Intermolecular reactions hydride termination

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

As a typical intermolecular carbopalladation and termination, hydroarylation of alkynes are carried out extensively in the presence of HCO2H as a hydride source. Formation of regioisomers is observed in the reaction of asymmetric alkynes, and ratios depend on the nature of the substituents. High regioselectivity was observed in the reaction of the tertiary propargylic alcohol 14 to give 15 as a major product [5]. The (Z)-2-arylcinnamates 17, rather than 3-arylcinnamate 18, was obtained by the hydroarylation of methyl phenylpropiolate (16) [6]. 3-Substituled quinoline 21 was prepared by the regioselective hydroarylation of 19, followed by treatment of 20 with an acid without isolation [6]. 

Intermolecular oxidative addition of H—C usually involves activated H—C bonds. The weak acid HCN reacts with transition-metal complexes e.g., HCN and NiL lead to the hydride complexes HNi(CN)Lj (L = various phosphorus ligands). The versatile complex IrCl(CO)(PPh3)j adds HCN cleanly in CH Clj at RT to form HIr(CN)(Cl(PPhj)2. The zero-valent complexes Pt(PPhj) or Pt(PPh3)3 also add HCN to yield HPt(CN)(PPh3)j. Reactions of HMNp(dmpe)j (M = Fe, Ru, Os Np = 2-naphthyl dmpe = Me PCH CH PMej) with HCN and terminal acetylenes give HMR(dmpe)2 that contain new M—C bonds (R = — CN, — CjR ) . 

Another potentially powerfnl sequence arises by combining one or two intramolecular Heck-type couplings with an intra- or intermolecular Diels-Alder addition (for early examples of inter-intermolecular one-pot domino Heck-Diels-Alder reactions see Refs. [49] and [50]). An all-intramolecular version of such a sequence has been shown to proceed reasonably smoothly for terminally alkoxycarbonyl-substituted 2-bromotrideca-l,ll-dien-6-ynes under palladium catalysis at 130 °C. At 80 °C, the sequential reaction stops after the two consecutive Heck-type cyclizations and subsequent /3-hydride elimination to give a 1,3,6-triene apparently only the ( )-isomer undergoes the intramolecular Diels-Alder reaction, as the (Z)-l,3,6-triene is observed accompanying the tetracyclic system obtained at 130 °C (Scheme 36). 

Lewis acid complexes of p-substituted a,p-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one can not be induced to undergo ene reactions as acrolein and MVK do. 34 The presence of a substituent on the p-carbon stabilizes the enal- or enone-Lewis acid complex and sterically retards the approach of an alkene to the p-carbon. However, we have found that a complex of these ketones and aldehydes with 2 equivalents of EtAlQ2 reacts reversibly with alkenes to give a zwitterion. 34 This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane or undergoes two 1,2-hydride or alkyl shifts to irreversibly generate a p,p-disubstituted-o,p-unsaturated carbonyl compound (see Figure 19). The intermolecular addition of an enone, as an electrophile, to an alkene has been observed only rarely. The specific termination of the reaction by a series of alkyl and hydride shifts is also very unusual. 35 The absence of polymer is remarkable. 

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