The Catalyst Review June 2020 - 17

EXPERIMENTAL
a fact that clearly demonstrates that strain engineering is a viable strategy for improving electrocatalysts. Source: Ni W, Wang T,
Schouwink PA, et al. (2020). Angew. Chem. Int. Ed., doi.org/10.1002/anie.201916314.
Cobalt(II)-Catalyzed Stereoselective Olefin Isomerization: Facile Access to Acyclic Trisubstituted
Alkenes...
Trisubstituted alkenes are versatile building blocks in synthetic organic chemistry,
particularly in the fields of pharmaceutical and natural product research, a fact that has
prompted considerable attention to the stereoselective synthesis of these moieties.
However, traditional olefin synthesis via the Wittig reaction achieves only limited
stereoselectivity in the case of trisubstituted alkenes. Transition metal-catalyzed alkene
isomerization, making use of noble metals such as ruthenium, rhodium, palladium,
iridium and platinum, has emerged as an atom-economic transformation which can
readily convert terminal alkenes to internal alkenes.

Scheme 1. Typical mechanisms for olefin
isomerization and our strategy in the present
work.

Low reaction efficiency and poor stereoselectivity are significant problems in alkene
isomerization. These issues can be better understood by making use of the two major
proposed reaction pathways: (a) H radical initiated isomerization (Scheme 1 a); (b)
metal-hydride involved isomerization (Scheme 1 b). Herein, the authors report the
development of a novel Co-hydride catalyst by incorporating a phosphamide group
into the ligand (Scheme 1 c). The insitu generated metal-hydride catalyst was found to
readily overcome challenging isomerizations affording a wide range of trisubstituted
alkenes and dienes with up to 98% yield and 130/1 (E/Z) stereoselectivity. The mild
reaction conditions and high efficiency allows scalable access to acyclic trisubstituted
alkenes with high stereoselectivity. Moreover, preliminary studies indicate that the
catalyst is moderately recyclable.
The authors suggest that there are four main pathways associated with Figure 1. Proposed catalytic cycle.
transition-metal catalyzed olefin isomerization: (a) radical mechanism;
(b) metal hydride mechanism; (c) 1,3-hydrogen shift mechanism ; (d)
oxidative cyclization mechanism. These mechanisms can be classified
into two types of pathways according to the means of H-transfer:
stepwise (pathways a and b) and concerted (pathways c and d). The
results of their experimental investigation indicate that the metalhydride pathway is the most plausible route. Moreover, based on
experimental observations and the single-crystal structure of the
precatalyst, the authors propose a Co-hydride catalytic mechanism, as
shown in Figure 1. The active cobalt-hydride catalyst (Co-H) initiates
the reaction through a double bond insertion giving an alkyl cobalt
intermediate I-1. On the determining step. Then the alkyl cobalt species
(I-1) experiences a stereoselective β-H elimination through a transition
state TS-1. In the transition state, π−π stacking effects between the pyridine (or quinoline) ring and an aromatic ring in the substrate,
as well as the steric influence of the phosphamide moiety in the catalyst, plays an important role in the high E/Z selectivity of the
reaction. Source: Zhang S, Bedi D, Cheng L, et al. (2020). J. Am. Chem. Soc., https://dx.doi.org/10.1021/jacs.0c02101.

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June 2020

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The Catalyst Review June 2020

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