- Hydrogen shifts in cyclohexylcarbenes. Spatial dependence of activating power and of primary deuterium isotope effects
-
The conformationally biased ketones 4-t-butyl-cis-2-methylcyclohexanone (1c) and 4-t-butyl-cis-2-trans-6-dimethylcyclohexanone (7a; and its 2,6-dideuterio derivative 7c) were converted into p-toluenesulfonylhydrazone Li salts. Thermolysis or photolysis generated putative singlet carbenes, which underwent competitive axial vs equatorial H shift (or D shift in the case of 7c) to give alkenes. Product analysis showed that a bystander Me(eq) substituent promotes a geminal H shift several times more efficiently than does a bystander Me(ax). This geometry-dependent activating power parallels behavior noted earlier for OMe and Ph bystander groups; but as Me groups are rotationally symmetric and possess no lone pair or π electrons this phenomenon cannot be attributed solely to rotameric considerations or to effects involving mobile electron clouds. For the trans-dimethylcarbenes 10a and 10c the primary deuterium isotope effect (k(H)/k(D)) for axial migration (I(ax)) was determined to be ca. 1.5 times larger than that for equatorial migration (I(eq)). This finding invalidates the common assumption that I(ax) = I(eq) and suggests that published data on deuterium isotope effects and on H(ax)/H(eq) migration selectivities need to be adjusted.
- Kenar, James A.,Nickon, Alex
-
p. 14871 - 14894
(2007/10/03)
-
- Bi+3-Catalyzed Regeneration of Carbonyl Compounds from Hydrazones under Microwave Irradiation
-
A new and efficient method for the hydrolytic cleavage of C=N bond of dimethyl and tosylhydrazones 1, to yield their corresponding carbonyl compounds 2, with catalytic quantities of bismuth trichloride in wet tetrahydrofuran under microwave irradiation has been developed.
- Boruah, Anima,Baruah, Bipul,Prajapati, Dipak,Sandhu, Jagir S.
-
p. 1251 - 1252
(2007/10/03)
-
- Axial/equatorial proportions for 2-substituted cyclohexanones
-
Axial-equatorial conformational proportions have been measured for 2- substituted cyclohexanones in chloroform by the Eliel method for F, Cl, Br, I, MeO, MeS, Me2N, MeSe, and Me. For the first seven of these, at least five experimentally independent measurables were used and the resulting conformational preferences appear to be accurate to within 10%. Systematic errors degraded the results for MeSe and Me. For Me2N, the conformational preference also was measured for the first time at slow exchange in the low- temperature 13C spectrum in several solvents. In chloroform, steric and polar effects contribute to the conformational preferences, with steric effects dominant for large groups such as I and MeS.
- Basso,Kaiser,Rittner,Lambert
-
p. 7865 - 7869
(2007/10/02)
-
- Alkyl Migration in Competition with Phenylthio Migration in the Acid-catalysed Rearrangement of Alcohols
-
Sulfonate derivatives of conformationally rigid syn-2-phenylthiocyclohexanols, which are prevented from phenylthio migration by stereochemistry, rearrange slowly by alkyl migration or ring contraction.In contrast to other electronegative groups, phenylthio slows the reaction down but allows migration of other groups.
- Hannaby, Malcolm,Warren, Stuart
-
p. 3007 - 3014
(2007/10/02)
-
- Activation of Reducing Agents. Sodium Hydride Containing Complex Reducing Agents. 36. Stereoselective Reduction of Alkylcyclohexanones and Rigid Ketones by MCRA's
-
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.
- Fort, Yves,Feghouli, Abdelhafid,Vanderesse, Regis,Caubere, Paul
-
p. 5911 - 5915
(2007/10/02)
-
- Stereochemistry of Nucleophilic Reductions of 2-Methyl-4-t-butylcyclohexanones. Further Support for the Linear Combination of SSC and PSC Stereochemical Models
-
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
- Fang, Jim-Min,Sun, Shih-Feng,Rei, Min-Hon
-
p. 747 - 750
(2007/10/02)
-
- Kinetics of Decarboxylation of the Two Epimers of 5-tert-Butyl-1-methyl-2-oxocyclohexanecarboxylic Acid: Lack of Stereoelectronic Control in β-Keto Acid Decarboxylation
-
Rates of decarboxylation of the two epimers of 5-tert-butyl-1-methyl-2-oxocyclohexanecarboxylic acid have been measured under both acidic and basic conditions at 25 deg C.The decomposition of isomer 1e (methyl and tert-butyl trans) is more rapid than that
- Kayser, Robert H.,Brault, Margaret,Pollack, Ralph M.,Bantia, Shanta,Sadoff, Scott F.
-
p. 4497 - 4502
(2007/10/02)
-
- Fluoride-Mediated Reactions of Enol Silyl Ethers. Regiospecific Monoalkylation of Ketones
-
Treatment of enol silyl ethers with alkyl halides in the presence of benzyltrimethylammonium fluoride and molecular sieves at room temperature gives the corresponding monoalkylated products with high regiospecificity.In most cases no polyalkylated products formed in the reaction.The alkylation reaction is highly chemospecific: esters, epoxides, and even ketones survive the reaction conditions.The reactions of various cyclohexanone derivatives proceed with the preferential axial attack of the electrophile.
- Kuwajima, Isao,Nakamura, Eiichi,Shimizu, Makoto
-
p. 1025 - 1030
(2007/10/02)
-
- 13 C shieldings in syn and anti aldimines and ketimines
-
The 13 C chemical shifts in a variety of aldimines and in both acyclic and cyclic ketimines have been examined.A comparison of the shieldings at the α carbons in syn versus anti stereoisomers shows a consistent upfield shift of 8-11 ppm in the syn isomer.The greater magnitude found in imines vs. oximes or hydrazones is a clear indication of a steric origin for the differential shieldings.The shifts in the imines of cyclohexanones exhibited stereochemically dependent effects of methyl substitution, very similar to those in analogous oxime and hydrazone derivatives.Chemical shift differences in the syn isomers of the α-phenetylimines of chiral aldimines and ketimines allow a facile measurement of diastereomer ratios for such compounds.
- Fraser, Robert R.,Banville, Jacques,Akiyama, Fuminori,Chuaqui-Offermanns, Noemi
-
p. 705 - 709
(2007/10/02)
-
- α-Nitro Ketones. 3. Stereochemistry of the Nitration of 1-Acetoxycyclohexenes: Synthesis of 2-Nitrocyclohexanones
-
The nitration of enol acetates of substituted cyclohexanones in acetic anhydride at 15 deg C with concentrated nitric acid is a kinetically controlled process which can lead to cis-trans-substituted 2-nitrocyclohexanones in very good yields.The stereochem
- Oezbal, Hadi,Zajac, Walter W.
-
p. 3082 - 3087
(2007/10/02)
-
- -
-
The addition of allyl and butenyl Grignard reagents to 4-t-butylcyclohexanone affords a greater proportion of the equatorial alcohols than is formed from the corresponding saturated Grignard reagents (propyl and s-butyl). In the case of the allyl Grignard reagent, the equatorial alcohol is the major product. Lithium aluminium hydride reduction in ether at 0° of cis and trans 4-t-butyl-2-methyl-cyclohexanone and cis and trans 4-t-butyl-2-chlorocyclohexanone affords mixtures of epimeric alcohols in ratios of equatorial to axial OH 8.4:1, 20:1, 4.2:1, and >70:1 respectively. These results fit in well with the view that polar interactions and torsional strain are both important in determining the steric course of the reaction.
- Cherest
-
p. 1593 - 1598
(2007/10/02)
-