- Stereoselectivity of lithium aluminium hydride and lithium alkoxyaluminohydride reductions of 3,3,5-trimethylcyclohexanone in diethyl ether
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The stereochemistry of lithium aluminum hydride and lithium tert-butoxyaluminohydride reductions of 3,3,5-trimethylcyclohexanone in diethyl ether has been studied.An unusual nonmonotonic dependence of the percentage of trans-3,3,5-trimethylcyclohexanol formed upon the ratio of : is observed.These unusual results have led to the use of diol addends to LiAlH4 solutions in order to probe the role of disproportionation upon stereoselectivity.Lithium mono-tert-butoxyaluminohydride is proposed to have a radically different stereoselectivity from that of either LiAlH4 or LiAl(O-t-Bu)3H.
- McMahon, Robert J.,Wiegers, Karl E.,Smith, Stanley G.
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- Reaction of Lithium Aluminum Hydride with Hindered Phenols. Formation of Tricoordinate Aluminum Species
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Lithium aluminum hydride reacts readily with 3 molar equiv of 2,4,6-tri-tert-butylphenol in refluxing ether.It has been demonstrated that the products resulting from this reaction cause the conversion of axial to equatorial cyclohexanols in the presence of a ketone.This behaviour is attributed to the formation of tricoordinate aluminum species.An ether solution of bis(2,4,6-tri-tert-butylphenoxy)aluminum hydride was prepared, and infrared spectra provide evidence for the existence of this species in solution resulting from the reaction of LiAlH4 with 3 molar equiv of 2,4,6-tri-tert-butylphenol.
- Haubenstock, Howard,Mester, Theodore A.,Zieger, Herman
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- DIASTEREOSELECTIVE REDUCTIONS OF CYCLOHEXANONES WITH DIISOBUTYLALUMINUM 2,6-DI-t-BUTYL-4-METHYLPHENOXIDE
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Isomerization of aluminum alcoholates occurs in the reduction of cyclohexanones with diisobutylaluminum 2,6-di-t-butyl-4-methylphenoxide 1 in the presence of excess ketone.Factors affecting the diastereoselectivity of reduction of cyclohexanones with 1 were investigated.
- Haubenstock, Howard
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- On the Reaction of Lithium Aluminum Hydride with Alcohols
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Infrared spectra of the reaction products of lithium aluminum hydride (LiAlH4) with 3 molar equiv of isobutyl alcohol, 2-propanol, and the more sterically hindered 2,4-dimethyl-3-pentanol, 4, in diethyl ether were examined.Alcohol 4 reacts with LiAlH4 in diethyl ether, giving a clear solution that is highly stereoselective in the reduction of 3,3,5-trimethylcyclohexanone, 1.The reaction of LiAlH4 with 4 does not lead to tricoordinate aluminum species.On the basis of infrared spectra and the stereochemistry of reduction of 1, it is concluded that LiAlH4 is presentin solution after its reaction with 3 molar equiv of isobutyl alcohol.LiAlH4 is also the initial reducing species in solution after reaction with 2-propanol.On the other hand, LiAlH4 reacts with methanol, ethanol, and 4, giving lithium alkoxyaluminum hydride species.Possible pathways for the reaction of LiAlH4 with alcohols are discussed.
- Haubenstock, H.,Mester, T.
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- Reduction of Ketones by Tributyltin Hydride: The Effect of High Pressure on Steric Hindrance and Rearrangement Processes
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The reduction of sterically hindered ketones by tributyltin hydride under high pressure (1 GPa) afforded the corresponding alcohols in good yields without the need of free radical initiator or Lewis acid catalyst.Cyclopropyl ketones and α,β-epoxy ketones were also reduced in high yields under 1400 MPa with preservation of the three-membered ring.
- Degueil-Castaing, Marie,Rahm, Alain
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- Heterogeneous Hydroxyl-Directed Hydrogenation: Control of Diastereoselectivity through Bimetallic Surface Composition
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Directed hydrogenation, in which product selectivity is dictated by the binding of an ancillary directing group on the substrate to the catalyst, is typically catalyzed by homogeneous Rh and Ir complexes. No heterogeneous catalyst has been able to achieve equivalently high directivity due to a lack of control over substrate binding orientation at the catalyst surface. In this work, we demonstrate that Pd-Cu bimetallic nanoparticles with both Pd and Cu atoms distributed across the surface are capable of high conversion and diastereoselectivity in the hydroxyl-directed hydrogenation reaction of terpinen-4-ol. We postulate that the OH directing group adsorbs to the more oxophilic Cu atom while the olefin and hydrogen bind to adjacent Pd atoms, thus enabling selective delivery of hydrogen to the olefin from the same face as the directing group with a 16:1 diastereomeric ratio.
- Shumski, Alexander J.,Swann, William A.,Escorcia, Nicole J.,Li, Christina W.
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p. 6128 - 6134
(2021/05/29)
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- The Stereoselective Reductions of Ketones to the Most Thermodynamically Stable Alcohols Using Lithium and Hydrated Salts of Common Transition Metals
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A simple method is presented for the highly stereoselective reductions of ketones to the most thermodynamically stable alcohols. In this procedure, the ketone is treated with lithium dispersion and either FeCl2·4H2O or CuCl2·2H2O in THF at room temperature. This protocol is applied to a large number and variety of ketones and is both more convenient and efficient than those commonly reported for the diastereoselective reduction of five- and six-membered cyclic ketones.
- Kennedy, Nicole,Cohen, Theodore
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p. 8134 - 8141
(2015/09/02)
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- Selective reduction of organic compounds with Al- trifluoromethanesulfonyldiisobutylalane. Comparison of its reactivity with Al-methanesulfonyldiisobutylalane
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The new MPV type reagent, Al-trifluoromethanesulfonyldiisobutylalane (DIBAO3SCF3), has been prepared and its reducing characteristics in the reduction of selected organic compounds containing representative functional groups have been examined, and compared its reactivity with that of Al-methanesulfonyldiisobutylalane (DIBAO3SCH 3) in order to understand the fluorine-substituent effect on its reactivity. In general, the reactivity of DIBAO3SCF3 appears to be much higher than that of DIBAO3SCH3, apparently due to the acidity increase by the electron-withdrawing fluorine-substituent. The reagent reduced aldehydes and ketones readily, but showed a perfect selectivity in the reduction of α,β-unsaturated aldehydes and ketones to produce the corresponding allylic alcohols in an absolutely 100% purity. In addition, the reagent achieved the regioselective cleavage of phenyl-or/and alkyl-substituted epoxides to the less substituted alcohols in a perfect regioselectivity. Moreover, the reagent also showed an high stereoselectivity in the reduction of substituted cycloalkanones to produce the thermodynamically more stable alcohol epimers exclusively.
- Cha, Jin Soon
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experimental part
p. 219 - 224
(2011/11/06)
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- Selective reduction of organic compounds with Al- methanesulfonyldiisobutylalane
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The new MPV type reagent, Al-methanesulfonyldiisobutylalane (DIBAO 3SCH3), has been prepared and its reducing characteristics in the reduction of selected organic compounds containing representative functional groups have been examined in order to find out a new reducing system with high selectivity in organic synthesis. In general, the reagent is extremely mild, showing only reactivity toward aldehydes, ketones and epoxides. The reagent exhibits a unique reducing applicability in organic synthesis. Thus, the reagent can achieve a clean 1,2-reduction of α,β-unsaturated aldehydes and ketones to produce the corresponding allylic alcohols in 100% purity. In addition, the reagent shows an excellent regioselectivity in the ring-opening reaction of epoxides. Finally, DIBAO3SCH3 shows a high stereo-selectivity in the reduction of cyclic ketones to produce the thermodynamically more stable epimers exclusively.
- Cha, Jin Soon,Noh, Minyeong
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experimental part
p. 840 - 844
(2010/10/21)
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- Al-isopropoxydiisobutylalane: A study of the effect of solvent on the rate and stereoselectivity of cyclic ketone reduction
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The effect of solvent on the rate and stereoselectivity of cyclic ketone reduction by Al-isopropoxydiisobutylalane (DIBAiOPr) has been investigated. In dichloromethane, DIBAiOPr behaves as a bulky reducing agent, approaching the carbonyl group along an equatorial trajectory to produce the axial alcohol with > 10:1 stereoselectivity. In sharp contrast, reduction in toluene gives the complementary outcome, affording the thermodynamically more stable isomer with > 99:1 stereoselectivity.
- Bahia, Perdip S.,Jones, Matthew A.,Snaith, John S.
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p. 9289 - 9291
(2007/10/03)
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- Directive effect of the 2- and 3-axial hydroxy groups that appeared in the complex metal hydride reduction of cyclohexanones
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A directive effect of the 2-axial hydroxy group appeared in the LiAlH4, NaBH4, and Zn(BH4)2 reduction of cyclohexanone, while the 3-axial hydroxy group exhibited a steric hindrance. The distance between the carbonyl carbon and the hydroxy group interacting with the hydride reagent was responsible for such a difference. In the reduction of Na[B(OAc)3H], the 2- and 3-axial hydroxycyclohexanones gave the products obtained by the hydride approaching from the side of the hydroxy group. The key point of the stereoselectivity was the formation of Na[B(OAc)2(OR)H, which was more reactive than the parent hydride, by exchanging the acetate ion with the alkoxide. Although the reduction was performed under the condition that the hydride/substrate ratio was 1, the conversion of the hydroxy ketone to an alcohol were 4, NaBH4, and Zn(BH4)2 reductions in tetrahydrofuran. The conversions in the NaBH4 reduction in ethanol were > 90%.
- Senda,Kikuchi,Inui,Itoh
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p. 237 - 242
(2007/10/03)
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- Lithium hydride containing complex reducing agent: A new and simple activation of commercial lithium hydride
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Commercially available lithium hydride, an essentially inert metal hydride, can be activated as a hydride source by in situ generated lithium alkoxide in the presence of a nickel salt. The obtained new reagent exhibited reducing properties versus alkyl or aryl halides, ketones, ethylenic or sulfurated compounds as well as coupling properties versus aryl halides when prepared in the presence of a ligand.
- Fort, Yves
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p. 6051 - 6054
(2007/10/02)
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- The Diastereoselectivity of Zirconium Alkoxide Catalysed Meerwein-Ponndorf-Varley Reductions
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The new variation of the Meerwein-Ponndorf-Verley reduction using 1-ethanol (6) or 1-tetralol (12) (3 equiv.) as the reducting alcohols and Zr(OtBu)4 as the catalyst (0.2 equiv.) is kinetically controlled and highly stereoselective.Preferential axial attack is achieved with the sterically less bulky alcohol 6 in the case of 4-tert-butyl- and 3-methylcyclohexanones (1a -> 96percent axial and 3a -> 93percent axial attack).The combination tetralol (12)/Zr(OtBu)4 behaves as a very bulky reducting agent, and the thermodynamically less stable alcohols 2c (80percent) and 4c (92 - 96percent) are formed preferentially in the reduction of 2a and 4b.The fused bicyclic systems 1-methyl-2-tetralone (14a) and flavanone (15a) and the steroids 16a-18a are reduced with high stereoselectivity to the corresponding cis alcohols 14b and 15b and the β-alcohols 16b-18b.The stereochemical outcome is in agreement with the Felkin-Ahn model. - Key Words: Reduction / Meerwein-Ponndorf-Verley reduction / Catalysis / Zirconium tetra-tert-butoxide / Stereochemistry / Diastereoselectivity / Felkin-Ahn model
- Krohn, Karsten,Knauer, Birgit
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p. 1347 - 1352
(2007/10/02)
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- Stereoselective Introduction of Hydroxyl Groups via Hydrazones
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Reduction of tosylhydrazones by hydride reagents in wet alcohol gave predominantly alcohols whose stereochemistries are opposite to those of the major reduction products of the corresponding ketones with sodium borohydride.
- Tada, Masahiro,Chiba, Kazuhiro,Izumiya, Koji,Tamura, Mihoko
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p. 3532 - 3533
(2007/10/02)
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- Copper(II) exchanged cation exchange resin: Useful activator in the reduction of ketones
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A copper(II) exchanged cation exchange resin has been used as support for reduction of ketones with sodium borohydride. Supported Cu(II) ions activate the reduction of ketones to a large extent and control the stereochemistry of reductions of cyclic ketones resulting in preponderance of equatorial alcohol in most cases.
- Sarkar,Rao,Ram
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p. 291 - 296
(2007/10/02)
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- Organic reactions in a solid matrix-VII sodium on alumina: A convenient reagent for reduction of ketones, esters and oximes
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Sodium dispersed on alumina is described and evaluated as a convenient off-the-shelf reagent (in a wax-coated form) for reduction of ketones, esters and oximes. While isopropanol is the preferred proton donor for the reduction of ketones and oximes, t-butanol is the alcohol of choice for the reduction of esters.
- Singh, Satendra,Dev, Sukh
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p. 10959 - 10964
(2007/10/02)
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- Montmorillonite supported (-)N-dodecyl-N-methyl ephedrenium borohydride: An efficient reducing agent
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The clay supported reagent reduces ketones under phase transfer conditions efficiently. A two fold activation of the carbonyl group - one by the Lewis acid sites on clay and the other by the β-hydroxy group of ephedreium moiety, ensures facile eductions -maintaining a fair degree of stereoselectivity in reducing some of the cyclic ketones. Asymmetric inductions of 1-9.7% were observed in case of some prochiral ketones.
- Sarkar,Rao
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p. 1247 - 1250
(2007/10/02)
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- Reduction of ketones with polydibenzo-18-crown-6-borohydride
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Polydibenzo-18-crown-6-(P-DB-18-C-6), a condensation polymer of dibenzo-18-crown-6 and formaldehyde has ben used as support for borohydride ions in the reduction of few cyclic and acyclic ketones. The reagent has been found to be more stereoselective than dibenzo-18-crown-6-borohydride in the reduction of cyclic ketones. 4-t-butylcyclohexanone has been converted exclusively (100%) to trans-4-t-butylcyclohexanol.
- Sarkar,Rao, Br
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p. 2157 - 2160
(2007/10/02)
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- Activation of Reducing Agents. Sodium Hydride Containing Complex Reducing Agents. 36. Stereoselective Reduction of Alkylcyclohexanones and Rigid Ketones by MCRA's
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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
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p. 5911 - 5915
(2007/10/02)
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- REDUCTION OF ISOPHORONE WITH SODIUM BOROHYDRIDE: CHANGE IN REGIOSELECTIVITY WITH HYDROTROPES
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Preferential 1,4-reduction of isophorone by sodium borohydride occurs in the presence of hydrotropes such as polyalkylene glycols.An anomalous effect of temperature was also observed using sodium salicylate as hydrotrope.
- Laxman, Mahalaxmi,Sharma, Man Mohan
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p. 111 - 117
(2007/10/02)
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- MONTMORILLONITE SUPPORTED BOROHYDRIDE: A NEW REDUCING AGENT FOR REDUCTIONS UNDER PHASE TRANSFER CONDITIONS
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A few cyclic and acyclic ketones were reduced in dichloromethane, using reducing agents prepared from reusable montmorillonite clay supported phase transfer catalyst.A high percentage of axial attack (76-100percent) by hydrides on substituted cyclohexanones observed, has been rationalized on the basis of a four center cyclic transition state for the clay absorbed ketone.
- Sarkar, A,Rao, B R,Konar, M M
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p. 2313 - 2320
(2007/10/02)
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- ACTIVATION OF REDUCING AGENTS. SODIUM HYDRIDE CONTAINING COMPLEX REDUCING AGENTS 28. STEREOCHEMISTRY OF REDUCTION OF KETONES BY COMPLEX REDUCING AGENTS
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The stereochemistry of reduction of selected ketones by a variety of Complex Reducing Agents (MCRA) has been investigated.The selectivity depends on the nature of the CRA constituents and principally on that of the metal salt.
- Feghouli, A.,Fort, Y.,Vanderesse, R.,Caubere, P.
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p. 1379 - 1382
(2007/10/02)
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- Stereoselectivite de la radioreduction γ d'alkylcyclohexanones
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γ Radioreduction of substituted cyclohexanones mainly leads to diastereoisomeric cyclohexanols.The stereoselectivity of radioreduction depends on ketone concentration.The assumption of two pyramidal diastereoisomeric hydroxycyclohexyl radicals explains the observed stereoselectivity.A selective solvation process of the hydroxy group of the radical has been proposed to account for the variations of stereoselectivity with ketone concentration.
- Alipour, E.,Micheau, J. C.,Paillous, N.
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p. 503 - 506
(2007/10/02)
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- The Predominance and Quantification of Steric Effects in the Solvolysis of Secondary Aliphatic Esters
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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
- Schneider, Hans-Joerg,Becker, Norman,Schmidt, Guenther,Thomas, Fred
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p. 3602 - 3607
(2007/10/02)
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- Directing Effects in Homogeneous Hydrogenation with PF6
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The presence of a ligating group, e.g., OH, CO2Me, C=O, or OMe, on an olefinic substrate is shown to direct the attack of the hydrogenation catalyst PF6/H2/CH2Cl2 from the face of the molecule containing the directing group.Isomerization is a minor side reaction in the cases studied.The origin of this effect is discussed and a model intermediate isolated.
- Crabtree, Robert H.,Davis, Mark W.
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p. 2655 - 2661
(2007/10/02)
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- Application d'une methode de calcul au choix d'un modele cinetique en photochimie II. Intervention de radicaux pyramidaux dans la photoreduction des cyclohexanones
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The photreduction of cyclohexanones leads usually to the corresponding diastereomeric cyclohexanols.In ethers, photoadducts are also isolated.The stereoselectivity of photoreduction depends on experimental conditions (concetration of the various reactants, addition of radical scavengers).The variation of stereoselectivity can be interpreted at the radical step by assuming that the radical center is either planar or pyramidal.These two assumptions were compared by using the previously described method of numerical analysis.Thus the involvement of a pyramidal radical center carrying stereochemical information, can be postulated with a high degree of reliability.This hypothesis which implies that pyramidal radicals posses some configurational stability was confirmed by a selective deuteriation experiment.A pyramidalisation angle between 17 deg and 42 deg according to the invertomer in consideration, was measured by independent ESR measurements.The results definitively confirm the non-planar character of the hydroxycycloheyl radical showing the reliability of the "ACADEMIC" method.
- Micheau, J. C.,Paillous, N.,Lattes, A.
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p. 257 - 263
(2007/10/02)
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- STEREOCHEMICAL EVIDENCE FOR SINGLE ELECTRON TRANSFER MECHANISM IN THE REDUCTION OF CYCLIC KETONES WITH ALKOXYALUMINIUM DICHLORIDES
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The stereochemical results of the reduction of cyclic ketones with alkoxyaluminium dichlorides do not conform to the conventional polar cyclic mechanism and may be explained by a single electron transfer mechanism.
- Nasipuri, Dhanonjoy,Gupta, Mita Datta,Banerjee, Satinath
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p. 5551 - 5554
(2007/10/02)
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- Aminoborohydrides as Reducing Agents. 1. Sodium (Dimethylamino)- and (tert-Butylamino)borohydrides as Selective Reducing Agents
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Replacement of a hydride in borohydride by an electron-donating alkylamino group greatly enhances the reducing ability of the resulting reagents.Thus, sodium (dimethylamino)- and (tert-butylamino)borohydrides (1, NaDMAB, and 2, NaTBAB, respectively) not only reduce aldehydes and ketones to alcohols but also are effective for the conversion of esters to alcohols and primary amides to amines in good to excellent yields.Tertiary amides are reduced to alcohols (i.e., N,N-dimethylamides) or amines (i.e.N,N-diisopropylamides) depending on the steric bulk of the alkyl substituents on nitrogen.However, secondary amides are not reduced by the reagents allowing selective conversion of primary and tertiary amides in the presence of secondary amides.Nitriles are attacked by the reagents but do not afford synthetically useful amounts of amine products.Aryl halides are slowly converted to arenes, but alkyl halides and epoxides undergo unusual reactions with the amino portion of the reagents.
- Hutchins, Robert O.,Learn, Keith,El-Telbany, Farag,Stercho, Yuriy P.
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p. 2438 - 2443
(2007/10/02)
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- POLYNUCLEAR BORANE ANIONS AS MILD REDUCING AGENTS 2. THE OCTAHYDROTRIBORATE (1-) ANION IN THE PRESENCE OF PROMOTERS
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A variety of metal compounds are shown to be effective promoters for the tetra-n-butylammonium octahydrotriborate (1-) reduction of carbonyl compounds in THF at room temperature.
- Tamblyn, William H.,Aquadro, Ruth E.,DeLuca, Oneida D.,Weingold, David H.,Dao, Tuan V.
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p. 4955 - 4958
(2007/10/02)
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- Sodium Borohydride and Mandelic Acid Complex - A New Reducing Agent
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A new reducing agent (II) has been prepared from sodium borohydride by replacing two of its active hydrogens with alkoxy and acyloxy groups from a mandelic acid molecule (I).The reagent is milder than sodium borohydride.It reduces the unhindered cyclohexanones with the same stereoselectivity as lithium aluminium hydride giving 75-92percent of the equatorial alcohols.The hindered ketones, 3,3,5-trimethylcyclohexanone and camphor, on the other hand, are reduced with high but opposite stereoselectivity giving 26 and 2percent of the stabler alcohols respectively.The aromatic and unsaturated ketones are slow to reduce.The chiral reagent prepared from (S)-(+)-mandelic acid reduces acetophenone in boiling dioxane to (R)-(+)-methylphenylcarbinol with 11.5percent enantiomeric excess.The transition states of the reaction have been discussed and further modifications of the reagent suggested.
- Nasipuri, Dhanonjoy,Sarkar, Ashis,Konar, Samir K.,Ghosh, Amitabha
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p. 212 - 215
(2007/10/02)
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- Lanthanoids in Organic Synthesis. 6. The Reduction of α-Enones by Sodium Borohydride in the Presence of Lanthanoid Chlorides: Synthetic and Mechanistic Aspects
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Lanthanoid chlorides (LnCl3) are efficient catalysts for the regioselective 1,2-reduction of α-enones by NaBH4 in methanol solution.Optimal conditions of this reaction have been determined.A mechanistic interpretation depicting the role of the Ln3+ ions is given.The major effect of Ln3+ is the catalysis of BH4- decomposition by the hydroxylic solvent to afford alkoxyborohydrides, which may be responsible for the observed regioselectivity.The stereoselectivity of the process is also modified by the presence of the Ln3+ ions, in that axial attack of cyclohexanone systems is enhanced.
- Gemal, Andre L.,Luche, Jean-Louis
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p. 5454 - 5459
(2007/10/02)
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- SELECTIVITE DE LA REDUCTION DE CYCLOHEXEN-2 ONES PAR NBu4BH4 DANS DIVERS SOLVANTS ET PAR NaBH4 DANS DES CONDITIONS DE TRANSFERT DE PHASE
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The 2-cyclohexenone 1 and isophorone 2 reductions with NBu4BH4 in aprotic solvents lead to a highly preferential 1-4 attack; i.e. 85percent with 1 and 96percent with 2 in THF.These regioselectivities are nearly the same as those observed with LiBH4 in the presence of cryptand confirming thus the cation influence.This method which is inexpensive and easy to work up, seems to constitute a general way to reduction of α-enones to saturate alcohols while other reagents such as K(sec Bu)3BH are not able to reduce the carbon-carbon double bond of isophoron.Phase transfer catalysis conditions are not useful for selective reduction: large amounts of allylic alcohols are formed in liquid-liquid phase transfer conditions (60percent in toluene-water); a good regioselectivity is only obtained when a cryptand is used as a transfer agent in solid-liquid conditions.
- D'Incan, E.,Loupy, A.
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p. 1171 - 1180
(2007/10/02)
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- Reactions of Magnesium Hydride. 4. Stereoselective Reductions of Cyclic and Bicyclic Ketones by Lithium Alkoxymagnesium Hydrides
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A series of lithium alkoxymagnesium hydrides were prepared and allowed to reduce 4-tert-butylcyclohexanone (I), 3,3,5-trimethylcyclohexanone (II), 2-methylcyclohexanone (III), and camphor (IV).It was found that very bulky secondary cyclic alkoxy groups such as 2,2,6,6-tetramethyl and 2,2,6,6-tetrabenzylcyclohexoxy were very stereoselective in the reduction of these ketones.For example, LiMgH2(2,2,6,6-Me4-c-HxO) reduced ketone I to provide 89 percent of the axial alcohol compared to 83 percent for HMg(2,6-i-Pr2PhO) which previously had provided the greatest degree ofselectivity among the new hydrides of magnesium.The dialkoxyhydrides, LiMgH(OR)2 , were also found to reduce ketones I, II, III, and IV stereoselectively but to a lesser extent than the monoalkoxyhydrides, LiMgH2(OR).These reactions were also accompanied by more enolization than observed for the monoalkoxy reagents.
- Ashby, Eugene C.,Noding, Stephen A.,Goel, Anil B.
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p. 1028 - 1035
(2007/10/02)
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- STEREOCHIMIE-LII. CONTROLE ORBITALAIRE DE LA STEREOCHIMIE DES REACTIONS-III EFFETS DES GROUPES β-FLUORO ET β-CYANO SUR LA STEREOCHIMIE ET LA CINETIQUE DE LA REDUCTION DE CYCLOHEXANONES PAR LE TRITERTIOBUTOXYALUMINOHYDRURE DE LITHIUM
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The respective influences of β-fluoro and β-cyano groups on the reduction of ketones by Li(t-BuO)3AlH on the stereoselectivities of the reduction with and without added cryptands, and with and without added alkyl fluoride and nitrile, were compared with ab initio calculations using the frontier orbitals of analogous carbonyl compounds to give energy values
- Agami, C.,Kazakos, A.,Levisalles, J.,Sevin, A.
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p. 2977 - 2981
(2007/10/02)
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- Activation of Reducing Agents. Sodium Hydride Containing Complex Reducing Agents. 13. Selective Heterogeneous Hydrogenation of Polyfunctional Substrates over Nic
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Preparative-scale heterogeneous hydrogenation over Nic at room temperature and 1 atm are described.It is shown that these catalysts allow the selective hydrogenation of carbon-carbon double bonds in the presence of oxo groups without side reactions.Alkynes and functional alkynes are selectively hydrogenated to the corresponding cis alkenes in high yields.Carbonyl group hydrogenations were also performed in high yields.Selective hydrogenation properties of Nic were exemplified in the steroid series.Finally, it was demonstrated that this new catalyst-preparation concept is not limited to nickel and also applies to the preparation of cobalt and palladium catalysts.
- Gallois, Philippe,Brunet, Jean-Jaques,Caubere, Paul
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p. 1946 - 1950
(2007/10/02)
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- Kinetics and Activation Parameters for the Reduction of Alkylcyclohexanones by Lithium Tri-tert-butoxyaluminohydride
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The kinetics of reduction of 15 cyclohexanones by lithium tri-tert-butoxyaluminohydride in tetrahydrofuran solvent are reported.The data confirm that the reaction is well represented by a simple second-order kinetic process.Second-order rate constants determined at various temperatures are recorded and the activation parameters determined.Rate constants vary from 5.0E-3 to 4.4 l mol-1 s-1; these rate constants exceed those of reduction by NaBH4 by factors varying from 50 (unhindered cyclohexanones) to 450 (hindered cyclohexanones).The reductions appear to be nearly isoenthalpic, all but three of the values of ΔH(excit.) being in the range 6.8 +/- 0.8 kcal mol-1.Variations in rates between ketones are caused by changes in ΔS(excit.), and entropy is also more significant than enthalpy in the free-energy barrier to reaction.Mechanistic aspects of the reduction are discussed.
- Wigfield, Donald C.,Gowland, Frederick W.
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p. 653 - 658
(2007/10/02)
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