Simplifying Nickel(0) Catalysis: An Air-Stable Nickel Precatalyst for the Internally Selective Benzylation of Terminal Alkenes

Abstract

The synthesis and characterization of the air-stable nickel(II) complex trans-(PCy2Ph)2Ni(o-tolyl)Cl is described in conjunction with an investigation of its use for Mizoroki Heck-type, room temperature, internally-selective coupling of substituted benzyl chlorides with terminal alkenes. This reaction, which employs a terminal alkene as an alkenylmetal equivalent, provides rapid, convergent access to substituted allylbenzene derivatives in high yield and with regioselectivity greater than 95:5 in nearly all cases. The reaction is operationally simple, can be carried out on the bench-top with no purification or degassing of solvents or reagents, and requires no exclusion of air or water during setup. Synthesis of the precatalyst is accomplished through a straightforward procedure that employs inexpensive, commercially available reagents, requires no purification steps, and proceeds in high yield.

Recent advances in homogeneous nickel catalysis

“…Later, an air-stable nickel precatalyst ( 68 ) was developed for this reaction, which obviates the need for Ni(cod) 2 (and therefore the use of a glovebox) and increases reaction rates. 90 Finally, a branch-selective Heck reaction of the more typically used aryl electrophiles ( 70 ) with electronically unbiased olefins has recently been reported (Figure 11c). 91 A wide range of electrophiles including aryl triflates, chlorides, and other less reactive sulfonates (mesylates, tosylates, sulfamates) can be used, underscoring the utility of nickel to undergo oxidative addition to a broad range of cheap, stable, traditionally unreactive electrophiles.…”

Nickel‐Catalyzed Mizoroki–Heck Reaction of Aryl Sulfonates and Chlorides with Electronically Unbiased Terminal Olefins: High Selectivity for Branched Products

“…[5] Recently, our group demonstrated some of these features by showing that benzyl chlorides could react with ethylene and terminal olefins in a highly selective manner (≥19:1 in most cases) to afford branched products using Ni(cod) 2 and PCy 2 Ph. [6] This report represented the first catalyst-controlled example of a highly branched-selective Heck reaction of electronically unbiased alkenes. Herein, we report the expansion of this highly regioselective reaction to more widely used aryl electrophiles.…”

“…Building on our understanding of the Ni-catalyzed cationic Heck reaction, [6a,15] we set out to directly access this pathway with phenyl triflate ( 2a ) and 1-octene. The ligand previously used for benzyl chlorides, PCy 2 Ph ( 5 ), produced only a moderate yield and rr of the desired product even after optimization of reaction conditions (Table 1, entry 1).…”

“…Comparing the proposed intermediates in Figure 1, we see that a benzyl electrophile can transition from η 1 to η 3. [6a] However, the ligand sphere for aryl electrophiles is quite different, presumably containing two ligated phosphines, and providing a fresh challenge for inducing high levels of regiocontrol. Extensive investigation of monodentate ligands of varying electronic and steric properties provided little improvement in yield or br/ln selectivities; thus the search was expanded to bidentate ligands.…”

“…Though the cost of Ni(cod) 2 is not insignificant, we hope to continue to develop alternative catalysts or pre-catalysts from inexpensive Ni sources. [6b, 22] These developments continue to show the promise of the Ni-catalyzed Heck reaction as a viable, highly selective alternative to its Pd-catalyzed counterpart.…”