Eliminations from Olefins

Abstract

increase in the rate of reaction with increasing dielectric constant of the solvent found in the acid-catalysed hydrolysis of esters [1,221. VI. Concluding RemarksDiscussion of the importance of the mobility of protons in the course of chemical reactions has been attempted in this paper, using acid-catalysed esterification as an example. Conditions are considered which are significant, not only in esterification, but in proton-catdlysed [22] K. J. Laidler and P. A. Lnndskroener, Trans. Faraday SOC. 52, 200 (1956).reactions generally. Our observations are necessarily incomplete and have the character of an approximation. However, we believe that the great importance of protons in chemistry justifies our attempted interpretation.We wish to thank Prof. G. Scheibe for his helpful interest in this work. We are also indebted to thz Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Indw strie, and the Badische Anilin-und Sodafabrik for financially and materially supporting our investigations on "Protonic States in Chemistry".Eliminatiotts from olefiny are very ofren initiated by bases, and usually lead to formation of acetylene derivatives. In view of the numerous side-reactions and subsequent additions or rearrangements 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 exis fence of these can be denioiistrated by trapping them. The mechanisms known for @-elimination ( E 2 , E l and ElcB) occur also in the case of olefins; howeverdue to the sp2-hybridization of the carbon atoms taking partthey are realized with diflerent rates of reaction relative to saturated corm pounds. a-Eliminations from olefines having aryl residues in the @-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 presentday knowledge carbenes are involved in r-eliminations only when both @-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-chlorornethylenefuorene) . With organolithium compounds a number of dehydrohalogenations, which are formally @-eliminations, actually proceed via deprotonation on the halogenbearing carbon atcm (u-metallation). a-Metallations are the rate-determining steps when ether is used as solvent, but proceed quickly in tetrahydrofuran even at low iemperatures. Compounds of the type >C=C(Li)CI may be prepared by this method and are recognized as intermediates in u-eliminations.

The Gilch Synthesis of Poly(p‐phenylene vinylenes): Mechanistic Knowledge in the Service of Advanced Materials

“…If formed in trans-configuration F, the second elimination of HX will follow soon, but cis-configurated moieties G are quite persistent. [144] It is exactly this portion of cisconfigurated CH¼CX motifs G that we believe holds back halides in the PPVs very persistently. Moreover, this argument offers a rationalization as to why the literature correlates ageing and fatigue of devices with the concentration of TBB defects in the applied PPV: the concentration of halide-containing moieties G in a PPV that survives synthesis and work-up procedure is certainly correlated with the experimentally observable TBB content (more precisely: with the concentration of C C and CH 2 ÀCH 2 motifs in the final material), and there is no doubt that halides play a very destructive role in the context of the ageing and fatigue of devices.…”

Preparation of fluorohalomethylmagnesium halides using highly active magnesium metal and their reactions

“…Preparation of fluoroorganolithium compounds by direct lithiation is not feasible [7,8] and therefore transmetallation of the fluoroalkyl halide with an appropriate organolithium reagent has to be employed [1,9]. Transmetallation of perfluoroalkyl iodides with an organomagnesium reagent at À40 8C is also the preferable method for the generation of perfluoroalkylmagnesium compounds [2,3].…”

“…Early attempts to prepare fluoro-or fluorohalomethyl organometallic compounds using more electropositive metals, such as lithium or magnesium were unsuccessful due to immediate decomposition of the fluoroorganometallics formed to the corresponding carbenes and metal halides [6] and hence the name ''carbenoids'' was introduced for this class of compounds [7,8]. A major breakthrough was achieved by Kuroboshi et al, who successfully generated dibromofluoromethyllithium under Barbier conditions from tribromofluoromethane and butyllithium at À130 8C and reacted it with a series of electrophiles [9].…”

Carbozincation Reactions of Carbon–Carbon Multiple Bonds

“…A third possible reaction pathway of chlorocarbenoids, namely the FritschButtenberg-Wiechell (FBW) rearrangement, has also been exploited (Scheme 50). 90 However, when R 1 and R 2 are alkyl groups, the yield of alkynes is less than 10% because of competitive C-H insertion reactions to form cyclopentenes. 91 An example of a zincated species undergoing the FBW rearrangement is shown in Scheme 51.…”