Non‐conventional Behavior of a 2,1‐Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

Abstract

The titled 2,1-benzazaphosphole (1) (i. e. ArP, where Ar=2-(DippN=CH)C 6 H 4 , Dipp=2,6-iPr 2 C 6 H 3 ) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC�CR (R=CO 2 Me, C 5 F 4 N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3, that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5. Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH) 2 (R=Me, tBu, Ph) resulting in the addition products 7-9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [μ-ArP(AuCl) 2 ] (10), [(μ-ArP) 4 Ag 4 ][X] 4 (X=BF 4 (11), OTf ( 12)) and [μ-ArP(Co 2 (CO) 6 )] ( 13). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.

Probing Limits of a C=C Bond Activation by N‐Coordinated Organopnictogen(I) Compounds

“…The structural data are summarized in Table 1 along with already published data for Sb-1Me, Sb-1tBu, Sb-1Ph (exo), [13] P-2Me and P-2Ph (endo). [14] The particular orientation of the maleimide in the products corresponds to that established in solution and dihedral torsion angles HC7À C8H are found in the range 71-78°and 42-49°(Table 1) for exo and endo forms, respectively. In all cases, the addition of maleimides resulted in the formation of new covalent bonds E1À N1, C7À C8 and E1À C9 and detected bond lengths well correspond to the sum of covalent radii of the bonding partners.…”

“…ΔG°and ΔH°values for both P-2 and As-2 approach or are even more negative in comparison with the tBu-pincers E-1 and show the same trend that the substrate is more tightly coordinated by the phosphorus than the arsenic compounds. More importantly, despite the presence of the bulkiest R' (Dipp) group in P-2 and As-2 used in the study, the values of ΔG°are significantly more favouring the h-DA adduct formation in comparison with those determined for P-1' or As-1' (entries [11][12][13][14][15][16].…”

“…Treatment of pincer compounds P-1, P-1', As-1 and Sb-1 [13] as well as C,N-chelated compounds P-2 [14] and As-2 with the maleimides RN(C(O)CH) 2 (R=Me, tBu, Ph) smoothly provided expected h-DA products of the C=C bond activation (Scheme 2). The reaction of As-1' and Sb-1' with maleimides is reversible at room temperature and shifted significantly to the starting materials that thwarted an isolation of respective products.…”

“…To provide insights into the relationships between the trends discussed above and the electronic structures of the relevant molecules, comprehensive theoretical studies of the systems bearing the Ph-substituted maleimide have been carried out and compared with published examples. [13,14] Structures of the reactants, products and transition states (TS) were optimized taking into account solvation effects in benzene solution. Both exo and endo orientations of the maleimide Scheme 3.…”

“…[12] These compounds exhibit similar reactivity toward activated C=C double bonds in maleimides (Scheme 1D). Nevertheless, the preliminary tested antimony (Sb-1) [13] and phosphorus (P-2) [14] systems differ significantly both in the reversibility of the alkene binding and in its orientation (endo/exo) in the final products (Scheme 1D).…”