Organic Reactions in the Gas Phase. Part I: Decomposition of Isomeric Cis 6,7-dimethyl-2,3-diazabicyclo[3.2.0]hept-2-enes in the Gas Phase. Part II: The Wall-Less Reactor. Part III: Thermal Isomerization of Cyclopropenes

Citation

Keppel, Robert Andrew (1975) Organic Reactions in the Gas Phase. Part I: Decomposition of Isomeric Cis 6,7-dimethyl-2,3-diazabicyclo[3.2.0]hept-2-enes in the Gas Phase. Part II: The Wall-Less Reactor. Part III: Thermal Isomerization of Cyclopropenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9ske-ad80. https://resolver.caltech.edu/CaltechTHESIS:09042024-184628054

Abstract

Part I: Decomposition of Isomeric Cis 6,7-dimethyl-2,3-diazabicyclo[3.2.0]hept-2-enes in the Gas Phase

The thermal decomposition of the isomeric cis 6,7-dimethyl-2,3-diazabicyclo[3.2.0]hept-2enes was studied. In order to account for the products observed, dual pathways for the decomposition are proposed. The formation of isomeric cis 2,3-dimethyl-bicyclo[2.1.0] pentanes is explained by a mechanism involving a diradical intermediate. The formation of 2,3-dimethylpenta-1,4-dienes is best explained by a stereospecific retro-1,3-dipolar cleavage to an acyclic diazo compound which loses nitrogen to form a carbene. The absence of a thermal "Di-π-methane pathway is indicated from the products.

Part II: The Wall-less Reactor

A wall-less reactor was constructed based on one designed previously at Kent State. The reactor was modified and used to study the unimolecular decomposition of strained hydrocarbons. Cyclopropane was found to isomerize to propylene with a rate law of log k = 14.6 -65.1 kcal per mole/θ over the temperature range of 936 to 1026°K. The wall-less reactor was also used to study the isomerization of 3,3-dimethylcyclopropene (see abstract of Part III).

Part III: Thermal Isomerization of Cyclopropenes

The thermal rearrangement of 3,3-dimethylcyclopropene (6) was studied in the wall-less reactor over a temperature range of 608 to 668°K. The rate law for the isomerization was found to be log k = 12.6 -36.2 kcal per mole/θ. 6 isomerized to 86.2% 3-methyl-1-butyne, 12.9% 1-methyl-1,3-butadiene and 0.9% 3-methyl-1,2-butadiene. The data were in accord with published results of the isomerization of 6 in a static thermal reactor.

The thermal rearrangement of 1-methylcyclopropene (14) was investigated by entering the energy surface for the reaction by two routes: thermal isomerization of the cyclopropene and pyrolysis of a vinylcarbene precursor, 3-methyl-3-vinyldiazirine (12). The acyclic C₄H₆ products from both reactions are identical, however the proportions vary greatly. Greater than 99% of the product from the diazirine is the cyclopropene, and the remainder of the products is 1,3-butadiene, 2-butyne and 1,2-butadiene in the ratio of 1:0.6:0.2, respectively. The products from the isomerization of 1-methyl-cyclopropene are the same but in the ratio of 5:93:2, respectively. From the data it is concluded that two intermediates exist for the isomerization, a planar vinylcarbene and a 90° rotamer, designated a diradical.

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