Alkynes rate constants with

Addition reactions of ArS to alkynes produce cis- and trans-olefins, which seem to be controlled kinetically [42]. To understand stereospecific addition processes, kinetic studies are important. Some representative data obtained by the flash photolysis method are shown in Table 5 [36]. For each pair of acetylene and olefins (Scheme 8), the slopes of the Hammett plots for the rate constants with different substituents in ArS do not vary much, suggesting that the reactivities of these acetylenes and olefins are mainly determined by the resonance stability of the transition state of the reaction, but not by the polar nature of the transition state. Thus, the resonance stability of the C atom-centered radicals in... 

Addition of phosphonyl radicals onto alkenes or alkynes has been known since the sixties [14]. Nevertheless, because of the interest in organic synthesis and in the initiation of free radical polymerizations [15], the modes of generation of phosphonyl radicals [16] and their addition rate constants onto alkenes [9,12,17] has continued to be intensively studied over the last decade. Narasaka et al. [18] and Romakhin et al. [19] showed that phosphonyl radicals, generated either in the presence of manganese salts or anodically, add to alkenes with good yields. 

The reactions of 1,2,3-triazolium 1-imide (277) with a range of alkene and alkyne dipolarophiles give rise to a variety of new ring systems (Scheme 54). Compounds (276) and (278) are obtained from (277) by reaction with acrylonitrile and DMAD, respectively. These reactions are tandem 1,3-dipolar (endo) cycloadditions and sigmatropic rearrangements which are regio- and stereospecific <90JCS(Pl)2537>. Kinetic and mechanistic studies show that these reactions are dipole-HOMO controlled. The second-order rate constants are insensitive to solvent polarity, the reaction indicates... 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

The only significant loss of alkynes is reaction with OH, for which a pressure dependence is observed. Table 6.15 gives the high-pressure limiting rate constants for the OH reactions with acetylene, propyne, 1-butyne, and 2-butyne. The reaction of acetylene approaches the high-pressure limit at several thousand Torr (see Problem 5). However, for the larger alkynes, the reactions are essentially at the high-pressure limit at 1 atm (and room temperature). 

TABLE 6.15 High-Pressure Limiting Rate Constants (k,) for the Reaction of OH Radicals with Alkynes at 298 K ... 

From the Fischer rate study, it appears that primaiy ester-substituted radicals are not electrophilic but ambiphilic and the borderline between ambiphilic and electrophilic radicals is not at all clear. Consider our results68 (Scheme 16) on the atom transfer additions of ester-substituted radicals to alkynes (with the caution that it may be dangerous to compare yields in place of rate constants). The primary ester-substituted radical adds more efficiently to 1-heptyne but the tertiary ester-substituted radical prefers ethyl propiolate. 

A listing of alkyne-nucleophile systems whose substitution kinetics have been studied is given in Table 24 for each of these systems. Rate constants and enthalpies and entropies of activation, if available, are tabulated. In order to compare the reactivity of haloalkynes with other organic halides we have also included in Table 24 related rate data for vinylic, aromatic and alkyl halides. 

The reactions of ozone with alkynes are essentially slower than those with alkenes. This is shown by the values of rate constants, which are lower in the case of alkynes due to the higher activation energy. Alkanes react very slowly with O3 and thus, they are unimportant in atmospheric processes. 

In Eq. (13.4), the silane reaction is first order with a rate constant of 4.2 x 10 4s 1, and facile exchange of the BH and SiH hydrogens in the silane-borane product occurs even at - 30°C. This lability and exchange dynamics are typical of true a complexes and favor reactivity such as the hydroboration process in Eq. (13.5).29 The alkyne borane is unstable at - 30°C and reacts with diphenylacetylene... 

As part of their study of the relative reactivity of alkenes and alkynes, Modena and coworkers determined the reactivity ratio toward carbocations. These cations were generated by interaction of either diphenylmethyl chloride or 1-phenylethyl chloride with zinc chloride. Absolute rate constants were not determined. In general, the reactivities of alkynes and alkenes are of the same order of magnitude (see also Section II. A). 

R" = COOMe to 22 dm mol s" for R = H and R" = Ph. The rate constants for addition of PhCF were slightly lower. The rate constants decreased with increasing Ti-ionization potential of the alkyne, except for very electron-deficient alkynes such as dimethyl acetylenedicarboxylate. The correlation indicated that in these additions the carbenes generally behave as electrophiles, whereas with the acetylenedicarboxylate carbenic nu-cleophilicity comes into play. The rate constant of addition of phenylchlorocarbene to 3-hexyne was determined as a function of temperature. The reaction appeared to be entropy controlled Ea = 8.8 + 0.4 kJ mol and AS = -82 J mol" K The corresponding alkenes have rate constants and activation parameters in the same order of magnitude. 

Boron is the prime metal in the area of stoichiometric interactions between metals and unsaturated bonds. Especially, boron hydride additions have been investigated, in particular by H. C. Brown and his students. Nowadays, these addition reactions are well-established text book subjects. A number of reviews on hydroboration have appeared . The development of a clear mechanistic picture lagged far behind the applications in synthesis. It was also the group of Brown that contributed to mechanistic understanding by performing careful kinetic measurements using 9-borabicyclo[3.3.1]nonane, abbreviated as 9-BBN-H, as reagent. Reactive alkynes such as 1-hexyne and 3-methyl-1-butyne exhibited first-order kinetics in 9-BBN-H with a rate constant equal to that of reactive... 

Free-radical additions to alkynes generate vinyl radicals (equation 1) and information about the structure of these intermediates has been obtained either by spectroscopic or chemical means. Vinylic intermediates are generally cr-type radicals (1), in which the unpaired electron is in an orbital with substantial s character. The degree of bending and the inversion barrier depends on the a-substituent. That is, for vinyl (R=H) the rate constant for the inversion lies between 3 x 10 and 3 x 10 s at -180 °C, whereas 1-methylvinyl inverts somewhat more slowly. Electronegative substituents, such as alkoxy, increase the barrier of inversion. ... 

Gilbert and coworkers employed ESR spectroscopy in studies of radical intermediates formed in the reaction of alkynes. In particular, they study the addition of a variety of alkyl, a-hydroxyalkyl and aryl radicals to butynedioic acid. Rate constant at room temperature for the reaction of isopropyl and hydroxymethyl radicals with butynedioic acid are estimated to be 3 x 10 s and 1 x 10 M s" respectively . The first-... 

Hydroxy radical initiated oxidation of alkynes is important from the point of view of both atmospheric and combustion chemistry. Hatakeyama and coworkers have measured rate constants for the reaction of HO with acetylene, propyne and 2-butyne under atmospheric conditions. It has been suggested, based on product studies, that the jS-hydroxyvinyl radicals further react with molecular oxygen to form the corresponding peroxyl radicals and their subsequent reactions give carboxylic acid, a-dicarbonyl compounds and acyl radicals. 

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