National Science Foundation of China [20801046, 20872123, J0630429, 20705029

Building bridges: The first m-metallapyridine and the first metallapyridyne were synthesized under mild reaction conditions. The two complexes are metal-bridged polycyclic metallabenzenoid aromatics, in which the transition-metal center is shared by both six-membered rings. The synthetic method permits the use of metallabenzene as a starting material to access higher π-electron metallaaromatics.

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.

Details of the preparation and characterization of 4 and 5 can be found in the Supporting Information.

The strongly electrophilic borane derivative amino-CHCHCH-B(CF)6 was α-CH deprotonated with LiTMP to give the borata-alkene {[amino-(CH)-CH[double bond, length as m-dash]B(CF)][Li]}9 which underwent facile [2 + 2] cycloaddition reactions with benzophenone or fluorenone to yield the respective 1,2-oxaboretanides 11a,b. Compounds 9 and 11 were characterized by the X-ray diffraction. Thermolysis or hydrolysis of compounds 11a,b gave the corresponding borata-Wittig olefination products 12a,b. A variety of R-CH-CH-B(CF) boranes (conveniently generated by hydroboration of terminal alkenes R-CH[double bond, length as m-dash]CH with Piers' borane [HB(CF)]) were analogously deprotonated to give the respective borata-alkenes 16a-e (R: Ph-CH-, CH, Bu, Cy, PhCHCH-). They underwent "non-classical" borata-Wittig olefination reactions with ethylformate to give the respective enolether carbonylation products, or their C-elongated aldehydes (after hydrolysis). The borata-alkene [Ph-(CH)-CH[double bond, length as m-dash]B(CF)] [LiHTMP] (16a) gave the respective "non-classical" borata-Wittig olefination products, the enolethers 25a,b and 27, respectively, upon treatment with methyl- or ethyl acetate or γ-butyrolactone.