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The cobalt-catalyzed crossed [4+2]-benzanullation of C4-substituted 1,3-enynes 29/41with disubstituted buta-1,3-diynes 38 was realized by the use of the HILTcatalyst consisting of the pre-catalyst CoBr2 (dppp), the reducing agent zinc powder, the LEWIS acid zinciodide and methylene dichloride as solvent.
First, reactions between C4-substituted 1,3-enynes 29a-h/41and symmetrical buta-1,3-diynes 38a-f were investigated resulting in 1,2,3-trisubstituted benzene derivatives 106a-l as product.
The reactions proceeded in absolute control of regioselectivity and products with the alkyne functionality in C2-position were formed exclusively.
The present reaction tolerated a wide range of functional groups, affording the 1,2,3-trisubstituted benzenes
106a-l in acceptable to good yields.
Moreover, the crossed [4+2]-benzannulation of the phenyl-substituted 1,3-enyne 41 and various unsymmetrical substituted buta-1,3-diynes 38j-p was investigated using the cobalt-catalyst system.
The reactions resulted in the formation of two regioisomeric 1,2,3-trisubstituted benzene derivatives
117 and 118 in acceptable to good yields and good to very good regioselectivities.
As major products the 1,2,3-trisubstituted benzene derivatives 117 were obtained, with the sterically less hindered substituent (R=alkyl, aryl) at the annulated benzene ring and the sterically more hindered substituent (TMS) at the alkyne moiety.
The comparison of these results with those of the literature known and complementary palladium(II)-
catalyzed crossed [4+2]-benzannulation of YAMAMOTO and GEVORGYAN showed, that under palladium-
catalysis the opposite regioisomeric 1,2,3-trisubstituted benzene derivatives 118 were formed predominantly, with the sterically more hindered substituent (TMS) at the annulated benzene ring and the sterically less hindered substituent (R=alkyl, aryl) at the alkyne moiety.
Consequently, the use of the cobalt-catalyzed system resulted in the unprecedented synthesis of 1,2,3-
trisubstituted benzenes of type 117 as major product in the crossed [4+2]-benzannulation, which broadened the scope of this reaction leading to a new type of products not easily accessible by other methods.