Metal catalysts, addition alkynes

Hydrosilylations. Addition of triethylsilane across multiple bonds occurs under the influence of a large number of metal catalysts. Terminal alkynes undergo hydrosilylations easily with triethylsilane in the presence of platinum, rhodium, ruthenium, osmium, or iridium catalysts. For example, phenylacety-lene can form three possible isomeric hydrosilylation products with triethylsilane the (Z)-/3-, the and the a-products... 

Alkynes are reduced to alkanes by addition of TI2 over a metal catalyst. The reaction occurs in steps through an alkene intermediate, and measurements indicate that the first step in the reaction is more exothermic than the second step. 

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

A direct addition of cydoethers to terminal alkynes has been discovered by Zhang and Li (Scheme 6.136) [271]. The best results were obtained when the reactions were run without additional solvent and in the absence of additives such as transition metal catalysts, Lewis acids, or radical initiators. Typically, the cycloether was used in large excess (200 molar equivalents) as solvent under sealed-vessel conditions. At a reaction temperature of 200 °C, moderate to good yields of the vinyl cycloether products (as mixtures of as and trans isomers) were obtained. The reaction is proposed to follow a radical pathway. 

The addition of organodichalcogenides to alkynes does not necessarily require the aid of transition metal catalysts. Indeed, the reaction proceeds via different mechanisms under various conditions (radical, In, CsOH,183 SnCl4,184 and phase-transfer catalysts185). Treatment of a mixture of (PhS)2 and terminal alkynes with GaCl3 affords (E)-products (E Z=>20 1).186 The reaction is assumed to involve a thiirenium ion as the intermediate (Scheme 38). 

Phosphonium salts can be synthesized by the transition-metal-catalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh3, CH3SO3H, and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh3)4 directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)]2... 

These critical aspects of the classical fluorous biphasic catalysis led in recent works to the development of protocols for the conversions with modified catalyst systems in non-fluorinated hydrocarbons as solvents. As part of the BMBE lighthouse project, Gladyzs and coworkers appHed this concept to C - C coupHng reactions (Suzuki reaction) and other metal-catalyzed addition reactions (hydrosilylation, selective alcoholysis of alkynes), which have direct relevance for the synthesis of fine chemicals and specialties [74]. 

The author hopes that this chapter has convinced the readers of the value of homogeneous catalysis for the synthesis of organophosphorus compounds and for organo-heteroatom compounds in a broader sense. Hydrosilylation and hydroboration are indispensable modern synthetic reactions in this category. The H-P addition reactions herein described joins them as a third member. Although this chapter does not cover, the addition reactions of the S-P and Se-P bonds in thiophosphates [39] and selenophosphates [40] to alkynes also proceed in the presence of transition metal catalysts. In view of the wide use of phosphorus compounds, the new procedures will find practical applications. 

Silylformylation, defined as the addition of RsSi- and -CHO across various types of bonds using a silane R3SiH, CO, and a transition metal catalyst, was discovered by Murai and co-workers, who developed the Co2(CO)8-catalyzed silylformylation of aldehydes, epoxides, and cyclic ethers [26]. More recently, as described in detail in Section 5.3.1, below, alkynes and alkenes have been successfully developed as silylformylation substrates. These reactions represent a powerful variation on hydroformylation, in that a C-Si bond is produced instead of a C-H bond. Given that C-Si groups are subject to, among other reactions, oxidation to C-OH groups, silylformylation could represent an oxidative carbonylation of the type described in Scheme 5.1. 

The most notable point of this reaction is that the internal sp-c xhon is selectively carbonylated to form (Z)-14a predominantly, although the ZjE ratio is likely to depend on reaction temperature, time, and catalyst precursor. It is revealed that the stereochemistry of the transition metal-catalyzed addition to alkynes is intrinsically cis. Isomerization from (Z)-14a to ( )-14a proceeds as an independent event from silylformylation. This feature sharply contrasts to the results observed in hydrosilylation of 13 with Me2PhSiH (Equation (3)). ... 

Addition of hydrogen atoms in the presence of a metal catalyst to double or triple bonds is known as hydrogenation or catalytic hydrogenation. Alkenes and alkynes are reduced to alkanes by the treatment with H2 over a finely divided metal catalyst such as platinum (Pt—C), palladium (Pd—C) or Raney nickel (Ni). The platinum catalyst is also frequently used in the form of Pt02, which is known as Adams s catalyst. The catalytic hydrogenation reaction is a reduction reaction. 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

Requires a transition metal catalyst, Pt, Pd, Ni, etc. Addition is supra-facial from least hindered side. Rearrangements can occur. Alkynes are reduced to c/ s-alkenes over Lindlar catalyst, Pd-Pb (Section 11-2). 

What was evident for macrocyclization involving metal carbene addition to alkenes is even more so for addition to alkynes [110]. However, here the chiral dirhodium(II) catalyst Rh2(4.S -IBAZ)4 exhibits the highest degree of enantiocontrol, superior even to Cu(MeCN)4PF6 (Eq. 5.22). 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

The rate of hydroboration with catecholborane and pinacolborane can be tremendously increased by the addition of transition metal catalysts. Hydroboration of pinacolborane 49 with alkenes 50 <1996JA909> and terminal alkynes 52 <19950M3127> proceeds with high regioselectivity in the presence of catalytic HZrCp2Cl furnishing the terminal boronates 51 and vinylboronates 53, respectively (Scheme 8). 

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

Abstract Metathesis-based polymerizations of 1-alkynes and cyclopolymerizations of 1,6-heptadiynes using late transition metal catalysts are reviewed. Results obtained with both binary, ternary, and quaternary catalytic systems and well-defined molybdenum- and ruthenium-based catalysts are presented. Special consideration is given to advancements in catalyst design and mechanistic understanding that have been made in this area over the last few years advancements that have facilitated tailor-made syntheses of poly(ene)s. In addition, the first supported ruthenium-based cyclopolymerization-active systems are summarized. Finally, selected structure-dependent properties will be outlined where applicable. 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

We and others recently reported the metal-catalyzed addition of diboron compounds to alkenes and alkynes.15,16 Subsequent work has improved the catalysts and extended the scope of the substrate to include disubstituted alkenes, enynes, and dienes.17 Stoichiometric reactivity studies support a mechanism which involves oxidative addition of the B-B bond to the metal center, followed by insertion of the substrate into the M-B bond, and product-forming B-C reductive elimination (Scheme l).18... 

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