A new concept of highly chemo- and regioselective formation of the benzene ring by a palladium-catalyzed formal [2 + 2 + 2] sequential intermolecular trimerization of alkynes is proposed. Homodimerization of terminal alkynes and subsequent [4 + 2] benzannulation with diynes gives tetrasubstituted benzenes in moderate to good yields. The introduction of two different alkynes (terminal and internal) in the first step of the sequence allows for construction of pentasubstituted benzenes from three different acyclic acetylenic units. In all cases the tetra- and pentasubstituted benzenes are formed as a single reaction product without being accompanied by any of regio- or chemoisomers. A significant acceleration of the sequential trimerization reaction in the presence of Lewis acid/phosphine combined system was observed. Mechanistic studies reveal that the Lewis acid assisted isomerization of the E-enyne formed in the first step of the sequence to the more reactive Z-isomer is responsible for the observed acceleration effect. The proposed methodology provides a conceptually new and synthetically useful route to multifunctional aromatic compounds.
The enyne-diyne [4+2] cross-benzannulation proceeded smoothly in the presence of Pd(0) catalyst to give 1,2,4-trisubstituted benzenes 8a-j, 1,2,3-trisubstituted benzenes 9a-i, 1,2,3,5-tetrasubstituted benzenes 11a-k, 1,2,3,4-tetrasubstituted benzenes 11l,m, and 1,2,3,4,5-pentasubstituted benzenes 11n-p in moderate to quantitative yields. In all cases the reaction was found to be regiospecific with regard to substitution at the benzene ring. The palladium-catalyzed [4+2] cycloaddition of monosubstituted conjugated enynes 3a-c and 5a with unsymmetrical diynes 13a-j and triynes 16a-g also proceeded with perfect regiochemistry with regard to substitution at the benzene ring. However, since two differently substituted triple bonds are present in the enynophiles 13 and 16, the enyne preference toward either triple bond in the enynophiles became an issue, and in most cases, two regioisomeric products were obtained. The detailed deuterium-labeling studies were performed, and the mechanism of this regiospecific [4+2] cycloaddition between enynes and diynes was proposed.
[reaction: see text] The simple combination of two angular tritopic pyridine donor linkers (109 degrees bond angle) with three ditopic platinum acceptors (90 degrees bond angle) leads to essentially quantitative formation of self-assembled M(3)L(2) trigonal-bipyramidal (TBP) supramolecular species.
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