Title

Syntheses, thermal stereomutations, and (1,3)-sigmatropic isomerizations of cis-(1S,2R)- and trans-(1S,2S)-1-(E)-propenyl-2-methylcyclobutanes

Date of Award

2001

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

John E. Baldwin

Keywords

Sigmatropic isomerizations, Stereomutations, Propenyl-2-methylcyclobutane, Dimethylcyclohexene

Subject Categories

Chemistry | Organic Chemistry | Physical Sciences and Mathematics

Abstract

Samples of trans -3,4-dimethylcyclohexene ( (±)-12 ), cis -3,4-dimethylcyclo-hexene ( (±)-13 ), trans -3,6-dimethylcyclohexene ( (±)-59 ), cis -3,6-dimethylcyclo-hexene ( 60 ), trans -1-( E )-propenyl-2-methylcyclobutane ( (±)-10 ), cis -1-( E )-propenyl-2-methylcyclobutane ( (±)-11 ), trans -1,5-octadiene ( 40 ); cis -1,5-octadiene ( 39 ), 3-ethyl-1,5-hexadiene ( 41 ), and trans -1,6-octadiene ( 80 ) were synthesized and their structures have been correlated with gas chromatographic peak elution order using standard columns. Racemic samples of trans -1-(E)-propenyl-2-methylcyclobutane ((±)-10) and cis -1-(E)-propenyl-2-methylcyclobutane ( (±)-11 ) were heated at 275°C and their thermal stereomutations, fragmentations, and isomerizations to dimethylcyclohexenes were studied. From (±)-10 the ratio of trans -3,4-dimethylcyclohexene ( (±)-12 , ( k si + k ar ), "allowed") to cis -3,4-dimethylcyclohexene ( (±)-13 , ( k sr + k ai ), "forbidden") was determined to be 64:36. From (±)-11 the ratio of trans -3,4-dimethylcyclohexene ( (±)-12 , ( k sr + k ai ), "forbidden") to cis -3,4dimethylcyclohexene ( (±)-13 , ( k si + k ar ), "allowed") was determined to be 71:29.

Samples of trans -(3 S ,4 R )-dimethylcyclohexene ( (-)-(3 S ,4 R )-12) , cis -(3 R ,4 R )-dimethylcyclohexene ( (+)-(3 R ,4 R )-13) , trans -(3 R ,6 R )-dimethylcyclohexene ( (+)(3 R ,6 R )-59) , (1 S ,2 S )- trans -2-methylcyclobutanemethanol ( (+)-(1 S ,2 S )-47) , and (1 S ,2 R )- cis -2-methylcyclobutanemethanol ( (-)(1 S ,2 R )-48) were synthesized and the samples were used to correlate absolute stereochemistry with gas chromatographic peak elution order using columns with chiral stationary phases. Samples of trans -(1 S ,2 S )-1-( E )-propenyl-2-methylcyclobutane ( (+)-(1 S ,2 S )-10) and cis -(1 S ,2 R )-1-( E )-propenyl-2-methylcyclobutane ( (-)-(1 S ,2 R )-11) were synthesized and their thermal stereomutations, fragmentations, and isomerizations to dimethylcyclohexenes were studied at 275°C. From (+)-(1 S ,2 S )-10 the relative importance of the four distinct reaction paths were determined to be 58% si , 5% ar , 33% sr , and 4% ai . From (-)-(1 S ,2 R )-11 the relative importance of the four distinct reaction paths were determined to be 18% si , 11% ar , 51% sr , and 20% ai . These results show that the vinylcyclobutane rearrangements of (+)-(1 S ,2 S )-10 and (-)-(1 S ,2 R )-11 are not under orbital symmetry control. These vinylcyclobutane rearrangements most likely proceed through a short-lived conformationally flexible diradical traversing relatively flat transition regions under dynamic control much like the vinylcyclopropane rearrangement.

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