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Other enantioselective reactions catalyzed by transition metals
Published in Ilya D. Gridnev, Pavel A. Dub, Enantioselection in Asymmetric Catalysis, 2016
Olefin hydroboration, which is the addition of a B–H bond across C=C bond, was first discovered by H. C. Brown in 195692 and Köster in 1958.93 Typically, the reaction does not require a catalyst and the simple borane reagent (e.g., B2H6, BH3·THF, BH3·SMe2, BH2Cl·Et2O, thexylborane, disiamylborane, and 9-BBN) or boranes bearing electron-withdrawing substituents (e.g., Piers’s borane B(C6F5)294) react rapidly even at room temperature to afford, after oxidation, the linear anti-Markovnikov products. The reaction can be remarkably C=C/C=O chemoselective for terminal alkenes containing carbonyl groups.95 For hydroborating reagents in which boron atom is bonded to heteroatoms such as oxygen or nitrogen (e.g., catecholborane, pinacolborane, and ephidrineborane) elevated temperatures are needed for reaction to occur.96–98 Although the uncatalyzed hydroboration–oxidation of alkenes usually affords the anti-Markovnikov products, the catalyzed versions can be induced to produce either Markovnikov or anti-Markovnikov products.99 On the contrary to metal-free, the metal-catalyzed anti-Markovnikov reaction with diorganyloxyboranes proceeds to completion within minutes at room temperature.100 The regioselectivity and enantioselectivity obtained with a catalyst depend on the ligands attached to the metal, form of the metal precursor used (i.e., neutral or cationic complex), nature of the metal used (e.g., Rh or Ir), steric and electronic properties of the reacting alkene, nature of the borane used (e.g., catecholborane vs. pinacolborane), additives, and other parameters. For example, catalytic hydroboration of perfluoroalkylalkenes with cationic101,102 and neutral101 rhodium complexes allows for selective access to Markovnikov and anti-Markovnikov alcohol products, respectively, after hydroboration with catechol- and pinacolboranes, followed by oxidation with alkaline hydrogen peroxide. Catecholborane usually affords Markovnikov product, whereas pinacolborane affords a mixture of Markovnikov and anti-Markovnikov products in the cationic Rh-BINAP-catalyzed hydroboration–oxidation of vinylarenes.103 In contrast, the selectivities are similar for these reagents, when cationic Rh-QUINAP complexes are used (still, however, catecholborane gives better regio- and enantioselectivity).103 Substitution of Rh by Ir in the hydroboration of vinylarenes104 or meso substrates105 leads to complete reverse of the sense of enantioinduction and/or regioselectivity.
NaSH-HCl mediated reduction of sulfoxides into sulfides under organic solvent-free reaction conditions
Published in Green Chemistry Letters and Reviews, 2020
Several reductions by using reducing agents in combination with different catalysts have been reported including PPh3 (1.5 eq.) in the presence of up to 5 mol% of MoO2(O2)(phox)2 in THF and in DCE (17), boranes (2.5 eq.)/5 mol% of Zn(OTf)2 in toluene (18), 1,3-dithiane (1.1 eq.) in the presence of electrophilic bromine as catalyst (19), mercaptopropyl-functionalized silica gel in the presence of 2% of Mo catalyst in toluene (20), PPh3 (1.1 eq.) in the presence of 5 mol% of ionic-liquid-molybdenum complexes in ionic liquid/toluene (21), NaHSO3 (1.1 eq.)/10 mol% of I2 in chloroform (22), catecholborane (2 eq.)/5 mol% of Rh catalyst in C6D6 (23), pinacol (2 eq.)/2 mol% of MoO2Cl2(DMF)2 under solvent-free reaction conditions (24), several equivalents of glycerol in the presence of 2.5 mol% of MoO2Cl2–(DMF)2 (25), boranes/5 mol% of MoO2Cl2 (26), benzyl alcohol (1.25 eq.)/2 mol% of carbon-supported dioxo-molybdenum catalyst (27), excess of isopropanol/5 mol% of hydroxyapatite-supported ruthenium nanoparticles in toluene (28), and pinacolborane (2 eq.)/1 mol% of magnesium hydridotriphenylborate in THF (29).
Exploring the potential role of heavy pnictogen elements in ligand design for new metal-ligand cooperative chemistry
Published in Journal of Coordination Chemistry, 2022
W. M. Hollingsworth, E. A. Hill
To date, two examples of transfer catalysis have been reported that have been directly linked to cooperativity between an NHP ligand and coordinated metal center. The Cr complex reported by Gudat, 3 P-Cr(H)(CO)3-3P(H) was also found to hydrogenate styrene under photocatalytic conditions. Finally, the Thomas group has shown that a the putative 1 P-CoH complex generated by reduction of 1 P(Cl)-CoCl2 using KBEt3H was competent for the stereoselective hydroboration of olefins using HBpin (HBpin = pinacolborane) [50]. These transfer reactions were some of the first reported cases of MLC with a metal-phosphorus bond and suggest that the NHP scaffold will continue to produce new catalytic chemistry.