Soft Propargylic Deprotonation: Designed Ligand Enables Au-Catalyzed Isomerization of Alkynes to 1,3-Dienes

Abstract

By functionalizing the privileged biphenyl-2-ylphosphine with a basic amino group at the rarely explored 3′ position, the derived gold(I) complex possesses orthogonally positioned “push” and “pull” forces, which enable for the first time soft propargylic deprotonation and permit the bridging of a difference of >26 pKa units (in DMSO) between a propargylic hydrogen and a protonated tertiary aniline. The application of this design led to efficient isomerization of alkynes into versatile 1,3-dienes with synthetically useful scope under mild reaction conditions.

Buchwald‐Type Ligands on Gold(I) Catalysis

“…Thus, the group of Zhang reported the isomerization of alkynes 91 to 1,3dienes 93. [105] In this reaction, the propargylic hydrogen is acidified upon activation of the alkyne by gold(I) and the strategically positioned weak Brønsted base functions as a proton shuttle in the isomerization process, which leads to the formation of allenyl intermediate 92. Similarly, the isomerization of ynamides and allenamides, [106] propargylic esters [107] and allyl ynoates [108] has been reported.…”

Isomerization of Cyclooctadiene to Cyclooctyne with a Zinc/Zirconium Heterobimetallic Complex

“…[4] Consistent with these data, the microscopic reverse reaction, alkyne-todiene isomerisation, can be catalyzed by rhodium/BINAP, [5,6] ruthenium hydride, [7] or gold(I) complexes. [8] Herein, we show that the isomerization of cyclooctadiene to cyclooctyne is possible within the coordination sphere of an unusual Zn/Zr heterobimetallic complex. Thef avorable binding of cyclooctyne provides at hermodynamic driving force for the isomerization.…”

“…[13] Thebridging hydrides H m and H m' in Mg·Zr are chemically and magnetically inequivalent, and at 298 Ki n[ D 8 ]toluene they resonate at d = À2.00 (dd, 2 J H-H = 13.0, 6.6 Hz) and À2.87 (dd, 2 J H-H = 6.6, 6.6 Hz) ppm, respectively. [14,15] Both couple to the terminal hydride (H t )f ound at d = 1.55 (dd, 2 J H-H = 13.0, 8 ]toluene (273-353 K) provide no evidence for disintegration of the heterobimetallic complexes in solution, site exchange between the terminal hydride (H t )a nd the distal bridging hydride (H m' )w as observed at all temperatures.I nc ontrast, Al·Zr forms reversibly and readily establishes an equilibrium with the parent hydrides. [9] Ther eaction of Zn·Zr with 1,5-COD in C 6 D 6 at 80 8 8C resulted in more than 50 %c onversion of the hydrocarbon within 24 ha nd formation of 1,3-COD (22 %b ased on 1,5-COD), cyclooctene (80 %b ased on Zn·Zr), the heterobimetallic complex 1 (85 %b ased on Zn·Zr), and trace cyclooctane (Scheme 1).…”

Bifunctional Phosphine Ligand Enabled Gold‐Catalyzed Alkynamide Cycloisomerization: Access to Electron‐Rich 2‐Aminofurans and Their Diels–Alder Adducts

“…In 2014, we reported ag old-catalyzed conversion of arylalkynes and certain types of aliphatic alkynes into dienes (Scheme 2). [6] In this reaction, allenes are the reaction intermediates,a nd the double proton migrations are both enabled by ad esigned bifunctional biphenyl-2-ylphosphine ligand (i.e., L1)featuring a3'-amino group,which acts as the proton shuttle.N otably,t he gold catalysis permits the deprotonation at the propargylic CÀHb ond (pK a in DMSO > 30) by amildly basic aniline (pK a in DMSO % 4), owing to metal ligand cooperative catalysis. [7] Our later work [8] expanded the scope of the ligands basic functionality to include differently positioned remote basic tertiary amines (see L2-L4 in Table 1footnote) and engaged frustrated Lewis pairs in gold catalysis.…”