The Catalyst Review July 2020 - 17

EXPERIMENTAL
initial selectivity to n-octene and methyl heptane, the co-cations increase the rate of
1-butene isomerization to 2-butene which, in turn, leads to a rapid increase in the
formation rate and selectivity to dimethyl hexene.

Scheme 1. Reaction pathways for the formation
of the different products (octene, methylheptene
and dimethylhexene) with 1-butene and 2-butene
catalyzed by a Ni-H from Cossee-Arlman site.

In the absence of BAS, the authors suggest that 1-butene is dimerized in a CosseeArlman type mechanism with the adsorption of 1-butene on Ni-H with subsequent
insertion and coordination of another butene molecule into the Ni-C bond. During
the formation of n-octene and methylheptene, the initial adsorption takes place as a
1,2-insertion into the Ni-H (Scheme 1, left pathway). The subsequent coordination of
another 1-butene to the Ni-C bond determines whether n-octene (by 2,1-insertion)
or methylheptene (by 1,2-insertion) is formed. The initial 2,1-insertion of 1-butene
leads to a branched dimer, but since dimethylhexene is not a primary product in
the presence of alkali and alkali earth cations, this route can be ruled out. Source:
Ehrmaier A, Löbbert L, Sanchez-Sanchez M, et al. (2020). ChemCatChem, 12: 1-8.
Transition-Metal-Catalyzed Cross-Coupling with Ketones or Aldehydes via N‑Tosylhydrazones...
New types of ketone- or aldehyde-based C−C bond-forming transformations
have recently emerged wherein these moieties are first converted to their
corresponding N-tosylhydrazones, which are then employed as reaction
partners in various transition-metal-catalyzed carbene-based crosscoupling reactions. Herein, the authors provide a perspective wherein
C−C bond-forming carbene coupling reactions are used to highlight new
applications of ketones and aldehydes in organic synthesis mediated by
their corresponding N-tosylhydrazones.
The characteristic features of this type of coupling are the formation of a
diazo compound in situ, followed by metal carbene formation and carbene
migratory insertion. N-tosylhydrazones, in combination with a suitable
transition metal catalyst and coupling partners, show versatile reactivity
in the construction of C−C bonds. These constructs can be divided into
four categories based on the feature of the newly formed C−C bonds
(Scheme 1): (a) Ketones which serve as alkenyl coupling partners, forming
C−C single bonds to produce poly-substituted olefins. (b) New C=C double
bonds constructed on the carbonyl functionality, which can be regarded
as carbonyl olefination transformations. (c) Ketones which serve as alkyl
coupling partners, forming C−C single bonds to produce the corresponding
alkylation products. (d) Ketones which serve as amphiphiles, in which two
C−C single bonds are formed on the carbonyl carbon.

Scheme 1. Carbene Coupling with N-Tosylhydrazones for C−C
Bond Formation

Scheme 2. Allene Formation via Cu(I)-Catalyzed Coupling of
N-Tosylhydrazones with Terminal Alkynes

The authors provide an in-depth review of each of these categories and give
numerous examples of how ketones and aldehydes can become versatile
coupling partners in a wide range of C−C bond-forming cross-coupling
reactions (Scheme 2). It should be noted, however, that current reaction
modes are quite limited in the case that the intermediates generated from
palladium carbene migratory insertion cannot contain β-hydrogen. In this
context, copper, nickel, or cobalt catalysis would be promising candidates
because the β-hydride elimination could be curbed in the alkyl metal
intermediates. However, current examples are limited in coupling only with
some acidic C−H bonds. The use of ketones or aldehydes as alkyl coupling
partners to construct chiral sp3 stereocenters is under development.
The construction of two C−C bonds at the same carbon is a unique
application of a carbene coupling reaction. However, there are only limited
and specific examples of this application, and the construction of two C−C
bonds on ketone-derived N-tosylhydrazone to form quaternary carbon
centers has not yet been achieved. Source: Xia Y and Wang J. (2020). J. Am.
Chem. Soc. https://dx.doi.org/10.1021/jacs.0c04445.
The Catalyst Review

July 2020

https://dx.doi.org/10.1021/jacs.0c04445

The Catalyst Review July 2020

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