Catalytic Hydroalkylation of Allenes

Article abstract of DOI:10.1002/anie.201709144

We have developed a catalytic method for the hydroalkylation of allenes using alkyl triflates as electrophiles and silane as a hydride source. The reaction has an excellent substrate scope and is compatible with a wide range of functional groups, including esters, aryl halides, aryl boronic esters, sulfonamides, alkyl tosylates, and alkyl bromides. We found evidence for a reaction mechanism that involves unusual dinuclear copper ally complexes as catalytic intermediates. The unusual structure of these complexes provides a rationale for their unexpected reactivity.

Full text of DOI:10.1002/anie.201709144

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Accepted Article
Title: Catalytic Hydroalkylation of Allenes
Authors: Mitchell Lee, Mary Nguyen, Chance Brandt, Werner
Kaminsky, and Gojko Lalic
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709144
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10.1002/anie.201709144
Angewandte Chemie International Edition
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Catalytic Hydroalkylation of Allenes**
Mitchell Lee, Mary Nguyen, Chance Brandt, Werner Kaminsky and Gojko Lalic*
Abstract: We have developed a catalytic method for hydroalkylation
of allenes using alkyl triflates as electrophiles and silane as a
hydride source. The reaction has an excellent substrate scope and is
compatible with a wide range of functional groups, including esters,
aryl halides, aryl boronic esters, sulfonamides, alkyl tosylates, and
alkyl bromides. We found evidence for the reaction mechanism that
involves unusual dinuclear copper ally complexes as catalytic
intermediates. The unusual structure of these complexes provides
rationale for their unexpected reactivity.
coupling partners.[15] However, hydroalkylation reactions with
simple σ-electrophiles remain limited to intramolecular reactions
of specific classes of functionalized allenes,[16] with no general
methods for intermolecular hydroalkylation of simple allenes.
In
this
communication,
we
report
copper-catalyzed
hydroalkylation of allenes using alkyl triflates as electrophiles.
We describe the reaction development and the exploration of the
substrate scope. We propose a reaction mechanism and
describe the isolation, structure, and reactivity of a new catalytic
intermediate.
Using our previous work on hydroalkylation of alkynes as a
starting point,[12] we developed a method for hydroalkylation of
allenes shown in Scheme 1. The best results in the reaction
were obtained using alkyl triflates as electrophiles, SIPrCuOTf
as a catalyst, (Me2HSi)2O as a hydride source and CsF as a
turnover reagent. The hydroalkylation product was formed as a
single regioisomer.
Reductive cross-coupling reactions of unsaturated compounds
have been extensively studied over the last decade and have
been developed into a powerful tool for the formation of carbon-
carbon bonds. These reactions obviate the need for the
stoichiometric
intermediates and allow direct use of unsaturated compounds as
coupling partners.[1] As
result, reductive cross-coupling
preparation
of
reactive
organometallic
a
reactions often provide a more efficient alternative to the
traditional cross coupling of organometallic reagents and organic
electrophiles.
Most reductive cross-coupling reactions involve π-electrophiles,
such as aldehydes,[2] ketones,[3] imines,[4] and activated
alkenes.[5] π-Electrophiles can be generated in situ from suitable
precursors, as demonstrated by Krische et al. For example,
primary[6] and secondary[7] alcohols can be used as coupling
partners in reactions with a range of unsaturated compounds.
Reactions with σ-electrophiles are generally significantly more
challenging and less common. Notable early examples of such
reactions are the reductive cross-coupling of alkynes with
epoxides developed by Jamison et al,[8] and the reductive cross-
coupling of functionalized alkenes with alkyl zinc reagents
developed by Sigman et al.[9] More recently, several inter
molecular hydroalkylation reactions have been developed using
alkyl halides of sulfonate esters as coupling partners.[10] In 2015,
Kambe et al. reported copper-catalyzed hydroalkylation of 1,3-
dienes.[11] The same year, our group reported E-selective
hydroalkylation of terminal alkynes,[12] while the Z-selective
hydroalkylation of aryl alkynes was reported by Hu et al.[13] In
2016, Fu et al reported Markovnikov hydroalkylation of terminal
alkynes using a nickel catalyst.[14] Despite these developments
in hydroalkylation of alkynes and certain alkenes,
hydroalkylations of allenes are still relatively rare.
Scheme 1. Copper-catalysed hydroalkylation of allenes
In the process of the reaction development we made several
observations summarized in Table 1. The changes in the
stoichiometry of the substrates are well-tolerated, and a high
yield (91%) of the desired product can be obtained even when
neither coupling partner is used in excess. IPrCuF can also be
used as a catalyst with a relatively small decrease in yield.
The use of (Me2HSi)2O is essential for the success of the
reaction, and even closely related silanes provide lower yields of
the hydroalkylation product. High yields are obtained when fewer
equivalents of silane or CsF are used in the reaction, while less
than 5% of the product is obtained with KF as a turnover reagent.
The reaction can be performed at a high concentration of alkyl
triflate without decrease in yield. Finally, in addition to 1,4-
dioxane, the reaction can also be performed in Et2O, while the
use of other common organic solvents leads to lower yields.
Table 1. Reaction development.
The most general method for reductive cross-coupling of allenes
is a recently reported hydroallylation with allylic chlorides as
Entry
Change from best conditions
Yield[a]
[*]
M. Lee, M. Nguyen, C. Brandt, W. Kaminsky, G. Lalic
Department of Chemistry, University of Washington, Seattle, WA
98195 (USA)
E-mail: lalic@chem.washington.edu
We thank NSF (CAREER award no 1254636) and University of
Washington for financial support.
1
2
3
4
5
6
7
none
99%
99%
91%
81%
91%
66%
91%
1.0 equiv of allene and 1.4 equiv of ROTf
1.0 equiv of allene
[**]
IPrCuF instead of SIPrCuF
(EtO)3SiH instead of (Me2HSi)2O
PMHS instead of (Me2HSi)2O
2.0 equiv of (Me2HSi)2O
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201709144
Angewandte Chemie International Edition
COMMUNICATION
reactions,[18] which require alkyl organometallic reagents as
coupling partners. The reversed polarity of the hydroalkylation
reaction allows the alkylation with substrates that carry highly
electrophilic functional groups, such as alkyl bromides (8) and
sulfonate esters (11). To provide a better understanding of the
underlining chemistry and enable further development of the
hydroalkylation reaction, we explored the mechanism of the
reaction.
8
2.0 equiv of CsF
98%
1%
9
KF instead of CsF
10
11
12
13
14
0.5 M ROTf vs 0.1 M ROTf
99%
96%
46%
77%
66%
Et2O
THF
CH2Cl2
toluene
[a] GC yields are reported. All reactions performed on 0.1 mmol scale.
There are two plausible mechanisms of the hydroalkylation
reaction (Scheme 2). The mechanism shown in Scheme 2a is
based on the generally accepted mechanism for most copper-
catalyzed hydrofunctionalization reactions reported so far.[19]
According to this proposal, copper hydride formation is followed
by hydrocupration of the allene and alkylation of the allyl copper
intermediate. An alternative mechanism featuring dinuclear
cationic complexes [(NHCCu)2(µ-X)]OTf (X=F, H, allyl) (22, 23,
24) is based on the mechanism of a related hydroalkylation of
Hydroalkylation can be accomplished with a range of allenes
and alkyl triflates. In all cases, only one regioisomer of the
product is obtained. The reaction can be performed in the
presence of a variety of functional groups, including esters (7),
silyl ethers (9), aryl iodides (4), aryl bromides (13), aryl boronic
esters (6), alkyl bromides (8), tosylates (11), and sulfonamides
(10, 12). 1,1-Disubstituted allenes provide compounds
containing a quaternary carbon center (16). With silyl-substituted
allenes, the alkenyl silane product is obtained in high yield, as a
single diastereoisomer (17).[17] Finally, we found that on the
large preparative scale (4 mmol) the hydroalkylation product is
obtained in good yield (see 3, Table 2).
alkynes recently reported by our group.[12b] [20]
The two plausible
,
mechanisms shown in Scheme 2 are closely related, and the
major difference is that all mononuclear copper complexes that
are intermediates in the first proposal are replaced by cationic
dinuclear intermediates in the second proposal.
Table 2. Substrate scope[a]
Scheme 2. Plausible mechanisms
One of the key differences between the two mechanisms is the
difference in the reactivity of the copper hydride responsible for
the hydrocupration of the allene. We have previously established
that dinuclear [(NHCCu)2(µ-H)]OTf (23) does not react with alkyl
triflates, while the NHCCuH reacts quickly to give alkanes.[12b] As
a result, an important implication of the first proposal is that
hydrocupration of allenes needs to be faster than the reduction
of alkyl triflates by SIPrCuH (20). We performed a stoichiometric
competition experiment shown in Scheme 3a, and observed the
formation of the hydroalkylation product (3) in 84% yield and the
reduction of the alkyl triflate in just 6% yield. This result confirms
that the hydroalkylation of allenes is faster than the reduction of
Overall, the exploration of the substrate scope demonstrates
that the new hydroalkylation reaction provides access to a wide
range of substituted terminal alkenes and vinyl silanes, and is
compatible with
a
variety of functional groups. The
hydroalkylation complementary to SN2’ allylic substitution
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COMMUNICATION
alkyl triflates and confirms the plausibility of the mechanism
shown in Scheme 2a.
alkenyl)]OTf complex does not react with alkyl triflates under a
variety of conditions.[12b]
Scheme 3. Stoichiometric experiments (mechanism a)
Next, we explored the formation of the allyl copper intermediate
and its reaction with alkyl triflates. We isolated the SIPrCu(allyl)
Scheme 4. Stoichiometric experiments (mechanism b)
complex 25 from
a stoichiometric hydrocupration reaction
(Scheme 3b). It is interesting to note the Z configuration of the
alkene in the X-ray crystal structure of the copper allyl complex
shown in Figure 1. In a stoichiometric reaction, SIPrCu(allyl)
complex 25 reacts with alkyl triflate to produce the desired
product in 94% yield (Scheme 3c). The SE2' regioselectivity of
the alkylation is surprising in light of the SE2 reactivity of
allylcopper complexes with allylic chlorides previously reported.
The results of experiments shown in Scheme
3 and 4
demonstrate the feasibility of key elementary steps of both
catalytic cycles shown in Scheme 2. However, we favor
mechanism b shown in Scheme 2b. In previous work, we have
shown that in related reduction of alkyl triflates and
hydroalkylation of alkynes the catalyst turnover (step IVa in
Scheme 2a) is rate limiting, most likely because of the slow
phase transfer of the fluoride. One consequence of the slow
entry of fluoride into the catalytic cycle is a relatively high
concentration of NHCCuOTf, which forces the formation of the
dinuclear intermediates. Considering that the elementary steps
involved in mechanism a (hydrocupration and alkylation of the
organocopper intermediate) are faster with allenes than with
alkynes, the formation of SIPrCuF is most likely still rate limiting
and would force the shift towards mechanism b.
The mechanism shown in Scheme 2b features dinuclear copper
allyl complexes (24) as key intermediates. These complexes are
related to the cationic dinuclear alkenyl complexes previously
described by Sadighi and later by us. However, they have never
been reported or characterized before. Attempts to obtain
crystals of 26 were not successful, and we focused on
crystallization of a closely related complex that contains a para-
bromobenzene moiety (27, Fig. 2a). After considerable effort, we
obtained an X-ray crystal structure of 27, which shows surprising
structural features distinct from those found in dinuclear alkenyl
copper complexes.[21]
Figure 1. ORTEP of mononuclear copper allyl complex closely related to 25
with thermal ellipsoids at the 50% probability level.
In the stoichiometric alkylation of 25, we noticed that while the
first 50% of the product is formed within the first 6 min, it takes 4
h to reach 87% yield. The significant decrease in the rate of the
reaction is consistent with the idea that the SIPrCu(allyl)
complex is sequestered by SIPrCuOTf formed as a byproduct of
the alkylation reaction. Such formation of the dinuclear
[(SIPrCu)2(µ-allyl)]OTf (26) complex from SIPrCuOTf with
SIPrCu(allyl) (25) can be replicated in a stoichiometric reaction
(Scheme 4a). The same complex can also be prepared by a
reaction of dinuclear [(SIPrCu)2(µ-H)]OTf with allene 2 (Scheme
4b). Access to the dinuclear allyl complex allowed us to explore
its reactivity toward alkyl triflates. The stoichiometric reaction of
26 with alkyl triflate 1 yields the expected alkylation product at a
significantly lower rate than the reaction of the mononuclear
complex under the same reaction conditions. The successful
alkylation of the dinuclear [(SIPrCu)2(µ-allyl)]OTf complex is
surprising considering that the analogous [(NHCCu)2(µ-
The crystal structure of 27 (Fig. 2a) contains two configurations
(Fig. 2b and 2c), with the Z double bond preserved in both. It is
1
interesting to note that the H NMR of 26 and 27 features broad
signals consistent with
a dynamic structure. The minor
configuration in the X-ray structure contains symmetrically
arranged metal centers bonded to a π-allyl fragment from the
opposite faces (Fig. 2b). This structural arrangement is very
unusual for late transition metal complexes, with no other
examples of ally fragments sandwiched between two metal
centers available in The Cambridge Crystallographic Data
Centre.
In the major configuration (Fig. 2c), the two metal centers distort
from the symmetrical arrangement towards the opposite ends of
the allyl fragment. In the resulting arrangement of copper atoms,
one is σ-bonded to the allyl fragment, while the other is π-
bonded to the alkene. This bonding arrangement contrasts with
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dinuclear alkenyl copper complexes, in which metal centers do
not interact with the alkene and instead form a three-center-two-
electron bond with the alkenyl carbon. This structural difference
is consistent with the difference in reactivity of two types of
dinuclear complexes. The nonsymmetrical configuration of the
dinuclear allyl copper complex with the σ-bonded allyl copper
fragment offers a plausible explanation for the much higher
reactivity of these complexes towards alkyl triflates relative to
the reactivity of analogous alkenyl complexes.
The results of our studies of the hydroalkylation of allenes and
alkynes point to several important differences in the reactivity of
these classes of compounds. First, while the hydrocupration of
allenes by SIPrCuH is faster than the reduction of alkyl triflates,
the opposite is true for the reaction of alkynes. This is in line with
the generally higher reactivity of allenes. The second difference
is the much higher reactivity of dinuclear [(SIPrCu)2(µ-allyl)]OTf
complexes, which react with alkyl triflates. The analogous
[(NHCCu)2(µ-alkenyl)]OTf complexes do not react with alkyl
triflates under a wide range of reaction conditions. These
differences in the reactivity of allenes and alkynes point to
opportunities for taking different approaches to the development
of reactions for their hydrofunctionalization.
a)
b)
Acknowledgements
Financial support by NSF is greatfully aknowldeged (CAREER
Award #1254636). Karl Haelsig is anknoweledged for providing
preliminary results for this project.
Keywords: allene 1 • copper 2 • catalysis 3 • alkylation 4 •
cross-coupling 5
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level. c) ORTEP of the major configuration with thermal ellipsoids at 40%
probability level.
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[17] Several other classes of disubstituted allenes provided either low
diastereoselectivity or low yield of desired hydroalkylation products. For
details, please see the Supporting Inofrmation
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[21] For details of the synthesis and crystalization of 27, please see
Supporting Information. The details of the interpretration of the X-ray
difraction data are also discussed in the Supporting Information.
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Mitchell Lee, Mary Nguyen, Chance
Brandt, Werner Kaminsky, Gojko Lalic*
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Catalytic Hydroalkylation of Allenes
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