Formation of a Bowl‐Shaped, Pentacyclic Phosphonium Cage by Methylation of a Nucleophilic Phosphinidene

Abstract

Dedicated to Professor Gottfried Huttner on the occasion of his 68th birthdayPhosphinidenes, that is, carbene-analogous compounds of monovalent phosphorus (phosphanylidenes, RÀP) represent a simple class of valuable electrophilic building blocks in organophosphorus chemistry, which are usually highly reactive transients and therefore difficult to isolate. [1] However, one simple chemical trick that aids the preparation of a room-temperature stable phosphinidene is the intra-or intermolecular addition of a donor group D to the initially electron-deficient phosphorus atom (D!PR); this affords an electron-rich (nucleophilic) phosphinidene, having eight or more valence electrons at the phosphorus center. Several types of donor-stabilized phosphinidenes have already been isolated and used as versatile precursors for the preparation of "free" phosphinidenes such as phospha-Wittig reagents (R 3 P = PR') [2] and related phosphinidene metal complexes (L n M = PR).[3] The intramolecular donor-stabilization of a P(+1) atom is also the reason for the intriguing stability of the nucleophilic phosphinidene 1 (Scheme 1), which has a planar, T-shaped three-coordinate phosphorus atom with 10 valence electrons (10-P-3 system).[4] Although 1 is a promising versatile building block for the synthesis of novel electronically tunable organophosphorus ligands, its reactivity has only been sparingly investigated.[5] The relatively high nucleophilicity at the P atom prompted us to investigate whether alkylation of phosphorus with classical alkylation reagents RX (R = alkyl; X = anionic leaving group) leads to a nucleophilic phosphenium salt A, phosphonium species B, or neutral phosphorane C as possible valence isomers, depending on the electronic nature and steric demand of R and X, respectively (Scheme 1).Here we report the surprising formation of the unusual phosphonium cage cation in 2, which results from a domino cyclization of two molecules of 1 in the presence of methyl trifluoromethylsulfonate (MeOTf). When a solution of MeOTf in CH 2 Cl 2 was added to a solution of 1 in CH 2 Cl 2 in the molar ratio of 1:1 at 20 8C, a rapid reaction occurs ( 31 P NMR monitoring), affording the unexpected phosphonium salt 2 [Eq. (1)]. The latter is insoluble in hydrocarbon solvents and other nonpolar solvents and can be isolated in the form of a colorless solid in 35 % yield. The yield can be increased up to 78 % by changing the molar ratio of the starting materials MeOTf and 1 to 1:2. Interestingly, compound 2 results also exclusively even if a solution of 1 in CH 2 Cl 2 is slowly added to a very large molar excess or even by using neat MeOTf at room temperature or below (À10 8C). The composition and constitution of 2 was established by EI-FAB and ESI (electrospray ionization) mass spectrometry Scheme 1. The possible valence isomers A, B, and C formed by alkylation of 1 with RX (R = alkyl, X = anionic leaving group; ET = electron transfer.

Recent developments in the chemistry of non-trigonal pnictogen pincer compounds: from bonding to catalysis

“…21 I was also shown to react with MeOTf which induced a rapid dimerization of the starting material and the P-methylated intermediate to give bowl shaped 30, which features ve 5membered rings within its structure (Scheme 14). 92 DFT computations suggests that the initial methylation proceeds at the phosphorus atom. This species is subsequently trapped by the starting material, despite the predicted thermodynamic preference for methylation in a-position to the nitrogen donor atom.…”

A Proton‐Triggered Cascade Reaction Involving a Heavy p‐Block Multiple Bond: Transformation of the Diphosphene C5Me5P=PC5Me5 into the Cationic Cage [C10Me10P2H]+

“…Although cascade reactions have played, and will undoubtedly continue to play, an important role in natural product syntheses, the reaction 4 → 5 (Scheme ) represents a new type of process where a homonuclear heavy p‐block multiple bond is an integral component of the process, allowing the construction, in one synthetic operation, of the C 10 P 2 cage structure present in 5 ; this provides an interesting contrast with the multistep syntheses of the corresponding hydrocarbon systems 12. Within the wider context of cascade reactions in inorganic chemistry, we note the report by Driess and co‐workers of the formation of a phosphonium cage through methylation of a phosphanylidene 13…”

Pincer‐Type Ligand‐Assisted Catalysis and Small‐Molecule Activation by non‐VSEPR Main‐Group Compounds

“…The availability of two lone pairs on the central phosphorus atom and facile nucleophilic addition of 1 to methyl triflate was experimentally established by Driess's group. [19] Radosevich and co-workers further revealed that compound 1 has significant electrophilic character at phosphorus center, and this property is uncommon to the typical Lewis base of σ 3 À P (C 3V point group) compounds. [17] In 2012, Radosevich et al revealed that compound 1 could serve as a platform for the catalytic reduction of azobenzene via a P(III)/P(V) redox cycle mechanism (Scheme 3).…”