o-Hydroxyarylphosphanes: Strategies for Syntheses of Configurationally Stable, Electronically and Sterically Tunable Ambiphiles with Multiple Applications

o-Hydroxyarylphosphanes are fascinating compounds by their multiple-reactivity features, attributed to the ambident hard and soft Lewis- and also Brønstedt acid-base properties, wide tuning opportunities via backbone substituents with ±mesomeric and inductive, at P and in o-position to P and O also steric effects, and in addition, the configurational stability at three-valent phosphorus. Air sensitivity may be overcome by reversible protection with BH3 , but the easy oxidation to P(V)-compounds may also be used. Since the first reports on the title compounds ca. 50 years ago the multiple reactivity has led to versatile applications. This includes various P-E-O and P=C-O heterocycles, a multitude of O-substituted derivatives including acyl derivatives for traceless Staudinger couplings of biomolecules with labels or functional substituents, phosphane-phosphite ligands, which like the o-phosphanylphenols itself form a range of transition metal complexes and catalysts. Also main group metal complexes and (bi)arylphosphonium-organocatalysts are derived. Within this review the various strategies for the access of the starting materials are illuminated, including few hints to selected applications.

A General Strategy for Increasing the Air Stability of Phosphines Including Primary Phosphines

A general approach for increasing the air-stability of various primary phosphines in the absence of kinetic stabilization is presented that contrasts with previous interpretations, which were limited to specific phosphines. This contribution shows the synthesis of a series of air-stable primary phosphines Fc(CH2 )n PH2 , where n=0,1,2,3; and Fc=ferrocenyl, and their corresponding isolable primary phosphine oxides. It was demonstrated that the ferrocene moiety exerts an antioxidant effect on the primary phosphine group, which is intermolecular, solvent dependent and increases with the electron density on the ferrocene moiety. Furthermore, we demonstrated that the presence of ferrocene in solution also inhibits the oxidation of other secondary and tertiary phosphines in air. Together our findings suggest that quenching of singlet oxygen is the actual reason for the antioxidant effect; this was experimentally confirmed by using other established singlet oxygen quenchers, thus demonstrating a key role of singlet oxygen in the aerobic oxidation of phosphines.

Luminescent 1H-1,3-benzazaphospholes

2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ²P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a-c offer insights into the interplay between structure-function relationships affecting excited state relaxation processes.

Copper mediated C(sp2)-H amination and hydroxylation of phosphinamides

Copper mediated C(sp2)-H amination and hydroxylation of arylphosphinic acid are accomplished by adopting phosphinamide as the directing group. This method is distinguished by its wide substrate scope and excellent functional group tolerance, thus allowing for the rapid preparation of organophosphorus compounds in organic synthesis.

The anionic Fries rearrangement: a convenient route to ortho-functionalized aromatics

The scope and mechanism of anionic (hetero-) Fries rearrangements are summarized for various migrating groups and arenes, including applications and computational studies.

Synthesis of Furan-Annelated BINOL Derivatives: Acid-Catalyzed Cyclization Induces Partial Racemization

In this account, we describe the synthesis of a series of BINOL-based bis- and trisphosphoric acids 11d/e/f, which commonly feature an unusual phosphoric acid monoester motif. This motif is generated by an acid-catalyzed 5- endo- dig cyclization of the 3-alkynyl-substituted BINOL precursors to give the corresponding Furan-annelated derivatives, followed by phosphorylation of the remaining phenolic alcohols. In the cyclization reaction, we observed an unexpected partial racemization in the bis- and tris-BINOL scaffolds, leading to mixtures of diastereomers that were separated and characterized spectroscopically and by X-ray crystal structure analyses. The cyclization and racemization processes were investigated both experimentally and by DFT-calculations, showing that although the cyclization proceeds faster, the barrier for the acid-catalyzed binaphthyl-racemization is only slightly higher.

Controlling the Emissive Activity in Heterocyclic Systems Bearing C═P Bonds

The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the C═N bonds of benzoxazoles are replaced by C═P bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X-C═P), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.

"Doping" pentacene with sp(2)-phosphorus atoms: towards high performance ambipolar semiconductors

Recent research progress in black phosphorus sheets strongly encourages us to employ pentacene as a parent system to systematically investigate how the "doping" of sp(2)-phosphorus atoms onto the backbone of pentacene influences its optical and charge transport properties. Our theoretical investigations proved that increasing the contribution of the pz atomic orbital of the sp(2)-phosphorus to the frontier molecular orbital of phosphapentacenes could significantly decrease both hole and electron reorganization energies and dramatically red-shift the absorption of pentacene. The record smallest hole and electron reorganization energies of 69.80 and 95.74 meV for heteropentacene derivatives were obtained. These results suggest that phosphapentacenes (or phosphaacenes) could be potential promising candidates to achieve both higher and balanced mobilities in organic field effect transistors and realize a better power conversion efficiency in organic photovoltaics.

Luminescent materials containing multiple benzoxaphosphole units

A series of bisbenzoxaphospholes and a trisbenzoxaphosphole have been prepared and characterized.

Oxaphospholes and bisphospholes from phosphinophosphonates and α,β-unsaturated ketones

The reaction of a {W(CO)5 }-stabilized phosphinophosphonate 1, (CO)5 WPH(Ph)P(O)(OEt)2 , with ethynyl- (2 a-f) and diethynylketones (7-11, 18, and 19) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)3 Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5-dihydro-1,2-oxaphospholes 3 and 4. When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)3 Si or (Et)3 Si acetylene terminus is present, as in 7, 8, and 19, an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene-decorated oxaphospholes 14, 15, 24, and 25. Diacetylenic ketones 10 and 11, with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl-bridged bisphospholes 16 and 17. Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene-decorated oxaphospholes is more exothermic for the trimethylsilyl-containing substrates. The pathway to the latter compounds contains a 1,3-shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3-shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high-lying transition state TS(E'-F')R=Ph of 37 kcal mol(-1) is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl-terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17.

Phosphoryl-related directing groups in rhodium(III) catalysis: a general strategy to diverse P-containing frameworks

Herein, a rhodium(III)-catalyzed oxidative C-H activation of simple arylphosphonates and phosphonamides with subsequent coupling with alkenes (olefination), internal alkynes (hydroarylation and oxidative cyclization), or simple arenes to give access to diverse P-containing functional frameworks is reported.

Phosphorus as a carbon copy and as a photocopy: New conjugated materials featuring multiply bonded phosphorus

Phosphaalkenes (RP=CR2) and diphosphenes (RP=PR) are main group analogues of alkenes (R2C=CR2). Molecules featuring such multiply bonded phosphorus functionalities often display structural features and chemical reactivities that mimic their purely organic counterparts, lending credence to the claim that these compounds are “carbon copies”. We have been expanding this analogy to include oligomers and polymers with extended conjugation that directly involve P=C and P=P units. Many of these materials, however, display little or no photoluminescence (PL). This article summarizes our efforts to understand P=C and P=P photobehavior and to produce materials having significant PL that mimic or “photocopy” the PL properties of the phosphorus-free systems. Recent materials based on benzoxaphospholes (BOPs), benzobisoxaphospholes (BBOPs), and higher analogues having significant fluorescence quantum yields are covered.