Enantioselective Michael addition to vinyl phosphonates via hydrogen bond-enhanced halogen bond catalysis

Abstract

An asymmetric Michael addition of malononitrile to vinyl phosphonates was accomplished by hydrogen bond-enhanced bifunctional halogen bond (XB) catalysis.

N‐Heterocyclic Iod(az)olium Salts – Potent Halogen‐Bond Donors in Organocatalysis

“…[11][12][13][14] The vast majority of XB-donors are based on monodentate iodine(I) derivatives with either a polyfluorinated or a N-heterocyclic backbone, for example, the triazolium derivative 1 or the pyridinium derivative 2 (Figure 1a), with cationic species being typically more reactive than neutral derivatives. [15][16][17][18][19][20][21][22][23] Bidentate XB-donors such as imidazolium 3 have been described as well and were found to have a significantly higher reactivity than monodentate analogues. [24] Besides iodine(I) species, hypervalent iodine(III) derivatives, in the form of (cyclic) diaryliodonium salts, received growing attention as XB-donors, due to the high Lewis acidity of the hypervalent iodine atom.…”

“…We turned our focus towards the Diels-Alder cycloaddition of cyclopentadiene (19) and methyl vinyl ketone (20) as another benchmark reaction. [17,53] Here, iodolium salt 4 b has been previously investigated.…”

“…Figure 8. Yield-vs.-time profile for the XB-mediated Diels-Alder reaction between cyclopentadiene(19) and MVK(20) over the course of 170 min employing several NHISs as potential activators in CD 2 Cl 2 . Yields determined via 1 H NMR spectroscopy using SiEt 4 as the internal standard.…”

“…Next, we investigated the more challenging Diels-Alder reaction between cyclohexadiene (22) and MVK (20), in which the bidentate iodolium salts 5 was reported to be the so far only active XB-donor and nearly approached the activity of the strong Lewis acid BF 3 . [34] In contrast to the literature report, we started the investigation with a catalyst loading of 15 mol % instead of 30 mol %.…”

“…[ 11 , 12 , 13 , 14 ] The vast majority of XB‐donors are based on monodentate iodine(I) derivatives with either a polyfluorinated or a N‐heterocyclic backbone, for example, the triazolium derivative 1 or the pyridinium derivative 2 (Figure 1 a), with cationic species being typically more reactive than neutral derivatives. [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ] Bidentate XB‐donors such as imidazolium 3 have been described as well and were found to have a significantly higher reactivity than monodentate analogues. [24] …”