Reactivity olefin structure

Methyl 4-[2-(ethylthiocarbonyl)ethenyl]cinnamate (3 SMe) crystallizes into a typical a-translation-type packing structure in which the distances between the ethylenic double bonds are 3.988 A and 4.067 A, respectively. However, the 3 SMe crystal is entirely photostable even though it should be photoreactive based on the topochemical rule (Sukegawa, 1991). Several examples of exceptionally photostable diolefin crystals have been found in compounds having a thioester moiety. Such anomalous behaviour of crystals such as 2 OMe and 3 SMe cannot be explained simply in terms of the topochemical rule since this rule involves only the positional relationship between the reactive olefin pair. 

Luef, W., Strained Olefins Structure and Reactivity ofNonplanar Carbon-Carbon Double Bonds, 20, 231. 

The dichlorophenyl group also functions as a sufficiently strong anchor to allow formation of solid solutions from pairs of structurally similar potentially reactive olefins. A series of such pairs are 170 to 173. All were found to yield the expected (3-truxinic-acid-type photodimers, 75 (135). 

The gas-phase reactivity of various terpenes has been measured. Stephens and Scott were the first to include two terpenes (pinene and a-phel-landrene) with their study of the relative reactivity of various hydrocar ns. Both monoterpenes showed the high reactivity predicted by their olefinic structure. Conversion of nitric oxide to nitrogen dioxide in e presence of isoprene is at a rate intermediate between those for ethylene and trans-2-butene, and Japar et al, reported rate constants for the a-pinene and terpinolene-ozone reactions. Grimsrud et a/. measured the rate con-... 

The nature of vinylcyclopropane radical cations was elucidated via the electron transfer induced photochemistry of a simple vinylcyclopropane system, in which the two functionalities are locked in the anri-configuration, viz., 4-methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 39). Substrates, 39 and 47 are related, except for the orientation of the olefinic group relative to the cyclopropane function trans for 39 versus cis for 47. The product distribution and stereochemistry obtained from 39 elucidate various facets of the mechanism and reveal details of the reactivity and structure of the vinylcyclopropane radical cation 19 . 

Strained Olefins Structure and Reactivity of Nonplanar Carbon-Carbon Double... 

Due to the large differences in reactivities of the comonomers the chains are mostly composed of isobutene units with minor amounts of 1-butene and traces of the even less reactive Z-2-butene. They are linear and present several types of unsaturations. Spontaneous termination and transfer involving proton abstraction lead to the expected and largely predominant exo/endo terminal double bonds (A, B) but some other tri- and tetrasubstituted olefinic structures (C, D) together with internal vinyli-denes were also detected by H and nC NMR spectroscopy [30-34]. 

In Section IV,H, we described how selectivity depends on the transport-limited arrival of reactants and removal of reactive olefins within catalyst pellets. Specifically, we showed how FT synthesis selectivity can be described accurately and entirely by a structural parameter regardless of whether olefin removal [Eq. (15)] or reactant arrival [Eq. (25)] is the controlling diffusive resistance. The preceding two sections described how transport-limited removal of olefins from catalyst pellets can enhance the rate of secondary reactions. Here, we show how such transport limitations can be controlled by varying the liquid composition within catalyst pellets. In the discussion that follows, we refer to the experimental and simulation results of Fig. 20, where we showed how Cs+ selectivity depends on the value of the structural parameter ( ). 

The most important result of this study is that Tl(OAc)2 is similar to TP in oxidizing olefins. The absolute values of their rates of oxidation are quite close, and the effect of olefin structure on relative rates (with respect to ethylene as one) and product distribution are almost identical. Thus, complexing Tl(III) with two acetates does not change the nature of its oxidation of olefins. Higher complexing does inhibit its reactivity, but Pd (II) complexed with more than two chlorides is likewise not reactive. It is interesting that the activated complexes in the two systems [Tl(III) in aqueous acetate and Pd(II) in aqueous chloride] resemble each other in that both contain one metal ion, one ethylene, and either two chlorides or acetates. Pd(II), of course, also contains a hydroxyl. 

Because the rate of alkene hydrogenation generally decreases as the number and size of the substituents on the double bond increase, the least hindered double bond should be hydrogenated preferentially in the competitive hydrogenation of olefin mixtures. This selectivity, however, is not always observed either because of diffusion constraints or the presence of concurrent double-bond isomerization. Isomerization modifies the olefin structure, which changes the alkene reactivity and makes reaction selectivity difficult to attain. 

Competitive hydroborations with diborane in diglyme established that the reaction is relatively insensitive to the structure of the olefin. The most reactive olefin studied, 2-methy 1-1-butene, is separated by a factor of only 20 or 30 from the least reactive ones, 2,4,4-trimethyl-2-pentene and 2,3-dimethyl-2-butene. In hydrobora-tion with BMB a factor of 10,000 separates reactive 1-octene from cyclohexene, one of the least reactive olefins studied the study could not be extended to still more inert structures such as 2,4,4-trimethyl-2-pentene. 1-Hexyne and 3-hexyne are more reactive than the most reactive olefins studied. 

STRAINED OLEFINS STRUCTURE AND REACTIVITY OF NONPLANAR CARBON-CARBON DOUBLE BONDS 231... 

Evidence for this mechanism is given by (a) a correlation of reactivity with structure of R, (b) the analogous reactivity of esters, ethers, and olefins, (c) an isotopic tracer study of the reactions of alcohols labeled with O, and (d) the stereochemical result of the reactions of optically active alcohols, esters, and ethers. This necessitates the postulation of a concomitant Sni mechanism in the reaction of some alcohols and esters. 

Olefins are more reactive with oxygen than are paraffins, and therefore olefins can be converted over milder catalysts and with much better selectivity. The main points to be considered here are the influence of olefin structure on the reactions, the influence of catalyst composition, and evidence regarding mechanisms. Since the catalysts are of paramount importance in obtaining selective oxidation reactions, we have subdivided the material according to catalyst types. The types are defined by the nature of the products obtained. Under each catalyst type the influence of olefin structure will be discussed to some extent. Therefore some preliminary remarks about the reactivity of olefins are in order. 

Relative reactivities of olefins were determined by Adams (132) by feeding a mixture of the olefin to be tested with 1-butene. The rate of oxidation was found to be a strong function of the olefin structure, being inversely related to the strength of the allylic C—H bond. These results will be discussed in a later section. 

Luef, W. and Keese, R. (1991). Strained Olefins Structure and Reactivity of Nonplanar Carbon-Carbon Double Bonds, in Topics in Stereochemistry, Volume 20 (eds E. L. Ehel and S. H. 61en), John Wiley, Sons, Inc., Hoboken, NJ, USA. 

The less reactive olefins such as cyclohexene, 1-methylcyclohexene, and 2,3-dimethyl-2-butene exhibit rates that are slower and vary with concentration and the structure of individual olefins. The kinetics establishes these reactions to be first order in olefins and one-half order in the 9-BBN dimer (Eq. 4.2). The calculated three-halves-order rate constants also do not change as the reaction proceeds. The rate constants observed both for the first-order and three-halves-order kinetics are summarized in Table 4.2 [1]. 

Apart from pre-vulcanized latex where the rubber molecules have been chemically crosslinked by sulfur, the chemical nature of the rubber molecules of the other latices described above remain chemically intact during and after the process. There are several other latexes available on the market in which the rubber molecules of the latex have been chemically modified. The chemical reactivity of the rubber molecules arises from the olefinic structure of the cis-1,4-isoprene unit within the molecule, which can undergo rapid reactions with, for example, halogens, ozone and hydrogen chloride. Some of these will be described in this section. They are prepared to serve niche applications. 

Louie J, Grubbs RH. Metathesis of electron-rich olefins structure and reactivity of electron-rich carbene complexes. OrganometalUcs. 2002 21(11) 2153—2164. 

In principle, all olefins participate in the hydroformylation reaction however, their reactivities vary markedly. I. Wender et aL [199] made a systematic investigation of the reaction rate as a function of the olefin structure and found a variation of a factor of 50 (see table 7). 

Branching of the olefin structure always decreases the reaction rate. The strongest decrease occurs when one of the carbon atoms of the double bond is substituted. Branching at more distant carbons gives a less marked, but still significant reduction in reactivity. The slowest rates occur with internal, double bond substituted olefins. 

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