- The Walling, El-Taliawi, and Zhao "Carnonyl Effect" in Radical Brominations Is an Example of HBr Reversal. It Is Not Relevant to ? and ? Radical Chemistry.
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Walling, El-Taliawi, and Zhao claim that our "S? chemistry" is reproduced in photobrominations employing Br2 in the presence of carbonyl compounds (no NBS).Photobromination of alkanes by N-bromosuccinimide (NBS) shows selectivities that vary, but which show limiting values for significant ranges of rection conditions.We have attributed these different limiting values to three reaction paths which differ in involving either Br., S?, or S? as the intermediate hydrogen-abstracting radical.Walling, El-Taliawi, and Zhao (WEZ) have reported selectivities they belive are attributable to a carbonyl-bromine intermediate rather than our S?.Evidence is now presented to show that the WEZ results are due to HBr in varying amounts with or without carbonyl compounds present.This represents a fourth type of selectivity and one that is sensitive to HBr concentrations and probably arises in their systems from the following reaction: There is no carbonyl effect.This fourth type had been deliberately excluded from our work, so that reactions involving HBr reversal are not relevant to our published work on succinimidyl chemistry.Thus, the selectivities (per H basis) for the neopentane/methylene chloride competitions stand without modification, as published earlier: 0.067, 1.0 and 17 for B., S?, and S?, respectively.
- Skell, P. S.
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- Radical Chain Carriers in N-Bromosuccinimide Brominations
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Relative reactivities per hydrogen (r's) for neopentane-CH2Cl2 have been redetermined for photobromination by Br2, r = 0.03-0.4, NBS + Br2, r = 1-2.5, and NBS + 1,1-dichloroethylene (DCE) as a Br. trap, r > 20.In the last case β-bromopropionyl isocyanate (BPI) is the major product.In Br2 reactions r's vary with experimental conditions and are increased significantly by a number of carbonyl and succinimide derivatives.BPI is also a major product in the brominaton of CHCl3 in the presence of DCE and in the UV photolysis of NBS in CD2Cl2 even in the presence of Br2.For CH2Cl2-CD2Cl2 mixtures, kH/kD ca. 10 for both NBS-Br2 and NBS-DCE systems.The kinetics of decomposition of NBS in CH2Cl2 in the presence of Br2, initiated by UV light, azobis(isobutyronitrile) and benzoyl peroxide are zero order in NBS, with k for attack of the chain-carrying radical on CH2Cl2 estimated as 55 at 50 deg C.In the presence of DCE, benzoyl peroxide initiated reactions in CH2Cl2 are also zero order, with rates corresponding to a rate constant for ring opening kd ca. 1660.Addition of cyclohexane produces an unexpected increase in rate corresponding to kd ca. 1.5*104, and k for reacton of the succinimide radical with cyclohexane ca. 104.If these rate constants are valid, they require a radical lifetime of ca. 10-4 s, and this and their low value appear inconsistent with a high-energy excited state of the succinimide radical.It is proposed that the chain carrier in the NBS-Br2 system is a reaction product of NBS and a bromine atom, for which some possible structures are suggested.
- Walling, Cheves,El-Taliawi, Gamil M.,Zhao, Chengxue
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- Tandem cyclopropanation with dibromomethane under Grignard conditions
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(Chemical Equation Presented) Tertiary Grignard reagents and dibromomethane efficiently cyclopropanate allylic (and certain homoallylic) magnesium and lithium alcoholates at ambient temperature in ether solvents. Lithium (homo)allyl alcoholates are directly cyclopropanated with magnesium and CH 2Br2 under Barbier conditions at higher temperatures. The reaction rates depend on the substitution pattern of the (homo)allylic alcoholates and on the counterion with lithium giving best results. Good to excellent syn-selectivities are obtained from α-substituted substrates, which are in accord with a staggered Houk model. In tandem reactions, cyclopropyl carbinols are obtained from allyloxylithium or -magnesium intermediates, generated in situ by alkylation of conjugated aldehydes, ketones, and esters as well as from allyl carboxylates or vinyloxiranes. Using this methodology, numerous fragrance ingredients and their precursors were efficiently converted to the corresponding cyclopropyl carbinols.
- Brunner, Gerhard,Eberhard, Laura,Oetiker, Juerg,Schroeder, Fridtjof
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p. 7543 - 7554
(2008/12/22)
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- Scavenger assisted combinatorial process for preparing libraries of tertiary amine compounds
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This invention relates to a novel solution phase process for the preparation of tertiary amine combinatorial libraries. These libraries have utility for drug discovery and are used to form wellplate components of novel assay kits.
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- PHOTOSTIMULATED REACTIONS OF NEOPENTYL IODIDES WITH CARBANIONS IN DMSO BY THE SRN1 MECHANISM
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Neopentyl iodide, 1, reacted under photostimulation with several carbanionic nucleophiles in DMSO.With acetone enolate ion only reduction and dimerization occured, but good yields of substitution products have been obtained with acetophenone, 5, and anthrone, 9, anions as nucleophiles.Nitromethane anion, 7, does not react with 1 under irradiation, but good yields of the substitution products are obtained when the photostimulated reaction is carried out in the presence of acetone enolate ions (entrainment reaction).Inhibition experiments by p-dinitrobenzene and by the radical trap TEMPO, suggest that these reactions occur by the SRN1 mechanism of nucleophilic substitution.The photostimulated reaction of 1,3-diiodo-2,2-dimethylpropane, 15, with 5 gave the disubstitution product 17 and the reduced monosubstitution product 18.It has been found that the monosubstitution product 16 (in which iodine is retained) is not an intermediate of these reactions. 1-iodoadamantane, 12, is more reactive (ca. 4.9 times) than 1 in competitive experiments toward 5 and under photostimulation.
- Penenory, Alicia B.,Rossi, Roberto A.
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p. 605 - 610
(2007/10/03)
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- Preparation and properties of metallacyclobutanes of nickel and palladium
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Bis(phosphine)-3,3-dimethylnickela- and palladacyclobutanes have been prepared by intramolecular C-H insertion reaction of the corresponding dineopentyl metal complexes.Nickelacyclobutane complexes decompose when heated thereby undergoing competitive carb
- Miyashita, Akira,Ohyoshi, Mayumi,Shitara, Hiroaki,Nohira, Hiroyuki
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p. 103 - 112
(2007/10/02)
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- SELECTIVE HYDROBROMINATION OF BRANCHED ALCOHOLS USING PHASE TRANSFER CATALYSIS
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In the presence of quaternary ammonium phase transfer catalysts hydrobromination of branched alcohols proceed via selective SN2 mechanism practically without rearrangements.
- Dakka, Gihad,Sasson, Yoel
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p. 1223 - 1224
(2007/10/02)
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- REACTIONS OF BrCl WITH ALKYL RADICALS.
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It is demonstrated that photohalogenation of low reactivity substrates with BrCl occurs mainly with Cl. selectivity.With tertiary or benzylic hydrogens in the substrate, mainly Br. selectivity is observed.These observations are rationalized, taking into account the relative concentrations of halogen atoms and their respective rates of hydrogen abstractions.The resultant radicals react with BrCl to make (RBr/RCl) in ratios between 1 and 15.
- Skell, P. S.,Baxter, H. N.,Tanko, J. M.
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p. 5181 - 5184
(2007/10/02)
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- Ground- and Excited-State Succinimidyl Radicals in Chain Reactions: A Reexamination
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The succinimidyl radical chemistry reported earlier and previously attributed to the ? state is reproduced, leaving unsettled only its assignment to the ? and ? state.This chemistry, observed in the presence of small amounts of alkenes which scavenge bromine, includes the following: (1) additions to alkenes, (2) additions to arenes, (3) Cl-like substitution selectivities, and (4) ring-openings.The chemistry previously reported under the title "?" is neither as simple nor conclusive as thought earlier.As reported earlier, ring-opening is suppressed by inclusion of bromine, benzene, bromotrichloromethane, or larger amounts of alkenes.These observations indicate the presence of a different chain carrier, formerly labeled as S?.We show now that the S? explanation was not correct with addends benzene, bromotrichloromethane, and olefins, and we identify the specific competitive processes.Reactions in the presence of bromine still show evidence for a third hydrogen abstractor besides an imidyl radical and a bromine atom.At this time there is no unambiguous identification of this third chain intermediate.
- Skell, Philip, S.,Luening, Ulrich,McBain, Douglas S.,Tanko, James M.
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p. 121 - 127
(2007/10/02)
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- Electrophilic cleavages in (CH3)3SnCH2M(CH3)3 (M = Sn, Ge, Si, C). 1. Product distribution
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The extent to which Sn-CH2 and/or Sn-CH3 cleavage occurs in (CH3)3SnCH2M(CH3)3 (M = Sn, Ge, Si) in reactions with several electrophiles has been determined. With iodine and with
- Hawker, Darryl W.,Wells, Peter R.
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p. 821 - 825
(2008/10/08)
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- Pyrylium Mediated Transformations of Neopentylamine
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A series of 2,4-diphenyl-5,6-dihydrobenzochromenylium salts have been prepared.They react with neopentylamine to form the corresponding 1-neopentylquinolinium salts which on thermolysis afford neopentyl halides, trifluoracetate, and thiocyanate, in good yields without rearrangements.
- Katritzky, Alan R.,Singh, Jagadish N.
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p. 421 - 424
(2007/10/02)
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- Quasiphosphonium Intermediates. Part 3. Preparation, Structure, and Reactivity of Alkoxyphosphonium Halides in the Reaction of Neopentyl Diphenylphosphinite, Dineopentyl Phenylphosphinite, and Trineopentyl Phosphite with Halogenomethanes and the Effect of Phenoxy-substituenst on the ...
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The reactions of neopenthyl diphenylphosphinite with chloro-, bromo-, or iodo-methane and of dineopentyl phenylphosphphonite with bromo- or iodo-methane yield crystalline alkoxyphosphonium halides.In deuteriochloroform these intermediates decompose by a first-order process which involves rate-determining collapse of the phosphonium halide ion-pair with SN2-type fission of the alkyl-oxygen bond.Rates for chloride, bromide, and iodide are similar.In a more ionising medium (deuterioacetonitrile) dissociation leads to stabilisation of the intermediates and to deviation from first-order decomposition.Previously determined X-ray diffraction data for the bromides, together with relative rates of decomposition in deuteriochloroform for intermediates in the series Phn(RO)3-nP+MeX- (n = 0,1, or 2) suggest that the stability and reactivity of alkoxyphosphonium intermediates are determined largely by inductive rather than mesomeric effects of ligands.The presence of phenoxy-substituents on phosphorus may cause a tendency towards SN1-type fission of the alkyl-oxygen bond in certain circumstances.
- Hudson, Harry R.,Kow, Aloysius,Roberts, John C.
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p. 1363 - 1368
(2007/10/02)
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- SELECTIVITIES OF pi - AND sigma -SUCCINIMIDYL RADICALS IN SUBSTITUTION AND ADDITION REACTIONS. APPENDIX: RESPONSE TO WALLING, EL-TALIAWI, AND ZHAO.
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A new method for studies of pi -succinimidyl (S// pi ) radicals is described, one that makes possible the study of reactions of this radical with a variety of substrates not accessible by the use of Br//2-NBS. NBS systems containing BrCCl//3 at mole fractions greater than 0. 3 show all the characteristics associated with S// pi behavior, and they function in the presence of olefins which serve as Br//2 scavengers. If CCl//4 is substituted for BrCCl//3, the system is clearly S// sigma . The S// pi behavior is contrasted with S// sigma and Br multiplied by (times) reactivities for H abstractions from a variety of substrates and for additions to tert-butylethylene, isobutylene, and 1,3-butadiene. In early-transition-state systems, for H transfer, the strength of the bond being broken and the strength of the bond being made are not the major factors in determining reactivities. The behavior in late-transition-state systems is influenced by both bond strengths. The S// pi radical shows intermediate behavior. These conclusions are supported by primary deuterium isotope effects for methylene chloride and chlororoform. The Appendix addresses a number of questions raised by the preliminary study of NBS reactions by Walling et al.
- Skell,Tlumak,Seshadri
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p. 5125 - 5131
(2007/10/02)
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- DESIGN AND REACTIVITY OF ORGANIC FUNCTIONAL GROUPS: THE HIGHLY CRYSTALLINE 2-ALKOXY N,N'-DIPHENYL-1,3,2-DIAZAPHOSPHOLANES AND THEIR FACILE CONVERSION INTO ALKYL HALIDES
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Functionalization of alcohols as 2-alkoxy N,N'-diphenyl-1,3,2-diazaphospholanes affords highly crystalline derivatives useful for characterization purposes.These tervalent phosphorus derivatives undergo facile and mild conversion into the corresponding alkyl halides with inversion of configuration.
- Hanessian, Stephen,Leblanc, Yves,Lavallee, Pierre
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p. 4411 - 4414
(2007/10/02)
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- Excited-State ? Succimidyl and Glutarimidyl Radicals: Reversible Ring Opening
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The free-radical isomerization of N-bromosuccinimide to β-bromopropionyl isocyanate has been examined.Of the two varieties of succinimidyl radical (S? or S?), only the ? excited state undergoes the ring opening to the β propionyl isocyanatyl radical.The conversion optimally takes place in >95percent yield.The dependence of NBS concentration along with results obtained from deuterium labeling studies indicate that the ring opening of S? is a reversible process.This explains the failure of N-chlorosuccinimide to produce β-chloropropionyl isocyanate, as well as the increase in ring-opened product for N-bromosuccinimides upon methyl substitution at the 2- and/or 3-position of the succinimidyl ring, since the open-chain radical intermediates are more stable.In the N-bromoglutarimide system, methyl groups on the 2-position are required for the glutarimidyl radicals to undergo the isomerization, ultimately producing isocyanates.The radical-chain nature of these systems is confirmed.
- Tlumak, Robert L.,Day, James C.,Slanga, Joseph P.,Skell, Philip S.
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p. 7257 - 7267
(2007/10/02)
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- Reactions of a Graded Set of Radicals with N-Bromosuccinimide; Two Transition States
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The reactions of N-bromosuccinimide with a series of radicals have been studied.These reactions fall into two categories, the more reactive radicals producing ?-succinimidyl and the less reactive radicals producing ?-succinimidyl.The threshold for the changeover from one reaction domain to the other occurs with radicals less reactive than secondary alkyls.These results are interpreted with two transition states, an in-line transition state for the more reactive radicals and an out-of-plane transition state for the less reactive radicals. An upper limit of 18 kcal/mol is established for the enthalpy difference, HS? - HS?.Two new methods for generating S? radicals are indicated.
- Tlumak, Robert L.,Skell Philip S.
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p. 7267 - 7274
(2007/10/02)
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- Thallium(I) Carboxylate Modification of the Hunsdiecker Reaction
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Treatment of a thallium(I) carboxylate with 1 molar equivalent of bromine gives the corresponding thallium(III) carboxylate dibromide.High yields of primary alkyl bromides are obtained if the thallium(III) carboxylate dibromide is treated with 0.5 mol equiv. of bromine in refluxing carbon tetrachloride.Pyrolysis of the thallium(III) derivative in the absence of added bromine gives a low yield of the corresponding alkyl bromide.The use of thallium(I) carboxylates for the preparation of alkyl bromides in high yields is limited to the salts of primary carboxylic acids.A simple procedure for the preparation of alkyl bromides from carboxylic acids using thallium(I) carbonate has been developed.
- Cambie, Richard C.,Hayward, Rodney C.,Jurlina, Jeffrey L.,Rutledge, Peter S.,Woodgate, Paul D.
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p. 2608 - 2614
(2007/10/02)
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- Arbuzov Reaction of Alkyl and Silyl Phosphites with Halogens involving Four- and Five-co-ordinate Intermediates
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Low temperature 31P n.m.r. spectroscopy and chemical data have been applied to elucidate the mechanism of the Arbuzov-type reaction between phosphites and halogens.Simple and substituted trialkyl, alkyl 1,2-phenylene, and trisilyl phosphites have been allowed to react with chlorine, bromine, and iodine.In some cases intermediate halogenophosphonium salt (2) and in others halogenophosphoranes (3) are observed which then decompose into the corresponding pure highly reactive phosphorohalidates (4).It was possible to prepare stable phosphonium salts from halogenophosphonium salts (2) and halogenophosphoranes (3).
- Michalski, Jan,Pakulski, Marek,Skowronska, Aleksandra
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p. 833 - 836
(2007/10/02)
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- Mechanism of Formation of Grignard Reagents. The Rate of Reaction of Cyclopentyl Bromide with Magnesium Is Transport Limited in Diethyl Ether
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Three observations indicate that the rate of reaction of cyclopentyl bromide with metallic magnesium in several ethereal solvents is limited by the rate of its encounter with the metal.First, the rate of disappearance of cyclopentyl bromide is dependent on the first power of the organic halide concentration, the magnesium surface area, and rate at which the solution is stirred, and is inversely dependent on the solution viscosity (-d/dt infinite 1, A1, ω1, η-1).Second, the activation energy for the reaction in diethyl ether (Ea ca. 2.3 kcal/mol) is consistent with mixing and diffusion as rate limiting.Third, a number of representative alkyl halides react with magnesium in diethyl ether at very similar rates; no organic halides examined react faster than this rate.The dependence of the rate of reaction of neopentyl bromide on solution dielectric constant illustrates transition from transport-limited to non-transport-limited rates, and suggests appreciable charge separation in the chemical step which breaks the carbo-bromine bond.
- Rogers, Harold R.,Deutch, John,Whitesides, George M.
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p. 226 - 231
(2007/10/02)
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