Diphenylacetylene, conversion

A selective hydrogenation catalyst for alkynes was obtained with the PdCl2 complex of such immobilized pyridine. Diphenylacetylene was hydrogenated under 0.44 MPa H2 in ethanolic solution. At full conversion, the following selec-tivities were observed cis-stilbene 80.7%, trans-stilbene 16.1%, and only 3.2% 1,2-diphenylethane [90]. 

Some li t is shed of these proces s by recent studies of the reactions of cyclo-pentadienyl-cobalt(I) species with acetylenes. For example, XXXVII reacts with diphenylacetylene to give XXXVIII, but relatively high temperatures (around 120 C) are necessary. Some of the intermediates involved have now been isolated and their reactions studied (Scheme The conversion of XL to XLF is a typical sym-... 

An early procedure used triethyl phosphate directly on the diketone, but better yields are obtained by the oxidation of the corresponding dihydrazone 42 The oxidant may be mercury(n) oxide, which is rather expensive 43 alternatively copper(i) chloride in dichloromethane and pyridine is oxidised with oxygen gas, and the derived complex is then used to oxidise the dihydrazone to the acetylenic group with the evolution of nitrogen.44 The reaction is illustrated by the conversion of benzil dihydrazone into diphenylacetylene (Expt 5.25). 

The double-addition products of type XVI are surprisingly stable. Complex 36 i, e.g., can be crystallized from CH2Cl2/MeOH in the presence of hydrochloric acid without substantial decomposition. The most striking property of 36i is the conversion into the cyclobutadiene complex 33 (R = Ph), which is more conveniently prepared from 28a and PhC2Ph at 150 °C. As already mentioned, the reaction proceeds by a stepwise mechanism through complexes XIV—XVI. This reaction offers a new, facile preparation of 33. The cyclo-dimerisation of alkynes other than diphenylacetylene could not been substantiated, however. 

Treatment of the cyclopalladate complex (61) with with diphenylacetylene (DPA) gives the new complex (62), which on treatment with AgBF4 and subsequent thermolysis in chlorobenzene yields the dibenzo[M]thiepine salt (63) (Scheme 9). Longer treatment of the salt in chlorobenzene results in conversion to the thiepine (64) <95JOci005>. 

An excess of a,a-dichlorotoluene and potassium tert-butoxide is necessary to ensure high conversion of diphenylacetylene. When 1 equiv of potassium tert-butoxide are used, only 60% conversion of diphenylacetylene is achieved and the remainder is recovered during the workup. Nonetheless, the yield corrected for recovered diphenylacetylene is still more than 90%. 

An example of a general synthesis of acetylenes is the conversion of benzit via the dihydrazone (and the unisolated diazide) to diphenylacetylene. ... 

For instance, propanol is converted into propyl propionate (conversion 93%, selectivity 98%) in acetone at 145°C with Ru3(CO)l2 and diphenylacetylene, the catalytic turnover being 140 (124,125). This reaction can be extended to other alcohols with excellent yields and selectivity (125). The intermediacy of the unsaturated mononuclear complex Ru(CO)2(C4Ph4CO) seems to be the key step in this catalysis (126) 1,4-and 1.5-diols give rise to lactones while 1.6- and 1,10-diols are polymerized in this reaction (127). 

Oxidation of allenes and alkynes. The oxidation promoted by cetylpyri-dinium peroxotungstophosphate in an alcoholic solvent produces a-alkoxy ketones from allenes and a,/3-epoxy ketones from alkynes (along with a,/3-unsaturated ketones). Diphenylacetylene gives benzil (93% yield at 45% conversion). 

Clusters 19 22 and 24 have also been tested as catalyst precursors for the hydrogenation of diphenylacetylene (Table 1, entries 16, 18, 20). Z-Stilbene (kinetic product) and f-stilbene (thermodynamic product) are formed with higher conversions (70-100% after 90 min) than in the case of terminal alkynes. As with tert-butylacetylene, similar activities and selectivities are observed for the five cluster complexes, suggesting the intermediacy of common catalytic species in solution. Compound 27 (which arises from the reactions of 21 or 22 with diphenylacetylene) and compound 28 (which arises from the reaction of 24 with the same alkyne) (Fig. 4) have been proposed as catalytic intermediates in these hydrogenation reactions. ... 

The conversion of diphenylacetylene into 1,2,3-triphenylazulene under catalytic condi-tions is well documented, but it has not previously been observed for the radical cation. 

Unlike diphenylcyclopropenone, the thione when heated gives only traces of diphenylacetylene, although the latter compound is formed by photolytic decomposition [153]. The mass spectrum of the thione, unlike that of the eyelopropenone, shows a molecular ion peak [153], Photolytic conversion of diphenylcyclopropenethione into a tetra-phenylthiophenothiophene has also been reported [154]. The thione is reconverted into diphenylcyclopropenone by reaction with lead tetra-acetate [155],... 

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