Acetylene‐Expanded Dendralene Segments with Exotopic Phosphaalkene Units

Bis‐TMS protected C , C ‐diacetylenic phosphaalkene (A 2 PA) 1 (Mes*PC(C≡CTMS) 2 ; Mes*=2,4,6‐ t Bu 3 Ph) 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 A 2 PA 1 and dimeric Mes*PC(C≡CR 1 )C 4 (R 2 C≡C)CPMes* ( 3 b , R 1 =R 2 =Ph; 6 , R 1 =R 2 =TMS), and 10 (R 1 =R 2 =C≡CPh) verifies that the stereochemistry across the PC bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis / trans isomerization in an iodo‐substituted A 2 PA intermediate 4 (Mes*PC(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 A 2 PA 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.