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

Halodecarboxylation reaction of ferrocenylacrylic acid 1 and ferrocenyldien oic acid 3d with N-bromo- and N-iodosuccinimide in the presence of catalyti c tetrabutylammonium trifluoroacetate at -40 degrees C and -78 degrees C af fords the corresponding beta-halovinylferrocenes 2a, 2b and delta-haloferro cenyldiene 4 in 37-72% yields. Heck reaction of beta-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 degrees C affords the corr esponding ferrocenyldienes 3a-3f in 50-81% isolated yields. Similar reactio n of delta-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 an d trienes show excellent all-E stereoselectivity (vide NMR). The electronic , redox, and nonlinear optical properties of ferrocenylpolyenes have been e valuated. 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-1/2) decreases, and (c) the HRS-derived second-order NLO response (beta) increases. From the insights d erived from semiempirical calculation (ZINDO/1), a mechanism for the halode carboxylation reaction has been proposed suggesting the prior formation of tetrabutylammonium salt of ferrocenylacrylic acid I. Attack of the halogeni um atom at the pi(c=c) in I leads to the formation of intermediate II, and the latter triggers the elimination of carbon dioxide.