Iridium-Catalyzed Reduction of Alkyl Halides by Triethylsilane

Abstract

A highly active cationic Ir(III) hydride complex catalyzes the reduction of primary, secondary, and tertiary chlorides, bromides, and iodides as well as certain fluorides by Et3SiH. The catalytic cycle appears to operate by a unique process in which an electrophilic iridium silane complex acts as a silylating reagent to produce a silyl-substituted halonium ion which is then readily reduced by the nucleophilic dihydride formed following silyl transfer.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…[13,14] These catalysts are effective for hydrosilation and reduction reactions involving a range of substrates (aldehydes and ketones, [69] esters, [10] amides, [11] CO2, [12] ethers, [70] and alkyl halides [71] ) that rival the breadth of those studied in B(C6F5)3-catalyzed hydrosilations. The earliest of these iridium studies was published a few years after the first reports implicating ionic hydrosilation with rhenium catalysis.…”

Reduction of Alkyl Halides by Triethylsilane Based on a Cationic Iridium Bis(phosphinite) Pincer Catalyst: Scope, Selectivity and Mechanism

“…[7] Complex 1 is significantly more stable than 4 due to the strong coordinating ability of acetone and can be stored in a dry box at room temperature for at least 1 year. Exposure of 1 to Et 3 SiH results in rapid hydrosilation of acetone and forms noncoordinating (CH 3 ) 2 CHOSiEt 3 and the highly reactive solvated complex which initiates reduction reactions [Eq.…”

“…Relative reduction rates of 18 RX vs. 28 RX were determined by carrying out head-to-head competition experiments in one flask. These results were previously described in detail [7] and provided the following relative reactivities: 8 [e] 0.5 60 2.3 > 99…”

“…Using primary halides, relative reactivities were found to be RI:RCl = 1200:1, RBr:RCl = 260:1, RI:RBr = 80:1. [7] Only traces of CD 2 HCl were observed during the reduction of CD 2 Cl 2 by 1/Et 3 SiH which shows that CD 2 HCl is reduced much faster than CD 2 Cl 2 , a result which is also inconsistent with a radical mechanism. Qualitative comparisons of selectivities of radical-based reduction of alkyl halides and reductions employing 1/ Et 3 SiH are shown in Scheme 3.…”

“…As a part of our continuing interest in incorporation of Si À H bond activation chemistry into catalytic cycles we have discovered and recently communicated [7] that the cationic iridium bis(phosphinite) pincer complex 1 ( Figure 1) is a highly efficient catalyst for the reduction of a broad range of alkyl halides by tri-A C H T U N G T R E N N U N G ethylsilane reagents. Our preliminary studies argue against a radical pathway and the catalytic cycle appears to operate by a unique process involving an unprecedented highly electrophilic h 1 -silane complex, 2 [8] (Figure 1).…”