Reactivity of alkenes and alkynes

Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source. 

The double and triple bonds of alkenes and alkynes are composed of strong a bonds and relatively weak tt bonds. We have seen that the reactivity of alkenes and alkynes is the result of an electrophile being attracted to the cloud of electrons that constitutes the TT bond. 

Similarly, Gonzdlez and Houk predicted that the Diels-Alder reaction of acetylene with 2- abutadiene is more than 2 kcal/mol higher in activation energy than the corresponding ethylene reaction [93]. In the same paper, they also investigated the substituent effects on the reactivities of alkene and alkyne multiple bonds, and the effect of Lewis acid catalysis on these reactions. Another reaction studied computationally was the Diels-Alder reaction of acetylene with a-pyrone [94]. 

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). 

Since the HOMO of alkynes are also 7r-orbitals, it is not surprising that there is a good deal of similarity between the reactivity of alkenes and alkynes toward electrophilic reagents. The fundamental questions about additions to alkynes include the following how reactive are alkynes in comparison with alkenes what is the stereochemistry of additions to alkynes and what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions... 

Further, when double and triple bonds are both present in the same substrate (vide infra), their relative reactivity appears to depend upon what is being added. Thus, generalizations regarding the relative reactivities of alkenes and alkynes can be made only within narrow Omits. 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

Thus, whereas intermolecular carbozincations of alkenes and alkynes appear restricted in scope and limited to the more reactive di(terf-butyl)zinc, the zinc-induced cyclization of... 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

Chlorofluorination of alkenes and alkynes using the A -chlorosaccharin-hydrogen fluoride/ pyridine system (Method G)50 results in vicinal chlorofluoro-substituted alkanes or alkenes, respectively. Since this system is more reactive than with other A -chloroamides, its use allows shorter reaction times and milder reaction conditions to prepare chlorofluorinated products in higher yields. With phenyl-substituted alkenes, Markovnikov-type adducts are formed exclusively. The additions proceed predominantly anti, with varying degrees of stereoselectivity. 

Alkylboranes formed in the hydroboration of alkenes and alkynes seldom are isolated for the most part they are used as reactive intermediates for the synthesis of other substances. In the reactions of alkylboranes, the B-C bond... 

Imido selenium compounds Se(NR)2, where R = Bu or Ts, were first noted to give allylic amination of alkenes and alkynes.232 Formally the NR function is inserted into the allylic C—H bond yielding the C—NHR moiety. Related reactivity was also found for the sulfur imides, S(NR)2.233 Reactions between 1,3-dienes and Se(NTs)2 give [4 + 2] adducts which, in the presence of TsNH2, react to generate 1,2-disulfonamides.234... 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

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