N,N-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

Abstract

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes(2)P-C(6)H(10)-B(C(6)F(5))(2)] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO-CH(2)Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes(2)P-CHR(1)-CHR(2)-B(C(6)F(5))(2)] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO-CHMePh 6b derived from 1-bromoethylbenzene undergoes NO-C bond cleavage at 120 °C with an activation energy of E(a) = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C(2)-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Synthesis of Pyridine- and 2-Oxazoline-Functionalized Vinyl Polymers by Alane-Based Frustrated Lewis Pairs

“…More drastically, replacing Al(C 6 F 5 ) 3 with B(C 6 F 5 ) 3 or AlEt 3 led to a completely inactive polymerization system (runs 8 and 9). Polymerizations employing the preformed monomer-LA adduct, (2-VP)·Al(C 6 F 5 ) 3 , afforded the results similar to those by the toluene adduct, (toluene) 0.5 ·Al(C 6 F 5 ) 3 (runs 10-13 vs. runs [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Also noteworthy is that, when carried out in CH 2 Cl 2 or DMF, the above polymerizations were much less effective or completely shut down, attributable to rapid deactivation of Al(C 6 F 5 ) 3 in CH 2 Cl 2 15 or impeding of the monomer activation by virtue of strong coordination of DMF to Al(C 6 F 5 ) 3 .…”

“…5 Erker et al described the radical polymerization of styrene mediated by the persistent FLP-NO aminoxyl radical derived from N,N-cycloaddition of a cyclohexylene-bridged intramolecular P→B FLP to nitric oxide. 6 We have reported the addition polymerization of biorenewable α-methylene-γ-butyrolactones by borane B(C 6 F 5 ) 2 -based intramolecular, interacting FLPs and B(C 6 F 5 ) 3 -based CLPs. 7 Herein we communicate the first example of polymerization of 2-vinyl pyridine (2-VP) and 2-isopropenyl-2-oxazoline (iPOx) by Al(C 6 F 5 ) 3 -based FLPs into medium to high molecular weight, pyridineand 2-oxazoline-functionalized vinyl polymers.…”

Frustrated Lewis Pair Catalyzed Hydroamination of Terminal Alkynes

“…[1] While initial reports described a limited substrate scope, recent advances have included the demonstration of metal-free hydrogenation of olefins, [2] anilines, [3] N-heterocyclic compounds, [4] and alkynes, [5] as well as enantioselective hydrogenation of imines. [7] To date, the majority of the reported reactions of FLPs with small-molecule substrates is stoichiometric. [7] To date, the majority of the reported reactions of FLPs with small-molecule substrates is stoichiometric.…”

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

“…This C-H activation is believed to proceed via a radical mechanism involving a transient "frustrated radical pairs (FRPs)." This result is reminiscent of the activation of NO by FLPs as described by Erker and coworkers that yield the FLP-NO radical which can also activate C-H bonds [50]. …”