Heterogeneous‐Gold‐Catalyzed Acceptorless Cross‐Dehydrogenative Coupling of Hydrosilanes and Isocyanic Acid Generated in situ from Urea

Abstract

Cross-coupling reactions (e.g., Suzuki, Negishi, Heck, Stille, Kumada, and Buchwald-Hartwig reactions) are of paramount importance and have proven useful for the design of molecules. [1] Although cross-coupling reactions can precisely construct new C À C, C À X, and X À X bonds (X = heteroatom), they usually utilize preactivated substrates (e.g., halides, tosylates, and triflates) and require transmetalation steps, which concurrently generate at least stoichiometric amounts of metal salts as waste. [1] Recently, cross-dehydrogenative coupling reactions by direct activation of CÀH or XÀH bonds have been emerging as synthetic tools because they are more atom efficient and environmentally benign than classical cross-coupling reactions. [2] To date, several efficient crossdehydrogenative coupling reactions using hydrogen acceptors (oxidants) such as tert-butyl hydroperoxide, [3] hydrogen peroxide, [4] and molecular oxygen [5] have been developed. Acceptorless cross-dehydrogenative coupling reactions have also been developed. [6] Quite recently, we have also reported C À N (terminal alkynes and amides), [7] P À N (H-phosphonates and amides), [8] SiÀN (hydrosilanes and indoles), [9] and SiÀC (hydrosilanes and terminal alkynes) [10] bond-forming reactions by cross-dehydrogenative coupling strategy. Herein, we successfully developed a novel green synthetic route to silyl isocyanates through heterogeneous-gold-catalyzed, acceptorless cross-dehydrogenative coupling of hydrosilanes and isocyanic acid (generated by in situ thermolysis of urea [11] ) [Eq. (1)].

Reductive cyanation of organic chlorides using CO2 and NH3 via Triphos–Ni(I) species

“…4). Considering the thermolysis of urea to release isocyanic acid and ammonia, the formation of active silyl isocyanate intermediates is possible 51 , which is consistent with the results using TMS-NCO as shown in Fig. 5.…”

“…Subsequently, silyl isocyanates formation 51 is followed by nickel-carbon insertion to generate presumably transient imidate species D. Further transformation yields the cyano product via a plausible 1,3-silyl N-to-O migration, whereby nickel siliconate intermediate E is released and further reduced by hydrosilane to regenerate species A. As discussed before, the formation of aryl-Ni(I) species is critical, while species C is stabilized by the tridentate phosphine ligand Triphos.…”

“…This is followed by oxidative addition of the aryl chloride to A to form Ni(II) halide B , which is reduced by silane and Zn to afford highly nucleophilic Ni(I) intermediate C . Subsequently, silyl isocyanates formation 51 is followed by nickel–carbon insertion to generate presumably transient imidate species D . Further transformation yields the cyano product via a plausible 1,3-silyl N-to-O migration, whereby nickel siliconate intermediate E is released and further reduced by hydrosilane to regenerate species A .…”