- Factors Influencing Conformational Preferences in Cyclohexenes
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Conformational preferences have been measured for the first time for 4-substituted cyclohexenes in a solvent of low polarity.Measurements were made for the substituents Cl, Br, I, OH, OSiMe3, and CN and were compared with conformational preferences in cyclohexyl and exo-methylenecyclohex-3-yl.In the nearly nonpolar solvent CF2Cl2, in which intramolecular interactions are maximized, there is a much larger axial population for cyclohexen-4-yl than in cyclohexyl or exo-methylenecyclohex-3-yl.In particular, the dipolar interaction of the endocyclic double bond is reduced from that of the exocyclic double bond.This observation is confirmed by the almost negligible effect of symmetrizing the endocyclic double bond through 1,2-dimethyl substitution, in contrast with the large effect of symmetrizing the exocyclic double bond through 7,7-dimethyl substitution.Polar solvents increase the proportion of the axial conformer to a smaller extent for the endocyclic than for the exocyclic system, again in agreement with a lower dipolar effect in the endocyclic case.These results emphasize the anisotropic nature of the steric effects of double bonds.
- Lambert, Joseph B.,Marko, Dale E.
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p. 7978 - 7982
(2007/10/02)
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- MECHANISMS OF EPOXIDATIONS AND CHLORINATIONS OF HYDROCARBONS BY INORGANIC HYPOCHLORITE IN THE PRESENCE OF A PHASE-TRANSFER CATALYST.
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Inorganic hypochlorite in the presence of a quarternary ammonium salt (phase-transfer catalyst) not only epoxidizes several arenes to arene oxides in high yields but also converts toluene to alpha -chlorotoluene, anisole to ring chlorinated anisoles, and alkenes to a complex mixture of chlorinated and oxidized products, including the epoxide. More detailed studies with this system indicate the following: (1) the high-yield conversion of toluenes to benzyl chlorides proceeds with a deuterium isotope effect of 3. 6 and a rho ** plus value of minus 1. 7: (2) p-chloroanisole is the major product from anisole and is formed in a 22-fold greater quantity than o-chloroanisole; (3) the epoxidation of cis- and trans-alkenes is stereoselective but not completely stereospecific; (4) the chlorination of saturated hydrocarbons occurs with a selectivity that is experimentally identical with that of chlorine monoxide.
- Fonouni,Krishnan,Kuhn,Hamilton
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p. 7672 - 7676
(2007/10/02)
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- The photochemical addition of N-haloamides to olefins: the influence of various factors on the competition between 1,2-addition and hydrogen abstraction
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In the photodecomposition of N-haloamides (ZCONRX) in the presence of olefins, the 1,2-addition chain competes with the hydrogen abstraction chain(s) leading to the parent amide (the quantum yields for these processes are greater than unity).The following factors were shown to have an influence on this competition as measured by the yield of 1,2-addition and the yield of parent amide in methylene chloride solutions: (i) the N-halogen (higher yields of addition with X=Cl than with X=Br); (ii) the electronegativity of Z (increase of the yield of addition as the electronegativity of Z increases); (iii) the temperature (higher yields of addition at lower temperatures, and at -70 degC, better yields of addition (>90percent, R=H) for X=Br than for X=Cl); and (iv) the size of R (dramatic decrease of the yield of 1,2-addition in going from R=H to R=CH3).Surprisingly, the presence of a scavenger for HX had no influence on the yield of 1,2-addition.Both the size and electronegativity of Z had an effect on the stereochemistry of 1,2-addition to cyclohexene.High yields of addition to a variety of olefins were obtained with N-chloroamides such as ClCH2CONHCl, C2H5OCONHCl, CF3CONHCl.Their addition to enol ethers at -70 degC led to the synthesis of α-amido acetals or ketals (aldehydes or ketones) and to an α-amido glycoside in good yields.
- Lessard, Jean,Mondon, Martine,Touchard, Daniel
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p. 431 - 450
(2007/10/02)
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