Observation of a Thermally Induced Bora‐Nazarov Cyclization at a Phosphole Framework

Abstract

The reaction of the bis(enynyl)phosphanes 6 a,b with the electrophilic borane reagents RB(C6 F5 )2 (R=C6 F5 , CH2 CH2 Ph, CH3 ) gave phospholes cleanly in a 1,1-carboboration reaction sequence. Depending on the steric bulk, the resulting 2,5-alkenylphospholes underwent a thermally induced bora-Nazarov type cyclization. The equilibrium situation of these examples of a bora-Nazarov type cyclization was investigated in detail by NMR spectroscopy, X-ray crystal structure analysis, and DFT calculations.

Heavier element-containing aromatics of [4n+2]-electron systems

“…Thermolysis of the phosphirenium borates resulted in the formation of the 1,1-carboboration products 6-8-2 or phospholes 6-8-3. [298][299][300][301][302][303][304][305] In 2016, Erker and co-workers demonstrated the reaction of allene 6-9-1 with Piers' borane, HB(C6F5)2 to give zwitterionic phosphonium borate a3CP13 (Scheme 6-9). 306 Heating of a3CP13 for 6 days in toluene at 110 °C underwent internal nucleophilic aromatic substitution to form 6-9-2.…”

Reactivity of Alkynyl Phosphines with Lewis Acids for the Synthesis of Allenic Phosphonium Borate Zwitterions

“…These results signify that arene groups may play an essential role in promoting the methoxide abstraction process which initiates the observed rearrangement. Finally, bisalkynylphosphines with ionizable -OMe substituents in the γ-position of both alkyne substituents (7)(8)(9) were found to undergo transformations with B(C 6 F 5 ) 3 or Al(C 6 F 5 ) 3 resulting in a single -OMe activation, forming the analogous zwitterionic species. Under no circumstances was double activation or cyclization of the alkyne substituents observed.…”

“…The question we asked ourselves was could the ionizable group promote or preclude cyclization? Therefore, three new bisalkynylphosphines were synthesized bearing two fluorene substituted alkyne with phenyl (7), mesityl (8), and cyclohexyl ( 9) substituents at the phosphorus centre (Scheme 4). These compounds were prepared in an analogous fashion to the monoalkynyl phosphines via lithiation of 9-ethynyl-9-methoxyfluorene and subsequent addition of a dichloro alkyl-or arylphosphine.…”

“…This reaction was again proposed to proceed through a phosphirenium borate intermediate similar to the one shown in Scheme 1, C. This discovery was subsequently expanded to construct more complex phosphole frameworks; the reaction of bis(enynyl)phosphines with B(C 6 F 5 ) 3 led to the formation of the corresponding 2,5-alkenylphospholes which could undergo a thermal bora-Nazarov type cyclization. [8] In 2021, our group synthesized the first alkynyl phosphines that incorporated a secondary Lewis donor, in the form of an oxygen containing methoxide substituent in the �-carbon relative to the phosphorus atom (Scheme 1, D). [9] These molecules behave differently than the well-established reactivity of alkynylphosphines with B(C 6 F 5 ) 3 .…”

Cobalt- or rhodium-catalyzed synthesis of 1,2-dihydrophosphete oxides via C–H activation and formal phosphoryl migration

“…Indeed, metal-catalyzed coupling of dialkynylphosphine oxides/sulfides delivered various powerful strategies to create complex phosphacycles. 13 The C C bond in the dialkynylphosphine is expected to readily participate as a special π-bond in C–H activation reactions, and the resulting alkenyl phosphine intermediate may also undergo rearrangement to give complex structures. While significant achievements have been made in C–H bond activation catalyzed by transition metals such as Pd, Rh, Ru, Ir, Mn, and Co, 14 P-containing substrates are generally less explored either as the arene or the coupling reagent.…”