Carbocyclizations of diynes

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. 

The cobalt-catalyzed cooligomerization of diynes with nitriles allows a simple one-step synthesis222 of condensed pyridine derivatives including difficultly accessible 5,6,7,8-tetrahydroisoquinolines223 The synthesis is a versatile one in that pyridines condensed with five- and seven-membered carbocyclic rings can also be achieved in moderate yield in similar fashion. Additional attractive features of this simple synthesis are the formation of condensed isoquinolines by the use of functionalized nitriles and the pronounced regioselectivity observed when dissymmetrical diacetylenes are employed (Scheme 148).222... 

Novel carbonylative carbocyclizations of 1,6-diynes promoted by Ru3(CO)i2/P(hex-c)3 in the presence of HSiMc2Bu-Z give bicyclic o-catechol derivatives by incorporating two carbon monoxide molecules as the 1,2-dioxyethenyl moiety (equations 148 and 149)346. This reaction is tolerant of functional groups such as ester, ketone, ether and amide. The disilylated product 366 is formed through dehydrogenative silylation of the initially formed mono-silyl product 365 under the reaction conditions. 

The pure E stereochemistry of the vinylsilane 21 clearly shows that the carbocylization proceeds in a totally stereoselective fashion to give, under these conditions (20 °C, 5 min) the configurationally stable (Z)-alkenylzinc bromide [21] 20 via a syn addition of the allenic zinc bromide to the acetylenic moiety. This carbocyclization represents a new desymmetrization of diynes such as 18 which serves to deliver functionalized exo-... 

Hydrophosphorylative carbocyclization of a,major product in most cases was reported by Kanada et al. (Scheme 75). ... 

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

Das A, Chang H-K, Yang C-H, Liu R-S (2008) Platinum- and Gold-catalyzed hydrative carbocyclization of oxo diynes for one-pot synthesis of benzopyrenes and bicyclic spiro... 

Carbocyclization of 1,5- and 1,6-diynes has been reported leading to benzopyrones [142] and Z-cyclopentylidenes [143, 144], respectively. Furthermore, 1,4-diynes 44 react in the presence of gold-catalysts to form seven-membered ring heterocycles 45 by an endo-cyclization (Scheme 18) [145]. 

Carbocyclization of a,co-diynes was observed in Pd-catalyzed addition reaction (Scheme 8.36) [98]. Utilization of chelate phosphine ligand was of key importance to carry out this transformation only 5% of product was observed with PPh3, while 70% with dppe (100 °C, 1 h). 

Kanada J, Yamashita K, Nune SK, Tanaka M (2009) Pd-catalyzed addition-carbocyclization of a,co-diynes with H-P(0)R2 compounds. Tetrahedron Lett 50 6196-6199... 

Cascade silylcarbocyclization reactions tiave been developed based on the fact that it is possible to realize successive intramolecular carbocyclizations, as long as the competing reductive elimination is slower than the carbometallation. For example, the reaction of dodec-6-ene-l,ll-diyne 67 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at 50°C under 1 atm CO gives bis(exo-methylenecyclopentyl) 68 in 55% yield [44]. The reaction is stereo-specific that is, (6 )- and (6Z)-dodec-6-ene-l,ll-diynes, ( )-67 and (Z)-67, afford R, R )-68 and (S, R -68 respectively. A possible mechanism for this reaction is outlined in Scheme 7.20. It should be noted that none of the tricyclic product is formed even though a third carbocyclization in the intermediate III.2c is conceptually possible. 

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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