Photochemical Nozaki–Hiyama–Kishi Coupling Enabled by Excited Hantzsch Ester

Abstract

This work reports the first photochemical Nozaki−Hiyama−Kishi coupling enabled by bioinspired Hantzsch ester. The salient feature of this process is that commercially available and low-cost organic photoactive Hantzsch ester can serve as both an electron and a proton donor to reduce Cr/Ni to lowvalent species and hydrolyze the Cr III -alkoxy bond, thus bypassing the use of stoichiometric metallic reductants and additives such as TMSCl and Cp 2 ZrCl 2 . The mild conditions and operationally easy method showed broad compatibility with various alkenyl triflates and aldehydes, including electron-poor pentafluorobenzaldehyde which failed under previous conditions.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

“…HEH has been widely used as an organic hydrogen reductant in hydrogenation reactions. 14–16 Herein, we use the XH/HB pair strategy to obtain thermodynamically equivalent hydrogen reductants to HEH. The Gibbs free energy of HEH releasing two hydrogen ions, Δ G PH − (HEH), is determined as 83.1 kcal mol −1 in acetonitrile by combining Δ G H − D (HEH) (64.4 kcal mol −1 ) 3 d and p K a (HE + ) (13.65).…”

“…As seen from Scheme 8, the Δ G PH − (XH/HB) scale of 14 472 (216 XHs × 67 HBs) XH/HB pairs spans a very wide range of 107.2 kcal mol −1 , and all kinds of XH/HB pairs with different reduction abilities from 42.1 to 149.3 kcal mol −1 could be designed to hydrogenate various unsaturated substrates. Since HCO 2 H (70.1 kcal mol −1 ), 18 H 2 (76.0 kcal mol −1 ), 12 HEH (83.1 kcal mol −1 ) 3 d and i PrOH (84.3 kcal mol −1 ) 19 are excellent hydrogen reductants to be extensively applied in hydrogenation reactions, 14–16,20–22 it is convincing to deduce that many XH/HB pairs (42.1–149.3 kcal mol −1 ) are thermodynamically better hydrogen reductants than common excellent hydrogen reductants to be their better alternatives in hydrogenation reactions.…”

“…Hantzsch ester (HEH) is one of the most extensively used organic hydrogen reductants in hydrogenation reactions, and it is an excellent hydrogen carrier to release two hydrogen ions, HEH → HE + H − + H + (Scheme 1d). 14–16 Unlike N -alkyl-substituted organic hydrides, there are some unique characteristics of the HEH/HB pairs. After HEH releases a hydride, the generated HE + , a protonated pyridine, is an organic acid too.…”

“…Based on the above analysis, it can be inferred that Δ G PH − (XH/HB) is an important thermodynamic parameter to quantitatively evaluate and design XH/HB pairs as novel hydrogen reductants. Without a doubt, the XH/HB pair strategy not only enormously expands the molecular library of existing hydrogen reductants, examples include but are not limited to Hantzsch ester (HEH), 14–16 benzothiazoline (BTH), 23 dihydrophenanthridine (PDH) 24 and alcohols, 22,25 but also conveniently improves and regulates the thermodynamic abilities of novel hydrogen reductants releasing hydrogens.…”

“…26 Generally, HEH, H 2 and HCO 2 H, which are excellent hydrogen reductants in chemical synthesis, are used as hydrogen reductants to hydrogenate imines. 14–16,20,21 In this work, the hydrogenation process of N ,1-diphenylmethanimine (IM) by the BNAH/TsOH pair is considered from the view of thermodynamics (Scheme 12).…”