Photoredox‐Controlled β‐Regioselective Radical Hydroboration of Activated Alkenes with NHC‐Boranes

Abstract

In this Communication, we report an unprecedented β‐regioselective radical inverse hydroboration (compared with ionic hydroboration) of α,β‐unsaturated amides with NHC‐BH3 enabled by photoredox catalysis. Density functional theory (DFT) calculations show that the unique photoredox cycle is a key factor to control the β‐regioselective radical hydroboration, by lowering the energy barrier in comparison with other pathways. This protocol provides a general and convenient route to construct a wide range of structurally diverse β‐borylated amides in synthetically useful yields under mild conditions.

Copper‐Photocatalyzed Hydroboration of Alkynes and Alkenes

“…The reaction on electron deficient alkenes (i.e. Michael acceptor) provided the β‐borylated sulfone, ester and ketone ( 51 – 53 ) in excellent yields [21] …”

Photoredox‐Controlled β‐Regioselective Radical Hydroboration of Activated Alkenes with NHC‐Boranes

“…[10b, 11a] Two paralleled pathways may account for a higher yield in the presence of thiol (27 % vs. 77 %). Subsequently, the radical addition of the NHC-boryl radical to the unsaturated carboncarbon double bond can afford radical intermediate (12), which further accepts one electron from Ir II to complete the photoredox cycle and generates intermediate (13), which can get one proton from (7 a) or t BuSH to afford the desired product 8 a. Indeed, the photoredox catalysis is a key factor to achieve b-regioselective radical hydroborylation of unsaturated amides compared to Wangs work.…”

“…[12a] Fortunately, when a,b-unsaturated amide (7 a) was employed, the b-borylated amide can be obtained in 62 % yield. [13] It is interesting to find that this kind of NH-free amide functional group (7 a) is a crucial factor for a successful radical hydroboration.…”

DFT Investigation of Hydrogen Atom Abstraction from NHC-Boranes by Methyl, Ethyl and Cyanomethyl Radicals—Composition and Correlation Analysis of Kinetic Barriers

“…Stationary points were fully optimized using the B3LYP [ 42 , 43 , 44 ] and Truhlar’s M06-2X [ 45 ] functionals with the McLean–Chandler 6-311G basis set [ 46 ], which has polarization and diffusion functions on hydrogen and heavier second-row atoms (i.e., 6-311++G(d,p)). The two functionals were selected based on available reports [ 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 ] and our tests. The theoretical methods used often in the available computations of NHC-boranes, involving H-atom abstraction, addition, cyclization and complexation reactions, are B3LYP [ 47 , 48 , 49 , 50 , 51 ] and M06-2X [ 52 , 53 , 54 , 55 ].…”

“…The two functionals were selected based on available reports [ 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 ] and our tests. The theoretical methods used often in the available computations of NHC-boranes, involving H-atom abstraction, addition, cyclization and complexation reactions, are B3LYP [ 47 , 48 , 49 , 50 , 51 ] and M06-2X [ 52 , 53 , 54 , 55 ]. The M06-2X functional, a high-nonlocality functional with double the amount of nonlocal exchange (2X), was recommended for use in main-group thermochemistry, thermodynamic kinetics, noncovalent interactions and so forth.…”