Steric Acceleration Research Articles

Kinetics and mechanisms of the Bamberger rearrangement. Part 4. Rearrangement of sterically hindered phenylhydroxylamines to 4-aminophenols in aqueous sulphuric acid solution

The rates of the Bamberger rearrangement of sterically hindered phenylhydroxylamines have been determined in aqueous sulphuric acid solution and the substituent effects (in particular, the steric effects) are discussed. The rate constants for phenylhydroxylamines with 2-substituents (Me, Cl, l) satisfied the Taft equation: log krel=ρ*σ*–δES with ρ*– 1.93 and δ–1.16. The result shows that steric hindrance of the substituents, in addition to the electron-donating effect, has an accelerating effect on the rates of the Bamberger rearrangement. The rate constants for 3-substituted 2-methylphenylhydroxylamines were generally greater than those for 5-substituted 2-methylphenylhydroxylamines. The difference was attributed to the ‘buttressing effect’ of neighbouring 3-substituents. This is the first example of steric acceleration of the Bamberger rearrangement.

Preparation, structure and properties of polyesters from substituted β-lactones

The anionic polymerization reactions of a series of a,a-disubstituted-3propiolactones were investigated for the effect of the o-substituents and reaction conditions on the rate and mechanism of the propagation reactions and for the effect of polymer structure on physical properties, especially on crystalline properties. An unexpected, apparent steric acceleration was observed in the rate studies, and all polymers, even those prepared from racemic mixture of chiral monomers, were found to be crystalline with melting points having an unexplicable, direct relationship with the size ratio of the two a-substituents. The preparation and properties of a new polymer, poly-o-methylene-B-propiolactone, is also reported.

Nucleophilic substitution at sulphonyl sulphur. Part 2. Hydrolysis and alcoholysis of aromatic sulphonyl chlorides

Kinetics of hydrolysis, methanolysis and ethanolysis of furan-2- and -3-, thiophen-2- and -3-, and benzene-sulphonyl chlorides have been measured. Fair correlations with Taft σ* values for heterocycles are found; more satisfactory trends are observed by applying the two-parameter (polar and steric) Taft–Pavelich equation, particularly for the hydrolysis reaction including data for aliphatic sulphonyl chlorides. In this case the negative δ value, which is related to the steric parameter, is consistent with steric acceleration due to relief of strain in the transition state. Alcoholysis rates of substituted thiophen-2-sulphonyl chlorides (5-CH3, 5-Cl, 4-NO2, and 5-NO2) have been also measured in order to compare the substituent effects with those already observed for hydrolysis. The data are in accord with previous findings, that an SN2 type mechanism takes place which is shifted toward an SN1 process (looser transition state) or an SAN process (tighter transition state) in the hydrolysis and alcoholysis reactions, respectively.

Steric acceleration of reductive desulfonation of 1,8-naphthalenedisulfonate by an NADH model compound

1,8-Naphthalenedisulfonate was reductively desulfonated by N-benzyl-1,4-dihydronicotinamide, whereas no desulfonation was observed for the 1,6-isomer. The high reactivity of the 1,8-isomer is attributed to the steric-strain that enforces the shift of the initial state with the sp 2 -carbon close to the transition state with the sp 3 - carbon.

Nucleophilic displacement of N-benzyl groups: effect of pyridinium on rates and mechanism

Steric acceleration by α-substituents in pyridine leaving groups is quantitatively assessed. Constraint of α-phenyls to near planarity forms superior leaving groups for S N2 displacements. α-t-Butyl groups significantly increase S N1 dissociation.

Mechanisms for the solvolytic decompositions of nucleoside analogues— I: Acidic hydrolysis of 2-substituted 1-(1-ethoxyethyl)benzimidazoles

A few 2-substituted 1-(1-ethoxyethyl)benzimidazoles have been prepared and the rate constants for their hydrolysis measured at various temperatures and oxonium ion concentrations. The formal kinetics followed and the effect of varying the 2-substitution on the hydrolysis rate suggest that the acid-catalyzed cleavage of these compounds involves a rapid initial protonation of the benzimidazole ring and a subsequent rate-limiting heterolysis of the protonated substrate to form a free nitrogen base and an ocarbenium ion derived from the ethoxyethyl group. The values obtained for the entropy of activation are consistent with the assumed unimolecular nature of the rate-limiting step. The effects of 2-substituents on the acidities of the protonated substrates and their heterolysis rates have been compared. The latter partial reaction has been suggested to be the subject of steric acceleration.

Assessment of the methoxy substituent effect in the gas phase elimination of methoxyalkyl acetates

The gas phase elimination of 4-methoxybutyl acetate in the temperature range of 370–420°C and pressure range of 67–216 Torr is homogeneous, unimolecular and follows a first order kinetics. The rates have been measured in a static system and the reactions carried out in seasoned vessels. Steric acceleration seems to be an important factor when the methoxyalkyl chain of the ester is increased.

ChemInform Abstract: EFFECT OF SP2 AND SP HYBRIDIZED ATOMS ON BARRIERS TO ROTATION IN HIGHLY SUBSTITUTED ETHANES. STERIC ACCELERATION OF CONFORMATIONAL PROCESSES

Chemischer InformationsdienstVolume 10, Issue 2 Physical Organic Chemistry ChemInform Abstract: EFFECT OF SP2 AND SP HYBRIDIZED ATOMS ON BARRIERS TO ROTATION IN HIGHLY SUBSTITUTED ETHANES. STERIC ACCELERATION OF CONFORMATIONAL PROCESSES J. E. ANDERSON, J. E. ANDERSONSearch for more papers by this authorC. W. DOECKE, C. W. DOECKESearch for more papers by this authorH. PEARSON, H. PEARSONSearch for more papers by this authorD. I. RAWSON, D. I. RAWSONSearch for more papers by this author J. E. ANDERSON, J. E. ANDERSONSearch for more papers by this authorC. W. DOECKE, C. W. DOECKESearch for more papers by this authorH. PEARSON, H. PEARSONSearch for more papers by this authorD. I. RAWSON, D. I. RAWSONSearch for more papers by this author First published: January 9, 1979 https://doi.org/10.1002/chin.197902060Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume10, Issue2January 9, 1979 RelatedInformation

Steric acceleration in the decomposition of pyridinium cations: a safe alternative to the diazonium conversion of aryl amines into aryl thiocyanates

The diphenyl-dihydronaphthopyrylium thiocyanate (8) reacts with aryl amines to give the corresponding N-aryl-dihydrobenzoquinolinium thiocyanates (9)(average yield 88%) which thermolyse smoothly at ca. 200 °C to the aryl thiocyanates (average yield 69%).

The transition state in ester pyrolysis. Part 9. On the ‘surface-catalysed’ mechanism for the elimination

The ‘surface-catalysed’ mechanism of ester pyrolysis, recently proposed to account for the difference between observed alkene product distributions, and those predicted on the basis of alkene thermodynamic stability, is unsoundly based. Not only can all the known product patterns be accounted for primarily in terms of steric and statistical factors, the electronic contribution to thermodynamic stability being unimportant but, in addition, the classical demonstration of the supposed importance of the thermodynamic effect is shown to be experimentally incorrect.Moreover where it has been examined the elimination pattern is the same in reactors with both active and deactivated surfaces which further rules out surface catalysis as an explanation of these patterns. Steric acceleration is emphasized as a factor governing the elimination pattern and is demonstrated by the marked increase in reactivity (per β-hydrogen) along the series: ethyl acetate < 3-methylbutyl acetate < 3,3-dimethylbutyl acetate.

Steric Effects in Forward and Reverse Menschutkin Reactions of Some Pyridines, Quinolines and Thiazoles

The effects of ortho alkyl substituents on the rates of methylation of pyridine and demethylation of N-methylpyridinium iodides are compared (some quinolines are included as 'substituted pyridines'). For monosubstituted compounds, substituent steric effects are twice as important in the methylation (forward) reaction as in the demethylation reaction. In contrast, the ratio is 3 : 1 for disubstituted compounds, because the substituent effects are additive in the reverse reaction but greater than additive in the forward reaction. Some thiazole results are also reported and, here, steric effects on the forward reaction are small while no steric acceleration of the reverse reaction is evident.

Effect of sp 2 and sp hybridised atoms on barriers to rotation in highly substituted ethanes. Steric acceleration of conformational processes

Barriers to rotation about the central bond of the 1,2,2-trimethylbutyl (triptyl) group, i.e. barriers to rotation of the t-butyl group, are reported for a series of triptyl compounds But–C(CH3)2X, where the atom in X bonded to the triptyl group is an sp2 or sp hybridised carbon. Barriers in some similar compounds are also reported. Results are discussed in terms of the interactions arising during rotation, and of steric acceleration of conformational processes.

The mechanism of the gas-phase pyrolysis of esters. Part 7. The effects of substituents at the acyl carbon

Some t-butyl esters of substituted acetates (RCH2CO2But) have been prepared and pyrolysed at temperatures between 620 and 519 K. The rate data give an approximate correlation with σI values (ρ=ca. 1.4 at 600 K), deviations being most probably due to steric acceleration of elimination when the substituent at the acyl carbon is bulky. This is also indicated by the fact that isopropyl privalate and t-butyl pivalate each pyrolyse more rapidly than their corresponding acetates whereas on electronic grounds alone the reverse reactivity order should obtain. An alternative explanation of these anomalies is that hyperconjugation leads to increased nucleophilicity of the carbonyl oxygen, and this may in particular account for the high reactivity of t-butyl trimethylsilylacetate.

Kinetics of base hydrolysis of cis-aminebromobis(ethylenediamine)-cobalt(III) Complexes in aqueous solution at 25 °C

Rate constants, kOH, are reported for the base hydrolysis of 11 penta-aminebromocobalt(III) complexes, cis-[CoBr(en)2(NH2R)]2++[OH]–→[Co(OH)(en)2(NH2R)]2++ Br–, at 25 °C and I= 0.1 mol dm–3. For the straight-chain amines (R = Me, Et, Prn, Bun, n-pentyl, or n-hexyl) the values of KOH are essentially constant at 82 ± 3 dm3 mol–1 s–1. These results are in contrast to previous reports that the Prn derivative undergoes base hydrolysis 5–6 times faster than the methyl and ethyl derivatives. For the straight-chain amines the value of KOHBr/KOHCl is ca. 62. Activation parameters for the base hydrolysis of the Prn derivative are ΔH‡= 89 ± 2 kJ mol–1 and ΔS‡= 96 ± 4 J K–1 mol–1 at 298 K. The positive ΔS‡ is consistent with an SN1(CB) mechanism. Bulky amines such as cyclopropylamine and 2,2-dimethoxyethylamine lead to significant increases in the rates of base hydrolysis, probably due to steric acceleration of the ionisation step. With α, ω-aminoalkanols there is evidence for specific hydrogen-bonding effects leading to increased base-hydrolysis rates.

Configurational Isomerization of α-aryl-vinyllithium: kinetics, mechanism, and steric acceleration

Configurational Isomerization of α-aryl-vinyllithium: kinetics, mechanism, and steric acceleration

Steric acceleration of lactonization reaction: an analysis of "stereopopulation control"

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSteric acceleration of lactonization reaction: an analysis of "stereopopulation control"Claudia Danforth, Allen W. Nicholson, John C. James, and Gordon M. LoudonCite this: J. Am. Chem. Soc. 1976, 98, 14, 4275–4281Publication Date (Print):July 1, 1976Publication History Published online1 May 2002Published inissue 1 July 1976https://doi.org/10.1021/ja00430a044RIGHTS & PERMISSIONSArticle Views300Altmetric-Citations31LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (850 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: STERIC ACCELERATION IN ALKYLDIAZENE (AZOALKANE) DECOMPOSITIONS. EVIDENCE FOR A CONCERTED MECHANISM

Chemischer InformationsdienstVolume 7, Issue 9 Preparative Organic Chemistry ChemInform Abstract: STERIC ACCELERATION IN ALKYLDIAZENE (AZOALKANE) DECOMPOSITIONS. EVIDENCE FOR A CONCERTED MECHANISM A. W. GARNER, A. W. GARNERSearch for more papers by this authorJ. W. TIMBERLAKE, J. W. TIMBERLAKESearch for more papers by this authorPAUL S. ENGEL, PAUL S. ENGELSearch for more papers by this authorROBERT A. MELAUGH, ROBERT A. MELAUGHSearch for more papers by this author A. W. GARNER, A. W. GARNERSearch for more papers by this authorJ. W. TIMBERLAKE, J. W. TIMBERLAKESearch for more papers by this authorPAUL S. ENGEL, PAUL S. ENGELSearch for more papers by this authorROBERT A. MELAUGH, ROBERT A. MELAUGHSearch for more papers by this author First published: March 2, 1976 https://doi.org/10.1002/chin.197609072Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume7, Issue9March 2, 1976 RelatedInformation

The mechanism of the gas-phase pyrolysis of esters. Part IV. Effects of substituents at the β-carbon atom

Rates of gas-phase thermal decomposition of some 2-substituted ethyl acetates and 3-substituted propyl acetates into acetic acid and substituted alkenes have been measured between 368.7 and 429.2 °C. The data for 2-aryl-ethyl a acetates give a Hammett correlation with ρ+0.2 at 650 K, which taken along with previous data for the effect of α-aryl substituents (ρ–0.63 at 625 K) confirm that electron return to the β-carbon does not compensate for the electron withdrawal from the α-carbon during the transition state for the reaction so that the process is not fully concerted. These data, and comparison of rates of elimination of 2-substituted ethyl acetates with those of the corresponding 3-substituted propyl acetates provides the first unified and consistent interpretation of the effects of β-substituents on the rates of pyrolysis of esters. The data show that the conjugative effects of 3-substituents play a major role in determining the overall rate of elimination for compounds with β-substituents, and that the anomalous activating effect of β-alkyl substituents is not due to the stabilising effect upon the incipient α-carbonium ion as reported in the literature, but rather to steric acceleration.

Lignans and related phenols. Part XV. Remote substituent effects on the rates and products of some reactions of aryltetrahydronaphthalenes

An unambiguous example of the steric acceleration of a simple solvolysis at an uncongested site by a remote substituent is given. Other evidence of remote control of product formation has been obtained and related to the design of cancer-inhibitory podophyllotoxins. The findings have been applied to the synthesis of a peltatin (VII) which is stable and physiologically active in both acid and alkaline media. Evidence of extreme compression, ready extrusion of substituents, and encagement of other molecules is presented.

Studies of chelation IV. Steric influences on the chelation of ditertiary phosphine complexes of group VI metal carbonyls

The preparation of the complexes [M(CO) n (dcpe)] [M  Cr, Mo, W; n  4, 5; dcpe is ((cyclo-C 6H 11) 2PCH 2) 2] is reported. Attempts to prepare [M(CO) 2(dcpe) 2] by many different methods gave only cis-[M(CO) 4(dcpe)] and [M(CO) 5(dcpe)]. Heating cis-[M(CO) 4(dcpe)] with (Me 2PCH 2) 2(dmpe) gives cis-[M(CO) 2(dmpe) 2] only. These observations are explained in terms of unfavourable intramolecular non-bonded interactions between substituents at phosphorus. The rate of chelation of [M(CO) 5(dcpe)] to give cis-[M(CO) 4(dcpe)] has been measured at various temperatures in the range 360–420 K. The activation parameters indicate the dominance of a dissociative process leading to the observed steric acceleration in the chelation step. The rate of chelation is correlated satisfactorily with the ligand cone angle; the operation of an apparent saturation effect is noted.