A Phosphinine‐Derived 1‐Phospha‐7‐Bora‐Norbornadiene: Frustrated Lewis Pair Type Activation of Triple Bonds

Abstract

Salt metathesis of 1‐methyl‐2,4,6‐triphenylphosphacyclohexadienyl lithium and chlorobis(pentafluorophenyl)borane affords a 1‐phospha‐7‐bora‐norbornadiene derivative 2 . The C≡N triple bonds of nitriles insert into the P−B bond of 2 with concomitant C−B bond cleavage, whereas the C≡C bonds of phenylacetylenes react with 2 to form λ 4 ‐phosphabarrelenes. Even though 2 must formally be regarded as a classical Lewis adduct, the C≡N and C≡C activation processes observed (and the mild conditions under which they occur) are reminiscent of the reactivity of frustrated Lewis pairs. Indeed, NMR and computational studies give insight into the mechanism of the reactions and reveal the labile nature of the phosphorus–boron bond in 2 , which is also suggested by detailed NMR spectroscopic studies on this compound. Nitrile insertion is thus preceded by ring opening of the bicycle of 2 through P−B bond splitting with a low energy barrier. By contrast, the reaction with alkynes involves formation of a reactive zwitterionic methylphosphininium borate intermediate, which readily undergoes alkyne 1,4‐addition.

Borane Adducts of Aromatic Phosphorus Heterocycles: Synthesis, Crystallographic Characterization and Reactivity of a Phosphinine‐B(C6F5)3 Lewis Pair

“…The compound forms by a fast and regioselective [4+2] cycloaddition of styrene to the aromatic 1‐ R ‐phosphininium heterocycle. It should be mentioned that the reaction of a related 1‐methyl‐1‐phospha‐7‐bora‐norbornadiene with phenylacetylene has recently been described by us [30] …”

“…It should be mentioned that the reaction of a related 1-methyl-1-phospha-7-bora-norbornadiene with phenylacetylene has recently been described by us. [30] Finally, we also converted rapidly, quantitatively and selectively a mixture of 6, ester 11 and Me 3 SiÀ C�CH towards the corresponding 1-R-phosphabarrelenium salt 15 (Scheme 8), which we could also characterize crystallographically (Figure 10).…”

Frustrated Lewis pair chemistry of alkynes

“…32 More recently, Erker has also reported that the sterically encumbered phosphine/borane heterocycle (C 12 H 5 Et 8 ) PCH 2 CH 2 B(C 6 F 5 )(CHMe) 33 reacts with an alkyne to effect deprotonation affording the corresponding phosphonium alkynyl borate (Scheme 5). In 2020, Wolf and coworkers 34 developed a phosphininederived 1-phospha-7-bora-norbornadiene which behaves as an intramolecular FLP in reactions with alkynes (Scheme 5). While these reactions provide λ 4 -phosphabarrelenes, they are thought to proceed via alkyne addition to the B/P fragment followed by a migration of the B(C 6 F 5 ) 2 fragment (Scheme 5).…”

“…In 2020, Wolf and coworkers 34 developed a phosphinine-derived 1-phospha-7-bora-norbornadiene which behaves as an intramolecular FLP in reactions with alkynes (Scheme 5). While these reactions provide λ 4 -phosphabarrelenes, they are thought to proceed via alkyne addition to the B/P fragment followed by a migration of the B(C 6 F 5 ) 2 fragment (Scheme 5).…”

Application of nonmetallic frustrated cations in the activation of small molecules

“…The reactivity of 2 + towards acetonitrile, however, resembles the reactions of FLPs, where the incorporation of a CN moiety between the Lewis acidic B atom and Lewis basic P atom is observed, leading to the formation of new B-N and P-C bonds. [39][40][41] Regarding neutral B-P systems activating nitriles, the Lerner group reported a reaction of phosphaboradibenzofulvene with acetonitrile. As a first step of this reaction, the formation of a four-membered P-B-N-C cycle was postulated, which was further rearranged to stable cycloiminoborane.…”