Platinum acetylene compounds

Several aliphatic diols and hydroxy ethers have been made by catalytic hydrogenation of the triple bond in the corresponding acetylenic compounds. Both platinum and nickel catalysts are used. 

Cis addition is a characteristic feature of the hydrosilylation of acetylenic compounds in the presence of platinum catalysts. This is in marked contrast to the trans addition giving cis adduct on using free radical initiators (193). Free radical hydrogermylation (23) and hydrostannyhtion (194) also proceed with trans stereochemistry. 

In contrast to the above, tetrakis(methyldiphenylphosphine)platinum and tetrakis(dimethylpentafluorophenylphosphine)platinum react with hexa-fluorobut-2-yne to give the acetylene complexes (117) in good yield. The complex with L = PPhjMe reacts with mercuric chloride or bromide in ethanol to give the vinyl-platinum-mercury compounds (118). ... 

The oxidative addition reactions of organic halides to platinum phosphine compounds easily proceed. For example, reactions are shown in eqs. (21.22) and (21.23) [26,34]. The vinyl bromide derivative obtained by the reaction shown in eq. (21.22) is liable and it gives PtBr2(PMePh2)2 and acetylene by elimination [34]. 

The acetylene complex of zero valent platinum (R—C=C—R)Pt (PPh3)2 is a useful reagent for the extraction of acetylenic compounds from natural products. 

R SiH and CH2= CHR interact with both PtL and PtL 1. Complexing or chelating ligands such as phosphines and sulfur complexes are exceUent inhibitors, but often form such stable complexes that they act as poisons and prevent cute even at elevated temperatures. Unsaturated organic compounds are preferred, such as acetylenic alcohols, acetylene dicarboxylates, maleates, fumarates, eneynes, and azo compounds (178—189). An alternative concept has been the encapsulation of the platinum catalysts with either cyclodextrin or in thermoplastics or siUcones (190—192). 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Platinum on carbon did almost exactly the same thing but required a temperature of about 100°C to do so. With excess acetylene, only III formed. With tcrt-butylacetylene no II formed, probably because of steric hindrance, but I and III formed readily. 3-Hexyne reacted more slowly, required heat with chloroplatinic acid, and formed exclusively c/s-3-di-chlorosilyl-3-hexene. Trichlorosilane with platinum on carbon also added (57) to 1-alkynes or to phenylacetylene exclusively by cis addition to give only trans adducts. Later works (55) indicate that chloroplatinic acid and other soluble catalysts also give exclusively cis addition with a wide variety of Si—H compounds. 

Species D is most likely an ethylidyne complex which forms from self-hydrogenation on the palladium surface. Such species along with species E have been suggested to be part of the compounds formed from platinum and acetylene. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

It was somewhat puzzling that while olefin compounds of platinum(II) were well established, acetylene complexes were virtually unknown. Acetylene gives red intractable materials on reaction with potassium chloroplatinate(II) in aqueous solution, presumably acetylides and dimethylacetylene does not react, in marked contrast to ethylene, which forms [PtCl3(C2H4)] . However, Gel man, Bukhovets, and Meilakh (44)... 

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