Dialkynes

J-unsaturated ester is formed from a terminal alkyne by the reaction of alkyl formate and oxalate. The linear a, /J-unsaturated ester 5 is obtained from the terminal alkyne using dppb as a ligand by the reaction of alkyl formate under CO pressure. On the other hand, a branehed ester, t-butyl atropate (6), is obtained exclusively by the carbonylation of phenylacetylene in t-BuOH even by using dppb[10]. Reaction of alkynes and oxalate under CO pressure also gives linear a, /J-unsaturated esters 7 and dialkynes. The use of dppb is essen-tial[l 1]. Carbonylation of 1-octyne in the presence of oxalic acid or formic acid using PhiP-dppb (2 I) and Pd on carbon affords the branched q, /J-unsatu-rated acid 8 as the main product. Formic acid is regarded as a source of H and OH in the carboxylic acids[l2]. 

The replacement of rhodium from a wide range of rhodacycles to form condensed furans, thiophenes, selenophenes, tellurophenes and pyrroles has been widely explored and a range of examples is shown in Scheme 97. The rhodacycles are readily generated from the appropriate dialkyne and tris(triphenylphosphine)rhodium chloride. Replacement of the rhodium by sulfur, selenium or tellurium is effected by direct treatment with the element, replacement by oxygen using m-chloroperbenzoic acid and by nitrogen using nitrosobenzene. 

Palladium(O) or readily reduced paUadium(II) complexes were the most efficient catalysts, giving higher yields than analogous Pt catalysts. The Markovnikov product was formed with high regioselectivity. In dialkynes, both C=C bonds could be hy-drophosphorylated, while the C=C double bond in a cyclohexenyl alkyne subshtuent did not react. With trimethylsilylacetylene, unusual anti-Markovnikov selectivity was observed. 

Chung and coworkers [280] combined a [2+2+1] with a [2+2+2] cycloaddihon for the synthesis of multi-ring skeletons, angular triquinanes, and fenestranes. For the preparation of tetracyclic compounds such a 6/4-17, these authors used diynes as 6/4-16 and CO as substrates (Scheme 6/4.5). Fully substituted alkynes gave low yields, and 1,5- as well as 1,7-dialkynes, did not react... 

The same arsenal of preparative methods has been applied successfully for the corresponding dinuclear derivatives of ethyne HC CH and dialkynes HC C-X-C CH, where X can be virtually any spacer unit.50-52,54 55 57 61 62 71 76-83 As mentioned in the introduction to this chapter, ethyne is readily converted into polymeric explosive AuC=CAu and its complexes (L)AuC=CAu(L), of which the families with L = R3P84 and L = RNC are particularly large (Chapter 7). With the unit X in (L)AuC=CXC=CAu(L) being an alkylidene spacer, flexible complexes are obtained, while with alkenylidene, alkynylidene, or arylidene units,57 rigid molecules (L)AuC=CXC=GA11(L) are generated. Specific examples are presented below in the context with the structural patterns of extended systems. 

A variety of boron-substituted stannoles such as those shown in formulas 10 and 12 have been prepared by Wrackmeyer s 1,1-organoboration of dialkynes.234... 

Arene-solvated cobalt atoms (U) and (1, obtained by reacting Co vapor and arenes, have been found by Italian workers to promote the conversion of a,d)-dialkynes and nitriles to alkynyl-substituted pyridines [87JOM(326)C33] (Scheme 4). 

Uracil derivatives can also participate in cobalt-mediated [2-f2-f2] cycloaddition reactions with dialkynes under photochemical conditions, to give cobalt complexed dihydrobenzo[g]quinazoline derivatives 482 <1999CEJ3549>. 

Reaction of carbanions with dialkynic ketones, the so-called skipped diynes, can produce pyranones through an initial Michael condensation. It should be noted however that diynones are vulnerable to attack at several sites and that mixed products can be formed. Addition of the anions derived from diethyl malonate and ethyl cyanoacetate to hepta-2,5-diyn-4-one (313 R1 = Me) gives the pyranones (314 R2 = C02Et or CN Scheme 91) (74JOC843). The former carbanion reacts similarly with the diynone (313 R1 = Bun) (68T4285). The second alkyne moiety appears to have little effect on the course of the reaction, which parallels the synthesis of pyranones from monoalkynic ketones. 

The Ojima group has extended their studies of silylformylation to include more complex substrates, such as alkenyne, dialkyne, alkynyl nitrile, and ethynyl pyrrolidinone. Use of rhodium or rhodium-cobalt metal complexes catalyzes the silylformylation of these substrates with high chemoselectivity, as the other functionalities present are inert to the reaction.122b,c d... 

Cationic sulfonamides of the form (29) [obtained from dialkynes (28) upon treatment with acids or halogens] undergo stereoselective reduction of the sulfonamide group to generate a sulfinamide (31) under very mild conditions.279 Indirect 1H NMR evidence suggests that the reaction proceeds via the intermediate (30). Further reactions occur in the presence of I2, including the reduction of the sulfinamide to a sufenamide. 

This dialkyne metal salt, which exists in associated form, is shock sensitive. 

Alkyne-alkene carbonylative coupling. Intramolecular carbonylative coupling of dialkynes catalyzed by Fe(CO)3 provides a route to cyclopentadienones (equation I). The more difficult carbonylative alkyne-alkene coupling to provide cyclopen-tenones (Pauson-Khand reaction) can also be effected with Fe(CO)s, but in modest yield. In an improved coupling, acetone is treated with Fe2(CO)9 to form Fe-... 

The dialkyne/carbon dioxide copolymerisation is controlled by the relative rate of inter- and intramolecular cyclisations of the dialkyne the former is favoured when the number of methylene groups in the monomer R C C (CH2)X C = C—R is equal to 3,4 or 5 (x 3—5), but the intermolecular cyclisation of the dialkyne is favoured to effect 1 1 cycloaddition copolymerisation of the dialkyne and CO2 to a poly(2-pyrone) when the number x has other values [91 96]. 

Give reasons why the cyclopolymerisation of a, co-dialkyne such as 1,6-heptadiyne proceeds via both five-membered and six-membered ring closure. 

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