- Aerobic Oxidative Cleavage and Esterification of C(OH)–C Bonds
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C(OH)–C bonds are widely distributed in naturally renewable biomass, such as carbohydrates, lignin, and their platform molecules. Selective cleavage and functionalization of C(OH)–C bonds is an attractive strategy in terms of producing value-added chemicals from biomass. However, effective transformation of alcohols into esters by activation of C(OH)–C bonds has not been achieved so far. Herein, for the first time, we report selective cleavage and esterification of C(OH)–C bonds, catalyzed by inexpensive copper salts, using environmentally benign oxygen as the oxidant, to afford methyl esters in excellent yields. A diverse range of phenylethanol derivatives that contain C(OH)–C bonds were effectively converted into methyl benzoates. Detailed analysis revealed that the high efficiency and selectivity resulted mainly from the fact that, in addition to the major esterification reaction, the side products (e.g., olefins and acids) were also transformed in situ into esters in the reaction system. C(OH)–C bonds are widely distributed in naturally renewable biomass. In the context of developing future biorefineries, selective cleavage and functionalization of C(OH)–C bonds are crucial and represent an attractive strategy in terms of producing value-added chemical compounds from biomass resources. In the current manuscript, we report, for the first time, an effective and selective method for the cleavage and esterification of C(OH)–C bonds of alcohols to produce esters, by using environmentally benign O2 as the terminal oxidant and inexpensive commercially available copper salts as catalysts. Furthermore, a detailed mechanistic study revealed that, in addition to the major esterification route, side products (e.g., olefins and acids), which are inevitably generated under oxidative and basic conditions, were also simultaneously converted into esters, thus significantly improving the final yields of target ester products. Native lignin represents the only naturally sustainable aromatic resource. Transformation of native lignin into valuable aromatics would make a great contribution to our planet. We report, for the first time, the effective transformation of alcohols into esters by esterification of C(OH)–C bonds, which offers a new way for the simultaneous degradation and functionalization of lignin. This reaction promotes new explorations for biomass valorization.
- Liu, Mingyang,Zhang, Zhanrong,Yan, Jiang,Liu, Shuaishuai,Liu, Huizhen,Liu, Zhaotie,Wang, Weitao,He, Zhenhong,Han, Buxing
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supporting information
p. 3288 - 3296
(2020/10/20)
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- Intermolecular oxyarylation of olefins with aryl halides and TEMPOH catalyzed by the phenolate anion under visible light
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The phenolate anion was discovered as a new photocatalyst with strong reduction potentials. Under visible light irradiation, the phenolate anion enabled the reduction of (hetero)aryl halides (including electron-rich aryl chlorides) to (hetero)aryl radicals through single electron transfer. Based on this new photocatalyst, a novel and efficient photocatalytic protocol for the intermolecular oxyarylation of olefins with aryl halides and TEMPOH was developed. The developed three-component coupling reaction proceeded under redox-neutral reaction conditions with stable and readily available synthons and exhibited broad substrate scope. The utility of this process was further highlighted by the diversified chemical manipulation of the resulting oxyarylation products and the late-stage modification of active pharmaceutical ingredients.
- Li, Xipan,Liang, Kangjiang,Liu, Qian,Shen, Lei,Wei, Delian,Xia, Chengfeng,Zheng, Liyan
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p. 6996 - 7002
(2020/07/23)
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- Nitrogen Dioxide Catalyzed Aerobic Oxidative Cleavage of C(OH)–C Bonds of Secondary Alcohols to Produce Acids
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Stable organic nitroxyl radicals are an important class of catalysts for oxidation reactions, but their wide applications are hindered by their steric hinderance, high cost, complex operation, and separation procedures. Herein, NO2 in DMSO is shown to effectively catalyze the aerobic oxidative cleavage of C(OH)?C bonds to form a carboxylic group, and NO2 was generated in situ by decomposition of nitrates. A diverse range of secondary alcohols were selectively converted into acids in excellent yields in this transition-metal-free system without any additives. Preliminary results also indicate its applicability to depolymerize recalcitrant macromolecular lignin. Detail studies revealed that NO2 from nitrates promoted the reaction, and NO2 served as hydrogen acceptor and radical initiator for the tandem oxidative reaction.
- Liu, Mingyang,Zhang, Zhanrong,Song, Jinliang,Liu, Shuaishuai,Liu, Huizhen,Han, Buxing
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supporting information
p. 17393 - 17398
(2019/11/11)
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- Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese
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We have developed unprecedented methods for the direct transformation of primary alcohols to alkenes in the presence of hydrazine, and for the synthesis of mixed alkenes by the reaction of alcohols with hydrazones. The reactions are catalyzed by a manganese pincer complex and proceed in absence of added base or hydrogen acceptors, liberating dihydrogen, dinitrogen, and water as the only byproducts. The proposed mechanism, based on preparation of proposed intermediates and control experiments, suggests that the transformation occurs through metal–ligand cooperative N?H activation of a hydrazone intermediate.
- Das, Uttam Kumar,Chakraborty, Subrata,Diskin-Posner, Yael,Milstein, David
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supporting information
p. 13444 - 13448
(2018/09/14)
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- Chemoselective Benzylation of Aldehydes Using Lewis Base Activated Boronate Nucleophiles
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A benzylation of aldehydes using primary and secondary benzylboronic acid pinacol esters is reported. Activation of the boronic ester with s-butyllithium rendered it nucleophilic toward aldehydes. The activated nucleophile chemoselectively transfers the benzyl group over the sec-butyl group, providing excellent yields of the benzylated products. 11B NMR experiments were performed to study the mechanism of this transformation.
- Hollerbach, Michael R.,Barker, Timothy J.
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p. 1425 - 1427
(2018/05/24)
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- Enantioselective 1,2-Anionotropic Rearrangement of Acylsilane through a Bisguanidinium Silicate Ion Pair
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Highly enantioselective bisguanidinium-catalyzed tandem rearrangements of acylsilanes are reported. The acylsilanes were activated via an addition of fluoride on the silicon to form a penta-coordinate anionic silicate intermediate. The silicate then underwent alkyl or aryl group migration from the silicon atom to the neighboring carbonyl carbon atom (1,2-anionotropic rearrangement), followed by [1,2]-Brook rearrangement to provide the secondary alcohols in high yields with excellent enantioselectivities (up to 95% ee). The isolation of an α-silylcarbinol intermediate as well as DFT calculations revealed that the 1,2-anionotropic rearrangement occurred via a bisguanidinium silicate ion pair, which is the stereodetermining step. The chiral center formed is then retained without inversion through the subsequent [1,2]-Brook rearrangement. Crotyl acylsilanes were smoothly transformed into homoallylic linear crotyl alcohols with retention of E/Z geometry, and no branched alcohols were detected. This clearly suggested that the 1,2-anionotropic rearrangement occurred through a three-membered instead of a five-membered transition state.
- Cao, Weidi,Tan, Davin,Lee, Richmond,Tan, Choon-Hong
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p. 1952 - 1955
(2018/02/17)
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- Iodine-catalyzed transformation of aryl-substituted alcohols under solvent-free and highly concentrated reaction conditions
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Iodine-catalyzed transformations of alcohols under solvent-free reaction conditions (SFRC) and under highly concentrated reaction conditions (HCRC) in the presence of various solvents were studied in order to gain insight into the behavior of the reaction intermediates under these conditions. Dimerization, dehydration and substitution were the three types of transformations observed with benzylic alcohols. Dimerization and substitution reactions were predominant in the case of primary- and secondary alcohols, whereas dehydration prevailed in the case of tertiary alcohols. The relative reactivity of substituted 1-phenylethanols in I2-catalyzed dimerization under SFRC provided a good Hammett plot ρ+ = -2.8 (r2 = 0.98), suggesting the presence of electron-deficient intermediates with a certain degree of developed charge in the rate-determining step.
- Jereb, Marjan,Vra?i?, Dejan
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p. 747 - 762
(2018/01/17)
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- Pd-Catalyzed Conjunctive Cross-Coupling between Grignard-Derived Boron “Ate” Complexes and C(sp2) Halides or Triflates: NaOTf as a Grignard Activator and Halide Scavenger
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Catalytic enantioselective conjunctive cross-couplings that employ Grignard reagents are shown to furnish an array of nonracemic chiral organoboronic esters in an efficient and highly selective fashion. The utility of sodium triflate in facilitating this reaction is two-fold: it enables “ate” complex formation and overcomes catalytic inhibition by halide ions.
- Lovinger, Gabriel J.,Aparece, Mark D.,Morken, James P.
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supporting information
p. 3153 - 3160
(2017/03/11)
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- An unusual chemoselective oxidation strategy by an unprecedented exploration of an electrophilic center of DMSO: A new facet to classical DMSO oxidation
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A conceptually new dimethyl sulfoxide (DMSO) based oxidation process without the use of any activator has been demonstrated for the oxidation of active methylenes and benzhydrols. The developed protocol utilizes the electrophilic center of DMSO for oxidation, which was unexplored before. Mechanistic investigation has confirmed that the source of oxygen is DMSO.
- Chebolu, Rajesh,Bahuguna, Ashish,Sharma, Reena,Mishra, Vivek Kumar,Ravikumar
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supporting information
p. 15438 - 15441
(2015/10/20)
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- Highly Selective Addition of a Broad Spectrum of Trimethylsilane Pro-nucleophiles to N-tert-Butanesulfinyl Imines
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Addition of organotrimethylsilane reagents to chiral N-tert-butanesulfinyl imines can be achieved in good yields and with excellent diastereoselectivities by employing TMSO-/Bu4N+ as a Lewis base activator in THF. A variety of aliphatic, aromatic, heteroaromatic and organometallic chiral imines were utilised as electrophiles for the synthesis of enantioenriched N-tert-butanesulfinyl amides. Remarkably, the same sets of reaction conditions could be used with a highly diverse range of bench-stable organotrimethylsilane reagents, which highlights the generality and robustness of this methodology. Addition of organotrimethylsilane reagents to chiral N-tert-butanesulfinyl imines can be achieved in good yields and with excellent diastereoselectivities by employing TMSO-/Bu4N+ as a Lewis base activator in THF. A variety of aliphatic, aromatic, heteroaromatic and organometallic chiral imines were utilised as electrophiles for the synthesis of enantioenriched N-tert-butanesulfinyl amides, highlighting the generality and robustness of this methodology.
- Das, Manas,O'Shea, Donal F.
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supporting information
p. 18717 - 18723
(2016/01/25)
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- Redox-selective generation of aldehydes and H2 from alcohols under visible light
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Photosynthetic dehydrogenation: Potential usefulness of visible-light-induced dehydrogenation of alcohols in organic synthesis was demonstrated, in which aldehydes and H2 were afforded by using Ru/SrTiO3:Rh and water (see scheme). Water was essential for the reaction. High efficiency (TON: up to 15 400 based on Rh; H2 and aldehyde evenly generated) and high selectivity were achieved. Copyright
- Liu, Zijun,Caner, Joaquim,Kudo, Akihiko,Naka, Hiroshi,Saito, Susumu
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supporting information
p. 9452 - 9456
(2013/07/26)
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- Synthesis and application of benzyl-TMS derivatives as bench stable benzyl anion equivalents
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The regioselective benzylic metalation of toluenes using BuLi/KO tBu/TMP(H) (LiNK metalation conditions) and subsequent transmetalation to Si by reaction with TMSCl provides a general one-pot procedure for the synthesis of substituted benzyltrimethylsilanes. ArCH 2Si(Me)3 derivatives are bench stable reagents yet can serve as benzyl anion equivalents under mild reaction conditions. Following activation with fluoride they can successfully participate in a wide range of additions to both non-enolizable and enolizable carbonyls. In addition, their use in the synthesis of isochromanones and trifluoromethylated amines is illustrated. The broad synthetic scope and mild practical conditions of use for ArCH2Si(Me)3 reagents demonstrate their general potential as benzyl anion equivalents.
- Das, Manas,O'Shea, Donal F.
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p. 6448 - 6460
(2013/07/26)
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- Cu(acac)2-catalyzed synthesis of functionalized bis(arylmethyl)zinc reagents and their olefination reaction with aromatic aldehydes
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An efficient and facile copper(II) acetylacetonate catalyzed synthesis of functionalized bis(arylmethyl)zinc reagents from arylmethyl halides and their olefination reaction with aromatic aldehydes is reported. Aluminum trichloride was found to be the key ingredient in these reactions and (E)-stilbenes were obtained in high yields. Georg Thieme Verlag Stuttgart · New York.
- Fu, Ying,Hu, Xuemei,Chen, Yaojuan,Yang, Yanshou,Hou, Hongxia,Hu, Yulai
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scheme or table
p. 1030 - 1036
(2012/05/04)
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- A practical method to stereospecifically synthesize trans-stilbene derivatives
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A practical method to stereospecifically synthesize trans-stilbenes was developed via the one-pot benzylation-dehydration reaction of aromatic aldehydes with benzyltrimethylsilane (BTMS), which was driven by tetrabutylammonium fluoride (TBAF) in THF. At the same time a plausible description of the whole process was proposed and the effects of substituted groups on the reaction were investigated. Also this method was employed to synthesize three precursors of natural products with excellent yields, which demonstrated that this method is much efficient and practical in the synthesis of some natural products. A practical method to stereospecifically synthesize trans-stilbenes was developed via the one-pot benzylation-dehydration reaction of aromatic aldehydes with benzyltrimethylsilane, which was driven by tetrabutylammonium fluoride in THF. A plausible description of the whole process was proposed and the effects of substituted groups on the reaction were investigated. This method was employed to synthesize three precursors of natural products with excellent yields. Copyright
- Jian, Yujuan,Sun, Gaojun,Li, Jiaming,Su, Dan,Li, Chuanrun,Zhong, Guochen
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experimental part
p. 1423 - 1428
(2011/10/31)
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- Regioselective ring-opening reaction of unsymmetrical 2,3-diaryl epoxides via catalytic hydrogenolysis with Pd(0)EnCatTM
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A series of unsymmetrical 2,3-diaryl epoxides were hydrogenated under transfer-hydrogenation conditions using the polyurea-encapsulated palladium catalyst [Pd(0)EnCatTM]. The position of the cleaved C-O bond was determined by 1H NMR analysis of the ring-opened products derived from the epoxide, in which one of the two benzylic methyne protons is deuterated. The ratio of ring-opened products of the unsymmetrically substituted 2,3-diaryl epoxides was affected by the degree of the steric bulkiness on the aromatic ring. Georg Thieme Verlag Stuttgart New York.
- Kimachi, Tetsutaro,Nagata, Hiroyo,Kobayashi, Yusuke,Takahashi, Kaori,Torii, Eri,Ju-Ichi, Motoharu
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experimental part
p. 365 - 368
(2011/04/15)
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- The leaving group dependence in the rates of solvolysis of 1,2-diphenylethyl system
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1,2-Diphenylethyl chloride undergoes solvolysis by SN1 mechanism in aqueous organic solvents. The α-phenyl group of 1,2-diphenylethyl chloride enters into conjugation with the developing carbocationic centre. The β-phenyl group on the other hand was unable to extend its conjugation via neighbouring group participation due to steric inhibition of resonance in the formation of non-classical carbocation. 1,2-Diphenylethyl chloride thus behaves similar to 1-phenylethyl chloride in its solvolysis pattern. The solvolytic rate studies of chloride and methanesulphonate of 1,2-diphenylethyl alcohol in various aqueous organic solvents show that the dispersion observed in the Winstein- Grunwald plot is not due to a change in leaving group but due to the difference in solvation requirements of aromatic and aliphatic groups. Copyright
- Santhosh Kumara,Jayakumarb,Balachandranc
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experimental part
p. 783 - 788
(2011/05/05)
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- Ultrafast Grignard addition reactions in the presence of water
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The addition of allylmagnesium bromide and benzylmagnesium chloride to carbonyl compounds was investigated in the presence of protic reagents such as water and the rate of carbonyl addition was found to be comparable to the rate of protonation by the reagent. The Royal Society of Chemistry 2010.
- Osztrovszky, Gyorgyi,Holm, Torkil,Madsen, Robert
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scheme or table
p. 3402 - 3404
(2010/08/21)
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- Chemoselective and regiospecific suzuki coupling on a multisubstituted sp3-Carbon in 1,1-diborylalkanes at room temperature
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The palladium-catalyzed Suzuki-Miyaura cross-coupling on a multisubstituted sp3-carbon in 1,1-diborylalkanes was achieved at room temperature. The generation of a monoborate intermediate by virtue of the adjacent B atom could result in the chemoselective coupling reaction under ambient conditions.
- Endo, Kohei,Ohkubo, Takahiro,Hirokami, Munenao,Shibata, Takanori
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supporting information; experimental part
p. 11033 - 11035
(2010/09/16)
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- Organozinc generation via the titanium-catalyzed activation of alkyl halides
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[Chemical Equation Presented] A protocol for the generation of organozinc reagents via catalytic activation of alkyl halides is described herein. Subsequent nucleophilic addition to carbonyl derivatives provided the desired products in good to excellent y
- Fleury, Lauren M.,Ashfeld, Brandon L.
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supporting information; scheme or table
p. 5670 - 5673
(2010/03/01)
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- 1,2-Diarylethanols by alternative regioselective reductive ring-opening of 2,3-diaryloxiranes
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Non-symmetrical £rans-2,3-diaryloxiranes have been regioselectively opened by catalytic hydrogenation over Pd/C, NaBH4/Pd and [Cp 2TiCl]/H2O. Although in the catalytic hydrogenation reactions the epoxides were mainly opened at the β-carbon with respect to the substituted aryl ring in all cases, with the [Cp2TiCl]/H 2O system the regioselectivity was affected by the electronic properties of the aryl residues, the epoxides being opened on the carbon bearing the most electron-releasing or the least electron-withdrawing group. With the NaBH4/Pd system different regioisomers were obtained depending on the substituents. Starting from enantioenriched epoxides, no loss of optical purity was observed in the alcohols formed.
- Blasio, Nadia Di,Lopardo, Maria Teresa,Lupattelli, Paolo
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experimental part
p. 938 - 944
(2009/07/19)
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- Regioselectivity of the rhodium-catalyzed hydroboration of vinyl arenes: Electronic twists and mechanistic shifts
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Any substituent does it: The hydroboration of vinyl arenes with pinacol borane (HBPin) and cationic rhodium complexes selectively placed the boron proximal to the aryl rather than phenyl ring, regardless of whether this ring bears electron-donating or electron-withdrawing substituents. In competition experiments between styrene and various vinyl arenes, preferential hydroboration also occured at the substituted arene (see scheme). Hammett plots indicate a break in the mechanism. (Chemical Equation Presented).
- Edwards, David R.,Hleba, Yonek B.,Lata, Christopher J.,Calhoun, Larry A.,Crudden, Cathleen M.
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p. 7799 - 7802
(2008/09/18)
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- Kinetics of elimination reactions of 1-chloro-1-(4-methoxyphenyl)-2- phenylethane in acetonitrile
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1-Chloro-1-(4-methoxyphenyl)-2-phenylethane undergoes dehydrochlorination in acetonitrile to give trans-p-methoxy stilbene by E1 mechanism. The presence of electron donating p-methoxy group and the delocalisation of charge throughout the structure propose
- Santhosh Kumar,Balachandran
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p. 2751 - 2753
(2007/10/03)
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- Direct C-allylation of aryl-alkyl/glycosyl carbinols: Facile synthesis of C-linked carbo-β2-/γ2-/δ2- amino acids
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A facile ZrCl4-catalyzed direct allylation of the p-methoxyphenyl-alkyl/glycosyl carbinols at room temperature, and the conversion of the derived aryl-glycosyl-alkenes into hitherto unknown C-linked carbo-β2-/γ2-/δ2-amino acids is reported.
- Sharma, Gangavaram V. M.,Reddy, Katta Laxmi,Lakshmi, P. Sree,Ravi,Kunwar, Ajit C.
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p. 3967 - 3969
(2007/10/03)
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- A case of pictet-spengler revisited: Application to the synthesis of dihydroisocoumarins
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The reaction of Pictet-Spengler was revisited, and its modified conditions were applied to develop a general methodology toward the synthesis of dihydroisocoumarins containing a no-electron rich aromatic ring.
- Choukchou-Braham,Mostefa-Kara,Cheikh,Didi,Villemin
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p. 169 - 178
(2007/10/03)
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- Cadmium-mediated carbonyl benzylation in tap water
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Zn/CdCl2 has been developed as a mediator in the benzylation of various aldehydes in tap water affording the corresponding alcohols in moderate to good yields. The addition of a catalytic amount of InCl3 increases the yield of benzylation product significantly. It can selectively mediate the benzylation of aldehydes in the presence of ketones. A mechanism involving the formation of a cation π-complex is proposed based on the experimental facts. Georg Thieme Verlag Stuttgart.
- Zhou, Cunliu,Wang, Zhiyong
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p. 1649 - 1655
(2007/10/03)
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- Silver catalyzed zinc Barbier reaction of benzylic halides in water
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Benzylic chlorides react in aqueous dibasic potassium phosphate under silver catalysis with aromatic aldehydes in the presence of zinc dust to give 1,2-diaryl alcohols in moderate to good yields. Dimerization to bibenzyls and reduction of the halide are important side reactions. A wide range of substituted aromatic and heteroaromatic aldehydes and of substituted benzylic chlorides can be used. Aliphatic aldehydes and ketones are unreactive. A mechanism of two SET on the metal surface is discussed.
- Bieber, Lothar W.,Storch, Elisabeth C.,Malvestiti, Ivani,Da Silva, Margarete F.
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p. 9393 - 9396
(2007/10/03)
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- Side-chain fragmentation of arylalkanol radical cations. Carbon-carbon and carbon-hydrogen bond cleavage and the role of α- and β-OH groups
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A product analysis and kinetic study of the one-electron oxidation of a number of 1-arylpropanols, 1,2-diarylethanols, and some of their methyl ethers by potassium 12-tungstocobaltate(III) (abbreviated as Co(III)W) in aqueous acetic acid was carried out and complemented by pulse radiolysis experiments. The oxidations occur via radical cations which undergo side-chain fragmentation involving the C(α)-H and/or C(α)-C(β) bond. With 1-(4-methoxyphenyl)-2-methoxypropane (1), only deprotonation of the radical cation is observed. In contrast, removing the ring methoxy group leads to exclusive C-C bond cleavage in the radical cation. Replacing the side-chain β-OMe by β-OH, the radical cation undergoes both C-C and C-H bond cleavage, with both pathways being base catalyzed. C-C bond breaking in the radical cation is also enhanced by an α-OH group, as shown by 1-(4-methoxyphenyl)-2,2-dimethyl-1-propanol (7), where this pathway, which is also base catalyzed, is the only one observed. Interestingly, α- and β-OH groups exhibit a very similar efficiency in assisting the C-C bond cleavage route in the radical cations, as is evident from the kinetic and products study of the oxidation of 1-phenyl-2-(4-methoxyphenyl)ethanol (5) and 1-(4-methoxyphenyl)-2-phenylethanol (6) by Co(III)W, and from pulse radiolysis experiments on 5 and 6. C-C bond cleavage is the main reaction for both radical cations which exhibit a very similar rate of fragmentation (k = 2.0 and 3.2 x 104 s-1, respectively). In both fragmentation reactions a small solvent isotope effect, k(H2O)/k(D2O) (1.4 for 5.+ and 1.2 for 6.+) and negative activation entropies are observed. These data suggest that a key role in the assistance by α- or β-OH groups to C-C bond cleavage is played by hydrogen bonding or specific solvation of these groups. The kinetic study of the oxidations promoted by Co(III)W has also shown that when only one group, OH or OMe, is present in the side chain (either on C(α) or C(β)), the fragmentation step or both the electron transfer and fragmentation steps contribute to the overall oxidation rate. However, with an OH group on both carbons of the scissile bond, as in 1-(4-methoxyphenyl)-1,2-propanediol (9), the rate of C-C bond cleavage is so fast that the electron transfer step becomes rate determining.
- Baciocchi, Enrico,Bietti, Massimo,Putignani, Lorenza,Steenken, Steen
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p. 5952 - 5960
(2007/10/03)
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- A new, simple, efficient electrosynthesis of functionalized benzylic zinc species and their reactivity toward aromatic aldehydes
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Various stable substituted benzylic zincs are formed in acetonitrile upon reaction of the bromide derivatives, in good yields, using a catalytic electrogenerated zinc species associated with massive zinc. In the presence of chlorotrimethylsilone, these or
- Gosmini,Rollin,Gebehenne,Lojou,Ratovelomanana,Perichon
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p. 5637 - 5640
(2007/10/02)
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- Reactions of Carbonyl Compounds with Benzyltrialkylstibonium Bromide mediated by Different Strong Bases
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Reactions of carbonyl compounds with benzyltrialkylstibonium bromide mediated by alkyl-lithium gave homobenzylic alcohols, while mediated by lithium di-isopropylamide (LDA) gave a mixture of an alkene and an epoxide
- Huang, Yao-Zeng,Liao, Yi,Chen, Chen
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- Ceric Ammonium Nitrate Oxidative-Cleavage Reaction of Some Lignin Model Compounds: Role of the Benzylic Hydroxyl Group
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A series of β-1 lignin model compounds was synthesized and then oxidized with ceric ammonium nitrate (CAN) in aqueous acetonitrile.The compounds studied were 1,2-diarylethanols with either a p-hydroxy (1a-e) or a p-methoxy (2a-e) on the 1-ring and the following para substituents on the 2-ring: (a) H, (b) Cl, (c) Me, (d) OMe, and (e) NO2.Reactions of p-hydroxyl compounds 1a-e with CAN resulted in the formation of a red cerium complex that did not oxidize under the conditions of the reaction.In contrast all of the methoxy compounds 2a-e were rapidly oxidized by CAN atroom temperature to give p-anisaldehyde in near quatitative yield.The relative rates of these oxidative-cleavage reactions were found to be 0.11, 0.16, 1.00, 1.58, and 2.37 for the 1,2-diarylethanols 2e, 2b, 2a, 2c, and 2d, respectively.A Hammett treatment of this data revealed an excellent correlation with ?(ρ = -1.24).Methylation of the benzylic hydroxy group of 2a gave 1-methoxy-1-(4-methoxyphenyl)-2-phenylethane, which was found to be inert to CAN oxidation.
- Fisher, Thomas H.,Dershem, Stephen M.,Schultz, Tor P.
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p. 1504 - 1507
(2007/10/02)
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- Carbon-Hydrogen vs. Carbon-Carbon Bond Cleavage of 1,2-Diarylethane Radical Cations in Acetonitrile-Water
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Radical cations of 1,2-diarylethanes and 1-phenyl-2-arylethanes (Ar = phenyl, p-tolyl, p-anisyl) were generated in acidic 70percent acetonitrile-water by Cu2+-catalyzed peroxydisulfate oxidation.The radical cations fragment mainly by loss of benzylic protons (C-H cleavage) rather than by alkyl C-C bond cleavage.The radical cation of 2,3-dimethyl-2,3-diphenylbutane fragments efficiently by C-C cleavage, forming cumyl radical and cumyl cation.Oxidations of bibenzyl-bicumyl mixtures show selective oxidation of bicumyl dependent on total substrate concentration, providing evidence of equilibrating radical cations and showing that bicumyl fragments faster than bibenzyl loses protons.The effects of reaction conditions and substrate structure on reactivity are discussed.
- Camaioni, Donald M.,Franz, James A.
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p. 1607 - 1613
(2007/10/02)
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