摘要:

Abstractα‐Elimination denotes the removal of two substituents from the same carbon atom. Present‐day knowledge of these reactions, their intermediates, and end‐products is reviewed, with particular emphasis on the question of the occurrence of carbenes and the role of organometallic (carbanion) interme

ISSN:0570-0833    

DOI:10.1002/anie.196500011

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

摘要:

AbstractThe predictions of Hückel's rule gave the incentive to numerous investigations which led ultimately to a new definition of aromatic character, and which have added greatly to our knowledge of the properties and reactivity of aromatic compounds. Increasing use was made of physical measurements as criteria for the aromaticity of a compound. The first member (n = O) in the series of Hückel's (4n + 2) π systems is the cyclopropenylium cation. The predictions regarding the stability of this system have been confirmed at least qualitatively by the synthesis of cyclopropenylium salts. Properties and reactions of cyclopropenylium compounds and cyclopropenones were investigat

ISSN:0570-0833    

DOI:10.1002/anie.196500101

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

摘要:

AbstractKnowledge of the oxide halides has made significant advances in recent years as a result of new methods of preparation. Moreover, the systematic study of older methods has led to versatile syntheses which permit the preparation of many new oxide halides. A brief outline of the scope, limitations, and difficulties of the various processes is given together with a number of examples.

ISSN:0570-0833    

DOI:10.1002/anie.196500221

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

摘要:

AbstractThe empirical parameters of the polarity of solvents which have been determined recently are reviewed: Y‐values (Winstein, Grunwald), X‐values (Gielen, Nasielski), Ω‐values (Berson, Hamlet, Mueller), Z‐values (Kosower), ET‐values (Dimroth, Reichardt, Siepmann, Bohlmann), R‐ and S‐values (Brownstein). Their field of application and interrelationships are discussed. The theoretical principles of the solvent effect on the reaction rate and light absorption of organic compounds are briefly described, and mention is made of attempts to correlate the physical characteristics of the solvent with the empiri

ISSN:0570-0833    

DOI:10.1002/anie.196500291

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

摘要:

AbstractProtons can be very mobile in condensed phases; the length of time they are present in any one place is minute. The significance of proton mobility in the course of chemical reactions is considered in the case of acid‐catalysed esterification by the normal carbonyl addition mechanism. – The properties of the protons in the carboxylic acid/alcohol system can be discussed on the basis of a semiconductor term diagram. The high mobility of the catalysing protons in the semiconductor system suggests that formation of the esterification intermediates occurs in two reaction steps which proceed at different rates. In the first rapid step the transition state is reached by proton transfers via hydrogen bridges. The proton rearrangement involved proceeds so rapidly that the heavy molecular skeletons retain their mutual spatial positions (Franck‐Condon principle for proton‐catalysed reactions). In the second slower step, the intermediate is formed by rearrangement of the groups of atoms. – Decomposition of the intermediate, with formation of either the starting materials or the end products, proceeds via a mechanism similar to that for the

ISSN:0570-0833    

DOI:10.1002/anie.196500401

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

摘要:

AbstractEliminations from olefins are very often initiated by bases, and usually lead to formation of acetylene derivatives. In view of the numerous side‐reactions and subsequent additions or rearrangments observed, the nature of the base used is of considerable significance. Organometallic bases are very versatile. In addition to dehydrohalogenation, dehalogenation by metals, and also thermolytic and photolytic eliminations. (e.g. the retrodiene reaction with exchange of substituents) are discussed. Elimination from low‐membered cyclic olefins yields cycloalkynes having strained ring‐systems, and the existence of these can be demonstrated by trapping them. The mechanisms known for β‐elimination (E2, E1 and E1cB) occur also in the case of olefins; however – due to the sp2‐hybridization of the carbon atoms taking part – they are realized with different rates of reaction relative to saturated compounds. α‐Eliminations from olefines having aryl residues in the β‐position lead to formation of arylacetylenes by rearrangement of the carbon skeleton. The mechanism of this reaction, which is known as the Fritsch‐Buttenberg‐Wiechell rearrangement, is discussed in considerable detail, and several variations of the reaction are considered. According to present‐day knowledge carbenes are involved in α‐eliminations only when both β‐positions of the olefin are occupied by aliphatic substituents, or when they are occupied by aromatic residues where rearrangement to acetylenes is impossible for steric reasons (as for example with 9‐chloromethylenefluorene). With organolithium compounds a number of dehydrohalogenations, which are formally β‐eliminations, actually proceed via deprotonation on the halogenbearing carbon atom (α‐metallation). α‐Metallations are the rate‐determining steps when ether is used as solvent, but proceed quickly in tetrahydrofuran even at low temperatures. Compounds of the type\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$\raise1pt\hbox{$>$}{\rm C}\raise1pt\hbox{=\kern-3.45pt=}{\rm C(Li)Cl}$\end{document}may be prepared by this method and are

ISSN:0570-0833    

DOI:10.1002/anie.196500491

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.196500692

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.196500702

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.196500712

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY

ISSN:0570-0833    

DOI:10.1002/anie.196500722

出版商:Hüthig&Wepf Verlag    

年代:1965

数据来源: WILEY