Palladium-Catalyzed Homo-benzannulation Reaction

SCHEME 14.7 Pd-catalyzed [4 + 2] homo-benzannulation reaction of monosubstituted enynes. 

Homo-benzannulation reaction of di- or trisubstituted enynes is a less facile process [7b,c]. Thus, neither 1,2- nor 2,4-dialkyl enynes underwent homo-benzannulation 

However, a recently discovered catalytic system [8] based on Af-heterocyclic carbene-ligated palladium complex in combination with electron-rich phosphine ligand 27 enabled this transformation for different 2,4-disubstituted enynes, including 2,4-dialkylenynes possessing protected alcohol (26f) or amine (26g) moieties (Table 14.1). However, no examples of successful homo-benzannulation of tri- or tetrasubstituted enynes have been reported to date. 

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The reactivity trend of enynes for cross-benzanulation reaction remains consistent to that of the palladium-catalyzed homo-benzannulation of two enynes. However, in this case, the scope is expanded to 1,4-disubstituted and 1,2,4-trisubstituted substrates. 

Shortly after palladium-catalyzed [4-1-2] homo-benzannulation reaction of two enynes was discovered [4], a cross-benzannulation of enynes with diyne enynophiles was developed [13], thus substantially expanding the scope of this transformation. Similar to the homo-benzannulation reaction, the cross-benzannulation reaction occurs in a highly regioselective manner, offering easy access to densely substituted arylacetylenes 10 as solo regioisomers (Scheme 14.17) [13,14]. In most cases, the reaction is chemoselective, providing a product of cross-benzannulation reaction only. 

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