Phenyl acetylene, formation

The electrochemical allylation of carbonyl compounds by electroreductivc regeneration of a diallyltin reagent from allyl bromide and a Sn species leads to formation of homoallylic alcohols in yields of 70-90 % even in methanol or methanol/water (Table 7, No. 12) Bisaryl formation is possible also from aryl iodides or bromides in the presence of electro-generated Pd phosphane complexes (Table 7, No. 13) In the presence of styrenes, 1,3-butadienes, or phenyl acetylene the products of ArH addition are formed in this way (Table 7, No. 14) . The electroreductivc cleavage of allylic acetates is also possible by catalysis of an Pd°-complex (Table K No. 15)... 

Allene 68, formed in situ from l-(2,4,6-cydoheptatrien-l-yl)-2-phenyl-acetylene (67) in the presence of DBU in diethyl ether, underwent dimerization to give compounds 69,70, and 71 in 27, 43, and 4% yields, respectively (79CL171). Heating of compound 69 with DBU in benzene afforded a 2 1 isomeric mixture of azulenes 73 and 74 in 90% yield. The formation of the azulene derivatives (73 and 74) probably proceeds via the norcaradiene tautomer (72) of 69 (Scheme 2). 

A different way to obtain tra 5-alkenyls is the isomerization of the initial cA-alkenyl or its direct formation from a skewed intermediate such as that framed in Scheme 6.6. Interestingly, the insertion of phenyl acetylene into an Os-H bond affords a metallacyclopropene which has exactly that skewed structure, as determined by single crystal X-ray diffraction (Scheme 6.13). This unequivocally demonstrates the feasibility of that kind of structure as intermediates or transition states for the isomerization of transition metal alkenyls [63]. 

The same research group has also demonstrated a catalytic enantioselective tandem carbonyl yhde formation-cycloaddition of the a-diazo-j8-ketoester 91 using 0.5 mol% of Rh2(R-DDBNP)4 95, as catalyst to afford the cycloadduct 93 in good yields (Scheme 28) with up to 90% ee [ 109]. A detailed study on enantioselective reaction using a series of dirhodium tetrakiscar-boxylate and tetrakisbinaphtholphosphate catalysts under different solvent conditions has been described [56]. These studies indicate that dirhodium tetrakisbinaphtholphosphate catalysts are superior to the more commonly used carboxylates and carboxamidates in asymmetric transformations. Typically, the reaction [58] of the nitrophenyl-substituted diazodione 96 and phenyl acetylene in the presence of the binaphthyl catalyst 95 at 0 °C afforded the cycloadduct 97 with 76% ee (Scheme 29). 

If addition occurs across an acetylenic bond, there is the possibility of formation of cis and irons isomers. Phenyl acetylene reacts with ethanethiol in the presence of an alkali catalyst at 100-225°C. Progressively larger amounts of the trcms isomer were formed as the temperature increased, reaching a maximum of 71% irons at 200°C. A rapid cis-irons isomerism accompanies the vinylation reaction . 

Johnson and his collaborators have continued to refine their excellent procedures for steroid synthesis via cation-olefin cyclization. Their recent investiga-tions (Scheme 13) demonstrate that styrenes are superior to phenyl-acetylenes as terminators since the overall reaction is much more stereoselective. In addition, incorporation of an internal nucleophile into the styryl terminator greatly reduces the formation of undesired by-products. 

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