Dominant double rotation in the thermally induced 1,2,4-trimethylspiropentane geometric isomerization

Gajewski, Joseph J.; Weber, Robert J.; Chang, Ming Jing

doi:10.1021/ja00502a027

Cited by 6 publications

(5 citation statements)

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“…Nevertheless, at the CASPT2 level of theory, conrotation is the preferred pathway for ring opening of cis- 1,2-dimethylspiropentane ( 4 ) and of the trans isomer ( 7 ). The larger computed preference for double rotation over monorotatory ring opening in 4 than in 7 is in agreement with the experimental results of G&C 5a…”

Section: Discussionsupporting

confidence: 89%

“…Our finding that the s-cis , s-trans -(0,0) diradical conformation ( 6 ) is lower in energy than the s-trans , s-trans -(0,0) conformation ( 5 ) has profound implications for interpreting the experiments of G&C 5a. As discussed above, these experiments found that the ratio of double to single rotation is higher in 4 than in 7 , and G&C correctly interpreted this finding as being due to 4 undergoing ring opening to the more stable (0,0) diradical conformation.…”

Section: Resultsmentioning

confidence: 88%

“…We have also investigated why double rotation of C-1 and C-2 in the trans -1,2,4-trimethylspiropentanes is slower than epimerization at C-4. Herein we report the results of our calculations, which lead to a reinterpretation of the experimental results of G&C 5a…”

mentioning

confidence: 66%

“…Gajewski and Chang (G&C) investigated the pyrolyses of syn -4,4- 2 H 2 - cis -1,2-dimethylspiropentane ( 4- 2 H 2 ) and of optically active trans -1,2-dimethylspiropentane ( 7 ) at 290 °C 5a…”

mentioning

confidence: 99%

“…Nevertheless, when G&C reinvestigated the thermal isomerizations of the four 1,2,4-trimethylspiropentanes,5b using optically active trans compounds,5a they again found that, as in 4 and 7 , the ratio of double to single rotation is about a factor of 3 larger in each of the two cis stereoisomers than in each of the two trans compounds. This result was interpreted as additional evidence for the conclusion, drawn from the study of dimethylspiropentanes 4 and 7 , that disrotation is preferred to conrotation in spiropentane stereomutation reactions.…”

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confidence: 99%

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Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

1999

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The potential energy surfaces for stereomutation of spiropentane (1) and cis-and trans-1,2-dimethylspiropentanes (4 and 7) have been explored, using ab initio methods. In diradicals 2, 5, and 6, which are formed by cleavage of the peripheral bond between C-1 and C-2 in the spiropentanes, the weakly electrondonating cyclopropane ring results in the lowest energy pathway for stereomutation of all three being computed to be conrotatory. A larger preference for double over single rotation is computed in 4 than in 7, in agreement with the experimental results reported in 1980 by Gajewski and Chang. However, in contrast to the assumption made by these authors, our calculations find that the s-cis-methyl conformation in diradical 6 is lower in energy than the s-trans-methyl conformation in diradical 5, and moreover, 6 is statistically favored over 5 by a factor of 2. Thus, double rotation is both computed and found to be preferred by more in the stereomutation of 4 than of 7 because 4 undergoes conrotatory opening to 6, the lower energy diradical. A long-range attraction between the s-cis methyl group at C-1 and the nonbonding p-π AO at C-3 in 6 is shown to contribute to stabilizing this diradical.The first study of the stereomutation of spiropentane (1) was published 30 years ago. 1 Gilbert reported that, upon heating, cis-1,2-2 H 2 -spiropentane isomerizes to trans-1,2-2 H 2 -spiropentane, prior to undergoing structural rearrangement. 2 Subsequently, Gajewski and Burka pyrolyzed the proximal, medial, and distal stereoisomers of 1,4-dimethylspiropentanes, 3 and from the kinetics of interconversion that they observed, they concluded that fission of a peripheral bond, to form diradical 2, is preferred to cleavage of a radial bond, to form diradical 3. Stereomutation of 1 by exclusive formation of 1,1-dimethylenecyclopropane (2) is supported by the results of additional experiments by Gajewski. 4 Gajewski and Chang (G&C) investigated the pyrolyses of syn-4,4-2 H 2 -cis-1,2-dimethylspiropentane (4-2 H 2 ) and of optically active trans-1,2-dimethylspiropentane (7) at 290°C. 5a Coupled rotation was found to be slightly favored over monorotation in both spiropentanes.G&C based their assignment of the mode of coupled rotation that is preferred on the experimental finding that double rotation is favored over monorotation by a factor of 3.6 in the pyrolysis of 4-2 H 2 and by a factor of only 1.4 in the pyrolysis of 7. On the basis of the experimental results in the pyrolyses of other hydrocarbons, G&C assumed that "outward" rotation of a methyl group in 4 and 7 is sterically less demanding than "inward" rotation. Therefore, they expected formation of diradical 5 to provide a lower energy pathway for double rotation than formation of diradical 6. This assumption, together with the modes of coupled rotation that connect 4 and 7 to 5 and 6 (shown in Figure 1), resulted in G&C interpreting the experimental ratios of single to double rotation in 4 and 7 as indicating that disrotation is preferred to conrotation in the stereomutati...

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Section: Discussionsupporting

confidence: 89%

Section: Resultsmentioning

confidence: 88%

mentioning

confidence: 66%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

1999

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Reaktionen von Cyclopropylsulfonaten mit Nucleophilen: S _N 2‐Reaktionen an Cyclopropanen unter Inversion

Banert

1985

Chem. Ber.

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Die Umsetzung von Cyclopropyl-trifluormethansulfonat (3) rnit Tributylhexadecylphosphoniumazid (QN3) in aprotischen Lijsungsmitteln lieferte nahezu quantitativ in einer Reaktion 2. Ordnung Cyclopropylazid (6 Reactions of Cyclopropyl Sulfonates with Nucleophiles: S, 2 Displacements at Cyclopropanes with InversionTreatment of cyclopropyl trifluoromethanesulfonate (3) with tributylhexadecylphosphonium azide (QN3) in aprotic solvents almost quantitatively afforded cyclopropyl azide (6) in a secondorder reaction. Nucleophilic displacements with ring preservation were also achieved using QCN or QBr. The analogous reactions of the epimeric 2-methylcyclopropyl trifluoromethanesulfonates (16, 17) with QN, highly stereospecifically yielded the 2-methylcyclopropyl azides (24,25) with inversion in addition to the allylic azides 27, 28, and 29. The product fraction of cyclic azides decreased with growing steric hindrance in the order 3 > 16 > 17. The experimental results show that SN2 displacements take place with clean inversion at the cyclopropane carbon atom. SN2-Reaktionen an Cyclopropanen erwecken besonderes Interesse, seitdem MO-Berechnungen auf die Mdglichkeit einer retentiven Substitution an kleinen Ringen hinweisen2). Die experimentelle Uberpriifung wird zum einen durch die geringe Reaktivitat von Cyclopropylverbindungen3) ('5-strain"4)) erschwert; zum anderen md3 die S~z-ReaktiOn rnit der disrotativen, synchronens) Ringijffnung zum Allylkation konkurrierens-7). Untersuchungen fur Cyclopropylhalogenide liegen bisher nur fur intramolekulare SN2-Reaktionen an Bicyclen und Propellanen vors,9). Die hierbei beobachtete Inversion widerlegt aber keinesfalls das Konzept der retentiven SN2-Reaktion. Durch das starre Geriist der Substrate wird die Angriffsrichtung des intramolekularen Nucleophils festgelegt. Seine Annaherung in der Dreiringebene unter retentiver Substitution ist ausgeschlossen. i n der vorliegenden Arbeit werden reaktivere (monocyclische) Cyclopropylsulfonate rnit Nucleophilen umgesetzt. Durch die Wahl der Reaktionsbedingungen sollte die Ringoffnung zu allylischen Produkten zuruckgedrangt werden, so daR die Stereochemie der SN2-Reaktion an Cyclopropanen untersncht werden kann.

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ChemInform Abstract: DOMINANT DOUBLE ROTATION IN THE THERMALLY INDUCED 1,2,4‐TRIMETHYLSPIROPENTANE GEOMETRIC ISOMERIZATION

Gajewski¹,

Weber²,

Chang³

1979

Chemischer Informationsdienst

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Dominant double rotation in the thermally induced 1,2,4-trimethylspiropentane geometric isomerization

Cited by 6 publications

References 4 publications

Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

Reaktionen von Cyclopropylsulfonaten mit Nucleophilen: S _N 2‐Reaktionen an Cyclopropanen unter Inversion

ChemInform Abstract: DOMINANT DOUBLE ROTATION IN THE THERMALLY INDUCED 1,2,4‐TRIMETHYLSPIROPENTANE GEOMETRIC ISOMERIZATION

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Dominant double rotation in the thermally induced 1,2,4-trimethylspiropentane geometric isomerization

Cited by 6 publications

References 4 publications

Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

Ab Initio Calculations on Spiropentane Stereomutations Lead to a Reinterpretation of the Experimental Results

Reaktionen von Cyclopropylsulfonaten mit Nucleophilen: S N 2‐Reaktionen an Cyclopropanen unter Inversion

ChemInform Abstract: DOMINANT DOUBLE ROTATION IN THE THERMALLY INDUCED 1,2,4‐TRIMETHYLSPIROPENTANE GEOMETRIC ISOMERIZATION

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Reaktionen von Cyclopropylsulfonaten mit Nucleophilen: S _N 2‐Reaktionen an Cyclopropanen unter Inversion