Hydroboration of Phosphaalkynes by HB(C₆F₅)₂

Abstract

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Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

“…It is worth mentioning at this point that, in contrast to our observations, Longobardi et al. have shown previously that BCF is unreactive towards the phospha‐alkyne t BuCP . A comparison of the frontier orbitals of t BuCP and [B]OCP shows that the HOMO is C−P π‐based in both cases (Supporting Information, Figure S20), but the electron donating effect of the boryloxy substituent results in a ≈1.4 eV destabilisation (−6.45 eV in t BuCP vs. −5.03 eV in [B]OCP).…”

“…Analysis of single crystals obtained from this reaction revealed that 1,2‐carboboration of the C≡P bond of [B]OCP had taken place with formation of C−B and P−C bonds affording E ‐[B]O{(C 6 F 5 ) 2 B}C=P(C 6 F 5 ) ( 1 ). This bond formation is inverse to that observed in the related hydroboration of phosphaalkynes using Piers’ borane, HB(C 6 F 5 ) 2 , which forms C−H and P−B bonds due to the steric interaction of the C 6 F 5 groups and the tert ‐butyl of the phosphaalkyne . It is worth noting at this stage that previous reactions of phosphaethynolate salts with boranes were found to exclusively yield dimeric compounds with the boranes remaining intact …”

“…In contrast, the new PÀC aryl bond is only at an early stage of formation (2.39 vs. af inal value of 1.85 ), suggesting that the barriertothe 1,2reaction is associated primarily with the transfer of electron density from the P Ct riple bond to the boron centreo fB CF.I ti sw orth mentioning at this point that, in contrast to our observations, Longobardi et al have shown previously that BCF is unreactive towards the phospha-alkyne tBuCP. [15] Ac omparison of the frontier orbitals of tBuCP and [B]OCP shows that the HOMO is CÀP p-basedi nb oth cases (Supporting Information, Figure S20), but the electron donating effectoft he boryloxy substituent results in a % 1.4 eV destabilisation( À6.45 eV in tBuCP vs. À5.03 eV in [B]OCP). The greater nucleophilicity of [B]OCP is clearly important in stabilizing the dominant charget ransfer pathway leadingt oTS 1 .…”

“…[7,8] In the case of tris(pentafluorophenyl)borane, related reactions are only possible using more reactive substrates, such as allenyl ketones and isocyanates. [9][10][11][12] It is these latters tudies, as well as relatedw ork exploring the hydroboration of phosphaalkynes, [13][14][15] that prompted us to explore the reactivityo fB CF towardst wo isomericf orms of ab oryl-phosphaethynolate, [B]OCPa nd [B]PCO( where [B] = N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl). [16,17] Interestingly,t hese reactions give rise to three different borylatedp hosphaalkenes with the same molecular formula.…”

Borane‐Stabilized Isomeric Dimers of the Phosphaethynolate Anion

“…This species converts to the anion of 3 on warming, presumably via a [2+2] dimerization with a concurrent and low energy borane migration. Interestingly, there is no evidence of hydroboration of the P−C multiple bond as was previously observed with phosphaalkynes …”