A facile method for the efficient synthesis of 3H‐phosphaallenes, R−P=C=C(H)−R′, is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl‐alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe3)2 groups). Only supermesityl compounds are persistent at room temperature in solution. This simple method starting with easily accessible dialkynylphosphines and commercially available aluminium hydrides (HAlEt2, HAliBu2) allows the generation of transient species, which were trapped by coordination to transition metals. The η1‐coordination via a P−W bond was observed for tungsten, while the side‐on coordination via the P=C bond resulted with platinum. Decomposition of the mesityl derivative yielded an unprecedented product, which may be formed by 1,3‐H shift to the P atom, hydrophosphination of the P=C bond of a second phosphaallene and formation of a P−P bond.
The Al/P-based frustrated Lewis pair (FLP) Mes P-C(AltBu) =C(H)Ph (1; Mes=mesityl) reacted as an efficient two-electron reductant with benzil to afford a cis-enediolate that was coordinated to the FLP through P-O and Al-O bonds and the formation of a seven-membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide). The resulting enolates are bound to the FLP through P-C and Al-O bonds. Cyclopropenone gave an adduct (4) with the C=O bond coordinated by P and Al. Ynones gave a fascinating variety of different structures. 1,3-Diphenylprop-2-yn-1-one afforded a remarkable allene-type moiety with two cumulated C=C bonds (5); 3-hexyn-2-one yielded a ligand with two conjugated C=C bonds by C-H bond activation at the carbonyl methyl group (7); and 4-(trimethylsilyl)-3-butyn-2-one reacted by C-H bond cleavage, formation of an enolate group with a terminal C=C bond, and shift of the proton to the P atom (8). The C≡C bond was not affected. Allene compound 5 rearranged at elevated temperature and in daylight through the formation of a tricyclic compound by C-H bond activation and C-C bond formation. DFT calculations on this unusual rearrangement suggest insertion of the central allene C atom into the C-H bond of a methyl group and the intermediate formation of a C ring.
3H-Phosphaallenes are accessible
on a new and facile route and
show a fascinating chemical behavior. The thermally induced rearrangement
of Mes*PCC(H)R′ (R′ = tBu, Ad) afforded by C–H activation, isobutene elimination,
and C–C and P–H bond formation bicyclic 1-benzo-dihydrophosphetes
(2) with PC3 heterocycles. DFT calculations
suggest a mechanism with intramolecular nucleophilic aromatic substitution
and replacement of an alkyl group by the nucleophilic α-C atom
of the phosphaallene. These bicycles formed W(CO)5 complexes
(3) or afforded 1,2-dihydrophosphetes with P-bound alkenyl
groups by catalyst-free hydrophosphination of alkynes (4 and 5). The resulting bulky phosphines formed complexes
with IrCp*Cl2, RuCl2, AuCl, or CuO3SCF3. The Ru atom is coordinated by the P atom and a phenyl
group. Irradiation of TripPCC(H)tBu led by the insertion of the central C atom of the PCC
group into the α-C–H bond of an iPr substituent and by C–C
and P–C bond formation to a new isomer of phosphaallenes, 10, which features a strained PC2 heterocycle.
It formed adducts with M(CO)5 (M = Cr, Mo, W) and AuCl
and reacted with SO2Cl2 by cleavage of one of
the phosphirane P–C bonds to yield PC4 or PC5 heterocycles. Hydrolysis yielded a PC5 compound
with a P(O)Cl group.
3H-Phosphaallenes,R ÀP=C=C(H)CÀR' (3), are accessiblei namultigram scale on an ew and facile route and show af ascinating chemical reactivity.B H 3 (SMe 2)a nd 3a (R = Mes*, R' = tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7)w ith the Ba tom bound to two P=Cd ouble bonds.T his compound represents an ew FLP based on aB and two Pa toms. The increased Lewisa cidity of the Ba tom led to ad ifferent reaction course upon treatment of 3a with H 2 B-C 6 F 5 (SMe 2). Hydroboration of aC =Cb ond of af irst phosphaallene is followed in at ypicalF LP reaction by the coordination of as econd phosphaallene molecule via BÀCa nd PÀ Bb ond formation to yield aB P 2 C 2 heterocycle (8). Its BÀP bond is short and the B-bound Pa tomh as ap lanar surrounding. Treatment of 3a with tBuLi resultedi nd eprotonation of the b-C atom of the phosphaallene (9). The Li atom is bound to the Pa tom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl-SiMe 3 or Cl-PtBu 2 .T he thermally unstable phosphaallene PhÀP=C=C(H)-tBu gave au nique trimerics econdary product by PÀP, PÀCa nd CÀCb ond formation.I tc ontains aP 2 C 4 heterocycle and was isolated as aW (CO) 4 complex with two Pa toms coordinated to W(15).
While the range of accessible borylenes has significantly broadened over the last decade, applications remain limited. Herein, we present tricoordinate oxy‐borylenes as potent photoreductants that can be readily activated by visible light. Facile oxidation of CAAC stabilized oxy‐borylenes (CAAC)(IPr2Me2)BOR (R=TMS, CH2CH2C6H5, CH2CH2(4‐F)C6H4) to their corresponding radical cations is achieved with mildly oxidizing ferrocenium ion. Cyclovoltammetric studies reveal ground‐state redox potentials of up to −1.90 V vs. Fc+/0 for such oxy‐borylenes placing them among the strongest organic super electron donors. Their ability as photoreductants is further supported by theoretical studies and showcased by the application as stoichiometric reagents for the photochemical hydrodehalogenation of aryl chlorides, aryl bromides and unactivated alkyl bromides as well as the detosylation of anilines.
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