Carbopalladation 1,6-diynes

An inter-intra-intramolecular carbopalladation cascade, initiated by the alkenylpalladium bromide intermediate from /3-bromostyrene 74 inserting into one of the triple bonds of the diyne 73, yields the ring-annelated fulvene derivative 75 (Scheme 22). ... 

Tricyclic skeletons such as 85, 87, 89 with a central benzene ring are formed in the fully intramolecular Pd-catalyzed cascade cyclization of 2-bromo-l-ene-//,w-diynes 84, 86, 88 and analogs (Scheme 24). This process involves two alkyne relays in a row and a final 67r-electrocyclization or 6-endo-trig carbopalladation with ensuing / -dehydropalladation. 

There are at least two issues to be addressed regarding the Type lie circular cascade process shown in Scheme 42. One is the regioselectivity in the initial intermolecular carbopalladation. Since it is not very difficult to differentiate the two terminal positions of cYj -diynes, this is not a serious problem in most cases. A more serious problem is the exclusive formation of fulvene derivatives observed in a couple of cases [124] (Scheme 45). It is not very clear what the scope of the fulvene formation is and whether the course of the reaction could be altered to give benzene derivatives. 

The 1,6-diyne system 245 with an iodophenyl starter was converted to the tetracycle 248. Carbopalladation of 245 generates 246 and subsequent 5-exo-dig cyclization affords 247. The last step is attack on the aromatic ring to give the tetracycle 248 [67],... 

The octahydrophenanthrene skeleton is also formed from 2-bromotetradeca-l-ene-7,13-diynes with a trialkylsilyl-substituted terminal triple bond (Scheme 46, Eq. 1). With an unsubstituted terminal acetylene, the same bromoenediynes yield bisannelated ful-venes by a completely different cascade carbopalladation mode involving a 5-exo-trig cy-clization and a [l,5]-sigmatropic shift of the CH2PdBr group or a 3-exo-trig cyclization... 

The inter-intermolecular carbopalladation cascade, starting with the palladium intermediate from /3-halostyrene derivatives and two molecules of an alkyne, does not yield a benzene, but highly substituted fulvene derivatives, albeit in moderate yields only (Scheme 47, Eqs. 1 and Fulvene derivatives are also formed from 1-haloalkenes and 1,6-diynes (Schemed , Eq. 3). ... 

Alternatively, similar fused polycycles 178 can be synthesized from diynes 177 with aryl halides initiated by intermolecular carbopalladation, developed by Hu et al. 

FIGURE 8 J Compilation of chromans (in a deprotected form) generated by the dontino carbopaUadation/carbopalladation/cyclization sequence. Given are the yields of the domino sequence starting from the respective diynes. 

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