Elimination Reactions of α-Halogenated Ketones. V.1Kinetics of the Bromide Ion Promoted Elimination Reaction of 2-Benzyl-2-bromo-4,4-dimethyl-1-tetralone in Solvent Acetonitrile

1961 ◽  
Vol 83 (18) ◽  
pp. 3812-3815 ◽  
Author(s):  
Dennis N. Kevill ◽  
Norman H. Cromwell
Keyword(s):  
Elimination Reaction ◽  
Bromide Ion ◽  
Elimination Reactions

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct

2019 ◽  
Vol 123 (41) ◽  
pp. 8776-8786
Author(s):  
Timothy M. Brown ◽  
Blanton R. Gillespie ◽  
Mallory M. Rothrock ◽  
Anthony J. Ranieri ◽  
Melinda K. Schueneman ◽  
...  
Keyword(s):  
Elimination Reaction ◽  
Vibrationally Excited ◽  
Elimination Reactions

The Formation of Alkenes by the Thermal Elimination Reaction of N-Methyl-4-alkoxypyridinium Iodides. II. An Investigation of the Mechanism

1972 ◽  
Vol 50 (8) ◽  
pp. 1188-1191
Author(s):  
George H. Schmid ◽  
Aaron W. Wolkoff
Keyword(s):  
Elimination Reaction ◽  
Product Composition ◽  
Thermal Elimination ◽  
Leaving Group ◽  
Elimination Reactions ◽  
Leaving Groups

A comparison of the products from elimination reactions of a number of compounds containing various leaving groups with those containing the N-methyl oxypyridinium leaving group suggests that the elimination is not occurring by means of a simple E1 mechanism. Changing the anion of the salt from iodide to methyl-sulphate and tetrafluoroborate affects the product composition indicating that the anion is taking part in the reaction. The mechanism of this reaction appears to be on the E1-E2 borderline.

Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD3CD2CHF2

2015 ◽  
Vol 119 (17) ◽  
pp. 3887-3896 ◽  
Author(s):  
Leah N. Wormack ◽  
Meghan E. McGreal ◽  
Corey E. McClintock ◽  
George L. Heard ◽  
D. W. Setser ◽  
...  
Keyword(s):  
Elimination Reaction ◽  
Elimination Reactions

Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base

1992 ◽  
Vol 70 (4) ◽  
pp. 1195-1203 ◽  
Author(s):  
John W. Bunting ◽  
Andrea Toth ◽  
James P. Kanter
Keyword(s):  
Reaction Mechanisms ◽  
Substituent Effects ◽  
Hydroxide Ion ◽  
First Order ◽  
Pyridinium Cation ◽  
Rate Determining Step ◽  
Bromide Ion ◽  
Aqueous Base ◽  
Elimination Reactions ◽  
Pyridinium Cations

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH < 5.16 (slope = −0.30), and a change in rate-determining step to bromide ion departure for pKBH > 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.

Mechanisms of elimination reactions. 40. Attempted study of stereochemistry of elimination from 2-(p-nitrophenyl)ethyltrimethylammonium ion. Base-promoted cis–trans isomerization of p-nitrostyrene-β-d

1986 ◽  
Vol 64 (6) ◽  
pp. 1026-1030 ◽  
Author(s):  
Brent R. Dohner ◽  
William H. Saunders Jr.
Keyword(s):  
Amine Oxide ◽  
Quaternary Salt ◽  
Elimination Reaction ◽  
Trans Isomerization ◽  
Reversible Addition ◽  
Elimination Reactions

Stereospecifically deuterated ArCHDCHDNMe3+ I − and ArCHDCHDNMe2O have been prepared, where Ar=C6H5 and p-NO2C6H4. When Ar=C6H5, the elimination reaction of the quaternary salt with ethoxide in ethanol goes with >98% anti stereochemistry, and the Cope elimination of the amine oxide with >98% syn stereochemistry. When Ar=p-No2C6H4, however, both reactions lead to apparent 50:50 anti/syn product. Subjection of (E)-p-nitrostyrene-β-d to the conditions of both the ethoxide-promoted and Cope eliminations results in complete cis–trans equilibration. No loss of deuterium from p-nitrosryrene-α-d occurs under either set of conditions, excluding isomerization via an α-arylvinyl carbanion. The most likely mechanism for isomerization is reversible addition of ethoxide under E2 conditions and ArCHDCHDNMe2O under Cope conditions to the β-carbon of p-nitrostyrene. The cis–trans isomerization of the p-nitrosryrene is sufficiently rapid to preclude determination of the stereochemistry of base-promoted eliminations leading to it.

Stereochemistry and Mechanism of Reactions Catalyzed by Tyrosine Phenol-Lyase from Escherichia intermedia

1987 ◽  
Vol 42 (4) ◽  
pp. 307-318 ◽  
Author(s):  
M. M. Palcic ◽  
S.-J. Shen ◽  
E. Schleicher ◽  
H. Kumagai ◽  
S. Sawada ◽  
...  
Keyword(s):  
Bond Cleavage ◽  
Elimination Reaction ◽  
Carbon Bond ◽  
Single Base ◽  
Enzyme Substrate ◽  
Carbon Carbon Bond ◽  
Tyrosine Phenol Lyase ◽  
Elimination Reactions

Stereochemical studies on tyrosine phenol-lyase from Escherichia intermedia have shown that the α,β-elimination reactions of ʟ-serine and ᴅ- and ʟ-tyrosine proceed with retention of config­uration at C-β. Stereospecifically (β-tritiated ʟ-serine is slowly racemized at C-β Deuterium from the α-position of ʟ-tyrosine is partially transferred to C-4 of the phenol formed when the α,β- elimination reaction is carried out in H2O, although no transfer of α-1H in 2H2O was seen. The result favors tautomerization of the p-hydroxyphenyl to a cyclohexadienonyl moiety prior to carbon-carbon bond cleavage. In the conversion of ʟ- to ᴅ-alanine catalyzed by tyrosine phenol- lyase, some a-hydrogen recycling is observed, pointing to a single-base racemization mechanism. Attempts to demonstrate cofactor motion during racemization by NaBH4 reduction of [3H]PLP- enzyme: ᴅ- and ʟ-alanine complexes failed, but showed that, as in other PLP enzymes, the holoenzyme is reduced preferentially from the Re face with respect to C-4′ of PLP and enzyme- substrate complexes preferentially from the Si face.

Terpenoid biosynthesis and the stereochemistry of enzyme-catalysed allylic addition—elimination reactions

1991 ◽  
Vol 332 (1263) ◽  
pp. 123-129 ◽  
Keyword(s):  
Chain Elongation ◽  
Farnesyl Pyrophosphate ◽  
Reaction Intermediates ◽  
Elimination Reaction ◽  
Farnesyl Pyrophosphate Synthase ◽  
Internal Motions ◽  
Dimethylallyl Pyrophosphate ◽  
Elimination Reactions ◽  
Pentalenene Synthase ◽  
Intramolecular Transfer

Allylic addition—elimination reactions are widely used in the enzyme-catalysed formation of terpenoid metabolites. It has earlier been shown that the isoprenoid chain elongation reaction catalysed by farnesyl pyrophosphate synthase involving successive condensations of dimethylallyl pyrophosphate (DMAPP) and geranyl pyrophosphate (GPP) with isopentenyl pyrophosphate (IPP) corresponds to such an S E' reaction with net syn stereochemistry for the sequential electrophilic addition and proton elimination steps. Studies of the enzymic cyclization of farnesyl pyrophosphate (FPP) to pentalenene have now established the stereochemical course of two additional biological S E' reactions. Incubation of both (9R)-and (9S)-[9- 3 H, 4,8- 14 ] FPP with pentalenene synthase and analysis of the resulting labelled pentalenene has revealed that H-9 re of FPP becomes H-8 of pentalenene, while H-9 si undergoes net intramolecular transfer to the adjacent carbon, becoming H-l re (lH-loc) of pentalenene, as confirmed by subsequent experiments with [10- 2 H, 11- 13 C]FPP. These results correspond to net anti-stereochemistry in the intramolecular allylic addition—elimination reaction. The stereochemical course of a second S E < reaction has now been examined by analogous incubations of (4 S , 8 S )-[4,8- 3 H, 4,8- 14 C ]FPP and (4 R ,8R)-[4,8- 3 H, 4,8- 14 C ]FPP with pentalenene synthase. Determ ination of the distribution of label in the derived pentalenenes showed stereospecific loss of the original H-8 si proton. Analysis of the plausible conformation of the presumed reaction intermediates revealed that the stereochemical course of the latter reaction cannot properly be described as either syn or anti, since cyclization and subsequent double bond formation require significant internal motions to allow proper overlap of the scissile C—H bond with the developing carbocation.

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