Alkyne derivatives Stille reactions

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

Certain alkynes may not be suitable for the Sonogashira reaction. The highly sensitive ethoxyacetylene often fails, and use of its tin or zinc derivatives may be better. Alkynes conjugated to electron-withdrawing groups are frequently not effective partners, and a protected form may be required (Scheme 2.126), or the corresponding Negishi or Stille reactions may have to be used. Thus, propiolaldehyde is better replaced by its acetal 2.404, and propiolate esters are better replaced by the ortho-ester 2.406. 

The research field of ortho-carborane chemistry continues to expand rapidly mainly due to the commercial availability of ort/fo-carborane itself and the recent interest of these derivatives as agents for BNCT and other applications. Synthesis of these carboranes is still dominated by reactions of decaborane with various alkynes as shown in Table 19 (Figure 2). 3-Substituted derivatives are made by capping the /[Pg.71]

All the starting compounds in Scheme 8 have a sufficient potential for both catalytic and stoichiometric silylformylation, when Me2PhSiH and 1-alkyne are present in a reaction vessel at the same time. Stable mononuclear complex, RhH(GO)(PPh3)3, is far inferior in catalyst efficiency at 25 °G, though the efficiency is improved under practical operation at 100 °C (Table 6). Though 7 and 11 are derived from Rh4(GO)i2 under controlled conditions and work as an active catalyst of silylformylation, their position in the catalytic cycle is still a precursor of truly active species, because it takes a far longer induction period for activation than that for silylformylation. 

The cyclic carbopalladation-cross-coupling tandem reaction has been extensively developed over the past several years. Despite earlier favorable findings with A1 and Zr [84], these metals are still scarcely used. On the other hand, organometals containing Sn and B have been widely used, and favorable results have been obtained for the formation of ffve-membered car-bocycles and heterocycles containing N and O from halodienes [88] (Eqs. 1 and 2 in Scheme 29), haloenynes [89-92] (Eqs. 3-5 in Scheme 29), haloaryl-alkynes [94,95] (Eqs. 6 and 7 in Scheme 29), and allenene derivatives [93,96] (Eqs. 8 and 9 in Scheme 29). 

Tin-functionalized 1-phosphanorbornadienes can be prepared from tin-substituted alkynes using this type of methodology. Subsequent tin-halogen exchange affords the corresponding halo derivatives, which may be further elaborated using palladium-catalyzed reactions such as Stille coupling, for example (see Section 3.15.12.1.2). 

From the slow, noncatalyzed reactions, derivatives of 7-oxabicyclo[2.2.1]-hepta-2,5-diene (18) may be obtained. UV irradiation does not reverse the addition but allows an easy entry into the 3-oxaquadricyclane series (19)45 When furan adds to an acetylenic diester the product is still reactive enough to add another molecule of furan, a somewhat unexpected feature being that the product (20) is obtained only at lower temperatures. Above 100 C dissociation must occur and the less active double bond takes up the alkyne to give the thermodynamically more stable isomer (21)43 Whatever the addition, mixtures of endo and exo configurations result. The result is thought to reflect the fact that geometrical overlap of orbitals at the primary reaction centers is more important than secondary interactions as bond formation... 

Heteronuclear addition reactions to alkynes usually produce the corresponding trans-alkene derivatives. However, in the case of strained cyclic alkynes, these would be even more heavily strained than the starting alkyne. Thus, in most reactions, cis-products are observed. The initial addition of the electrophile may, however, still proceed in an anh -manner, as was shown by Krebs et al. [1 b] for the reaction of a cycloheptyne derivative with trichloromethylsulfenyl chloride (Scheme 8-21). This reaction produced the first seven-membered rm s-cycloalkene derivative to be isolable at room temperature. 

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