Enynes and 1,6-Dienes

Dienes and 1,5-enynes Regioselective insertion of zirconacyclopentenes by alkynylmetals leads, after hydrolysis, to acyclic products. 

Dienes and 1,5-Enynes via Pd-Catalyzed Homoallyl-Alkenyl Coupling and Homopropargyl-Alkenyl Coupling... 

Highly satisfactory alternate routes to the desired 1,5-dienes and 1,5-enynes have been systematically developed by the authors group through the use of homoaUyl-, homopropargyl-, and homobenzyhnetals [35a,b]. 

Negishi and his co-workers have described a new synthetic route to 1,5-dienes (or 1,5-enynes) which is particularly suitable for the construction of head-to-tail coupled isoprene units. The key step involves a regio- and stereo-selective palladium-catalysed cross-coupling of homoallylic (or homopropargylic) organozinc compounds with alkenyl halides (e.g. Scheme 95) homo-coupling... 

Negishi, E.-I. Rand, C. L. Jadhav, K. P. Highly selective and convenient method for the synthesis of 1,5-enynes and 1,5-dienes by the reaction of 1,3-dilifhiopropargyl phenyl sulfide with allylic halides. J. Org. Chem. 1981, 46, 5041-5044. 

Dienes and 1 -enynes. Negishi et al.9 have extended the synthesis of 1,3-enynes by palladium-catalyzed cross-coupling of alkynylzinc chlorides with an alkenyl halide (8, 472) to a similar stereospecific synthesis of 1,5-dienes or 1,5-enynes by coupling an alkenyl halide with a homoallyl- or homopropargylzinc chloride. An example is the synthesis of the 1,5-enync I (equation I). This reaction... 

With the exception of norbomene, internal olefins do not undergo hydrosilylation. Hydrosilylation of 3-phenylpropene with PhSiDj forms a unique product and the process tolerates a variety of fianctional groups, halides, ethers and acetals, despite the well known strong Lewis acidity of the catalysts. Cyclisation/silylation of 1,5-dienes or 1,6-enynes has been reported to give a single product (Scheme 14) [26]. In the case of metallocene complexes bearing a menthyl substituent, ee values near 70% were obtained for the asymmetric hydrosilylation of 2-phenyl-but-l-ene [31]. 

SCHEME 3.30 Ni-catalyzed synthesis of l,3-dien-5-ynes and 1,5-enynes. 

Dienes and l,S-enynn. pentenes (derived from alkynes. are treated with alkynylmetals. Pi w ith iodine gives 1,5-enynes. 

The rhodium A-heterocyclic carbine complex [Rh(IMes)(COD)] [IMes = A,A -bis(2,4,6-trimethylphenyl)imidazol-2-ylidine COD = cycloocta-1,5-diene] catalyses the 4 + 2 + 2-carbocyclization of 1,6-enynes (191) to carbocycles [(192) and (193)] (Scheme 54).226 Computational and experimental evidence of a new reaction pathway for the diastereospecific intermolecular rhodium-catalysed 4 + 2 + 2-carbocyclization (g) reactions of 1,6-enynes with r-components has been reported.227... 

Oraganosilicon compounds react with a wide variety of organic electrophiles in the presence of a palladium catalyst and a Lewis base activator such as a fluoride or hydroxy ion to give the corresponding coupled products. The reaction is applicable to synthesis of diynes, enynes, arylacetylenes, alkenylarenes, biaryls, allylarenes and alkylarenes in addition to 1,3-, 1,4- and 1,5-dienes with tolerance for various functional groups. 

Selective reduction of allyl sulfides implies that no scrambling of the carbon-carbon double bond occurs during the process. Effectively this has proved to be the case especially when lithium in ethylamine is used, and the method has allowed the regio- and stereo-selective synthesis of a large variety of 1,5-dienes including squalene (Scheme 25, entry a), mukapolide (Scheme 25, entry b), dendrolasin (Scheme 25, entry c), the basic nucleus of crassin acetate (Scheme 25, entry d) from 7,7-dialkylallyl sulfides and allyl halides, and also of 1,5-enynes " from propargyl sulfides and allyl halides (Scheme 34, entry b). 

Thiol-promoted cyclization of 1,6-dienes and analogous enynes in the presence of radical initiators has been studied. An equimolar amount of ethanethiol, in the presence of diphenyl disulfide as initiator, was added under photolytical conditions to dimethyl diallylmalonate an addition of thiyl radical followed by 5-hex-enyl cyclization took place [48] and cis- and fra 5-cyclopentanes (the ratio being... 

Inter- and intramolecular additions of alkenes and dienes to propargylic alcohols catalyzed by thiolate-bridged diruthenium complexes have been described. The processes, a kind of allenylidene-ene reaction, generate 1,5-enynes and dienynes by reaction of propargylic alcohols with 2-arylpropenes [196] and 1,3-conjugated dienes [197], respectively. The intramolecular version of this reac-ti(Mi has been developed to give diastereo- [196, 198] or enantioselective syn-substituted chromanes (Scheme 58) [199]. Recently, the results of DPT calculations indicated that nucleophilic attack of the olefinic Jt-electrons on a carbocationic... 

Scheme 58 Ru-catalyzed inter- and intramolecular additions of alkenes and dienes to propargylic alcohols to 1,5-enynes and dienynes...

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. 

Ruthenium hydride catalysts can also initiate a variety of cycloisomerizations of 1,5- and 1,6-enynes as well as dienes, as exemplified by the RuClH(CO)(PPh3)3-catalyzed reactions shown in Scheme 64.249... 

Cycloaddition.2 In combination with a triarylphosphine, 1 catalyzes the cyclization of a 1,6-enyne (2) to a 1,3-diene (3), and it effects [2+2+2] cycloaddition of 2 with an alkyne to give 4, which is isomeric with the product (5) of Diels-Alder addition of 3 with an alkyne. 

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