Alkenylation of alkenes

Although a reasonable working hypothesis has been established for the mechanism of the palladium-catalyzed arylation and alkenylation of alkenes (the Heck reaction), individual... 

CL Alkene-alkene coupling. The palladium(O) catalysed arylation or alkenylation of alkenes is known as the Heck reaction (equation l2l) and has found extensive applications in synthesis (Table 23). Several variations of the Heck reaction have also been reported. 

In an intramolecular Mizoroki-Heck-type alkenylation of alkenes, Wilkinson s catalyst [RhCl(PPh3)3] (84) proved superior than other rhodium compounds (Scheme 10.29) [56]. Notably, the selectivities of the cyclization of various 2-bromo-1,6-dienes were found to be improved with Wilkinson s catalyst when compared with those observed with palladium complexes. Thereby, the corresponding l,2-bis(methylene)cyclopentanes, which themselves are valuable substrates for further cycloaddition reactions, such as 1,3-diene 86, could be isolated in high yields. 

In 1985, Kamigata et al. [61] reported on ruthenium-catalysed alkenylations of alkenes, employing alkenesulfonyl chlorides. High yields of isolated product were obtained with [RuCl2(PPh3)3] (94) as catalyst in benzene as solvent (Scheme 10.34). Importantly, no base was required for the coupling process, for which a radical-based mechanism was proposed. 

The common feature of Mizoroki-Heck reactions is the palladium(O)-catalysed arylation or alkenylation of alkenes [1]. During recent years, several excellent overviews about this topic have been published [7-12]. The reaction is usually carried out under basic conditions. Stoichiometric amounts of base are needed to neutralize the acid which is formed during the reaction. 

Alkenylation of Alkenes, Ferrocenes, Heterocycles, and Phenol Derivatives with Ru(II) and Cu(II) Partners... 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

Alkynes react with many of the same electrophilic reagents that add to the carbon-carbon double bond of alkenes Hydrogen halides for example add to alkynes to form alkenyl halides... 

The double bond m the alkenyl side chain undergoes addition reactions that are typical of alkenes when treated with electrophilic reagents... 

Addition of Alkenes and/or Alkynes to Alkenes and/or Alkynes Hydro-alkenyl-addition... 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

The Heck reaction consists in the Pd(0)-catalysed coupling of alkenes with an aryl or alkenyl halide or triflate in the presence of a base to form a substituted alkene (Scheme 7.1). The reaction is performed in the presence of an organopalladium catalyst. The halide or triflate is an aryl or a vinyl compound and the alkene contains at least one proton. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

The Lewis acid catalyst 53 is now referred to as the Narasaka catalyst. This catalyst can be generated in situ from the reaction of dichlorodiisopropoxy-titanium and a diol chiral ligand derived from tartaric acid. This compound can also catalyze [2+2] cycloaddition reactions with high enantioselectivity. For example, as depicted in Scheme 5-20, in the reaction of alkenes bearing al-kylthio groups (ketene dithioacetals, alkenyl sulfides, and alkynyl sulfides) with electron-deficient olefins, the corresponding cyclobutane or methylenecyclobu-tene derivatives can be obtained in high enantiomeric excess.18... 

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

---