Aspects of Phosphaallene Chemistry: Heat-Induced Formation of 1,2-Dihydrophosphetes by Intramolecular Nucleophilic Aromatic Substitution and Photochemical Generation of Tricyclic Phosphiranes

Abstract

3H-Phosphaallenes are accessible on a new and facile route and show a fascinating chemical behavior. The thermally induced rearrangement of Mes*PCC­(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 TripPCC­(H)tBu led by the insertion of the central C atom of the PCC 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.

New Reactivity Patterns in 3H‐Phosphaallene Chemistry [Aryl‐P=C=C(H)‐tBu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation

“…The tricyclic phosphirane 5 is an isomer of ap hosphaallene and was obtained by rearrangement, CÀHb ond activation and PÀCb ond formation. [14] Compound 4 represents an unusual, asymmetricd imer with aP ÀP bond. [2] One Pa tom bears an alkynyl group, while the second one is boundt oa na lkenylg roup.…”

“…One of these crystallized from the reaction mixture and wasi dentified as compound 13 (Scheme 6) which contains af ive-membered AlP 2 C 2 heterocycle and resultsf rom the reactiono fa ni ntermediately formed FLP (2,S cheme 1) with the transient phosphaallene 3b. [15] Another secondary product (14)s howed resonances of three different Pa toms with three coupling con- 8, average valueso ftwo independent molecules : P1ÀP2 2.245, P1ÀC1 1.755, C1ÀC2 1.202, P1ÀC41 1.865 3 J PP coupling and indicates the presence of aP ÀPb ond. The concentration of 14 increased continuously, while the resonances of the phosphaallene 3b disappeared after two days.…”

“…Compound K,a st he analogue of the experimentally obtained trimer 14,i sf inally formed after proton shift and release of AlMe 3 .A si mpressively shown, the formationo fthe trimericphosphaallene requires acomplicated multistep process and proceedsv ia relativelyl ow activation barriers (maximum 12.4 kcal mol À1 )w hich are in accordance with the mild reaction conditions. Overall, the trimeric formula unit (14)i st hermodynamically highly favoured over the starting material (3b), but its selectiveformation requires activation by aLewis acid.…”

“…[7e, 12] The central Ca tom of the P=C=Cm oiety bears ap artial negative charge and is attacked by protons. [1,13,14] Recently,w ef ound facile accesst ov arious 3H-phosphaallenes( 3)i namultigram scale and excellenty ields by treatment of dialkynylphosphines (1) with commerciallya vailable dialkylaluminium hydrides (e.g. iBu 2 AlH;S cheme 1).…”

“…The phosphorus-bound Ha tom of 6 allowed catalyst-free hydrophosphination reactions and the generation of bulky phosphinel igands. [14] In this paper we focus on reactions at the vinylidene part of the phosphaallenes and report on the isolation of an ovel trimericproduct of an unstable derivative.…”