2-ethynyl-1,3-butadiene

Most interesting is the subsequent rearrangement of the dienyne to 2-ethynyl-1,3-butadiene at 500°C in a flow system." " The suggested mechanism involves the strained bisolefin, l,4-bicyclo[2.2.0]hexadiene, formed by a 2 -h 2 process, possibly by the diyl described above, which undergoes a six-electron shift preserving the central bond. This pathway is supported by the deuterium distribution from the 3-deuterio derivative (Scheme 7.20). 

Process Formaldehyde ethynylation Butadiene acetoxylation, Butadiene chlorination... 

The bonding pattern of the last molecule in Table 4 is rather different from that of 1,3-butadiene, a fact which is probably connected to the ethynyl substituent that allows a further delocalization of the --electrons in this molecule. 

At higher temperatures, 29 (360-500 °C, R = H) undergoes a formal 1,3-ethynyl shift yielding the cross-conjugated enyne 2-ethynyl-l,3-butadiene (344). Deuteriumlabeling studies indicate that species such as 343 or hydrocarbons derived therefrom could be intermediates in the process [143],... 

A similar route was patented by Ancel and Meilland [66], The ethynyl-retro-ionol was acetylated (Ac20-DMAP-Et3N) and this propargylic acetate was reacted with methyl butadiene acetate in the presence of BF3-Et20. The allenic-retinal, obtained in 61% yield was isomerised in retinal by HBr in acetone (yield 50%), Fig. (34). 

Reactions of compound 8a with butadiene, 2,3-dimethylbutadiene, and isoprene required more drastic conditions (150°C, autoclave) [24]. In the case of isoprene, a 60 40 regioisomeric mixture was obtained (the major product being the para-adduct). The influence of Lewis acid catalysis on the reaction of cyclopentadiene with different ethynyl aryl sulfoxides [Ar =p-Tolyl, o-nitrophenyl, and l(2-methoxynaphthyl)] has been recently reported [25].Dienophilicities of acetylenic sulfoxides were greatly enhanced by the presence of various Lewis acids (the effect being greater with stronger Lewis acid), but the stereoselectivity was scarcely affected by the catalysis. 

DIELS-ALDER REACTIONS Bis(methyl-thio)-l,3-ketadiene. 1,3-Bis(trimethyl-silyloxy)-l,3-butadiene. 2,5-Bis(tri-methylsilyloxyjfuranes. 5,5-Dimethoxy-1,2,3,4-tetrachlorocyclopentadiene. Di-peroxo-oxohexamethylphosphoramido-molybdenum(VI). Ethynyl p-tolyl sullbnc. liirane. (li)-l-Mcthoxy-1.3-... 

USB Catalyst in polymerization of acetylene to benzene and styrene, trimerization of ethynyl compds, cyclization of butadiene. 

The coupling of arylzinc halides with alkyl (E)-2,3-dibromoalkenoates occurs at C-3. 3-Arylindoles are obtained from 3-indolylzinc halides with haloarenes." A synthesis of 1,4-disubstituted butadienes starts from the hydrozirconation of ethynyl phenyl selenide. After Pd(0)-catalyzed coupling with alkenyl bromides, the seleno group is replaced via Grignard reaction in the presence of (Ph3P)2NiCl2. 

Danishefsky and coworkers have used the racemic P-phenylsulfinyl-a,P-unsaturated ketone (160) as an a,P-ethynyl carbonyl synthetic equivalent and found that the phenylsulflnyl group did not compete with the carbonyl group in determining the regioselectivity of cycloaddition with the l-methoxy-3-trimethylsilyloxy-1,3-butadiene (161), now known as Danishefsky s diene [140] (Scheme 5.54). Loss of methanol and phenyl sulfenic acid from the initial cycloadduct (162) gave the aromatic product (163). 

Pyrolysis of trans-l-ethynyl-2-vinylcyclobutane at 460°C gave a mixture of butadiene and vinylacetylene (36 %X isomeric bicyclo[3,3,0]octadienes (30.5%), 4-ethynylcyclohexene (29%), l-vinyl-l,3-hexadiene (3.5%), and a small amount of styrene.At higher temperatures, styrene was the main product. These products could be accommodated with a mechanism involving initial homolysis of the 1,2-bond of the cyclobutane ring, and fragmentation or recyclization of the resultant biradical. 

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