MERGED BIMOLECULAR SUBSTITUTION AND ELIMINATION<sup>1</sup>

Winstein, S.; Darwish, David; Holness, N. J.

doi:10.1021/ja01593a081

Cited by 50 publications

(22 citation statements)

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“…An equatorial confor~nation and, hence, a-configuration are indicated by the spectral properties of the 7-keto function. The 17 cm-I shift t o higher frequency in the infrared carbonyl absorption of the broino compound IXb as compared to the ketone (Vd) and the bathochroinic shift of only 5 mg in the ultraviolet absorption maximum of IXb from that of Vd are in agreement with the general understanding of the spectral behavior of equatorial a-bromoketones (23,24) and with the properties of a recently described P-bromo-a-tetralone ( 2 5 ) , a simple model of IXb. Finally, a priori consideration of the mode of formation of compounds of structure I X also is in favor of a 6a-bromo orientation.…”

Section: Dehydroabietane Derzvativessupporting

confidence: 83%

Derivatives of Dehydroabietic and Podocarpic Acids

Wenkert¹,

Beak²,

Carney³

et al. 1963

Can. J. Chem.

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The lithium a l u~n i n u~n hydride reduction of nitriles in the aroxnatic resin acid series to aldimines and the conversion of the products to other derivatives are described. The effect of the stereochemistry of the nitriles on the rate of reduction is noted. The transformation of dehydroabietane into a dehydro product is indicated. The syntheses of a desoxydecarboxy-5,6-dehydro-7-keto derivative of podocarpic acid and its optical antipode are discussed. The stereochemistry of hydrogenation of 5,6-dehydro derivatives of podocarpic acid is reported. The effect of a 7-keto group on the rate of hydrolysis of methyl esters of aromatic resin acids is illustrated. The reactions of 6-bromo-7-keto derivatives of methyl podocarpate with bases are portrayed.

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Section: Dehydroabietane Derzvativessupporting

confidence: 83%

Derivatives of Dehydroabietic and Podocarpic Acids

Wenkert¹,

Beak²,

Carney³

et al. 1963

Can. J. Chem.

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“…However, as stated above, the amine likely plays an important role in the breaking of the C-Br bond by solvating the developing cation through non-directive electrostatic bonding. Thus, in a formal sense, the mechanism of the reaction would resemble the merged biinolecular substitution and elimination inechanisin proposed by Winstein, Danvish, and Holness (19). The difference would be in the ionic character of the transition state.…”

Section: Discussion O F Iiesultsmentioning

confidence: 85%

THE MECHANISM OF THE DEHYDROBROMINATION OF TETRA-O-AGETYL-α-D-GLUGOPYRANOSYL BROMIDE

Lemieux¹,

Lineback²

1965

Can. J. Chem.

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The conversion of tetra-O-acetyl-a-D-glucopyranosyl bromide (I) to 2-acetoxy-D-glucal triacetate (11) was found to be catalyzed by secondary amines containing n-alkyl groups. Substitution a t the a-positions of the amine reduced the catalysis. Although triethylamine and tri-n-propylamine did not serve as useful catalysts, the less hindered and strongly basic 1,4-diazabicyclo[2,2,2]octane was more effective than diethylamine. The reaction is strongly promoted by polar solvents and added salts-especially tetra-n-butylammonium bromide. The course of the reaction involves direct elimination of hydrogen bromide and this reaction is in competition with the formation of acetylated N-glucosides. A kinetic study of the reaction using a variety of amines led to the conclusion that the first stage of the reaction is dissociation of I to glycosyloxocarbonium ion strongly solvated by the amine. Depending on the structure of the amine, the second stage of the reaction may involve transfer of the 2-proton to the amine t o form 11 a t a faster or slower rate than the formation of the N to C1 bqnd of the acetylated N-glucoside. Conditions were established for the near quantitative conversion of 1 t o I1 in about 1/100th the reaction time reported in the literature.

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“…The elimination reaction is a lesser known but well recognized part of the spectrum of E2 eliminations. When first reported by Winstein et al 23 with cyclohexyl substrates, it was called, for want of a better understanding, the 'merged' mechanism, connoting an elimination that occurred in a pathway destined for S N 2 substitution. Thus, this elimination accompanied S N 2 reactions of weakly basic, good nucleophiles, and gave rise to the classification, albeit somewhat controversially, E2C elimination.…”

Section: Reactions With Imentioning

confidence: 99%

“…Reactions with the cyclohexyl compounds 1b-d are particularly striking in that they parallel results reported years ago for cyclohexyl halides and tosylates. That is, Winstein et al 23 noted that cis-4-tert-butylcyclohexyl tosylate gave more ene than the trans isomer in reactions with I À , Br À and PhS À . Eliel and Haber 27 found in reactions with PhS À that the rate of elimination from cis-4-tert-butylcyclohexyl bromide was 66 times faster than from the trans isomer.…”

Section: Reactions With Imentioning

confidence: 99%

Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions

Liu

Shine

Zhao

1999

J. Phys. Org. Chem.

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Reactions of 5-(alkoxy)thianthrenium perchlorates (1) with weakly basic nucleophiles Br À , IÀ and PhS À (X À ) in MeCN and DMSO led to S N 2 substitution, E2C elimination, and reaction at sulfonium sulfur to extents depending on the structure of the alkoxy group (RO) in 1 and the nucleophile. Three types of reaction occurred with R = cyclopentyl (1a), cyclohexyl (1b), cis-(1c) and trans-4-methylcyclohexyl (1d) and cycloheptyl (1e), and X À = Br À and I À . That is, S N 2 reaction gave RX and thianthrene 5-oxide (ThO), E2C reaction gave cycloalkene and ThO and reaction at sulfonium sulfur gave X 2 , thianthrene (Th) and cycloalkanol (ROH). Earlier work with R = Me (1f) and Et (1g) and X À = I À , Br À had shown that only S N 2 reaction occurred. In contrast with reactions of halide ions, reactions of PhS À with 1b-g occurred only at sulfonium sulfur, giving Th, ROH and PhSSPh (DPDS). For comparison with 1, reactions of Ph 2 S OMe (2) with I À and PhS À were carried out. Reaction with I À gave only Ph 2 S=O and MeI (S N 2). Reaction with PhS À gave very little PhSMe (S N 2) but mainly Ph 2 S, MeOH, and DPDS from reaction at sulfonium sulfur. The differences in nucleophilic pathways (PhS À vs Br À and I À ) in reactions with 1 and 2 are attributed to differences in thiophilicities of the nucleophiles. The thiophilicity of PhS À dominates its reactions with 1 and 2. The direction toward products (Th, ROH and DPDS) in these reactions is compounded by the ease of displacement of alkoxide from 1 and 2 by PhS À , and the ease with which, subsequently, thiophilic PhS À attacks sulfenyl sulfur in the resulting phenylthiosulfonium ion.

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MERGED BIMOLECULAR SUBSTITUTION AND ELIMINATION¹

Cited by 50 publications

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Derivatives of Dehydroabietic and Podocarpic Acids

Derivatives of Dehydroabietic and Podocarpic Acids

THE MECHANISM OF THE DEHYDROBROMINATION OF TETRA-O-AGETYL-α-D-GLUGOPYRANOSYL BROMIDE

Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions

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MERGED BIMOLECULAR SUBSTITUTION AND ELIMINATION1

Cited by 50 publications

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Derivatives of Dehydroabietic and Podocarpic Acids

Derivatives of Dehydroabietic and Podocarpic Acids

THE MECHANISM OF THE DEHYDROBROMINATION OF TETRA-O-AGETYL-α-D-GLUGOPYRANOSYL BROMIDE

Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions

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MERGED BIMOLECULAR SUBSTITUTION AND ELIMINATION¹