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in situ activated by a highly reactive substrate with improved
reactivity and enantioselectivity in the hydrogenation of dialkyl
ketimines. However, in their early study,[27] the Ir–phosphinoox-
azoline (PHOX) complex generally suffered from low yields for
dialkyl ketimines but gave excellent enantioselectivities and
high turnover numbers for the hydrogenation of analogous
aryl alkyl N-arylketimines. On the basis of the mechanistic
study, the authors found that the addition of an equimolar
equivalent of acetophenone imine to Ir–phosphinooxazoline
(PHOX) complex resulted in the formation of an active catalyst
that displayed higher reactivities and better enantioselectivities
in the hydrogenation of both aryl alkyl N-arylketimines and
analogous dialkyl ketimines. The present study[26] provides an
excellent example on the substrate-type additive-activated
asymmetric catalysis as well as the catalyst–substrate interac-
tion-based reactivity enhancement.
presence of amide-derived olefin. Encouraged by this work, we
herein report our recent findings on the concept of spring-
board chemistry that highly reactive substrate act as a “reactive
tractor” to drive the transformation of poorly reactive sub-
strates. Thus we found that trifluoromethyl ketone could be
used as a very efficient additive in the nanosilver-catalyzed al-
kynylation of aldehydes. The new example of reactive additive-
enhanced transformations revealed that the springboard
chemistry would be an effective strategy and concept in the
enhancement of catalytic organic transformation.
On the basis of previous work on the highly efficient nano-
Ag@TiO2@PMHSIPN-catalyzed alkynylation of trifluoromethyl
ketones (ꢀ98% isolated yields), we felt that the use of an ap-
propriate trifluoromethyl ketone would allow us to enhance
the catalytic alkynylation of aromatic aldehydes. Fortunately,
using catalytic amounts of 2,2,2-trifluoro-1-phenylethanone
(1a, 10 mol%) as an additive when the coupling reaction of
2-naphthaldehyde (6a) and 1-ethynylbenzene (2a) was per-
formed with 5 mol% of nanoAg@TiO2@PMHSIPN, good yield of
the desired product 7a were achieved. In comparison to that
of the additive-free alkynylation, the yield was increased from
58% to 78% (Scheme 5 and Scheme 6, difference= +20%).
Therefore, this is a good example of highly reactive substrates
acting as a “reactive tractor” to initiate and drive the transfor-
mation of poorly reactive substrates. With this finding, we con-
tinued to examine the catalytic alkynylation of general aromat-
ic aldehydes in the presence of 1a (10 mol%) as well as the
nanosilver catalyst in water. As shown in Scheme 6, it was
found that all aromatic aldehydes we chose resulted in good
to excellent yields on these conditions. The isolated yields of
the aromatic aldehydes were increased by at least 13%. Espe-
cially for substrate 4-phenylbenzaldehyde (6d), the yield of 7d
could be increased from 24% to 67%. We reasoned that this
enhancement might result from the interchange between the
nanosilver–trifluoromethyl ketone intermediate (for example,
intermediate V-1 or IV-1 of Scheme 4) and the aromatic alde-
hyde. Similarly, the enhanced interaction between nanocatalyst
and substrate initiated by 2,2,2-trifluoro-1-phenylethanone led
to the decreasing of the activation energy for the 1,2-addition
reaction of the terminal alkyne to the aldehyde.
Inspired by previous results on additive-enhanced chemical
reactions,[25,26] we hypothesized that highly reactive substrates
could react with transition metal complexes to improve their
catalytic performance because of the formation of a new acti-
vated intermediate, thus the corresponding substrate could be
used as an additive to initiate the transformation of nonreac-
tive or poorly reactive analogous substrates. In other words,
the mechanistic procedure could be changed possibly by the
addition of highly reactive substrates through noncovalent in-
teractions between additive and catalyst. Thus the concept of
springboard chemistry that highly reactive substrate act as
a “reactive tractor” or “reactive springboard” to drive the trans-
formation of poorly reactive substrates would be a practical
principle to enhance or improve the activity of transition-metal
catalysts in many reactions (Figure 6).
Very recently, we have reported that substituted amide-de-
rived olefins could be used as a reactive springboard/ligand to
To clarify the dramatic increase in yield promoted by trifluor-
omethyl ketone, we performed a kinetic study of trifluoro-
methyl ketone promoted alkynylation of aromatic aldehydes
under different conditions. As shown in Figure 7, the time de-
pendence of the conversion of 4-bromobenzaldehyde 6i re-
veals that 1a has a pronounced promoting effect on the con-
version of 6i, especially in the initial stage. Thus, we demon-
strated the positive activation of aromatic aldehydes by tri-
fluoromethyl ketone for the proposed mechanism. As shown
in Scheme 7, we propose a “reactive tractor” like mechanism
for this trifluoromethyl ketone promoted silver-catalyzed addi-
tion of terminal alkynes to aromatic aldehydes. This proposed
mechanism is consistent with the observation in Figure 7 and
Figure 5 that both water and trifluoromethyl ketone are impor-
tant for the catalytic cycle owing to the existence of activated
intermediates (from I-1 to IV-1). Although the true mechanism
is not clear at present, this simple reaction path will provide
Figure 6. The hypothesis of springboard chemistry that highly reactive sub-
strates can act as a “reactive tractor” or “reactive springboard” to drive the
transformation of poorly reactive substrates.
promote the palladium-catalyzed hydrogenation of olefins and
reductive decarbonylation of acyl chlorides with hydrosilane, in
which the Z-to-E isomerization of substituted amide-derived
olefins initiated and enhanced the palladium catalyst to reach
a good level of catalytic performance in these reactions.[28] Al-
though the mechanism of isomerization-initiated transforma-
tion was not clear, the reaction results revealed that the in situ
generated hydride palladium complex was a key catalyst in the
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