Alkynes, reaction with platinum

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

The pure cis- and /ra t-l,2-disilacyclohexanes 10 each undergo platinum-catalyzed stereoselective insertion reactions with alkynes. Products from reactions of the air-isomer are shown in Scheme 1 <20010M3718>. 

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

The robustness and excellent turnover numbers of platinum complexes with terminal alkynes have made it the catalyst of choice for the synthesis of polymers and other macromolecular architechtures. Alkyne hydrosilylation with platinum has also served as a key element in the synthesis of dendrimers. Sequential reaction of an alkyne with HSiMeCl2 and lithiated phenylacetylene afforded the branching unit of a dendrimer synthesis which has been used to afford a large variety of structures at high generation.44,4411 441 ... 

By the example of 34 different alkynes, it was convincingly demonstrated that the product of the treatment of [PtCLJ with CO at 40-110 °C is a very powerful alkyne hydration catalyst some of the reactions are shown on Scheme 9.7 [25], The best medium for this transformation is THF containing 5 % H2O. The reaction can also be performed in a water-organic solvent two-phase system (e.g. with 1,2-dichloroethane), however in this case addition of a tetralkylammonium salt, such as Aliquat 336, is required to facilitate mass transfer between the phases. After the reaction with CO, the major part of platinum is present as H2[ Pt3(CO)6 n], but the catalytic effect was assigned to a putative mononuclear Pt-hydride, [PtHCl(CO)2], presumably formed from the cluster and some HCl (supplied by the reduction of [PtCU]). The hydration of terminal acetylenes follows Markovnikov s mle leading exclusively to aldehyde-free ketones. 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

On the other hand, the reaction of 1,2,3-selenadiazole 166 with 1 equiv of Pt(PPh3)4 in toluene at 140 °C (3 h) led to the formation of new selenoplatinum complex 52 in 35% yield (Equation 13) <2005TL1001>. This reaction may involve the insertion of di(triphenylphosphino)platinum into the selenadiazole ring, followed by 1,3-dipolar addition of an intermediate formed in situ by thermal elimination of dinitrogen with the elimination of triphenylphosphine, similar to the reaction with [Pd2(dba)3] and trialkylphosphine described above. The structure of complex 52 was established by X-ray analysis. Complex 52 is a selective catalyst for the hydrosilylation of terminal alkynes. 

Alkynes react with hydrogen gas in the presence of a metal catalyst and the process called hydrogenation. It is an example of a reduction reaction. With a fully active catalyst like platinum metal, two molecules of hydrogen are added to produce an alkane. 

Reactions of 250 with four electrophiles are recorded in Scheme 31. In general, the products are more stable than those from complexes with monodentate ligands. Reaction of 249 with CS2, 249 or 250 with alkynes, and 249-251 with ethylene gives products in which the C6H8 has been lost but its fate has not been determined attempts to trap free cyclohexyne failed.93 Loss of the organic ligand appears to occur more readily in nickel complexes than in those with platinum. 

In recent years, attention has been focused on alkyne carbonylation catalysts based on the metals nickel, palladium, and platinum, modified with a variety of tertiary (bi)phosphines [5]. TTie main goal has been to develop chemo- and regio-selective carbonylation catalysts for application to higher alkyne substrates for the synthesis of certain fine chemicals. Many of these catalysts do allow the carbonylation to proceed under milder conditions than those applied in the catalytic Reppe process, and some of these catalysts do provide the branched regioisomer product from higher alkynes with good selectivity. However, in all cases reaction rates are very low, i.e., below 100 (and in most cases even below 10) mol/mol metal per h, as are the product yields in mol/mol metal (< 100). These catalyst productivities are far too low for large-scale industrial application in the production of commodity-type products, such as (meth)acrylates. 

As outlined in previous volumes, the chemistry of ylides coordinated to noble metals, particularly palladium, platinum and gold continues to attract most attention. Refluxing (dppm)palladium(II) dichloride [dppm = bis(diphenylphosphino)methane] with alkynes in mixtures of 1,2-dichloro-ethane/l,4-dioxane provides a novel route to palladium-bound alkenyl phosphorus ylide complexes (53) (Scheme 1)P The reaction represents the... 

The reactions of [Pt2H2(//-H)(//-dppm)2]PFfi with alkynes serve to introduce a number of studies of the elimination of dihydrogen from this cationic complex. Addition of one of a range of neutral ligands causes H2 elimination and formation of a platinum(I) complex of the form [Pt2HL(/i-dppm)2]+ (108,109) ... 

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