Annulated 1,3,4‐Azadiphospholides: Heterocycles with Widely Tunable Optical Properties

Abstract

Phosphorus heterocycles find applications in the synthesis of p-conjugated compounds and as precursors for optoelectronic materials such as organic light-emitting diodes (OLEDs), electronic switches,a nd transistors.Ahigh-yield, one-pot synthesis of anionic annulated 1,3,4-azadiphospholides from Na(OCP) and 2-chloropyridines is presented. The synthesis proceeds without the use of transition metals and tolerates aw ide range of substrates.C yano-substituted compounds are especially deeply colored and have absorption maxima which range from l max = 525 to 596 nm. The optical properties are dominated by the spatial separation of an electron acceptor and donor unit within one molecule (pushpull chromophore). The anionic 1,3,4-azadiphospholides are silylated to neutral siloxy compounds with astrong blue-shifted absorption. This reaction can be reversed by addition of fluoride ions,w hich allows fluoride ions to be detected in optically lowc oncentrations.Numerous p-conjugated molecules have astrong impact in the studies of new organic semiconductors and materials,such as dyes in lasers, [1] sensors,l ight emitting diodes, [2] and components for solar cells.[3] Introduction of main group elements in the extended p-systems such as sulfur, [4] boron, [5] silicon, [6] arsenic, [7] and phosphorus [8] allow the physical and electronic properties to be tuned.[9] Unsaturated molecules with low-valent s 2 ,l 3 -phosphorus centers have at endencyt o oligomerize and/or form cage-compounds,l imiting their application.[10] But several successful attempts to use R 2 C= PR and RCPm onomers for the synthesis of conjugated polymers were reported. [11] Compared to the C = C p-bond, the P = Cdouble bond is significantly weaker and the gap between the p (HOMO) and p*(LUMO) orbitals of the C = Pb ond is much smaller. [9g, 12] Phosphorus-containing p-conjugated heterocycles are often easier to handle, [8] and sodium phosphaethynolate,N a(OCP), proved to be av ersatile building block for those (see selected examples in Scheme 1). )o r[ 2 + +4] cycloadditions (VI [15] ), respectively,w hich are likewise formed by incorporating ac omplete (OCP) À anion into the cycle.[14c]Thef ive-membered heterocycles VII-IX are special because they are formed in reactions between carboxylic acid chloride derivatives,RC(= X)Cl (X = NR, O, S) with two (OCP) À anions,but only one is fully included while the other serves as source of a" P À "i on after loss of CO.[17] Herein we report as imple access to annulated 1,3,4-azadiphospholes with aw idely tunable p-conjugated electronic structure, which have,i nc ontrast to well-studied diazaphospholes, [18] only rarely been investigated before. [19] These new compounds may find applications in electronic devices or chemical sensors.We first further improved the synthesis of Na(OCP), which can now be very easily prepared on aseveral-hundred gram scale in high yield by simply exposing as olution of NaOtBu and dimethylcarbonate in THF to PH 3 (Scheme 2).[20] Subsequently,N a(OCP) was reacted with various 2...

The Chemistry of the 2‐Phosphaethynolate Anion

“…[9] As with previous reactions affording five-membered rings, these reactions involve the formation of apyridylfunctionalized phosphaketene,w hich reacts with afurther equivalent of OCP À prior to adecarbonylative cyclization.T he resulting five-membered anionic heterocycles can be readily silylated with t BuMe 2 SiCl to afford neutral ring compounds functionalized at the exocyclico xygen atom. [9] As with previous reactions affording five-membered rings, these reactions involve the formation of apyridylfunctionalized phosphaketene,w hich reacts with afurther equivalent of OCP À prior to adecarbonylative cyclization.T he resulting five-membered anionic heterocycles can be readily silylated with t BuMe 2 SiCl to afford neutral ring compounds functionalized at the exocyclico xygen atom.…”

“…[9] Thes ynthesis uses phosphine gas (PH 3 )a safeedstock, and as imple deprotonation with NaO t Bu allows for the formation of NaPH 2 .The resulting phosphanide readily reacts with carbonates to afford sodium salts of the 2-phosphaethynolate anion, which can be precipitated out of solution using dioxane.While this synthesis will probably not be commonly employed on as mall laboratory scale due to the highly pyrophoric nature of PH 3 ,i ti sm uch more attractive on al arger industrial scale as it circumvents the use of solid reagents such as sodium and red phosphorus,which have the potential to foul chemical reactors. [9] Thes ynthesis uses phosphine gas (PH 3 )a safeedstock, and as imple deprotonation with NaO t Bu allows for the formation of NaPH 2 .The resulting phosphanide readily reacts with carbonates to afford sodium salts of the 2-phosphaethynolate anion, which can be precipitated out of solution using dioxane.While this synthesis will probably not be commonly employed on as mall laboratory scale due to the highly pyrophoric nature of PH 3 ,i ti sm uch more attractive on al arger industrial scale as it circumvents the use of solid reagents such as sodium and red phosphorus,which have the potential to foul chemical reactors.…”

Synthesis of Azaphosphinines by Directed Inverse‐Electron‐Demand Hetero‐Diels–Alder Reactions with Na(OCP)

“…The phosphaethynolate anion [PCO] − first described as its lithium salt in 1992 by Becker et al., remained unexplored until the recent novel one‐step preparation . Since then NaOCP has found applications in the synthesis of a range of previously unknown phosphorus heterocycles, such as V , VI , VII , or VIII . Importantly, Grützmacher et al.…”

Annulated 1,3,4‐Azadiphospholides: Heterocycles with Widely Tunable Optical Properties

“…The product was crystallized from aD ME solution layered with methyl-tert-butyl ether at À30 8 8C, which afforded single crystals suitable for X-ray diffraction analysis,w hich confirmed the product to be aa nnulated 1,3,4-azadiphospholide ( Figure 1). [26] Plots of the structures of Na[1a], Na[1b], and Si[1b]a re shown in Figure 1(see the Supporting Information for the others). Ther eaction between Na(OCP) and 2chloropyrimidine gives,a long with the major dark-purple anionic product Na[1d]( l max = 524 nm), am inor second phosphorus-containing product to which we assign the tetrameric oxidation product Na 2 (C 4 O 4 P 4 )o nt he basis of its 31 PNMR spectrum.…”

“…Crystal structure of compound {Na[1a(DME) 2 ]} 2 ,Na[1b(18- C-6)],and Si[1b] [26]. Anisotropic thermal displacement ellipsoids are shown at the 50 %probability level.…”