Reaction with alkenes and alkynes

The Fiiedel-Ciafts alkylation of aiomatics with the lesonance-stabihzed ttichloiocyclopiopenium ttiflate offers a synthetic pathway to ttiaiyl cyclopiopenium salts (26). The ttichloiocyclopiopenium ion has also been shown to undergo Friedel-Crafts reaction with alkenes and alkynes to give trivinyl and tri(halovinyl) cyclopiopenium ions. 

Quaternary ammonium tribromides can also be produced in situ from the quaternary ammonium bromide, sodium hypochlorite and sodium bromide and can be used, for example, in electrophilic addition reactions reaction with alkenes and alkynes. 

The theoretical studies showed that the [2 4-2] cydoaddition reactions with alkenes or alkynes were almost barrierless and very exothermic. The exothermicity for alkynes (—42.8 kcal mol ) was found to be greater than that for alkenes (—21.3 kcal mol ). Scheme 4.21b shows the calculated [2 4- 2] cydoaddition with polar unsaturated molecules, formaldehyde and FICN. The barriers (3.6kcalmol for formaldehyde and 9.0 kcal mol for HCN) are slightly higher than those in the reactions with alkenes and alkynes. The exothermicity is also significant (—26.5 for formaldehyde and —37.4 for HCN). 

Diels-Alder additions provide one of the more extensively studied reactions of these heteroles, and many examples are listed in the reviews given at the beginning of this section. Typical additions often require heating, giving the 7-hetero-norbornenes and -norbor-nadienes as products of reaction with alkenes and alkynes (Scheme 209). The report that (131) resulted at room temperature from tolane and (120) was therefore surprising. Subsequent investigation showed it to be a 1 1 crystalline adduct from which tolane could be sublimed (Scheme 210) (72CC690). 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Thiobenzoyl isocyanate is widely used as a starting material for the syntheses of TAs 23-28 both in condensation with ethyl sodiocyanoacetate (86KGS3) and in [4 + 2]-cycloaddition reactions with alkenes and alkynes containing electron-donor groups (81CB2713 85ZC324) (Scheme 4). It is established that the rate of cycloaddition increases from alkenes to alkynes and with the electron-donor properties of substituents. 

An important use of Cp2ZrHCl is in the so-called hydrozirconation reaction with alkenes and alkynes which generates a Zr-alkyl group which can then be converted to alcohols or other products.67 In the reactions of CO with Zr and Hf compounds the oxophilicity of these elements is important, because it leads to the formation of a variety of compounds that have M—O bonds. 

Several heteroatom nucleophiles, for example, amines, alcohols, thiols, carboxylates, and dialkylphosphines, undergo Michael addition reactions with alkene- and alkyne-substituted carbene complexes. Reaction of alkyne-substituted chromium carbenes with urea affords products derived from Michael... 

Since the initial demonstrations of the participation of substituted l,2,4,S-tetrazines ° and oxa-zoles ° ° in [4 + 2] cycloaddition reactions with alkene and alkyne dienophiles, the investigation and application of the Diels-Alder reactions of heteroaromatic systems possessing reactive azadienes have been pursued extensively. A number of general reviewshave treated the spectrum of heteroaromatic azadienes that participate in [4 + 2] cycloaddition reactions and many of the individual heteroaromatic systems have been reviewed separatcly. ° ° An extensive account was published recently and should be consulted for descriptions of the [4 + 2] cycloaddition reactions of the common heteroaromatic azadienes that have been observed to date. 

The ene and Prins reactions are not mechanistically distinct. Coverage will therefore be organized by the nature of the carbonyl compound, with intermolecular reactions presented first, followed by intramolecular reactions. The emphasis will be on material published since the field has been reviewed " and on examples demonstrating the stereo-, regio- and chemo-selectivity of these reactions. Coverage is restricted to the addition of carbonyl and thiocarbonyl compounds to simple alkenes. Addition of carbonyl compounds to vinylsilanes, allylsilanes and enol ethers is covered in the following chapters. Addition of imines and iminium compounds to alkenes is presented in Part 4 of this volume. Ene reactions with alkenes and alkynes as enophiles are covered in Volume 5, Chapter 1.1. Use of aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.6. 

The most intensively studied reactions of 1,2,4,5-tetrazines are cycloaddition reactions with alkenes and alkynes, not only due to theoretical interest but also due to its importance in synthesis. The 1,2,4,5-tetrazines are diene components in this [4 + 2]-cycloaddition reaction while the alkynes and alkenes are the dienophiles. Kinetic studies have shown that these reactions should be classified as Diels Alder reactions with inverse electron demand. Therefore, 1,2,4,5-tetrazines with electron-withdrawing substituents (C02Me, CF3) and dienophiles with electron donating substituents are the most reactive compounds. However, dimethyl... 

Reaction with alkenes and alkynes. In the carbosilylation of alkenes and dienes with carbon fragment that comes from a secondary or tertiary alkyl halide, promotion by BuMgBr/CpaTiCla is efficient. Primary alkyl halides are less suitable contributors unless the reaction is intramolecular. The regioselectvity of this process is such that the silyl group is branched out. ... 

Cyanoketenes, N=C—CH=C—O, have the tendency to react with alkenes and alkynes to yield a cycloaddition product. Because the ketenic carbonyl carbon has electrophilic character, the reagent will behave as an electrophile. The -butyl derivative, N=C—C( -Bu)=C=0, has received the most attention. Its reactions with alkenes and alkynes have been reviewed by Moore and Gheorghiu. The additions occur either in a 2n +2n concerted fashion or through a zwitterionic intermediate. Moore and coworkers also studied the cycloaddition of chlorocyanoketene to alkynes ". ... 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

Methyl-1,2-dihydropyridine behaves as an enamine rather than as a diene at — 10°C in its primary cycloaddition reactions with alkenes and alkynes. At higher temperatures, the [2 -I- 2] cyclo-adducts, e.g. (79), are unstable, and the more thermodynamically stable Diels-Alder endo- and exo-adducts, e.g. (80) and (81), are formed. ... 

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