Carbocyclization of diynes and enynes

The palladium-catalyzed stannylboration (90) [124] or silylboration (87) [109, 114] succeeds in the intramolecular carbocyclization of diynes and enynes (Scheme 1-27). It is interesting that a very strained four-membered cycUzation of hexa-l,5-diyne proceeds without any difficulties, similarly to five- or six-membered cycUzation. The boryl group is selectively introduced into the more reactive C=CH rather than C=C for enynes and into the terminal C=CH rather than the internal C=CR for diynes, again suggesting a mechanism proceeding through the first insertion into the Pd-B bond in preference to the Pd-Sn or Pd-Si bond. 

The palladium-catalyzed reaction allowed intramolecular five- or six-membered carbocyclization of diynes and enynes (Equation (40)).245,249 The boryl group was selectively introduced into the more reactive C=CH bond rather than C=C bond of enynes, and into the terminal C=CH bond rather than the internal C=CR bond of diynes. 

Carbocyclization of diynes and enynes took place with palladium catalysts and stannylborane 128 at room temperature (Equation (47)).260 The reaction was feasible for four-, five-, and six-membered cyclization. 

Transition metal-catalyzed carbocyclization of nitrogen- and oxygen-tethered 1, n-enynes and diynes to give five- or six-membered heterocyclic compounds 12CC10271. 

Transition metal-catalyzed carbocycUzation reactions of tethered diene, enyne, diyne, and vinylallene derivatives represent an important class of transformations in synthetic organic chemistry. This may be attributed to the abihty to significantly increase molecular complexity through the highly selective combination of acyclic components, thereby facilitating the synthesis of complex polycychc products. Recently, rhodium-catalyzed carbocyclization reactions have attracted significant attention due to their immense synthetic versatility and the unique selectivities observed over a range of different transformations. This chapter provides an account of recent developments in rhodium-catalyzed [4-1-2] and [4-i-2-t2] carbocyclization reactions. 

Diynes are also used to perform intermolecular enyne metathesis. With the objective of producing functionalized hetero- and carbocycles, a cascade diyne-alkene cross metathesis leading to five-membered cyclic products has recently been proposed [27] (Scheme 13). 

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