Reaction with alkenes and

Reactions of Propargylic Compounds Catalyzed by Pd(Q) 5.2 Reactions with Alkenes and Terminal 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. 

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. 

These intermediates undergo addition reactions with alkenes and aromatic compounds and insertion reactions with saturated hydrocarbons.254... 

In addition to undergoing cycloaddition reactions with alkenes and al-kynes, silenes readily undergo cycloaddition reactions with heteroatom multiple bonds such as C=0 and C=N, most commonly when the trapping reagent for the silene is either an aldehyde, ketone, or imine. In many... 

Reactions with alkenes and nonconjugated dienes have been described in many publications (101, 103, 106-111). Various alkenes, such as cycloalkenes as well as acyclic alkenes, up to tetrasubstituted derivatives, can react with nitroalkenes (42) (110). Only one double bond is involved in the reactions of heterodienes (42) with nonconjugated dienes (111), whereas the second double bond can be used in subsequent transformations of target nitronates (35). The reactions of heterodienes (42) with inactivated alkenes require the presence of LA as catalyst. 

The first stannenes were obtained as reactive intermediates that were identified by their reaction products.570,571 For example, Me2Sn=C(SiMe3)2 (Scheme 20) was prepared by the 1,2-elimination of LiBr, or by a retro-Diels-Alder reaction, and was characterized by cyclodimerization, by ene reactions with alkenes, and by cycloaddition with 1,4-dienes.572... 

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. 

Compared with the classical procedures, which employ chloroform and dry potassium /ert-butoxide, Makosza s method is several magnitudes superior, in spite of the normally recognized requirements that the dichlorocarbene should be produced under totally anhydrous conditions. Several early reports of the reactions of dichlorocarbene, generated by Makosza s procedure, led to suggestions that the activity of the carbene was considerably greater than that of the classically produced carbenes. This assumption was based on the overall higher yields of dichlorocyclopropanes derived from the reaction with alkenes, and upon the observation that weakly activated alkenes reacted with Makosza carbenes, but not with the classically produced carbenes. A consideration of the mechanism of formation of the carbenes under phase-transfer catalytic conditions exposes the fallacies in the assumptions. 

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). 

Where palladium(II) complexes are used as precursors in the above reactions, the palladium(O) species needed to initiate catalysis are generated by a stoichiometric reaction with alkene and base (equation 179). 

In addition to their reactions with alkenes and carbanions as nucleophiles benzhydryl cations react with hydride donors.282 284 These hydride transfer reactions show the same linear dependence of log k upon E as the reactions with alkenes and the same constant relative selectivity, that is with slopes of plots close to 1.0, for structures ranging from cycloheptatriene to the... 

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