TpRe cycloaddition reactions

Another early success of the (TpRe(CO)(MeIm) fragment was the promotion of Diels-Alder cycloaddition reactions with benzene and anisole. Complex 101 [TpRe(CO)(MeIm)( 2-benzene)] undergoes an endo-selective Diels-Alder reaction with N-methylmaleimide to afford the bound bicyclo[2.2.2]octadiene complex 102 in 65 % yield (Scheme 12) [40]. Oxidation of 102 yields the bicyclo[2.2.2]octadiene 103 and/or the bicyclo[2.2.2]octenone 104 depending upon the choice of oxidation conditions. 

Cycloaddition reactions, [3 + 2] or [2 + 2] depending on L, of TpRe(CO)(L) (f/ -furan) complexes have been reported. The electrophilic alkene tetracyano-ethylene (TCNE) and the alkyne dimethylacetylenedicarboxylate (DMAD) were used as dienophiles. For L = PMcs and BuNC, 1,3-dipolar cycloaddition of TCNE to / -furan gives [3 + 2] products via a carbonyl ylide (Scheme 47). Addition of... 

TpRe(CO)(MeIm)(f/ -anisole) reacts with DMAD to form an f/ -barrelene-complex (Scheme 54). Demetalation gives the barrelene and a trisubstituted benzene that is the product of retrocycloaddition involving elimination of ethyne. TpRe(CO)(MeIm)(5,6-f/ -anisole) also reacts with NMM to give a cycloadduct with the methoxy group at a bridgehead position. In addition, TpRe(CO) (MeIm)(f/ -l-methylpyrrole) undergoes a 1,3-dipolar cycloaddition reaction with dimethylfumarate to give 7-azabicyclo-[2.2.1]heptene (Scheme 54). 

Scheme 54. Rhenium-promoted cycloaddition reactions of f/ -aromatic ligands ([Re] = TpRe...

The more activated TpRe(CO)(MeIm)(r/2-anisole) complex has demonstrated a slightly broader range of reactivity by undergoing cycloadditions with both N-methylmaleimide (Scheme 13) and dimethylacetylene dicarboxylate (DMAD) (Scheme 14). Analysis of these reactions is complicated by the fact that the initial anisole complex exists as a 3 1 mixture of... 

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