Selective Oxidation of an Active Intramolecular Amine/Borane Frustrated Lewis Pair with Dioxygen

Abstract

Oxidation reactions are rarely encountered in frustrated Lewis pair (FLP) chemistry. We describe the selective reaction of an amine/borane FLP with molecular oxygen. The trimethylene-bridged amine/borane frustrated Lewis pair was prepared by hydroboration of N-allyl-tetramethylpiperidine with Piers' borane [HB(C6F5)2]. The intramolecular N/B system undergoes a variety of typical FLP reactions. It is a very active hydrogen splitting reagent at ambient conditions. Its reaction with dioxygen takes place rapidly at ambient conditions to give a tetrahydroisoxazolium/borate salt. A possible pathway of its formation is proposed and supported by DFT calculations.

CO/CO and NO/NO coupling at a hidden frustrated Lewis pair template

“…18,20,23,24 CO activation can occur by formation of Piers' borane carbonyl 25,26 [OC–B(H)(C 6 F 5 ) 2 ] 14 as we had recently shown (we had actually isolated 14 at low temperature and characterized this borane carbonyl by X-ray diffraction). FLP addition of the enamine nucleophile followed by hydridoborate reduction of the acyl group in 15 would then lead to the intermediate 16 which could add to a second equivalent of the borane carbonyl to eventually yield the observed CO coupling product 13a (see Scheme 3).…”

“…3), which contains the saturated [N]–CH 2 –CH– moiety, we must assume that the NO reaction started from the N/B FLP isomer 18a (see Scheme 5 and below). 20,23,24 NO addition to boron would give the radical 19 which might have rearranged to its isomer 20 . 20,27 Trapping with a second equivalent of NO would then straightforwardly lead to 17a .…”

“…20,23,24 NO addition to boron would give the radical 19 which might have rearranged to its isomer 20 . 20,27 Trapping with a second equivalent of NO would then straightforwardly lead to 17a . We must stress that this pathway which is depicted in Scheme 5 provides a mere possible rationalization for the observed formation of the NO/NO coupling product; so far none of the alleged intermediates has been observed directly.…”

“…We treated the substituted allyl amine precursor 8b 20,29 with one molar equiv. of HB(C 6 F 5 ) 2 in n -pentane.…”

“…18 We have now conveniently prepared an enamine/HB(C 6 F 5 ) 2 Lewis pair adduct containing the very bulky tetramethylpiperidinyl (TMP) moiety 19 by an isomerization route starting from the respective allylamine precursor. 20 The TMP-enamine/HB(C 6 F 5 ) 2 C/B adduct was shown to undergo CO/CO and NO/NO coupling reactions in rather differently oriented pathways. This opposing set of reactions will be described and discussed in this account.…”