Palladium/Copper‐catalyzed Oxidation of Aliphatic Terminal Alkenes to Aldehydes Assisted by p‐Benzoquinone

Abstract

The development of an anti‐Markovnikov Wacker‐type oxidation for simple aliphatic alkenes is a significant challenge. Herein, a variety of aldehydes can be selectively obtained from various unbiased aliphatic terminal alkenes using PdCl2(MeCN)2/CuCl in the presence of p‐benzoquinone (BQ) under mild reaction conditions. Isomerization of the terminal alkene to the internal alkene was suppressed via slow addition of the starting material to the reaction mixture. In addition to the Pd catalyst, CuCl and BQ were essential in order to obtain the anti‐Markovnikov product with high selectivity. Terminal alkenes bearing a halogen substituent afforded their corresponding aldehydes with high anti‐Markovnikov selectivity. The halogen acts as a directing group in the reaction. DFT calculations indicate that a μ‐chloro Pd(II)−Cu(I) bimetallic species with BQ coordinated to Cu is the catalytically active species in the case of a terminal alkene without a directing group.

Palladium-Catalyzed Anti-Markovnikov Oxidation of Aromatic and Aliphatic Alkenes to Terminal Acetals and Aldehydes

“…PdCl 2 (MeCN) 2 /CuCl/BQ was found to be an efficient catalytic system for the AM Wacker-type oxidation of simple aliphatic alkenes (Table 6). 59 Similar to the oxidative acetalization of aliphatic alkenes, slow addition of the substrates (alkenes and H 2 O) led to high AM selectivity (entries 1-4). ,-Unsaturated aldehyde 11a was also obtained as a byproduct by further oxidation (dehydrogenation) of the desired saturated aldehyde 10a.…”

“…The identity of the catalytically active Pd species was predicted based on DFT calculations [geometry optimizations: B3LYP/LANL2DZ for Pd, 6-311G(d) for Cu, and 6-31G(d) for all other atoms; frequency calculations: M06/SDD for Pd, 6-311G(d) for Cu, and 6-311+G(d,p) for all other atoms]. 59 Propene was adopted as an aliphatic terminal alkene without a directing group for simplicity ( Figure n…”

Realization of Anti‐Markovnikov Selectivity in Pd‐Catalyzed Oxidative Acetalization and Wacker‐Type Oxidation of Terminal Alkenes

“…Again, a PdCl 2 (MeCN) 2 /CuCl/electron‐deficient cyclic alkene was found to be an efficient catalytic system for the AM Wacker‐type oxidation of 1‐octene (Scheme 22). [65] In this reaction, dehydrogenation also occurred, affording 2‐octenal ( 15a ) from octanal ( 14a ). The AM selectivity of the products was lowered when 8 a or H 2 O was added at a time (without slow addition).…”

“…[56,63,64] Because the product yields were considerably low in the absence of CuCl (Scheme 18) in contrast to the case of aerobic AM oxidative acetalization (Scheme 6), the Pd(0) species is reoxidized by CuCl 2 rather than by O 2 directly in the AM Wacker-type oxidation. [65] The realization of AM selectivity in the Wacker-type oxidation of simple and unbiased aliphatic terminal alkenes is challenging. [21,31,32,44] The reaction exclusively affords ketones, except in a few cases, such as the catalytic systems (RCN) 2 PdClNO 2 /CuCl 2 /tertiary alcohol/O 2 (up to 70 % AM selectivity) [32] and PdCl 2 (MeCN) 2 /CuCl/t-BuOH/O 2 (84 % AM selectivity).…”

“…Because observation of the catalytically active Pd species was difficult, DFT calculations were performed for predictive purposes (Figure 1, geometry optimizations: B3LYP/ LANL2DZ for Pd, 6-311G(d) for Cu, and 6-31G(d) for all other atoms; frequency calculations: M06/SDD for Pd, 6-311G(d) for Cu, and 6-311 + G(d,p) for all other atoms). [65] Propene-coordinated complexes were used as model complexes. The μ-chloro Pd(II)-Cu(I) bimetallic complex D was more stable than the monometallic Pd complexes with either BQ (A), MeCN (B), or t-BuOH (C).…”