Metal-alkyne complexes palladium

Hydroaminations occurring by nucleophilic attack on ir-ligands are the oldest class of hydroamination and are discussed first. A mechanism for the hydroamination of alk-enes and alkynes catalyzed by palladium(II) complexes is shown in Scheme 16.16. By this pathway, coordination of the alkene or alkyne through the -ir-system occurs to generate a cationic or electron-poor, neutral metal-olefin or metal-alkyne complex. Nucleophilic attack of the amine on the coordinated olefin or alkyne then occurs. Nucleophilic attack on coordinated olefins and alkynes is presented in detail in Chapter 11. As noted in Chapter 11, this nucleophilic attack occurs at the internal position of an alkene or alkyne. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

In certain other systems, there is compelling evidence for the insertion into a metal-caiboxylate complex (equation 37). For example, in the synthesis of a-methylene-y-lactones from alkynic alcohols,70,71 no double bond rearrangement to a butenolide occurs, a reaction shown to take place in the presence of transition metal hydrides. The source of the vinyl proton (deuterium) on the a-methylene group is indeed the alcohol function. Finally, palladium carboxylate complexes containing alkynic (equation 40) or vinyl tails (equation 41) can be isolated and the corresponding insertion reaction can be observed. 

There seems no reason why any of the mechanisms discussed in Sections 3.4-3.6 cannot function in the conversion of alkynes to alkenes. The alkene route of hydrogenation is frequently encountered because alkynes complex more strongly to transition metals than alkenes and their complexes are formed preferentially in competition with the oxidative addition of dihydrogen. Internal alkynes coordinate to bis(arylimino)acenaphthene complexes of palladium and the fricoordinate species activate molecular hydrogen. Transfer of both atoms of hydrogen forms... 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

Cyclometallated complexes incorporating transition metals other than palladium react with alkynes and act as synthetic precursors to hetero-... 

Reaction of the cyclometallated derivative of phenyl-2-pyridylketone 73 leads to indenol-chelated, palladium-containing derivatives 74. Here, incorporating an electrophilic (CO) function in the starting palladacycle signifies that, following alkyne insertion in the Pd-C bond, an intramolecular attack of the vinyl palladated unit on the metal-bound, activated CO function occurs. This is in sharp contrast to the reaction described in Scheme 9 whereby incorporating a nucleophilic, masked, secondary amine function leads to indole derivatives 40 and to the azepinium synthesis from the metallated benzylpyridine complex 34. Therefore, these reactions are rather sensitive to the nature of other potentially reactive functions within the metallacyclic framework. 

In contrast, unactivated olefins and alkynes complexed to organopalladium species generated in situ by oxidative addition of an unsaturated halide to a palladium(0) complex react intramolecularly with stabilized nucleophiles. These reactions that require catalytic quantities of the metal result in overall difunctionalization of the olefinic or acetylenic substrates. 

Complexation of triynes to Pd(0) has been reported to give homoleptic palladium alkyne complexes that show a trigonal-planar arrangement with all of the alkyne carbons and Pd in the same plane. Complex 73 is a macrocyclic complex synthesized by reaction of the triyne with Pd(PPh3)4- Due to coordination to the metal, the alkyne carbons are shifted to the center of the cycle and their substituents deviate from linearity by about 22°. Complex 74 undergoes clean intramolecular cyclization at room temperature upon addition of PPh3 (Equation (24)). No intermediate complexes were detected in the course of this reaction, which is an example of the important cycloisomerization of alkynes and enynes catalyzed, among other transition metal complexes, by Pd(0) derivatives. 

Very few rr-bonded alkyne complexes of palladium(ll) have been reported in recent years. Except for two examples of monomeric complexes, most of them are polymetallic derivatives where a metal alkynyl acts as a liquid, as discussed above for some Pd(0) derivatives. These complexes adopt interesting stmctures that conform to the types shown below. They have been synthesized by reaction of a metal alkynyl with [PdCl(7/ -C3H5)]2 or Pd(C6Fs)2(THF)2. Platinum alkynyl complexes are the most common ligand frame used, as shown in structures 76-79, but other metal alkynyls can play the same role, such as the Rh and Ir alkynyls depicted 80 and 81. The 7 -alkyne moieties bound to palladium are rotated out of the Pd-coordination plane. The angles of the vectors and the normal to the... 

The insertion of alkynes into Pd-C or Pd-ER bonds (ER = SiR3, SnRs, BR2, etc.) of palladium(ll) derivatives is an important reaction involved in the mechanism of alkyne polymerization and in the numerous Pd-catalyzed additions to alkynes. It must occur by previous coordination of the alkyne to the metal, but the intermediate Pd( 7 -alkyne) complex is usually not detected either in catalytic studies or in examples of stoichiometric reactions reported in recent years,and those studies will not be collected here. Double insertion of alkynes into Pd-C bonds leads sometimes to <7-77 -enyl derivatives and some examples will be given in Section 8.06.6.5. Reviews are available dealing with palladium-catalyzed processes of interest where addition of two different groups to alkynes with Pd(ll) complexes takes place. 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

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