New strategy for synthesising conjugated hexatrienylferrocenes via cross-dimerisation

The first broadly applied strategy for the cross-dimerisation of an internal alkynylferrocene (1) with conjugated dienes (2) is revealed. The Ru(0)-catalysed cross-dimerisation of 1-propynylferrocene, FcCCMe (1a), with (E)-butadien-1-ylboronic acid pinacolato ester, CH2CH–CHCHB(pin) (2g), yields a conjugated triene, (1E,3E,5E)-FcCHCMe–CHCH–CHCHB(pin) (3ag), in a 92% yield. Subsequently, the Pd(II)-catalysed cross-couplings of 3ag with a series of aryl iodides yields corresponding conjugated polyenes, such as (1E,3E,5E)-FcCHCMe–CHCH–CHCHAr. The photo-electronic properties of the ferrocenyl conjugated polyenes are evaluated and their electronic properties are also evaluated by time-dependent density-functional theory calculations.