Novel catalytic Hunsdiecker-Heck (CHH) strategy toward all-E stereocontrolled ferrocene-capped conjugated push-pull polyenes

Abstract

Halodecarboxylation reaction of ferrocenylacrylic acid 1 and ferrocenyldienoic acid 3d with N-bromo- and N-iodosuccinimide in the presence of catalytic tetrabutylammonium trifluoroacetate at −40°C and −78°C affords the corresponding β-halovinylferrocenes 2a, 2b and δ-haloferrocenyldiene 4 in 37-72% yields. Heck reaction of β-iodovinylferrocene 2a with vinyl substrates (CH₂=CH-Z where Z=CO₂Me, CO₂Et, COMe, CO₂H, CONH₂, 4'-NO₂C₆H₄) in the presence of tri(4-tolyl)arsine/palladium acetate/lithium chloride/triethylamine in acetonitrile at 35-80°C affords the corresponding ferrocenyldienes 3a-3f in 50-81% isolated yields. Similar reaction of δ-iodoferrocenyldiene 4 with vinyl substrates (CH₂=CH-Z where Z=CO₂Me, CO₂Et, CO₂H, 4'-NO₂C₆H₄) affords the corresponding ferrocenyltrienes 5a-5d in 55-87% isolated yields. The ferrocene-capped conjugated dienes and trienes show excellent all-E stereoselectivity (vide NMR). The electronic, redox, and nonlinear optical properties of ferrocenylpolyenes have been evaluated. The data suggest that upon increasing the polyene chain length, (a) the absorption maxima shifts progressively to higher wavelength, (b) the oxidation potential of the Fc/Fc⁺ couple (E_½) decreases, and (c) the HRS-derived second-order NLO response (β) increases. From the insights derived from semiempirical calculation (ZINDO/1), a mechanism for the halodecarboxylation reaction has been proposed suggesting the prior formation of tetrabutylammonium salt of ferrocenylacrylic acid I. Attack of the halogenium atom at the π_C=C in I leads to the formation of intermediate II, and the latter triggers the elimination of carbon dioxide.