Allylic functionalization of unactivated olefins with grignard reagents

Article abstract of DOI:10.1002/anie.201309134

New advances in the functionalization of unactivated olefins with carbon nucleophiles have provided more efficient and practical approaches to convert inexpensive starting materials into valuable products. Recent examples have been reported with stabilized carbon nucleophiles, tethered carbon nucleophiles, diazoesters, and trifluoromethane donors. A general method for functionalizing olefins with aromatic, aliphatic, and vinyl Grignard reagents was developed. In a one-pot process, olefins are oxidized by a commercially available reagent to allylic electrophiles, which undergo selective copper-catalyzed allylic alkylation with Grignard reagents. Products are formed in high yield and with high regioselectivity. This was utilized to synthesize a series of skipped dienes, a class of compounds that are prevalent in natural products and are difficult to synthesize by known allylic alkylation methods. It all begins with olefins: Allylic functionalization with carbon nucleophiles is a powerful strategy for converting unactivated olefins into complex products. A general method for functionalizing olefins with aromatic, aliphatic, and vinyl Grignard reagents was developed. In a one-pot process, olefins are oxidized by a commercially available reagent to allylic electrophiles, which undergo selective copper-catalyzed allylic alkylation with Grignard reagents. Copyright

Full text of DOI:10.1002/anie.201309134

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Angewandte
Communications
DOI: 10.1002/anie.201309134
C–C Coupling
Allylic Functionalization of Unactivated Olefins with Grignard
Reagents**
Hongli Bao, Liela Bayeh, and Uttam K. Tambar*
Abstract: New advances in the functionalization of unacti-
vated olefins with carbon nucleophiles have provided more
efficient and practical approaches to convert inexpensive
starting materials into valuable products. Recent examples
have been reported with stabilized carbon nucleophiles,
tethered carbon nucleophiles, diazoesters, and trifluorometh-
ane donors. A general method for functionalizing olefins with
aromatic, aliphatic, and vinyl Grignard reagents was devel-
oped. In a one-pot process, olefins are oxidized by a commer-
cially available reagent to allylic electrophiles, which undergo
selective copper-catalyzed allylic alkylation with Grignard
reagents. Products are formed in high yield and with high
regioselectivity. This was utilized to synthesize a series of
skipped dienes, a class of compounds that are prevalent in
natural products and are difficult to synthesize by known
allylic alkylation methods.
Scheme 1. Allylic functionalization of unactivated olefins.
EWG=electron-withdrawing group.
selectivity. In contrast to most Heck reaction protocols, our
strategy exhibits broader scope in the coupling partner and
does not require activated olefins for high levels of reactivity
and selectivity in the formation of a single olefin isomer.[9]
Allylic functionalization with carbon nucleophiles usually
requires a stoichiometric oxidant to restore the starting
oxidation state of the metal in the catalytic cycle. We
envisioned a conceptually distinct approach, in which the
stoichiometric oxidant could oxidize the unactivated olefin in
the absence of a catalyst to a reactive intermediate, which
would be susceptible to metal-catalyzed allylic substitution
with Grignard reagents in the same reaction flask (Scheme 2).
T
he functionalization of unactivated olefins with carbon
nucleophiles is an attractive strategy for synthesizing value-
added small molecules from inexpensive and abundant
components of petrochemical feedstock (Scheme 1).[1,2] It is
quickly becoming an alternative approach to the Heck
reaction for the conversion of simple terminal olefins into
more complex products.[3] Elegant examples of allylic alkyl-
ations have been reported with stabilized carbon nucleo-
philes,[4] tethered carbon nucleophiles,[5] diazoesters,[6] and
trifluoromethane donors.[7] To complement these studies, we
were interested in functionalizing unactivated olefins with
Grignard reagents, which are a readily accessible and
affordable class of diverse carbon nucleophiles.
Herein, we describe a general method for functionalizing
unactivated olefins with aromatic, aliphatic, and vinyl
Grignard reagents.[8] This one-pot process is performed by
the sequential addition of a stoichiometric oxidant, copper
catalyst, and a Grignard reagent to unactivated terminal
olefins, which are converted into internal olefins with high E
Scheme 2. General approach to the allylic functionalization of unacti-
vated olefins with Grignard reagents. Bs=benzenesulfonyl.
[*] Dr. H. Bao, L. Bayeh, Prof. U. K. Tambar
Department of Biochemistry, The University of Texas
Southwestern Medical Center at Dallas
5323 Harry Hines Boulevard, Dallas, TX 75390-9038 (USA)
E-mail: Uttam.Tambar@utsouthwestern.edu
We were drawn to the metal-free allylic oxidation of
unactivated olefins 1 with the commercially available sulfur-
diimide reagent 2. This chemistry was initially explored for
the allylic amination of unactivated olefins through the [2,3]-
rearrangement of allylic sulfinamides (3).[10,11] Motivated by
early examples of metal-catalyzed couplings of allylic sulfides
and sulfoxides with Grignard reagents,[12] we were interested
in exploiting sulfinamides 3 as a new class of electrophiles for
metal-catalyzed allylic substitution. Herein, we report the
broad substrate scope and unique product selectivity exhib-
ited by this novel strategy for allylic alkylation.
Homepage: http://www4.utsouthwestern.edu/tambarlab/
[**] Financial support was provided by the W. W. Caruth, Jr. Endowed
Scholarship, the Robert A. Welch Foundation (Grant I-1748), the
National Institutes of Health (1R01GM102604-01), the Chilton
Foundation Fellowship (H.B.), the University of Texas System
Chemistry and Biology Training Grant (L.B.), and the Sloan
Research Fellowship (U.K.T.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309134.
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Our initial challenge in functionalizing olefins with
Grignard reagents via sulfinamide 3 was the suppression of
the facile [2,3]-rearrangement to allylic amines such as 5 in
favor of the desired coupling with Grignard reagent 6
(Table 1). Sulfinamide 3a was generated upon mixing olefin
(entry 9). In DME, sulfurdiimide 2 and sulfinamide 3a were
completely soluble, which led to significant [2,3]-rearrange-
ment to allylic amine 5 before the reaction mixture was
treated with the copper and Grignard reagent (entry 10). A
1:1 mixture of DME/Et2O produced the ideal environment
for the formation and stability of sulfinamide 3a, which was
smoothly converted in situ into internal olefin 4a (entry 11).
Under these conditions, the desired product was isolated in
72% yield with high E-olefin selectivity (15:1) and complete
regioselectivity.
Table 1: Optimization of allylic functionalization with Grignard reagents.
With optimal reaction conditions in hand for the func-
tionalization of unactivated olefins with Grignard reagents,
we explored the substrate scope of this process (Scheme 3). A
diverse range of aliphatic Grignard reagents could be utilized
in this transformation without affecting the overall efficiency
of the process (Scheme 3a, 4a–f), including Grignard
reagents with trimethylsilyl groups (4e) and remote stereo-
centers (4 f). A hindered tert-butyl Grignard reagent fur-
nished the neopentyl internal olefin 4d. When aryl Grignard
reagents were employed in the reaction, the in situ conversion
of sulfinamide 3a resulted in considerably lower yields,
presumably because of side reactions between the highly
reactive aryl Grignard reagents and byproducts from the
oxidation reaction. Optimal yields were obtained for the
coupling of these aryl Grignard reagents after isolating the
sulfinamide (3a) and adding catalytic amounts of the radical
scavenger TEMPO in the second copper-catalyzed step (4g–
i). Notably, products 4g–i were formed with high E-allylic
selectivity, a result distinct from both the mixture of E-allylic
and E-styrenyl products typically obtained through indis-
criminate b-hydride eliminations under traditional Heck
conditions,[3] and the high E-styrenyl selectivity observed
under the modified Heck conditions recently reported by the
Sigman group.[14] Regardless of the Grignard reagent utilized
in the reaction, we always isolated E-olefins as the major
products (> 20:1 in most cases).
Entry
MLn
Solvent
Yield [%]
[a]
1
2
3
4
5
6
7
8
[Pd2(dba)3]·CHCl3
[{Pd(C3H5)Cl}2][a]
[{Ir(cod)Cl}2][a]
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
<5
<5
<5
<5
<5
<5
22
6
25
53
72
[a]
[Ir(cod)2]BF4
[{Rh(cod)Cl}2][a]
[(C7H8)Mo(CO)3][a,b]
CuTc
CuCl
9
10
11
CuBr·SMe2
CuBr·SMe2
CuBr·SMe2
Et2O
DME
DME/Et2O
(1:1)
Reaction conditions. Step 1: Olefin 1a (1 equiv), sulfur diimide 2
(1.2 equiv), solvent (0.3m). Step 2: DME (0.2m), MLn (5 mol%),
Grignard reagent 6 (4 equiv). [a] Metal complex was pretreated with
10 mol% phosphine ligands, pyridine ligands, phosphoramidite ligands,
or no ligand. [b] C7H8Mo(CO)3 =cycloheptatriene molybdenum tricar-
bonyl, DME=1,2-dimethoxyethane, cod=cyclo-1,5-octadiene, dba=di-
benzylidenacetone.
1a and sulfurdiimide reagent 2 for 12 h at 48C in various
solvents. This oxidized intermediate was subsequently cooled
to À208C and treated with several low-valent metals that
typically generate electrophilic metal–allyl intermediates 7
from allylic acetates, carbonates, and halides. Palladium,
iridium, rhodium, and molybdenum cata-
We also examined an array of terminal olefins as
substrates for coupling with iso-butyl Grignard reagent
(Scheme 3b). Unsaturated hydrocarbons furnished internal
E-olefins in good yields (4j–l). This reaction was tolerant of
several functional groups in the olefin substrate, including
alkyl chlorides (4m), silyl ethers (4n), carbonates (4o), and
esters (4p). When terminal 1,1-disubstituted exocyclic olefins
were employed, we obtained trisubstituted olefins (4q,r).
To highlight the synthetic potential of the functionaliza-
tion of olefins with Grignard reagents, we synthesized a series
of skipped dienes (Scheme 4). Although methods have been
developed for the assembly of specific classes of skipped
dienes,[15] the allylic alkylation of unactivated olefins is not
considered a general strategy for accessing these structures.
This may be due to the difficulty in controlling the reactivity
of multiple olefins in the skipped diene product towards
further undesired allylic alkylations. We recognized that our
strategy for functionalizing olefins is uniquely suited to
synthesizing skipped dienes because we can control the
reactivity of a single olefin in the presence of other
unsaturated bonds through a selective oxidation reaction
with sulfurdiimide 2. We coupled terminal olefin 1a with
unsaturated Grignard reagents 8 and 10 to furnish skipped
lysts did not yield the internal olefin 4a in
either the presence or absence of various
classes of ligands (Table 1, entries 1–6).
Gratifyingly, copper(I) salts emerged as
promising catalysts for this process
(entries 7–11).[13] In the presence of copper(I) thiophene-2-
carboxylate (CuTc), internal olefin 4a was isolated in 22%
yield (entry 7). The source of copper impacted the yield of the
isolated desired product (entries 7–9). When CuTc was
replaced with CuBr·SMe2, olefin 4a was produced in slightly
greater yield (entry 9). Given the propensity for sulfinamide
3a to undergo a facile [2,3]-rearrangement to allylic amine 5,
we hypothesized that the proper selection of solvent for the
oxidation step was essential for the efficiency of the overall
process. In Et2O, sulfurdiimide 2 and sulfinamide 3a were
largely insoluble, resulting in an inefficient oxidation reaction
and subsequent allylic substitution with Grignard reagent 6
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Scheme 4. Synthesis of skipped dienes. [a] Yield calculated relative to
sulfurdiimide 2 as the limiting reagent.
Scheme 3. Substrate scope. Reaction conditions: 1) Olefin 1 (1 equiv),
sulfurdiimide 2 (1.2 equiv), DME/Et2O 1:1 (0.3m). 2) DME (0.2m),
CuBr·SMe2 (5 mol%), Grignard reagent (4 equiv). [a] Intermediary
adduct was filtered and isolated before treating with copper catalyst
and Grignard reagent. In addition, 8 mol% TEMPO was utilized in the
second step. TBS=tert-butyldimethylsilyl, TEMPO= 2, 2, 6, 6-tetrame-
thylpiperidine-N-oxyl.
lyzed functionalization of allylic sulfinamides 3 with Grignard
reagents. For example, while most copper-catalyzed Grignard
additions to allylic electrophiles favor the SN2’ pathway to
yield branched products,[16] the allylic alkylation of sulfina-
mides 3 favored the SN2 product under all the conditions we
examined. In addition, the use of excess Grignard reagent was
necessary to obtain a high yield of product (Scheme 5a). Most
notably, in the presence of 2 equivalents of Grignard reagent
6, we did not observe product formation, a result that suggests
that Grignard reagents play a multifunctional role in the
overall allylic substitution process. For example, one equiv-
alent of Grignard reagent may initially react with the allylic
sulfinamide (3) prior to allylic substitution.[12] Another
equivalent of Grignard reagent may also react with the
sulfonamide-containing byproduct produced after displace-
ment with the first equivalent of Grignard reagent.[17]
dienes 9 and 11, respectively (Scheme 4a). We also converted
dienes 12a–d into monoalkylation products 14a–d by selec-
tively controlling the monooxidation reaction of one of the
olefins in the substrates with one equivalent of sulfurdiimide 2
(Scheme 4b). Finally, we controlled the mono- and dialkyla-
tion of olefin 12c with iso-butyl Grignard reagent 15 by
altering the relative equivalents of the two substrates, which
resulted in the selective synthesis of skipped dienes 16 and 17
(Scheme 4c).
Given the distinct reactivity and product selectivity
exhibited by this novel class of electrophiles, we have
commenced a mechanistic examination of the copper-cata-
To probe the presence of free-radical intermediates in the
copper-catalyzed step, we isolated sulfinamide 3a and sub-
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Grignard reagents and allylic sulfinamide 3a.
jected this intermediate to the copper-catalyzed allylic sub-
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sulfinamide 3a was coupled with phenylmagnesium bromide,
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equivalents of TEMPO, with no trace of adduct 18. This
observation suggests that the productive pathway may not
proceed via free-radical intermediates (Scheme 5b).[18]
In conclusion, we have developed a one-pot functional-
ization of unactivated olefins with Grignard reagents. We
demonstrated the conversion of terminal olefins into sulfin-
amide intermediates that are susceptible to highly selective
nucleophilic substitutions with Grignard reagents. Overall,
the process exhibited a broad substrate scope for the
unactivated olefins as well as the Grignard reagents. The
internal olefin products were formed in high yield as
predominantly single-olefin regioisomers and stereoisomers.
Allylic sulfinamides 3 were unique in their ability to generate
predominantly SN2 products under all the conditions exam-
ined so far, presumably via dialkyl p-allylcopper(III) com-
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activation of olefins in polyunsaturated structures to synthe-
size a variety of skipped dienes, which are prevalent in natural
products and are difficult to synthesize by the known methods
for allylic alkylation. We are currently expanding this strategy
to include allylic functionalization with other nucleophiles to
develop a diverse array of chemical transformations.
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Received: October 18, 2013
Revised: November 19, 2013
Keywords: alkenes · allylic compounds · copper ·
.
Grignard reaction · homogeneous catalysis
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