Abstract:Bicyclo[3.1.0]hex-3-en-2-yl Cation1 Sir:Recently developed synthetic procedures2 for specific introduction of deuterium at the 2-and 6-endo positions of the parent bicyclo[3.1.0]hex-3-en-2-yl cation 1 make possible the study of the properties of this species by nuclear magnetic resonance (nmr) spectroscopy of Fellow, 1967Fellow, -1970 (5-FG1-GM-38,024-02).
“…These conclusions were further supported by Berson and Hasty from the photolysis of benzene at 254 nm in D 2 O (0.1 N D 3 PO 4 ), eq 3 . Analysis of the 1 H NMR of the isolated alcohol, 14 , indicated that the deuterium at C6 was exclusively endo.…”
Section: Introductionmentioning
confidence: 61%
“…The stereochemical assignments are made on the basis of coupling constants. When the cyano group is exo at C6 ( 22 and 23 ), H6 appears as a “triplet” with J 16 and J 56 in the range of 3−3.7 Hz, values consistent with a trans relationship to the hydrogens at C1 and C5; for endo cyano at C6 ( 24 and 25 ), H6 still appears as a triplet but with J 16 and J 56 ∼7.5 Hz, consistent with a cis relationship to H1 and H5. , Finally, when the TFE group is endo ( 22 and 25 ), J 12 is ∼6 Hz but when it is exo, J 12 approaches zero. ,, 1 …”
Section: Resultsmentioning
confidence: 81%
“…Rapid equilibration of bicylo[3.1.0]hex-3-en-2-yl cations has been observed previously. For instance, the heptamethyl compound has a free energy of activation for rearrangement 11 of only 9 kcal/mol at − 89 °C and even the barrier for the unsubstituted cation is quite small, 19.8 kcal/mol at −20 °C …”
Section: Resultsmentioning
confidence: 99%
“…As well, irradiation of benzene in acetic acid gave all four possible regio- and stereoisomers of the bicyclo[3.1.0]hex-3-en-2-yl addition products, 15 − 18 , R = acetyl, but the composition varied with percent conversion. In the earliest sample analyzed (after a 2-h irradiation), the ratio was approximately 40:41:4:16 (estimated from a plot in ref ). Assuming rapid endo to exo ground-state conversion of the products, i.e., 16 R = acetyl to 15 R = acetyl, the major primary photoproduct is the endo adduct.…”
Irradiation of benzonitrile in 2,2,2-trifluoroethanol (TFE) with 254-nm light from low-pressure mercury
lamps results in the formation of four addition products, the stereoisomers of 6-cyano-2-(2,2,2-trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22−25. The proposed mechanism begins by formation of the
6-cyanobicyclo[3.1.0]hex-3-en-2,6-diyl biradical/zwitterion from S1 followed by both endo and exo protonation
by TFE at C6. Deuterium labeling demonstrated that the resulting 6-cyanobicyclo[3.1.0]hex-3-en-2-yl cation
underwent a rapid degenerate 1,4-sigmatropic rearrangement with inversion of configuration at the migrating
carbon before being trapped by the nucleophilic solvent. Irradiation of p- and m-methylbenzonitrile in the
same way gave six major addition products. Three of them, 32, 33, and 35 were 6-cyano-2-(2,2,2-trifluoroethoxy)-4-methylbicyclo[3.1.0]hex-3-enes and the other three, 32, 33, and 35, were 6-cyano-2-(2,2,2-trifluoroethoxy)-2-methylbicyclo[3.1.0]hex-3-enes. The proposed mechanism is again by TFE endo and exo
protonation of the first formed biradical/zwitterion followed by trapping of the cations by the solvent. The
1,4-sigmatropic rearrangement of the cations now stops at the most stable structures, the endo- and exo-6-cyano-2-methylbicyclo[3.1.0]hex-3-en-2-yl cations, and all of the products are derived from them.
“…These conclusions were further supported by Berson and Hasty from the photolysis of benzene at 254 nm in D 2 O (0.1 N D 3 PO 4 ), eq 3 . Analysis of the 1 H NMR of the isolated alcohol, 14 , indicated that the deuterium at C6 was exclusively endo.…”
Section: Introductionmentioning
confidence: 61%
“…The stereochemical assignments are made on the basis of coupling constants. When the cyano group is exo at C6 ( 22 and 23 ), H6 appears as a “triplet” with J 16 and J 56 in the range of 3−3.7 Hz, values consistent with a trans relationship to the hydrogens at C1 and C5; for endo cyano at C6 ( 24 and 25 ), H6 still appears as a triplet but with J 16 and J 56 ∼7.5 Hz, consistent with a cis relationship to H1 and H5. , Finally, when the TFE group is endo ( 22 and 25 ), J 12 is ∼6 Hz but when it is exo, J 12 approaches zero. ,, 1 …”
Section: Resultsmentioning
confidence: 81%
“…Rapid equilibration of bicylo[3.1.0]hex-3-en-2-yl cations has been observed previously. For instance, the heptamethyl compound has a free energy of activation for rearrangement 11 of only 9 kcal/mol at − 89 °C and even the barrier for the unsubstituted cation is quite small, 19.8 kcal/mol at −20 °C …”
Section: Resultsmentioning
confidence: 99%
“…As well, irradiation of benzene in acetic acid gave all four possible regio- and stereoisomers of the bicyclo[3.1.0]hex-3-en-2-yl addition products, 15 − 18 , R = acetyl, but the composition varied with percent conversion. In the earliest sample analyzed (after a 2-h irradiation), the ratio was approximately 40:41:4:16 (estimated from a plot in ref ). Assuming rapid endo to exo ground-state conversion of the products, i.e., 16 R = acetyl to 15 R = acetyl, the major primary photoproduct is the endo adduct.…”
Irradiation of benzonitrile in 2,2,2-trifluoroethanol (TFE) with 254-nm light from low-pressure mercury
lamps results in the formation of four addition products, the stereoisomers of 6-cyano-2-(2,2,2-trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22−25. The proposed mechanism begins by formation of the
6-cyanobicyclo[3.1.0]hex-3-en-2,6-diyl biradical/zwitterion from S1 followed by both endo and exo protonation
by TFE at C6. Deuterium labeling demonstrated that the resulting 6-cyanobicyclo[3.1.0]hex-3-en-2-yl cation
underwent a rapid degenerate 1,4-sigmatropic rearrangement with inversion of configuration at the migrating
carbon before being trapped by the nucleophilic solvent. Irradiation of p- and m-methylbenzonitrile in the
same way gave six major addition products. Three of them, 32, 33, and 35 were 6-cyano-2-(2,2,2-trifluoroethoxy)-4-methylbicyclo[3.1.0]hex-3-enes and the other three, 32, 33, and 35, were 6-cyano-2-(2,2,2-trifluoroethoxy)-2-methylbicyclo[3.1.0]hex-3-enes. The proposed mechanism is again by TFE endo and exo
protonation of the first formed biradical/zwitterion followed by trapping of the cations by the solvent. The
1,4-sigmatropic rearrangement of the cations now stops at the most stable structures, the endo- and exo-6-cyano-2-methylbicyclo[3.1.0]hex-3-en-2-yl cations, and all of the products are derived from them.
“…7 When, after an independent synthesis was developed and benzvalene became available in "bounteous quantities", 8 these and other addition reactions were extensively studied. [9][10][11] Several features of the proposed intermediacy of benzvalene derivatives in the phototransposition reactions were problematic.…”
The phototransposition reactions in acetonitrile of p-, m-, and o-methylbenzonitrile have been studied. Any one of the three is converted to the other two by either a 1,2-or 1,3-isomerization in a primary photochemical step. However, the reactivities are quite different with the relative values for para:meta:ortho ) 32:4:1. For both the para and meta isomers, extended irradiations approach a calculated steady-state composition of para:meta:ortho ) 3:20:77. Quenching of the excited triplet state of the para and meta isomers with 2,4-dimethyl-1,3-butadiene indicates that these reactions are occurring from the excited singlet state. Irradiation of selectively labeled 2,6-dideuterio-4-methylbenzonitrile demonstrates that only the cyano-substituted carbon undergoes migration.
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