Room‐Temperature‐Stable Diazoalkenes by Diazo Transfer from Azides: Pyridine‐Derived Diazoalkenes

Abstract

Recently, stable diazoalkenes have received significant attention as a new substance class in organic chemistry. While their previous synthetic access was exclusively limited to the activation of nitrous oxide, we here establish a much more general synthetic approach utilizing a Regitz‐type diazo transfer with azides. Importantly, this approach is also applicable to weakly polarized olefins such as 2‐pyridine olefins. The new pyridine diazoalkenes are not accessible by the activation of nitrous oxide, allowing for a considerable extension of the scope of this only recently accessed functional group. The new diazoalkene class has properties distinct from the previously reported classes, such as photochemically triggered loss of dinitrogen affording cumulenes and not C−H insertion products. Pyridine‐derived diazoalkenes represent the so far least polarized stable diazoalkene class reported.

Diazoalkenes: From an Elusive Intermediate to a Stable Substance Class in Organic Chemistry

“…Recently, we could find a new route to stable diazoalkenes by dinitrogen transfer from azides such as p-TsN 3 (Scheme 9). [36] This finding allows a much broader synthetic access to stable diazoalkenes since only strong nucleophiles are capable of activating N 2 O. [37] We could show that upon reaction of polarized olefins such as mNHOs (40) or NHOs (42) with p-TsN 3 in the presence of additional base (KO t Bu) the previous diazoalkenes 41 or 44 are readily accessible (Scheme 9a).…”

“…The HOMO is centered on the CN 2 fragment and has π-type character at carbon while the HOMO-1 has σ-type character at carbon reminiscent of the frontier molecular orbitals typically encountered in carbon(0) compounds [41,42] and in general to base-supported zero-oxidation state main group species. [44] So far ten stable diazoalkenes have been reported featuring the 4-imidazole (38), [28] the 1,2,3-triazole (41), [32] the 2-imidazole (44), [29] and pyridine (47) [36] heterocyclic cores. Their characterization data are summarized in Table 1.…”

“…Representative X-ray solid-state structures of 38, [28] 41 A, [32] 44 B [29] and 47 B. [36] Thermal ellipsoids are shown with 50 % probability. Why have stable diazoalkenes been overlooked for decades?…”

“…Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements show that 38 , 41 and 44 are thermally stable compounds (decomposition >150 °C), while 47 decomposes rapidly and highly excergonically [decomposition enthalpies (Δ H D ≈−1600 J g −1 )] at ca. 90 °C in a typical energy range for small energetic diazo compounds [36, 54] …”

Synthesis and Reactivity of an Anionic Diazoolefin

“…[26] Coordination of the diazoolefin via the terminal nitrogen atom was observed in complex 9, which was formed in 67 % yield by combining IrCl(CO)(PPh 3 ) 2 with 3 in benzene (Scheme 3). Diazoolefins generally act as carbon donors, [12,13,27] although N-coordination was reported for the kinetic adduct of B(C 6 F 5 ) 3 and a neutral mesoionic diazoolefin. [13c] Thus far, complex 9 serves as the only example of an N-bound diazoolefin ligand in a transition metal complex.…”