2484-73-3Relevant academic research and scientific papers
An examination of hyperconjugative and electrostatic effects in the hydride reductions of 2-substituted-4-tert-butylcyclohexanones
Rosenberg,Abei,Drake,Fox,Ignatz,Kwiat,Schaal,Virkler
, p. 1694 - 1700 (2007/10/03)
To better understand electronic effects on the diastereoselectivity of nucleophilic additions to the carbonyl group, a series of 2-X-4-tert-butylcyclohexanones (X = H, CH3, OCH3, F, Cl, Br) were reacted with LiAlH4. Reduction of ketones with equatorial substituents yields increasing amounts of axial alcohol in the series for X {H 3 3}. These data cannot be explained by steric or chelation ·effects or by the theories of Felkin-Anh or Cieplak. Instead, an electrostatic argument is introduced: due to repulsion between the nucleophile and the X group, axial approach becomes energetically less favorable with an increase in the component of the dipole moment anti to the hydride approach trajectory. The ab initio calculated diastereoselectivities were close to the experimental values but did not reproduce the relative selectivity ordering among substituents. For reduction of ketones with axial substituents, increasing amounts of axial alcohol are seen in the series for X {Cl 3 3 H F}. After some minor adjustments are made, this ordering is consistent with both the electrostatic model and Felkin-Anh theory. Cieplak theory cannot account for these data regardless of adjustments. Ab initio calculated diastereoselectivities were reasonably accurate for the nonpolar substituents but were poor for the polar substituents.
Diastereofacial selectivity in reactions of substituted cyclohexyl radicals. An experimental and theoretical study
Damm,Giese,Hartung,Hasskerl,Houk,Hüter,Zipse
, p. 4067 - 4079 (2007/10/02)
The diastereofacial selectivity in reactions of a series of alkyl-substituted cyclohexyl radicals has been investigated. In additions of cyclohexyl radicals to alkenes, it has been found that only substituents bound at the olefinic center being attacked by the radical influence the equatorial-axial selectivity. Substituents bound to the radical center or axial substituents β to the radical center lead to increased axial attack. Equatorial β-substituents or axial γ-substituents increase the amount of equatorial attack. The same trends are observed for halogen and hydrogen abstraction reactions; the amount of axial reaction product is usually somewhat higher than in the addition reactions. The stereoselectivities can be explained with steric and torsional effects very similar to those suggested for nucleophilic addition reactions to cyclohexanones. A MM2 force field has been parameterized to gain further insight into the stereochemistry of the reaction.
Regioselective dehydration in cyclic system with triphenylphosphine-azodicarboxylate
Iimori,Ohtsuka,Oishi
, p. 1209 - 1212 (2007/10/02)
The regioselective dehydration of cyclohexanol derivatives was achieved by using the Mitsunobu reagent system. The reaction undergoes under mild and neutral conditions. The observed regioselectivity was explained by considering the importance of the orientation of the leaving group at the elimination stage.
Activation of Reducing Agents. Sodium Hydride Containing Complex Reducing Agents. 36. Stereoselective Reduction of Alkylcyclohexanones and Rigid Ketones by MCRA's
Fort, Yves,Feghouli, Abdelhafid,Vanderesse, Regis,Caubere, Paul
, p. 5911 - 5915 (2007/10/02)
The stereoselectivity of reduction of selected ketones by a variety of Complex Reducing Agents (resulting from the Aggregative Activation of NaH and symbolized MCRA's) has been investigated.The stereochemistry of reduction is shown to be dependent on the nature of the metal.Steric hindrance plays an important role and the apparent size of the reagents follows the trend MnCRA's > ZnCRA's, CdCRA's > NiCRA's, CoCRA's.On the other hand NiCRA's and CoCRA's have been found as very strong isomerizing reagents.Insertion of the metal species into the C-O bond with formation of metallacycles appears as one of the possible mechanisms intervening during the reduction of ketones by MCRA's.
Stereochemistry of Nucleophilic Reductions of 2-Methyl-4-t-butylcyclohexanones. Further Support for the Linear Combination of SSC and PSC Stereochemical Models
Fang, Jim-Min,Sun, Shih-Feng,Rei, Min-Hon
, p. 747 - 750 (2007/10/02)
trans-2-Methyl-4-t-butylcyclohexanone reacts with lithium aluminum hydride, lithium trimethoxyaluminium hydride and methyl-lithium with equal stereoselectivity of axial attack (i. e. 95 +/- 1 percent).This is attributed to the equal steric strain control
The Predominance and Quantification of Steric Effects in the Solvolysis of Secondary Aliphatic Esters
Schneider, Hans-Joerg,Becker, Norman,Schmidt, Guenther,Thomas, Fred
, p. 3602 - 3607 (2007/10/02)
The solvolysis rates of 35 tosylates in hexafluoroisopropyl alcohol are measured and compared to MM2 calculated strain energies, ΔSI, between weighted sp3 states and the lowest sp2 state.For unhindered (pseudo)equatorially substituted cycloalkyl tosylates a linear correlation, free from ambiguities involved, e.g., with the leaving group simulation, is obtained which shows a sensitivity of m=1.04+/-0.05, indicating an extremely late transition state or limiting behavior.Based on the corresponding equation, it is shown that alkyl substituents in the γ- and in the β-position do not promote significant rate increases, even when there is an antiperiplanar disposition between the leaving group and a migrating β-methyl substituent.Instead, these substituents can lead to substantial ΔG* increase (by up to 5 kcal/mol in comparison to the ΔSI prediction), which is related to steric hindrance of solvation and/or hindrance for elimination. 17-(Tosyloxy)androstanes show extremely large epimeric rate ratios of>30000; these are not due to anchimeric assistance but only to the exceedingly slow reaction of the hindered 17β isomer, whereas the fast reaction of the 17α tosylate (e.g. 200 times higher than cyclopentyl tosylate) is in line with the ΔSI calculation. endo-Bicycloheptane esters show evidence for steric hindrance; exo-norbornyl tosylate has, however, a ΔG* value lower by 4 kcal/mol than predicted. ks/kc values, obtained by rate comparison in 80percent ethanol and 97percent HFIP, vary between 0.5 and 300, mainly as a result of different steric hindrance to rearside nucleophilic subnstitution
Stereoelectronic Effects in Hydrogen-atom Transfer Reactions of Substituted Cyclohexyl Radicals
Beckwith, Athelstan L. J.,Easton, Christopher J.
, p. 661 - 668 (2007/10/02)
Thermolysis of the peroxyoxalates (1)-(7) and the diacyl peroxides (8)-(11) in cyclohexane at 100 deg C gives cycloalkenes and cycloalkanes by hydrogen-atom transfer reactions of the initially formed conformationally biased 4-t-butyl-, 4-t-butyl-cis,cis-2,6-dimethyl-, 4-t-butyl-cis,trans-2,6-dimethyl-, 4-t-butyl-cis-2-methyl-, 4-t-butyl-trans-2-methyl-, and 5-t-butyl-cis-2-methylcyclohexyl radicals (12)-(17).The composition of the product mixtures indicates that transfer of axial β-hydrogen atoms occurs more rapidly than does transfer of equatorial β-hydrogen atoms.These results support the hypothesis that homolytic fission of a C-H bond is favoured when it lies chlose to the plane of an adjacent semi-occupied orbital.
