Intermolecular Hydroamination of Ethylene and 1-Alkenes with Cyclic Ureas Catalyzed by Achiral and Chiral Gold(I) Complexes

Abstract

The intermolecular hydroamination of unactivated alkenes remains an important, unsolved challenge in catalysis.1 Hydroamination has been realized with alkali metal amides,2 lanthanide metallocene complexes,3 or acidic zeolites,4 but these approaches suffer from a number of limitations, most notably poor functional group compatibility. Ru(II),5 Rh(III),6 and Pt(II)7 complexes catalyze the hydroamination of ethylene and, in one case, 1-hexene8 with carboxamides or alkyl or aryl amines, but these transformations require forcing conditions and are of extremely limited scope.9 Although electrophilic gold(I)-10 and platinum(II) triflate11 complexes have been reported to catalyze the intermolecular hydroamination of unactivated alkenes with sulfonamides, these transformations are catalyzed with equal or greater efficiency by Brønsted acids and the metal-catalyzed reactions display behavior consistent with Brønsted acid catalysis.12 -14 Given the challenges associated with the intermolecular hydroamination of unactivated alkenes, it is not surprising that the enantioselective intermolecular hydroamination of unactivated alkenes remains unknown.15 , 16 Here we report the Markovnikov-selective gold(I)-catalyzed hydroamination of ethylene and 1-alkenes with cyclic ureas and the unprecedented enantioselective hydroamination of unactivated 1-alkenes with up to 78% ee.We have recently reported the room temperature intramolecular hydroamination of γ-and δ-alkenyl ureas catalyzed by a mixture of a gold(I) N-heterocyclic carbene (NHC) complex and AgOTf. 17 The mild reaction conditions and the absence of an acid-catalyzed reaction pathway17 pointed to the potential development of a corresponding intermolecular process. However, attempts to realize the hydroamination of ethylene with acyclic ureas catalyzed by gold NHC complexes were uniformly unsuccessful. Conversely, cyclic ureas, employed in combination with a gold o-biphenyl phosphine precatalyst led to efficient hydroamination of ethylene. As an example, treatment of 1-methyl-imidazolidin-2-one (1) (0.4 M) with ethylene (120 psi) and a catalytic 1:1 mixture of (2a)AuCl [2a = P(t-Bu) 2 o-biphenyl] and AgOTf (5 mol %) in dioxane at 100 °C for 24 h led to isolation of 1-ethyl-3-methyl-imidazolidin-2-one (3) in 99% yield (Table 1, entry 1). In addition to 1, a number of cyclic ureas and 2-oxazolidinone reacted with ethylene at 100 °C to give the corresponding N-ethyl derivatives in good yield (Table 1, entries 5,6,7,10). 18 Extension of gold(I)-catalyzed hydroamination to include 1-alkenes was encouraging, but also revealed the limitations of the (2a)AuCl/AgOTf catalyst system. Gold(I)-catalyzed reaction of propene or 1-butene with cyclic ureas at 100 °C led to Markovnikov hydroamination in good yield with high regioselectivity, but extended reaction time and/or higher catalyst loading was rwidenho@chem.duke.edu .

Gold‐Catalyzed Intermolecular Anti‐Markovnikov Hydroamination of Alkylidenecyclopropanes

“…[20][21][22][23][24] Drawing from our experience with the gold-catalyzed intermolecular hydroamination of 1-alkenes, [18] we similarly targeted imidazolidin-2-ones as nucleophiles for ACP hydroamination. An initial experiment was encouraging and reaction of a 1:1.1 mixture of 1-phenyl-2-methylenecyclopropane (1 a) and 1-methyl-imidazolidin-2-one (2) catalyzed by a 1:1 mixture of (P1)AuCl (P1 = P( t Bu) 2 (o-biphenyl)) and AgSbF 6 in dioxane at 100 8C for 18 hours led to complete conversion of the starting materials to form a 1:2.2 mixture of the trans-a-aminocyclopropane 3 a and allylic amines 4 (Table 1, entry 1).…”

“…As gold-catalyzed hydroamination of isobutylene occurs with exclusive Markovnikov regioselectivity, [18] it appears that the anti-Markovnikov selectivity of gold-catalyzed ACP hydroamination is due either to the ring strain or the sp hybridization of the ACP. By analogy, allenes also have an internal sp-hybridized carbon atom and undergo gold- [c] [a] Determined by GC analysis of the crude reaction mixture.…”

Catalytic Oxidative Cyclisation Reactions of 1,6‐Enynes: A Critical Comparison Between Gold and Palladium

“…This geometry makes facial selection challenging when attempting enantioselective catalysis. [13][14][15] Therefore, usually very specialised ligands/complexes are used, such as complex 4, [16] which possesses a highly modified NHC ligand, or the dimeric gold complex [(S)-3,5-t-Bu-4-MeOMeOBIPHEP](AuCl) 2 , [17][18][19][20][21] which possesses a phosphine ligand.…”

Cationic Iridium-Catalyzed Synthesis Initiated by the Cleavage of C-H, N-H, and C-O Bonds

“…A gold catalyzed reaction of cyclic ureas to unactivated alkenes is a successful example. 32 Ir catalyzed enantioselective hydroaminations have also been reported, but the scope of amines and alkenes was limited to aniline and norbornene derivatives, respectively.…”