Alkynes rate constants

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. 

A pre-exponential factor and activation energy for each rate constant must be established. All forward rate constants involving alkyne adsorption (ki, k2, and ks) are assumed to have equal pre-exponential factors specified by the collision limit (assuming a sticking coefficient of one). All adsorption steps are assumed to be non-activated. Both desorption constants (k.i and k ) are assumed to have preexponential factors equal to 10 3 sec, as expected from transition-state theory [28]. Both desorption activation energies (26.1 kcal/mol for methyl acetylene and 25.3 kcal/mol for trimethylbenzene) were derived from TPD results [1]. 

Ratio of second-order rate constants (alkene/alkyne)... 

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 ... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

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. 

Table I. Average CO Vibrational Energies and Absolute Rate Constants for CO Production at 300K for 0(3P) + 1-Alkynes.

The apparent chemoselectivity for the addition of the electrophilic S-centered radicals to the less electron-rich alkyne moiety in enyne 138 can be rationalized by the fact that addition of S radicals to both aUcenes and alkynes proceeds smoothly (the rate constants for addition of S radicals to alkenes are about three orders of magnitude larger than those for the addition to alkynes), but is also reversible. However, the reversibility is less pronounced for the radical addition to alkynes, due to the high reactivity of the vinyl radicals formed (compared to alkyl radicals), which undergo subsequent reactions at faster rates than undergoing fragmentation back to the S... 

Some catalysts suffer a different type of alkyne poisoning. Chlorotris(triphenylphosphine)rhodium(I) is an effective terminal alkyne polymerization catalyst. When this complex is used in the reduction of these alkynes, it gradually loses its activity because of the competing polymerization reaction. Even initially the rate of alkyne hydrogenation is much slower than that of the corresponding alkene because of the greater binding constant of the former substrate. 

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. 

Other radicals can add to alkenes, and the rate constant for the addition of methyl radicals to alkenes has been studied,and the rate of radical additions to alkenes in general has also been studied.The kinetic and thermodynamic control of a radical addition regiochemistry has also been studied. Alkynes... 

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