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

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posted on 20.06.2012, 00:00 by Muhammad Sajid, Annika Stute, Allan Jay P. Cardenas, Brooks J. Culotta, Johannes A. M. Hepperle, Timothy H. Warren, Birgitta Schirmer, Stefan Grimme, Armido Studer, Constantin G. Daniliuc, Roland Fröhlich, Jeffrey L. Petersen, Gerald Kehr, Gerhard Erker
The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes2PC6H10B­(C6F5)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CH2Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes2PCHR1CHR2B­(C6F5)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 Ea = 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 C2-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.