L,5-Diene-3-ynes

Examine the geometry of methylbenzyne. Measure carbon-earbon distances. Which 7C bonds are deloealized and whieh are localized Is there really a triple bond (Compare bond distance to triple bond in hexa-l,5-dien-3-yne and to partial double bonds in benzene). Are you able to draw a single Lewis structure whieh adequately represents the geometry of the molecule ... 

Bromoalkynes also couple with vinylstannanes readily to result in enynes. Synthesis of protected enynals via cross-coupling of vinylstannanes with 1-bromoalkynes in the presence of a catalytic amount of Pd(II) has been reported (equation 143)252. Hiyama and coworkers extended the Stille methodology for sequential three-component coupling of trimethylstannyl(trimethylsilyl)acetylene with a vinyl iodide in the first step and cross-coupling of the intermediate trimethylsilylethyne with another alkenyl iodide in the presence of tris(diethylamino)sulphonium trimethyldifluorosilicate in the second step to generate a dienyne (equation 144)253. Both steps occur under palladium catalysis, in one-pot, to result in stereodefined l,5-dien-3-ynes. 

Aryl group substituted butatrienes and hexapentaenes can be selectively reduced with Zn-ZnCl2-H20 to result in aryl-substituted 1,3-butadienes and hexa-l,5-dien-3-ynes, respectively (equations 191 and 192)315. 

Unsymmetrical dienynes react regioselectively with organolithium compounds at the less substituted double bond (Scheme 2.37). Thus, addition of n-butyllithium to 2-methylhexa-l,5-dien-3-yne (107) led after hydrolysis to vinylallene 108, whereas the corresponding carbolithiation of the linear isomer 109 furnished product 110 with 55% yield [68]. 

In enynes, both the double and the triple bond can compete for the carbenoid that is generated from alkyl diazoacetate under copper or rhodium catalysis. The chemoselectivity is sometimes not very pronounced, but but-l-en-3-yne is selectively attacked at the triple bond [methyl diazoacetate, RhjCOAc), 70% yield], whereas in 2-methylhexa-l,5-dien-3-yne only the double bonds accept the carbenoid (Table 12, entry 2). The vinylcyclopropene obtained by addition to the triple bond may dimerize to form a 3,6-dialkoxycarbonyltricyclo-[3.1.0.0 ]hexane.2 "... 

Some heterocyclizations have been used for the preparation of 3-perflu-oroalkylthiophenes. As an illustration, treatment of l,l,6,6-telrakis(ethylsulfanyl)-2,5-bis(trifluoromethyl)-hexa-l,5-dien-3-yne 135 with a mixture of trifluoroacetic acid and water for 2 h at 75 °C led to the thiophene derivative 136 in high yield. The starting compound 135 was obtained by reaction of perfluoroketene dithioacetal 134 with bis(trimethylsilyl)acetylene [76]. 

As indicated earlier, one of the diene double bonds can be substituted by a triple bond. The resulting cyclic product then has an additional double bond. When these new bonds are conjugated with other olefinic linkages, a further adduct is formed. Thus, 2,5-dimethyl-l,5-hexadiene-3-yne 23 reacts with 2 moles of MA to give product 24. ... 

Nonetheless, there are a small number of systems that do mediate such [2 -i- 2 -t- 2] cycloadditions. With allenes as the alkene , cycloaddition with both acetylene and terminal alkynes proceeds regio-selectively to give 3,5-dimethylenecyclohexenes using Ni catalysts, and mostly 3,6-dimethylenecyclo-hexenes using Ni° catalyst precursors (equation 19). Norbomadiene undergoes so-called homo-Diels-Alder cycloaddition with both alkenes and a ynes in the presence of nickel catalysts. Further elaboration of this chemistry with alkynes but not alkenes has been described using a Co/Al catalyst system (equation 20). Attempts to produce cyclohexenes via all-intramolecular [2 + 2 + 2] cycloaddition of l,13-dien-7-ynes or 1,1 l-dien-6-ynes have been unsuccessful. ... 

In more complex reaction cascades an additional alkyne-insertion step can occur. Thus starting with intramolecular carbopalladation of a vinyl iodide to a carbon-carbon triple bond, followed by two intramolecular alkene-insertion steps and termination with dehydropalladation, a palladium-catalyzed synthesis of l-(5 -methylbicyclo[3.1.0]hex-T-yl)-5,5-bis(carboethoxy)cyclo-hexadiene (52) starting from l-iodo-4,4-bis(carboethoxy)-ll-methyldodeca-l,ll-dien-6-yne (51) is achieved. ... 

The unidentified peak in the chromatogram (Figure 3) gives a negative test for an acetylenic hydrogen (ammoniacal cuprous chloride solution) and does not disappear after the sample is subjected to free radical catalysis for several hours. Thus, the open chain acetylenic isomers of benzene, hexa-l,3-diene-5-yne, 1,4-hexadiyne, 1,5-hexadiyne, and 1,5-hexadien-3-yne do not appear to account for this peak. The possibility of valence isomers of benzene such as bicyclo (2.2.0) hexa-2, 5-diene,... 

In 1948 [1] a versatile approach to various symmetrical Cio-building blocks (Scheme 1) was opened up by the synthesis of 2,7-dimethylocta-l,7-dien-4-yne-3,6-diol (1) [2-6]. This compound could easily be obtained (as a mixture of racemate and meso-form different m.p. have been reported for preparations of 1 [1,4]) by a Grignard reaction of acetylene dimagnesium dibromide (2) with 2-methyIprop-2-enal (3). A crystallized form of the acetylene glycol 1 was transformed into a crystalline dibromide 4 upon treatment with PBrs [4]. Reaction of 4 with potassium acetate and subsequent saponification of the intermediate diacetate yielded 2,7-dimethylocta-2,6-dien-4-yne-1,8-diol (5) [4]. 

A solution of PhaPAuOTf (5 mol%) was prepared by mixing PhaPAuCl (9.1 mg, 0.018 mmol) and AgOTf (4.6 mg, 0.018 mmol) in 1,4-dioxane (1.9 mL). To this solution was added 3-methyl-l,8-diphenylocta-l,2-dien-7-yne (100 mg, 0.37 mmol) at 100 °C, and then the mixture was stirred for 4 h. The resulting solution was filtered through a Celite (Sigma-Aldrich, St. Louis, MO) bed and eluted through a silica gel column... 

Dimethylvinylidene carbene generated from 3-chloro-3-methylbut-l-yne using aqueous KOH and a phase-transfer catalyst, has been added to cis,cis-cyclo-octa-l,5-diene to give the mono-adduct. ... 

Racemic isohumulone is prepared from the key product 2-methyl-2-penten-4-yne, which is obtained by a 1,4-elimination reaction from 1-bromo-4-methylpenta-1,2-diene, in the presence of copper(l) cyanide (85). Addition of the hydrocarbon to ethyl pyruvate, followed by hydrolysis, leads to 2,6-dimethyl-2-hydroxy-5-hepten-3-yne acid. The corresponding acid chloride is added to ethyl 3-oxo-5-methylhexanoate in the presence of magnesium methylate. The reaction product cyclizes in basic medium to... 

Various cycdization products have been observed in the cycloisomerization of 3,5-dien-l-ynes using [Ru(Tp)(PPh3)(CFl3CN)2]PF6 catalyst the cyclization chemos-eledivity is strongly dependent on the type of substrate structures, which alters the cycdization pathway according to its preferred carbocation intermediate. The reaction protocols are summarized below ruthenium vinylidene intermediates are responsible for these cyclizations (Scheme 6.10). 

In the preceding cyclization, one concern is the behavior of intermediate 24 if its benzyl cation is replaced with a tertiary cation to avoid forming a tertiary carbon, the 1,2-aryl migration of this intermediate would be unlikely to occur. When this cyclization was extended to 2,2-dimethyl(o-ethynyl)styrene 26, the 2-alkenyl-lH-indene product 28 [16] was obtained in 76% yield, as depicted in Scheme 6.14. In this transformation, the alkenyl double bond of the 3,5-dien-l-ynes is cleaved and a... 

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