Alkynyl migration

Irradiation of j8-phenylethynyl-a-diazo-jS-ketoester with 300 or 350 nm light causes regioselective Wolff rearrangement resulting in alkynyl migration (Scheme 102). ... 

The coordinated BF3 might then latch on to the tin of the alkynylstannane, creating an anionic tin center and thereby setting the stage for alkynyl migration ... 

Alkyl, alkenyl, or alkynyl migration from the metal center to the Ca. 

Reactions of the branched tetraalkynylsilanes 115 (RC=C)4Si (R = tBu, Ph, Me3Si) with the metallocene sources 1 or 2a lead, through twofold migration and C—C coupling of the alkynyl groups, to a novel type of dinuclear carbon-rich spiro-complexes 116 [57]. 

Alkenyl and alkynyl Fischer carbene complexes reacted with pyrrole imine 108 to give, through a 1,2- and 1,3-metal migration, respectively, indolizine derivatives at a different level of unsaturation (Scheme 29) <1999JA3065>. 

Metal-promoted 1,2-migration of silyl groups in silylalkynes results in the formation of silylvinylidenes which are subsequently readily desilylated. " Alkynyl-substituted silylvinylidenes have been obtained from silylated diynes and the Fe(N2)(CO)2 P(OMe)3 2/ Fe(CO)2[P(OMe)3]2 2(/t-N2) reagent and similar species are implicated in the reactions of several Group 8 metal complexes with mono- and bis-trialkylsilyl diynes. " ... 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

Treatment of the azido complexes 19 with CO leads also to the migration of the N3 ligand to the aiienyiidene unit (Scheme 2.9). Nevertheless, the initially formed azido-alkynyl compounds 20 are in this case thermally unstable, evolving slowly into the metallated acrylonitrile derivatives 23 via extrusion of N2. The mechanism of formation of 23 involves the migration of the azido moiety from Cy to the C atom of the alkynyl ligand to generate the allenyl intermediates 21, which by elimination of... 

Movassaghi et al. [21[ reported the synthesis of substituted pyridine derivatives via ruthenium-catalyzed cycloisomerization of 3-azadienynes. To avoid the isolation of the chemically active alkynyl imines, trimethysilyl alkynyl amines served as initial substrates, as shown in Scheme 6.19. The formation of ruthenium vinylidene intermediates is accompanied by a 1,2-silyl migration according to controlled... 

When these two types of coordination take place at the same metal center, the migration of the alkynyl ligand to the C -carbon atom of the vinylidene can occur, to form enynyl intermediates which, upon protonation by the terminal alkyne, lead to... 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

---