- A Photochemical Probe for Single Electron Transfer in Nucleophilic Aliphatic Substitution: Evidence for Geminate Radical Coupling in the Solvent Cage
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A major effort to establish single electron transfer (SET) as an important pathway in nucleophilic aliphatic substitution reactions has involved the use of cyclizable probes, e.g., 6-iodo-5,5-dimethyl-1-hexene.In order to examine the partition between cyclization and direct radical-radical recombination, we have investigated the ground-state and excited-state chemistry of the 9-phenylfluorenyl anion (9PF-) with neopentyl-type iodides.It has been shown that 9PF- does not react with Me3CCH2I, but it does undergo efficient reaction upon irradiation (Φ=1.0) to yield nucleophilic aliphatic substitution products.With the sterically analogous cyclizable probe 6-iodo-5,5-dimethyl-1-hexene, no ground-state reaction is observed.However, both cyclized and uncyclized products of substitution, 9PFRc and 9PFRu, are produced upon irradiation.Thus photoproducts clearly involving electron-transfer-induced radical intermediates can result without acommpanying free-radical cyclization.These results suggest that, although the observation of cyclized products in the reaction of a cyclizable radical probe with a nucleophile is evidence of a radical intermediate, the absence of such cyclized products does not require the absence of radical intermediates.
- Tolbert, Laren M.,Sun, Xiao-Jing,Ashby, E.C.
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- EVIDENCE FOR SINGLE ELECTRON TRANSFER IN THE REACTION OF A LITHIUM ENOLATE WITH A PRIMARY ALKYL IODIDE
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Evidence for a radical process in the reaction of the lithium enolate of propiophenone with a primary alkyl iodide was obtained by the observation of cyclization of an appropriate radical probe, by the trapping of the radical intermediate and by the comparison of the relative rates of reactions of the probe alkyl iodide with the corresponding bromide and tosylate.
- Ashby, E.C.,Argyropoulos, J.N.
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- Mechanism of Reaction of Geminal Dihalides with Lithium Naphthalenide (LiNp(.-)): Evidence for an Electron Transfer Mechanism. Similarities to the Mechanism of Reaction of Geminal Dihalides with Certain Nucleophiles and Other One-Electron Donors
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The reactions of the sterically hindered geminal dihalides 6,6-dichloro-5,5-dimethyl-1-hexene (1a) and 6,6-diiodo-5,5-dimethyl-1-hexene (1b) with lithium naphthalenide (LiNp(.-)) in THF were investigated in an attempt to compare the results of reactions involving a known one-electron donor (LiNp(.-)) with those involving nucleophiles believed to be one-electron donors.On the basis of radical-trapping studies, deuterium tracer studies, and product studies using cyclizable radical probes, it can be concluded that the reactions studied are very similar to those reported by us earlier involving magnesium metal, LiAlH4, and other nucleophiles.In addition to radical-derived products, the reaction of 1a with LiNp(.-) afforded hydrocarbons, in high yields, that were derived from a carbene intermediate.On the other hand, 1b, on reaction with LiNp(.-), did not yield any carbene-derived hydrocarbons.These results show that the formation of a carbene intermediate, derived from a radical, depends on the nature of the halogen present.It was also found that naphthalene can behave as a hydrogen atom donor toward a radical in the presence of known hydrogen atom donors, such as THF.
- Ashby, E. C.,Desphande, Abhay K.
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p. 4530 - 4535
(2007/10/02)
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- Electron Transfer in the Reactions of Geminal Dihalides with Ph2P(-). Evidence for the Formation of a Carbene Intermediate from a Radical Precursor
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The reactions of two sterically hindered geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (5a) and 6,6-diiodo-5,5-dimethyl-1-hexene (5b), with Ph2P(-) have been found to involve a single electron transfer (SET) pathway.Since the corresponding monochl
- Ashby, E. C.,Deshpande, Abhay K.
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p. 7117 - 7124
(2007/10/03)
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- Mechanism of Reaction of Geminal Dihalides with Magnesium. Evidence for the Formation of Carbenes from Radical Precursors. The Similarity in Reactions of Geminal Dihalides with Magnesium and LiAlH4
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The geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (2a) and 6,6-diiodo-5,5-dimethyl-1-hexene (2b) were allowed to react with Rieke activated magnesium (Mg*) in THF at 25 deg C.Both radical and carbene intermediates were identified by product analyses, by radical trapping experiments using DCPH and THF-d8, and by selective experiments in which isolated intermediates were shown to form the predicted products.Although carbene products predominated over radical products when the dichloride was allowed to react with Mg*, radical products predominated when the diiodide was allowed to react with Mg*.Evidence is presented that indicates that the carbene intermediate arises from a radical precursor.A mechanistic scheme (Scheme 5) is presented that is consistent with all of the observed data.A comparison of the reactions of 2b with Mg* and 2b with LiAlH4 shows that most of the products are common in both reactions thereby providing further evidence for LiAlH4 as a one-electron donor, as is known behavior of Mg*.
- Ashby, E. C.,Deshpande, Abhay K.,Doctorovich, Fabio
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p. 6223 - 6232
(2007/10/02)
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- Mechanism of Reaction of Geminal Dihalides with Lithium Aluminum Hydride. Evidence for Single Electron Transfer as the Major Reaction Pathway
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The reactions of a sterically hindered geminal dichloride 4a and the corresponding diiodide 4b with LiAlH4 (LAH) were found to involve an electron-transfer mechanism.Whereas the monochloro analog of 4a (8) is inert toward LAH, 4a was more reactive.Observation of radical-derived products in the reaction of 4a with LAH indicates that the presence of two chlorine atoms on the same carbon atom results in a favorable reduction potential for 4a, as compared to 8.These results show that LAH can function as a one-electron donor toward geminal dichlorides.It was found that the diiodo analog 4b is more reactive toward LAH than 4a due to the inherent favorable reduction potentials of alkyl iodides compared to chlorides.The reactivity of 4b toward LAH was also found to be greater than that of the monoiodo analog (1b) in keeping with the more favorable reduction potential of the diiodide (4b) compared to the monoiodide (1b).The rates of cyclization of the corresponding haloalkyl radicals generated from 4a and 4b were also determined and were found to be 7.4E5/s for the chloroalkyl and 5.5E5/s for the iodoalkyl radical at 25 deg C.The formation of small amounts of the carbene-derived product, 5, in the reactions of both 4a and 4b with LAH appeared to be preceded by a radical intermediate.
- Ashby, E. C.,Deshpande, Abhay K.
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p. 3798 - 3805
(2007/10/02)
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- Investigation of the Purity of Alkali Metal Diphenylphosphides and Their Reactions with Organic Halides. Evidence for Single Electron Transfer
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For the first time the purity of lithium, sodium, and potassium diphenylphosphide, prepared by various methods, has been evaluated using (31)P NMR spectroscopy.A method was developed to prepare each of the phosphides in a high state of purity.Highly pure potassium diphenylphosphide was then allowed to react with p-iodotoluene in order to determine the effect of purity on the SRN1 nature of this reaction.The results were then compared with literature reports which used less pure KPPh2.The mechanism of reaction of alkyl halides with pure alkali metal diphenylphosphides, using the radical probes 6-halo-5,5-dimethyl-1-hexenes and 1-halo-2,2-dimethylhexanes, was investigated.The results provide the first evidence to support single electron transfer (SET) in the reaction of an alkali metal diphenylphosphide with an alkyl halide.SET was found to be the major reaction pathway in the reaction of hindered alkyl iodides (neopentyl type).On the other hand, SET was found to be a minor pathway in the reaction of the corresponding alkyl bromides and chlorides with PPh2(1-).There was no evidence found for SET in the reactions of unhindered alkyl halides with PPh2(1-) although SET participation cannot be rigorously excluded.
- Ashby, E. C.,Gurumurthy, R.,Ridlehuber, R. W.
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p. 5832 - 5837
(2007/10/02)
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- Competing Radical, Carbanion, and Carbene Pathways in the Reactions of Hindered Primary Alkyl Halides with Lithium Dialkylamides
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A variety of methods were utilized to study the mechanism of reaction of 6-iodo-5,5-dimethyl-1-hexene and its bromo, chloro, and tosylate derivatives with LDA and several other lithium dialkylamides.In the reaction of 6-iodo-5,5-dimethyl-1-hexene with LDA in THF, radical, carbanion, and carbene pathways occured simultaneously.However, when the corresponding bromide was allowed to react with LDA, the radical pathway was minor and when the corresponding chloride or tosylate was allowed to react with LDA, no evidence for radical products was observed.This is the first time that competing radical, carbanion, and carbene pathways have been detected in the reaction of a primary alkyl halide with any nucleophile.
- Ashby, E. C.,Park, B.,Patil, G. S.,Gadru, K.,Gurumurthy, R.
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p. 424 - 437
(2007/10/02)
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- Reactions of 1,1,3,3-Tetramethylcyclobutane on Evaporated Metal Films
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Reactions of 1,1,3,3-tetramethylcyclobutane (TMCB)-hydrogen mixtures on evaporated metal films have shown that both ring scission to 2,2,4-trimethylpentane (TMP) and ring enlargement to 1,1,3-trimethylcyclopentane (TMCP) are dominant on sintered platinum films, while ring scission to TMP predominates on sintered palladium and on unsintered molybdenum films.Unsintered tantalum and molybdenum films gave a large production of iC4 hydrocarbon, particularly above ca. 500 K: ring enlargement was a minor reaction found particularly with tantalum.The homogeneous reaction of TMCB giving a large iC4 production sets in at 600 K.The possibility, suggested by the product distribution, that the reaction of TMCB on Mo and Ta is a metal-assisted free-radical reaction is examined.While it may not be completely excluded that the catalysed ring enlargement on Pt and Pd is a free-radical reaction it is argued that only the previously proposed Rooney-Samman bond-shift mechanism accommodates without added qualifications published facts on bond shifts, including those at quaternary carbon atoms, and also ring enlargements.
- Clarke, John K. A.,Hegarty, Bernard F.,Rooney, John J.
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p. 2511 - 2518
(2007/10/02)
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- Single Electron Transfer in Reactions of Alkyl Halides with Lithium Thiolates
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Single Electron Transfer (SET) in the reaction of lithium thiolates with trityl halides was studied in detail by (1) isolation and identification of all products, (2) studying the effects of the radical trap, dicyclohexylphosphine (DCPH), on product distribution, and (3) studying the effects of light and the presence of p-dinitrobenzene on the reaction rate.The reaction of lithium thiolates with the cyclizable alkyl iodide probe 2,2-dimethyl-1-iodo-5-hexene was also studied.Reactions carried out in the presence of the radical trap, DCPH, yielded up to 22percent hydrocarbon products, suggesting a significant contribution of a SET pathway.Direct spectroscopic detection of radical intermediates was made for reactions of lithium thiolates with well-known one-electron acceptors, such as diaryl ketones, polynuclear hydrocarbons, trityl halides, and 9-bromofluorene.
- Ashby, E. C.,Park, W. S.,Goel, A. B.,Su, Wei-Yang
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p. 5184 - 5193
(2007/10/02)
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- Single Electron Transfer in the Reaction of Enolates with Alkyl Halides
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Single electron transfer (SET) in the reaction of a model system consisting of lithiopropiophenone with primary neopentyl type alkyl halides and tosylate was investigated by (1) the use of an appropriate cyclizable alkyl radical probe, (2) observing the effect of varying the leaving group on reaction rate and product distribution, (3) studying the effect of light, di-tert-butyl nitroxyl radical, and p-dinitrobenzene on the rate of reaction, (4) observing the consequence of varying solvent composition on both the reaction rate and product distribution, and (5) studying the effects of the radical traps, dicyclohexylphosphine and 1,4-cyclohexadiene, on product composition.The results of these studies indicate that single electron transfer is the major reaction pathway involved in the reaction of the enolate with the alkyl iodide in HMPA and that the corresponding bromide and tosylate react by an SN2 process.
- Ashby, E. C.,Argyropoulos, J. N.
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p. 3274 - 3283
(2007/10/02)
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- REACTIVITY AND SELECTIVITY IN THE CYCLIZATION OF SILA-5-HEXEN-1-YL CARBON-CENTERED RADICALS
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A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri-n-butyltin hydride (deuteride).The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially exo-trig) cyclization compared to the all-carbon system.The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system.The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate via the expected exo-trig mode and from a significantly enhanced rate of hydrogen abstraction from TBTH.Both the α- and γ-silyl radicals cyclized via the endo-trig mode at rates close to that of the parent 5-hexen-1-yl radical itself.The cyclizations studied were demonstrated to be irreversible.The kinetic control thus shown by the preferred formation of endo cyclized product from the α- and β-silyl radicals is highly unusual and represents the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule (exo-trig cyclization preferred by 5-hexen 1-yl radicals).Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors.The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no exo mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet.Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.
- Wilt, James W.
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p. 3979 - 4000
(2007/10/02)
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- EVIDENCE FOR SINGLE ELECTRON TRANSFER IN THE REACTION OF ALKOXIDES WITH ALKYL HALIDES
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Evidence for a radical process in the reaction of lithium alkoxides with alkyl iodides was obtained by observation of cyclization of appropriate radical probes, by the trapping of radicals, and by EPR spectroscopic observations relating to the one electron donor properties of alkoxides.
- Ashby, E. C.,Bae, Dong-Hak,Park, Won-Suh,Depriest, Robert N.,Su, Wei-Yang
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p. 5107 - 5110
(2007/10/02)
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- Occurrence of Electron Transfer in the Reduction of Organic Halides by LiAlH4 and AlH3
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A variety of methods have been utilized to detect the occurrence of a single electron transfer pathway in the reduction of alkyl halides by LiAlH4 and AlH3, i.e., (1) product studies of reduction of cyclizable alkyl halides containing the 5-hexenyl group, (2) trapping of intermediate radicals by dicyclohexylphosphine and other trapping agents, (3) direct EPR observation of the trityl radical in the reduction of trityl bromide, and (4) stereochemical studies of the reduction of secondary halides by lithium aluminum deuteride.The extent of electron transfer was found to be a function of the solvent, the substrate, the leaving group, and the hydride reagent.For alkyl iodides, and to a lesser extent bromides, electron transfer was found to be the major reaction pathway; however, no evidence for electron transfer was found for the corresponding chlorides or tosylates.Reduction of (+)-2-octyl iodide by LiAlD4 was found to be much less stereospecific than the corresponding reduction of bromide, chloride, or tosylate, indicating intermediate radical formation in the reduction of the secondary iodide.
- Ashby, E. C.,DePriest, R. N.,Goel, A. B.,Wenderoth, Bernd,Pham, Tung N.
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p. 3545 - 3556
(2007/10/02)
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- Organometallic Reaction Mechanisms. 17. Nature of Alkyl Transfer in Reactions of Grignard Reagents with Ketones. Evidence for Radical Intermediates in the Formation of 1,2-Addition Product Involving Tertiary and Primary Grignard Reagents
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When a Grignard reagent reacts with an aromatic ketone, a radical anion-radical cation pair is formed which can collapse to give 1,2-addition product or dissociate to form a radical anion and a free radical within the solvent cage which in turn can collapse to 1,2-addition product or a conjugate addition product or escape the solvent cage to form pinacol.The 1,2-addition products, which form after dissociation of the radical anion-radical cation pair, show free-radical character as indicated by the cyclized 1,2-addition products formed from the reaction of a tertiary Grignard reagent probe with benzophenone in THF and from the reaction of a primary Grignard reagent probe (neooctenyl Grignard reagent) with benzophenone in ether.The 1,6-addition products, which come about after dissociation of the radical anion-radical cation pair, show free-radical character as evidenced by the cyclized 1,6-addition products formed in all of the reactions which involve the tertiary probe Grignard reagent (in all solvents studied) with benzophenone and 2-MBP and also in the reaction of the neooctenyl probe Grignard reagent with 2-MBP.
- Ashby, E. C.,Bowers, Joseph R.
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p. 2242 - 2250
(2007/10/02)
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- Evidence for Single Electron Transfer in the Reactions of Alkali Metal Amides and Alkoxides with Alkyl Halides and Polynuclear Hydrocarbons
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Evidence for single electron transfer as the major pathway in reactions previously considered to be classic SN1 and SN2 pathways has been obtained.In this connection, the reaction of KOBu-t with trityl bromide has been shown to proceed through the trityl radical, and the reaction of LiN(i-Pr)2 with a primary alkyl iodide probe gave evidence of proceeding by single electron transfer, as indicated by the cyclized nature of the product as a result of a radical intermediate.
- Ashby, E. C.,Goel, A. B.,DePriest, R. N.
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p. 2429 - 2431
(2007/10/02)
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- ELECTRON TRANSFER IN THE REDUCTION OF PRIMARY HALIDES BY METAL HYDRIDES
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The reactions of various main-group metal hydrides with 1-halo-5-hexenes and with 1-halo-2,2-dimethyl-5-hexenes produce both straight chain and cyclized reduction products.The formation of cyclic hydrocarbons clearly indicates the presence of radical intermediates during the course of these reactions.
- Ashby, Eugene C.,DePriest, Robert N.,Goel, Anil B.
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p. 1763 - 1766
(2007/10/02)
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