- Photochemical Organocatalytic Benzylation of Allylic C–H Bonds
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We report a radical-based organocatalytic method for the direct benzylation of allylic C–H bonds. The process uses nonfunctionalized allylic substrates and readily available benzyl radical precursors and is driven by visible light. Crucial was the identification of a dithiophosphoric acid that performs two distinct catalytic roles, sequentially acting as a catalytic donor for the formation of photoactive electron donor–acceptor (EDA) complexes and then as a hydrogen atom abstractor. By mastering these orthogonal radical generation paths, the organic catalyst enables the formation of benzylic and allylic radicals, respectively, to then govern their selective coupling. The protocol was also used to design a three-component radical process, which increased the synthetic potential of the chemistry.
- Le Saux, Emilien,Melchiorre, Paolo,Zanini, Margherita
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supporting information
p. 1113 - 1118
(2022/02/05)
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- Iron-Based Catalyst for Borylation of Unactivated Alkyl Halides without Using Highly Basic Organometallic Reagents
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The mild borylation of alkyl bromides and chlorides with bis(neopentylglycolato)diborane (B2neop2) mediated by iron-bis amide is described. The reaction proceeds with a broad substrate scope and good functional group compatibility. Moreover, sufficient ca
- Siddiqui, Sheema,Bhawar, Ramesh,Geetharani
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supporting information
p. 1948 - 1954
(2021/01/14)
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- Copper-catalyzed sp3-sp3 cross-coupling of turbo grignards with benzyl halides
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The aromatic ring in benzyl halides and sulfonates imparts unique reactivity at the benzylic carbon atom. Photoredox sp3-sp3 cross-coupling proved ineffective for coupling p-methoxybenzyl chloride (PMBCl), leading to a new strategy for the sp3-sp3 cross-coupling of benzyl halides and sulfonates. This strategy involved LiCl-accelerated synthesis of a Grignard reagent followed by a copper-catalyzed cross-coupling. The conditions worked well for PMBCl due to its exceptional reactivity but other benzyl bromides or sulfonates reacted poorly.
- Elahi-Mohassel, Synah,Girgis, Michael,Paige, Mikell,Petruncio, Greg
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supporting information
(2021/11/17)
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- Synthesis of Dibenzyls by Nickel-Catalyzed Homocoupling of Benzyl Alcohols
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Dibenzyls are essential building blocks that are widely used in organic synthesis, and they are typically prepared by the homocoupling of halides, organometallics, and ethers. Herein, we report an approach to this class of compounds using alcohols, which are more stable and readily available. The reaction proceeds via nickel-catalyzed and dimethyl oxalate assisted dynamic kinetic homocoupling of benzyl alcohols. Both primary and secondary alcohols are tolerated.
- Pan, Feng-Feng,Guo, Peng,Huang, Xiaochuang,Shu, Xing-Zhong
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p. 3094 - 3100
(2021/04/23)
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- Use of Isopropyl Alcohol as a Reductant for Catalytic Dehydoxylative Dimerization of Benzylic Alcohols Utilizing Ti?O Bond Photohomolysis
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Photohomolysis of Ti?O bonds is utilized in photocatalytic generation of titanium(III) species for dehydroxylative dimerization of benzylic alcohols under UV-light irradiation by using isopropyl alcohol (IPA) as a stoichiometric reductant. In this reaction, IPA works not as a single-electron donor as in the photo-redox catalyzed reactions but as an H-atom-donor. The reaction also proceeds under visible-light irradiation in the presence of thioglycolic acid as a ligand.
- Iwasawa, Nobuharu,Sumiyama, Keiichi,Toriumi, Naoyuki
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supporting information
p. 2474 - 2478
(2021/06/25)
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- Ni-Catalyzed Iterative Alkyl Transfer from Nitrogen Enabled by the in Situ Methylation of Tertiary Amines
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Current methods to achieve transition-metal-catalyzed alkyl carbon-nitrogen (C-N) bond cleavage require the preformation of ammonium, pyridinium, or sulfonamide derivatives from the corresponding alkyl amines. These activated substrates permit C-N bond cleavage, and their resultant intermediates can be intercepted to affect carbon-carbon bond-forming transforms. Here, we report the combination of in situ amine methylation and Ni-catalyzed benzalkyl C-N bond cleavage under reductive conditions. This method permits iterative alkyl group transfer from tertiary amines and demonstrates a deaminative strategy for the construction of Csp3-Csp3 bonds. We demonstrate PO(OMe)3 (trimethylphosphate) to be a Ni-compatible methylation reagent for the in situ conversion of trialkyl amines into tetraalkylammonium salts. Single, double, and triple benzalkyl group transfers can all be achieved from the appropriately substituted tertiary amines. Transformations developed herein proceed via recurring events: The in situ methylation of tertiary amines by PO(OMe)3, Ni-catalyzed C-N bond cleavage, and concurrent Csp3-Csp3 bond formation.
- Nwachukwu, Chideraa Iheanyi,McFadden, Timothy Patrick,Roberts, Andrew George
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p. 9979 - 9992
(2020/09/03)
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- Luminescent tungsten(vi) complexes as photocatalysts for light-driven C-C and C-B bond formation reactions
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The realization of photocatalysis for practical synthetic application hinges on the development of inexpensive photocatalysts which can be prepared on a large scale. Herein an air-stable, visible-light-absorbing photoluminescent tungsten(vi) complex which can be conveniently prepared at the gram-scale is described. This complex could catalyse photochemical organic transformation reactions including borylation of aryl halides, such as aryl chloride, reductive coupling of benzyl bromides for C-C bond formation, reductive coupling of phenacyl bromides, and decarboxylative coupling of redox-active esters of alkyl carboxylic acid with high product yields and broad functional group tolerance.
- Chan, Kaai-Tung,Che, Chi-Ming,Du, Lili,Liu, Yungen,Phillips, David Lee,To, Wai-Pong,Tong, Glenna So Ming,Wu, Liang-Liang,Yu, Daohong
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p. 6370 - 6382
(2020/07/15)
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- Photo-catalytic preparation method of bibenzyl compounds
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The invention relates to a preparation method of bibenzyl compounds. A compound represented by a formula (A) and a compound represented by a formula (C) carry out reactions under the action of an organic tungsten catalyst and an alkali in the presence of light to generate bibenzyl compounds represented by the formula (B). The method is simple and is easy to operate. The yield is high, and the application range is wide. Moreover, the invention also provides an application of a tungsten complex in organic chemical reactions as a photocatalyst.
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Paragraph 0049-0054; 0068-0071
(2020/02/27)
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- Photochemical Reductive C–C Coupling with a Guanidine Electron Donor
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The metal-free photoinduced reductive C–C coupling reactions of a number of substituted benzyl halides (15 examples) with the organic electron-donor 2,3,5,6-tetrakis(tetramethylguanidino)pyridine are evaluated. Depending on the substituents at the benzyl group, a C–C coupling product yield in the range 50–95 % is achieved. The photochemical benzyl-radical formation by homolytic N–C bond cleavage of the initially formed benzyl-pyridinium salts is the rate-determining step of these reactions. Electron-withdrawing as well as -donating substituents at the phenyl group increase the reaction rate. Quantum chemical computations did not reveal any correlation between either the enthalpy or Gibbs free energy of the N–C bond cleavage step and the experimentally determined first-order rate constants. Instead, the structural difference between the excited state generated by irradiation and the electronic ground state of the pyridinium ions could be used to rationalize the differences in the reaction rates.
- Wiesner, Sven,Walter, Petra,Wagner, Arne,Kaifer, Elisabeth,Himmel, Hans-J?rg
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p. 5045 - 5054
(2016/10/26)
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- Mechanism and Applications of the Photoredox Catalytic Coupling of Benzyl Bromides
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The photoredox catalytic coupling of halomethyl arenes to bibenzyl derivatives has been demonstrated. The catalytic protocol employed the Hantzsch ester, potassium phosphate, and a photoactive cyclometalated IrIIIcomplex catalyst. A photochemical quantum yield as high as 20 % was obtained. The catalytic mechanism was investigated in detail by performing photophysical and electrochemical measurements, as well as by quantum chemical calculations. The results suggest that two-electron mediation might be responsible for the improved photon economy. The reaction protocol was compatible with halomethyl arenes that contain a variety of functional groups. Finally, the synthetic utility of our protocol was demonstrated by the preparation of a natural dihydrostilbenoid, brittonin A.
- Park, Gyurim,Yi, Seung Yeon,Jung, Jaehun,Cho, Eun Jin,You, Youngmin
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p. 17790 - 17799
(2016/11/28)
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- A well-defined low-valent cobalt catalyst Co(PMe3)4 with dimethylzinc: a simple catalytic approach for the reductive dimerization of benzyl halides
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Herein, we report the first catalytic version of a cobalt-catalysed reductive homocoupling of benzyl halides which proceeds with low catalyst loadings (0.5 to 5 mol%). By synthetizing each cobalt intermediate we demonstrate that reaction proceeds through two single electron transfers (SET) and that dimethylzinc is only involved in the regeneration of the catalytic species.
- Fallon, Brendan J.,Corcé, Vincent,Amatore, Muriel,Aubert, Corinne,Chemla, Fabrice,Ferreira, Franck,Perez-Luna, Alejandro,Petit, Marc
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p. 9912 - 9916
(2016/12/07)
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- Cyclopentadienyl nickel(ii) N, C-chelating benzothiazolyl NHC complexes: Synthesis, characterization and application in catalytic C-C bond formation reactions
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Cyclopentadienyl (Cp) Ni(ii) complexes [CpNiL][PF6] containing hybrid N,C chelating benzothiazolyl NHC ligands (L1 = 1-(2-benzothiazolyl)-3-methylimidazol-2-ylidene, 3a; L2 = 1-(2-benzothiazolyl)-3-allylimidazol-2-ylidene, 3b; L3 = 1-(2-benzothiazolyl)-3-benzylimidazol-2-ylidene, 3c) have been synthesized and fully characterized. The catalytic activity of 3a-3c in some C-C bond formation reactions has been examined. They are efficient catalysts for the homo-coupling of benzyl bromide in the presence of MeMgCl at r.t. with good functional group tolerance. Complex 3a is active in the catalytic oxidative homo-coupling of Grignard reagents with 1,2-dichloroethane as an oxidant at r.t.
- Teo, Wei Jie,Wang, Zhe,Xue, Fei,Andy Hor,Zhao, Jin
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p. 7312 - 7319
(2016/06/01)
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- E-Selective Semi-Hydrogenation of Alkynes by Heterobimetallic Catalysis
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A unique cooperative H2 activation reaction by heterobimetallic (NHC)M′-MCp(CO)2 complexes (NHC = N-heterocyclic carbene, M′ = Cu or Ag, M = Fe or Ru) has been leveraged to develop a catalytic alkyne semi-hydrogenation transformation. The optimal Ag-Ru catalyst gives high selectivity for converting alkynes to E-alkenes, a rare selectivity mode for reduction reactions with H2. The transformation is tolerant of many reducible functional groups. Computational analysis of H2 activation thermodynamics guided rational catalyst development. Bimetallic alkyne hydrogenation and alkene isomerization mechanisms are proposed.
- Karunananda, Malkanthi K.,Mankad, Neal P.
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supporting information
p. 14598 - 14601
(2015/12/08)
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- Reversed electron apportionment in mesolytic cleavage: The reduction of benzyl halides by SmI2
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The paradigm that the cleavage of the radical anion of benzyl halides occurs in such a way that the negative charge ends up on the departing halide leaving behind a benzyl radical is well rooted in chemistry. By studying the kinetics of the reaction of substituted benzylbromides and chlorides with SmI2 in THF it was found that substrates para-substituted with electron-withdrawing groups (CN and CO2Me), which are capable of forming hydrogen bonds with a proton donor and coordinating to samarium cation, react in a reversed electron apportionment mode. Namely, the halide departs as a radical. This conclusion is based on the found convex Hammett plots, element effects, proton donor effects, and the effect of tosylate (OTs) as a leaving group. The latter does not tend to tolerate radical character on the oxygen atom. In the presence of a proton donor, the tolyl derivatives were the sole product, whereas in its absence, the coupling dimer was obtained by a SN2 reaction of the benzyl anion on the neutral substrate. The data also suggest that for the para-CN and CO2Me derivatives in the presence of a proton donor, the first electron transfer is coupled with the proton transfer. Reverse breakup: In the mesolytic cleavage of the radical anions of benzyl halides that are para-substituted by CN or CO2Me groups, the halogen departs, counterintuitively, as a radical and the benzyl system carries the negative charge (see figure).
- Yitzhaki, Offir,Hoz, Shmaryahu
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p. 9242 - 9248
(2015/06/16)
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- A clean and selective radical homocoupling employing carboxylic acids with titania photoredox catalysis
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A titania photoredox catalysis protocol was developed for the homocoupling of C-centered radicals derived from carboxylic acids. Intermolecular reactions were generally efficient and selective, furnishing the desired dimers in good yields under mild neutral conditions. Selective cross-coupling with two acids proved unsuccessful. An intra-molecular adaptation enabled macrocycles to be prepared, albeit in modest yields. (Chemical Equation Presented).
- Manley, David W.,Walton, John C.
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supporting information
p. 5394 - 5397
(2015/02/19)
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- Homocoupling of benzyl halides catalyzed by POCOP-nickel pincer complexes
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Two types of POCOP-nickel(II) pincer complexes were prepared by mixing POCOP pincer ligands and NiX2 in toluene at reflux. The resulting nickel complexes efficiently catalyze the homocoupling reactions of benzyl halides in the presence of zinc. The coupled products were obtained in excellent to quantitative yields.
- Chen, Tao,Yang, Limin,Li, Liang,Huang, Kuo-Wei
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experimental part
p. 6152 - 6157
(2012/08/29)
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- Reduction of diaryl alkenes by hypophosphorous acid-iodine in acetic acid
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A mixture of 50% aqueous H3PO2 and I2 (in catalytic amount) in HOAc efficiently reduces aryl alkenes to the corresponding alkanes in high yield. Addition of acetic anhydride to the medium results in ring-acetylation (or N-acetylation in the case of amines). H3PO2 costs only one-fifth as much as hydriodic acid on a mole basis and one mole of H3PO2 produces four moles of HI, resulting in a 20-fold cost advantage for H3PO2/I2 over aqueous HI as a source of HI.
- Fry, Albert J.,Allukian, Myron,Williams, Allison D.
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p. 4411 - 4415
(2007/10/03)
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- Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals
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Five aryl-substituted phenacetyl radicals (X = p-MeO, p-Me, H, p-Cl, p-CF3) were generated by laser photolysis of the corresponding dibenzyl ketones in n-hexane and acetonitrile. The decarbonylation reaction was monitored through the rise in time-resolved absorption of the benzyl radical chromophore at 317 nm. The decarbonylation rate constants were obtained by a numerical integration procedure, where second-order radical reactions were explicitly taken into account. Values of (2-3) × 106 s-1 in acetonitrile and (6-10) × 106 s-1 in n-hexane revealed a large solvent effect for all derivatives (by a factor of ~3). The electronic substituent effect indicates that both electron-withdrawing and electron-donating para substituents accelerate the decarbonylation slightly. The rate constants followed the order MeO > Me, Cl, CF3 > H. The substituent effects are interpreted in terms of the ability of the para substituent to stabilize the benzyl radical resulting from decarbonylation. Evidence for a polar effect was not obtained. The underlying reasons for the observed solvent effect are discussed. Copyright
- Zhang, Xiangyang,Nau, Werner M.
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p. 634 - 639
(2007/10/03)
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- Substituent effects on the photocleavage of benzyl-sulfur bonds. Observation of the "Meta effect"
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Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO2 derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between a values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO2 > 3-CF3 > 3-CH3 > 3-OCH3. These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH3 derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.
- Fleming, Steven A.,Jensen, Anton W.
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p. 7040 - 7044
(2007/10/03)
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- Photochemistry of substituted benzyl acetates and benzyl pivalates: A reinvestigation of substituent effects
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The photosolvolysis reactions, in methanol, of six substituted benzyl acetates (7a-f) and benzyl pivalates (8a-f) were studied. Five major benzylic products were formed from two critical intermediates. The ethers (9) were formed from the ion pair, 15, and all of the other products (10-14) were formed from the radical pair, 16. Quenching studies showed that only excited singlet state reactivity was important. The product yields were found to be highly substituent dependent. For instance, for the acetate esters, the yield of ether (9) varied from 2% for X = 4-OCH3 to 32% for X = 3-OCH3. Most of the differences in the yields could be attributed to ground state processes that occur after bond cleavage. The important competition is between electron transfer, converting the radical pair to the ion pair, and decarboxylation of RCO2*. The rates of electron transfer are shown to fit Marcus theory in both the normal and inverted regions. Direct heterolytic cleavage to form the ion pair is of minimal importance.
- Hilborn,MacKnight,Pincock,Wedge
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p. 3337 - 3346
(2007/10/02)
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- Photocleavage of Benzyl-Sulfide Bonds
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The nucleoside transport inhibitor 6--9-(β-D-ribofuranosyl)purine, NBMPR, has been used successfully in photoaffinity labeling.We have studied the mechanism for photocleavage of the benzyl-sulfur bond by using substituted benzyl phenyl sulfides as analogues of NBMPR.This has enabled us to enhance the photoreactivity of the benzyl-sulfur bond.We have also performed "radical clock" studies with a hexenyl side chain to trap reactive intermediates.The mechanistic interpretation from the substituent and side chain studies is that the benzyl-sulfur moiety is photocleaved via a homolytic pathway.
- Fleming, Steven A.,Jensen, Anton W.
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p. 7135 - 7137
(2007/10/02)
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- Photochemistry of phosphate esters: An efficient method for the generation of electrophiles
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The photochemical cleavage of benzyl diethyl phosphates, 1a-g, has been examined in tert-butyl alcohol, which produces the corresponding benzyl tert-butyl ether as the major solvolysis product upon direct irradiation. The multiplicity of the reactive excited state has been established as the singlet state. Evidence favoring an intermediate benzyl cation-phosphate ion pair formed from photofragmentation includes oxygen-18 scrambling, racemization of chiral benzyl phosphates, and substituent effects on the rate of reaction. The existence of an electrophilic intermediate is further supported by a linear free energy relationship for the rate of disappearance of 1, kdis, with Hammett σ, which gave a ρ value of -0.90 and for the rate of appearance of 2, kapp, a Hammett ρ value of -0.97. Possible mechanisms including an electron transfer either before or after homolysis or simple heterolysis of the ester bond are evaluated.
- Givens, Richard S.,Matuszewski, Bogdan,Athey, Phillip S.,Robert Stoner
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p. 6016 - 6021
(2007/10/02)
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- Photochemical Transformations. 26. Sensitized and Unsensitized Photoreactions of Some Benzyl Chlorides in tert-Butyl Alcohol
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Benzyl chloride and a variety of meta- and para-substituted derivatives have been irradiated in tert-butylalcohol at 254 nm or in acetone-tert-butyl alcohol mixtures at 300 nm.Quantum and chemical yields of photosolvolysis products (benzyl tert-butyl ether and/or benzyl alcohol), photohomolysis products (bibenzyl and 4-phenyl-2-methyl-2-butanol), and corresponding products from substituted benzylchlorides have been measured as functions of substrate concentration and medium composition.Sensitized reactions favor bond heterolysis, but direct irradiations favor homolysis, although reactions are not clean in either case.There is no correlation between quantum yields for photosolvolysis and Hammett ? or Brown ?+ constants, and no salt effects of added lithium perchlorate were observed, in either direct or sensitized reactions.
- Cristol, Stanley J.,Bindel, Thomas H.
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p. 951 - 957
(2007/10/02)
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