Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes

Abstract

Much of the recent work on catalytic hydroboration of alkenes has focused on simple alkenes and styrene derivatives with few examples of reactions of 1,3-dienes, which have been reported to undergo mostly 1,4-additions to give allylic boronates. We find that reduced cobalt catalysts generated from 1,n-bis-diphenylphosphinoalkane complexes [Ph 2 P-(CH 2) n-PPh 2 ]CoX 2 ; n = 1~5) or from (2-oxazolinyl)phenyldiarylphosphine complexes [(G-PHOX)CoX 2 ] effect selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to a variety of 1,3-dienes depending on the ligands chosen. Conditions have been found to optimize the 1,2-additions. The reactive catalysts can be generated from the cobalt (II)-complexes using trimethylaluminum, methyl aluminoxane or activated zinc in the presence of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate (NaBARF). The complex, (dppp)CoCl 2 , gives the best results (ratio of 1,2-to 1,4-addition >95:5) for a variety of linear terminal 1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)CoX 2 ] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted 1,3-dienes, but, surprisingly selective 1,2-additions with 2-substituted or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized (X-ray crystallography) Co(I)-complexes, (dppp) 3 Co 2 Cl 2 and [(S,S)-BDPP] 3 Co 2 Cl 2 , do not catalyze the reaction unless activated by a Lewis acid or NaBARF, suggesting a key role for a cationic Co(I) species in the catalytic cycle. Regio-and enantioselective 1,2-hydroborations of 2substituted 1,3-dienes are best accomplished using a catalyst prepared via activation of a chiral phosphinooxazoline-cobalt(II) complex with zinc and NaBARF. A number of common functional groups, among them,-OBn,-OTBS,-OTs, N-phthalimidogroups, are tolerated and er's >95:5

α- and β-Functionalized Ketones from 1,3-Dienes and Aldehydes: Control of Regio- and Enantioselectivity in Hydroacylation of 1,3-Dienes

“…Initial experiments quickly confirmed that, like the other [( L )­Co­(I)] + -catalyzed reactions we had reported, ,, there were strong solvent, counterion, and ligand effects in these reactions (Table ). While toluene and other hydrocarbons lead to a sluggish reaction, oxygenated solvents such as THF, ether, and ethyl acetate inhibit the reaction (entries 1–4).…”

“…Other chiral bis-phosphines (see Supporting Information), among them ( R )-Prophos, ( S,S )-Chiraphos, ( R,R )-DIOP, Ph-BIBOP, 2,2′-bis-phosphino-1,1-biaryls like ( S )-BINAP, ( S )-SegPhos, and ( R )-binaphane, and bis-phospholano ligands ( S,S )-Me-BPE, ( R,R )-Et-BPE, and ( R,R )-TangPhos, gave no detectable amount of hydro­acylation products (see Supporting Information, Table S2 for details and p S20 for structures of the ligands) . A number of sterically and electronically modified phosphino­oxazoline (PHOX) ligands which were successfully used elsewhere for Co­(I)-catalyzed hydro­boration and [2+2]-cyclo­addition reactions also failed in the hydro­acylation. Most notably, catalysts from several ligands completely consumed the starting 1,3-diene without producing any of the hydro­acylation products, presumably leading to high-molecular-weight oligomers (entries 4, 8, and 10)…”

Catalytic Enantioselective Synthesis of Cyclobutenes from Alkynes and Alkenyl Derivatives

“…In the initial studies we have recognized a prominent role for a cationic cobalt (I) species in these reactions. Control experiments (Eq 6 and the accompanying Table 3) confirm that no reaction ensues in the presence of isolated Co(I) complex (dppp) 3 Co 2 Cl 2 14 or a Co(I) complex formed by in situ reduction of L CoBr2 ( L = dppp or L1 ) using Zn as a reducing agent (See Supporting Information Table S5 for further details) in the absence of NaBARF (entry 4 and 9 versus 1 and 2). A possible unified mechanism that accounts for all products formed and the various experimental observations is shown in Figure 7.…”

“…Following many failed attempts to modify the reaction conditions described in Eq 1a, we wondered if the incompatibility of the Lewis bases in the substrates was the problem, and if so, could it be addressed by activation of the reagents by a cationic Co(I) species, an intermediate that was implicated in our recent 1,2-hydroboration of prochiral 1,3-dienes. 14 Initial scouting experiments (Table 1) were conducted with a prototypical alkyne, 4-octyne ( 4a ) and two acrylates, methyl acrylate ( 5a ) and 1,1,1-trifluoroethylacrylate ( 5b ). Concurrently, we also studied the corresponding reaction between a more reactive enyne and methyl acrylate (Table 2).…”

Versatile cobalt-catalyzed regioselective chain-walking double hydroboration of 1,n-dienes to access gem-bis(boryl)alkanes

“…Metal-catalyzed hydroboration of 1,3-dienes has been developed into selective approaches to prepare allylic or homoallylic organoboron compounds [30][31][32][33][34][35] . However, double hydroboration of 1,3-dienes has been barely studied, mainly because alkenylboron products from the first hydroboration contain an unactivated internal alkene that does not readily undergo the second hydroboration.…”