Acetylene-expanded dendralene segments with exotopic phosphaalkene units

Article abstract of DOI:10.1002/chem.201101358

Bis-TMS protected C,C-diacetylenic phosphaalkene (A₂PA) 1 (Mes*P=C(C=CTMS)₂; Mes=2,4,6-tBu₃Ph) has been used as a building block for the construction of butadiyne-expanded dendralene fragments in which phosphaalkenes feature as exotopic double bonds. Treatment of 1 with CuCl gives rise to a Cu^I acetylide that is selectively formed at the acetylene trans to the Mes* group. The cis-TMS-acetylene engages in similar chemistry, albeit at higher temperatures and longer reaction times. The differentiation between the two acetylene termini of 1 allows for the controlled synthesis of the title compounds by a variety of different Cu- and Pd-catalyzed oxidative acetylene homo- and heterocoupling protocols. Crystallographic characterization of A₂PA 1 and dimeric Mes*P=C(C=CR¹)C₄(R²C=C)C=PMes* (3 b, R¹=R²=Ph; 6, R¹=R²=TMS), and 10 (R¹=R²=C=CPh) verifies that the stereochemistry across the P=C bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis/trans isomerization in an iodo-substituted A₂PA intermediate 4 (Mes*P=C(C=CTMS)(C=CI). UV/Vis spectroscopic and electrochemical studies reveal that efficient π conjugation operates through the entire acetylenic phosphaalkene framework, even in the cross-conjugated dimeric structures. The P centers contribute considerably to the frontier molecular orbitals of the compounds, thereby leading to smaller HOMO-LUMO gaps than in all-carbon-based congeners. Phenyl- and/or ethynylphenyl substituents at the A₂PA framework influence the HOMO and LUMO to a varying degree depending on their relationship to the Mes* group, thus enabling a fine-tuning of the frontier molecular orbitals of the compounds.