Alkene-alkyne coupling

Mascarenas developed a synthetic method to 1,5-oxygen-bridged medium-sized carbocycles through a sequential ruthenium-catalyzed alkyne-alkene coupling and a Lewis-acid-catalyzed Prins-type reaction (Eq. 3.45). The ruthenium-catalyzed reaction can be carried out in aqueous media (DMF/H20 = 10 1).181... 

TABLE 24. Enynes through alkyne-alkene coupling ... 

Although a fair number of examples of the Ti-promoted intermolecular alkyne-alkene coupling reactions are known, those that display high pair selectivity and regioselectivity are still relatively limited. Some representative examples are shown in Scheme 18 51 53>53a... 

Scheme 18 Ti-promoted intermolecular alkyne-alkene coupling.

Scheme 19 Ti-promoted intramolecular alkyne-alkene coupling.

Scheme 21 Ti-promoted alkyne-alkene coupling and its applications to the synthesis of conjugated dienes, arenes, and pyridines.

Iridium-phosphine complexes were found to be efficient carbonylative alkyne-alkene coupling catalysts [62]. Although frequently applied in other transformations, the dimeric complex [ Ir( x-Cl)(cod) 2] appeared to be a very active catalyst in the coupling of silylated diynes with CO [63], giving bicyclic products with a carbonyl moiety (Scheme 14.12). 

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-... 

Lopez, F., Castedo, L. and Mascarenas, J.L. (2002) Atom-efficient assembly of 1,5-oxygen-bridged medium-sized carbocycles by sequential combination of a Ru-catalyzed alkyne-alkene coupling and a Prins-type cyclization. Journal of the American Chemical Society, 124, 4218-4219 Lopez, F., Castedo, L. and Mascarenas, J.L. (2005) Practical asymmetric approach to medium-sized carbocycles based on the combination of two Ru-catalyzed transformations and a Lewis add-induced cydization. Organic Letters, 7, 287—290. 

Fig. 24. Formation of a metallocyclopentene at a Ru center via alkyne-alkene coupling.

In the arena of C-C bond activation via p-C elimination, Ni shows complementary reactivity to Rh and in fact has unique characteristics (1) as a first row transition metal, Ni is usually more reactive than its second and/or third row counterparts when cyclometalation [43] is involved (2) Ni -catalyzed aldehyde and alkyne/ alkene coupling reactions have been developed [44],... 

Catalytic intermolecular coupling of alkene and alkyne is quite a challenging task. Nevertheless, cyclopentadienyl rutheniumcomplexes are able to catalyze alkyne-alkene coupling (an Alder-ene type reaction) to a mixture of the re-gioisomeric products 120 and 121 (Scheme 52). The most efficient catalysts are the complexes 78 or 53. The latter is more reactive. The scope of the reaction with respect to substituents attached to the both reactants is enormous ester, hydroxy, nitrile, ether, amino, and arylhalide groups are tolerated. Both terminal and internal alkynes and alkenes can be used. Some typical examples are summarized in Table 24 [67,69]. 

The authors postulated that these reactions can follow two different reaction pathways involving either alkyne-alkene or alkyne-alkyne oxidative couplings as the initial steps. The alkyne-alkene coupling is assumed to give high levels of stereocontrol, while the alkyne-alkyne coupling initiates low selectivity pathways. Therefore, it is anticipated that alkyne substituents which favour alkyne-alkyne couplings will afford low enantioselectivity levels. 

A palladium-catalysed carbometallation-alkyne cross coupling cascade process has been reported for the stereo- and regio-controlled synthesis of dibenzoxepines with substituted exocyclic alkene functionality <06OL1685>. 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

Several of the synthetic methods appropriate to these compounds have already been described as applicable to small rings (cyclization by metalloid hydride addition to alkenes and alkynes, and coupling through metal halide elimination). While much of the chemistry of these compounds is similar to that of acyclic derivatives, a few peculiarities are worthy... 

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