Acetylene‐Expanded Dendralene Segments with Exotopic Phosphaalkene Units

Abstract

Bis-TMS protected C,C-diacetylenic phosphaalkene (A(2)PA) 1 (Mes*P=C(C≡CTMS)(2); Mes* = 2,4,6-tBu(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*P=C(C≡CR(1))C(4)(R(2) C≡C)C=PMes* (3b, 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 P=C bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis/trans isomerization in an iodo-substituted A(2)PA 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 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.

Tuning the Electronic Properties of Acetylenic Fluorenes by Phosphaalkene Incorporation

“…In the case of 7 a , b , it is possible to protodesilylate the acetylene termini, which can then be further elaborated by desirable substituents to fine‐tune the photophysical and electrochemical properties of the F9Ps . The feasibility of such a strategy was exemplified by the deprotection of phosphaalkene 7 a with K 2 CO 3 and isolation of the terminal bis‐acetylene 8 a .…”

“…Compound 9 a was synthesized in analogy to the literature procedure . [Pd(PPh 3 ) 2 Cl 2 ] (0.03 mmol, 22 mg), CuI (0.06 mmol, 12 mg), and aqueous K 2 CO 3 (2 m , 1.5 mL) were added successively to a degassed solution of 7 a (0.32 mmol, 0.2 g) and 2‐iodothiophene (0.63 mmol, 0.13 g) in THF (20 mL), MeOH (10 mL), and Et 3 N (5 mL) under an argon atmosphere.…”

Enantiomerically Pure Phosphaalkene–Oxazolines (PhAk‐Ox): Synthesis, Scope and Copolymerization with Styrene

“…There is considerable interest in incorporating the P=C bond motif into structurally complex molecules and polymers with potential relevance in materials science. [4] In addition, phosphaalkenes containing metal-chelating functional groups have been shown to be quite effective ligands for application in catalysis (Scheme 2). [1b, c,e, 2, 5] Ligand classes that have garnered the most attention include those based on the diphosphinidenecyclobutene A (DPCB), [6] 4 Ox (1 g).…”

“…[4] In addition, phosphaalkenes containing metal-chelating functional groups have been shown to be quite effective ligands for application in catalysis (Scheme 2). [1b, c,e, 2, 5] Ligand classes that have garnered the most attention include those based on the diphosphinidenecyclobutene A (DPCB), [6] 4 Ox (1 g). To evaluate the PhAk-Ox compounds as prospective precursors to chiral phosphine polymers, monomer 1 a and styrene were subjected to radical-initiated copolymerization condi-…”

Mechanism of the Phospha‐Wittig–Horner Reaction

“…[5] In context of our previous work on the incorporation of low-valent phosphorus into unsaturated carbon scaffolds, [6] we were interested to see whether the scope of the pWH reaction could be extended to ketene substrates from which 1-phosphapropadienes would become accessible. Such phosphaallenes are the shortest members of an intriguing class of P-terminated cumulenes, which have however been synthetically highly challenging with only sporadic reports in the literature.…”