates of dehydrogenation of cyclohexenes resulting from Diels-Alder reactions between butadiene and olefins, VCH, HCH and MCH were earlier subjected to thermal reactions at 530- 665 C ( ). The main reactions in these cases were reverse Diels-Alder reactions and dehydrogenations. Dehydrogenations which are related to the productions of cyclohexa-diene and benzene homologues were 1 10 in selectivity as compared to that of the reverse Diels-Alder reaction. An interesting observation related to cyclic compound formation is that, in the case of MCH pyrolysis, cyclohexadiene and cyclopentene are formed at almost the same rates as butadiene and propylene. So that, in this case, about 60% of MCH is employed in the formation of cyclic compounds. 

Products from the pyrolysis of cellulose in the absence of a catalyst were found to be mainly derivatives of 1,6-anhydro-D-glucoseand other, unsaturated, products. One product, formed in 1.4% yield, was found to be 1,5-anhydro-4-deoxy-D- /yc ro-hex-l-en-3-ulose. This product was also formed during the pyrolysis of amylose, amylopectin, and laminarin. A further product from cellulose, probably formed as an oxidation product, was 3,5-dihydroxy-2-methyl-4/f-pyran-4-one. Sixty-three compounds were detected in the condensate of smoke produced by pyrolysis of cellulose. These compounds included furans, cyclic ketones, lactones, benzene derivatives, aliphatic ketones, and aldehydes. One of the major products was 2-hydroxy-3-methyl-cyclopenten-l-one. 

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