Unexpected synthesis and structure of a cage-like aza[3,3,3]propllane carbene

A stable, propeller-like N-heterocyclic cage carbene with a protective umbrella was unexpectedly synthesized in the hydrogen proton transfer catalytic hydrogenolysis. Its structural and spectral characteristics are obviously different from those of conventional imidazolin-2-ylidenes. Two nitrogen atoms of aza[3,3,3] propllenan-2ylidene ring are in a plane environment.

high symmetry and a maximum nitrogen content of this skeleton, it can be a core building block to satisfy high energy density requirements of high energy compound after debenzylation followed by nitration of 3a. So we studied systematically the debenzylation of compound 3a. It is surprising that the high resolution mass spectrum of white debenzylation product 4a of pentabenzylpentaaza [3.3.3]propellane was not the signal of expected debenzylation product, but the signal of losing a molecule of H2.
What is this product? How to get it? Fortunately, we have successfully obtained a single crystal of 4a, which shows that it is a new and very stable cage-like aza [3,3,3]propllenan-2-ylidene carbene. Here, we will report these results in detail.

Results and Discussion
In order to obtain a variety of multisubstituted azaspironanes, a modified synthetic strategy for suitable precursors [4]     The condition optimization indicated that the dehydrogenation of 3a was achieved in the catalyst of Pd(OH)2-C in DCM/MeOH, and protonated solvent was necessary, higher temperature was beneficial to the dehydrogenation. It is worth nothing that alkali was not essential, even argon protection was redundant (Table 1). To the best of our knowledge, this is a new and convenient synthetic method for N-heterocyclic imidazol-2-linene. We also used this method to synthesize the derivatives of 3a, such as 4b and 4c, and their structures were characterized by IR, NMR and mass spectrometry. The possible mechanism is shown in Figure 3. Firstly, the palladium catalyst is  It is worth noting that 4a is extremely stable, and its preparation and storage do not To get more insights into the stability of propeller-like NHC-carbene, the structure of 3a was optimized by using density functional theory (DFT) method with the long-range corrected hybrid exchange-correlation functional CAM-B3LYP and the polarized basis set 6-31g* [10]. Vibrational frequency analysis was performed at the same theoretical level to ensure that the optimized geometry was local minimum on the potential energy surface. All calculations were performed with the Gaussian 09 program package [11].
As shown in Figure 4, the five-membered ring containing the carbene C atom is almost planar, which is consistent with the X-ray result. The Mulliken charge on the carbene C atom is 0.195 e, while the two neighboring N atoms are negatively charged with Mulliken charge around -0.5 e. Interestingly, the highest occupied molecular orbital (HOMO) orbital of 3a is almost localized on the carbene C atom, while the lowest unoccupied molecular orbital (LUMO) orbital is mainly delocalized on the phenyl ring far away from the carbene C atom. Their HOMO-LUMO energy gap of 3a is up to 7.77 eV at CAM-B3LYP/6-31g*level, implying that the electron transfer from HOMO to LUMO is very difficult and the carbene is quite stable upon light. To the best of our knowledge, the HOMO-LUMO erengy gap of 7.77 eV is the highest value among those of NHC carbenes [12]. at the optimized structure of 4a.

Conclusion
A new aza [3,3,3]propellan-2-ylidene was accidentally obtained by a the synthetic method in the hydrogen proton transfer catalytic hydrogenolysis of fivebenzylfiveaza [3,3,3]propellane. This three-dimensional, fan-like NHC-carbene is a fair stable, and not deteriorate even if exposed to air. Its carbene imidazoline system is entirely in a flat environment, the bond angle of carbene carbon is larger than that of any five-membered NHC-carbene reported, the lengths of two C-N bonds in the carbene ring are obviously shorter than that of the saturated imidazolin-2-ylidene, and the chemical shift of the carbene center in 4a at δ 156.65 ppm is shockly high-fieldshift. Further studies on formation mechanism, catalytic application of 4a are ongoing.

Supporting Information
Supporting Information : Experimental procedures and characterization data