Co-catalyzed cross-coupling of alkyl halides with tertiary alkyl Grignard reagents using a 1,3-butadiene additive

Article abstract of DOI:10.1021/ja404285b

The cobalt-catalyzed cross-coupling of alkyl (pseudo)halides with alkyl Grignard reagents in the presence of 1,3-butadiene as a ligand precursor and LiI is described. Sterically congested quaternary carbon centers could be constructed by using tertiary alkyl Grignard reagents. This reaction proceeds via an ionic mechanism with inversion of stereochemistry at the reacting site of the alkyl halide and is compatible with various functional groups. The use of both 1,3-butadiene and LiI was essential for achieving high yields and high selectivities.

Full text of DOI:10.1021/ja404285b

Communication
pubs.acs.org/JACS
Co-Catalyzed Cross-Coupling of Alkyl Halides with Tertiary Alkyl
Grignard Reagents Using a 1,3-Butadiene Additive
Takanori Iwasaki,† Hiroaki Takagawa,† Surya P. Singh,‡ Hitoshi Kuniyasu,† and Nobuaki Kambe*,†
†Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡Inorganic and Physical Chemistry Division, CSIR − Indian Institute of Chemical Technology, Tarnaka, Hyderabad, 500607, India
S
* Supporting Information
coupled with tertiary alkyl Grignard reagents to afford branched
alkanes including quaternary carbon centers.
ABSTRACT: The cobalt-catalyzed cross-coupling of alkyl
(pseudo)halides with alkyl Grignard reagents in the
presence of 1,3-butadiene as a ligand precursor and LiI
is described. Sterically congested quaternary carbon
centers could be constructed by using tertiary alkyl
Grignard reagents. This reaction proceeds via an ionic
mechanism with inversion of stereochemistry at the
reacting site of the alkyl halide and is compatible with
various functional groups. The use of both 1,3-butadiene
and LiI was essential for achieving high yields and high
selectivities.
When 2 mol % of CoCl2 was added to a THF solution of n-
OctBr, t-BuMgCl (1.2 equiv), 1,3-butadiene (2 equiv), and LiI
(4 mol %) at −78 °C, followed by stirring at 50 °C for 5 h, the
cross-coupling product 3a was obtained in 84% yield without
any detectable byproducts arising from the isomerization of the
t-Bu group (Table 1, entry 1).9 In contrast, the use of NiCl2 and
a
Table 1. Cross-Coupling of n-OctBr and t-BuMgCl
ransition-metal-catalyzed cross-coupling is one of the
Tmost fundamental routes to the construction of carbon
skeletons. In the past decade, various efficient catalytic systems
have been developed for cross-coupling at sp3-hybridized
carbon centers.1 Although cross-coupling at the tertiary alkyl
carbons is a promising route for constructing quaternary carbon
centers, only limited examples are available using congested
tertiary alkyl nucleophiles,2−4 and the reactions often result in
low yields due to steric hindrance, facile β-hydrogen
elimination, and ease of isomerization of tertiary alkylmetals.5
In pioneering work by Kumada et al., the cross-coupling of t-
BuMgCl with β-bromostyrene catalyzed by a nickel complex
bearing a dppf ligand was demonstrated to proceed efficiently,
although undesired isomerization of the tertiary alkyl group as
well as the hydrodehalogenation of β-bromostyrene occurred
concomitantly.2 After this discovery, several examples of the
cross-coupling of sp2-hybridized organo halides with tertiary
alkylmetal reagents using various transition metals were
reported,3 however isomerization of alkylmetal reagents as
well as β-hydrogen elimination remains a serious problem to be
controlled.3g,h In contrast to the progress achieved using sp2-
hybridized organic halides, to the best of our knowledge, the
catalytic cross-coupling of alkyl halides with tertiary alkylmetals
has been attained only by Cu catalysts employing tert-butyl
Grignard reagent and continues to be a challenging issue in
organic synthesis.4
b
b
b
entry
MCl2
T (°C)
additive
3a (%)
4 (%)
5 (%)
1
2
3
CoCl2
NiCl2
PdCl2
CuCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
50
50
50
50
rt
LiI
LiI
84
7
1
<1
11
2
1
1
LiI
<1
19
nd
nd
18
27
31
60
<1
92
3
c
4
LiI
<1
1
5
6
LiI
6
0
LiI
2
<1
<1
2
7
50
50
50
50
50
50
none
LiCl
LiBr
MgI2
LiI
4
8
7
9
10
3
5
10
2
d
11
16
<1
<1
1
e
12
LiI
a
Reaction conditions: n-OctBr (1 mmol), t-BuMgCl (in THF, 1.2
mmol), MCl2 (2 mol %), additive (4 mol %), 1,3-butadiene (2 equiv)
at 50 °C for 5 h. Determined by GC analysis. 58% of 6 was obtained.
In the absence of 1,3-butadiene. Isoprene was used instead of 1,3-
b
c
d
e
butadiene. 6:
PdCl2, which are efficient catalysts for the coupling reaction of
alkyl halides with primary and secondary alkyl Grignard
reagents,6c resulted in the formation of 3a in poor yields
(entries 2 and 3). Under the present conditions using CuCl2, 3a
was obtained only in 19% yield accompanied by a 58% yield of
2,2-dimethyl-4-tetradecene (6), which was formed by the
carbomagnesiation of t-BuMgCl to 1,3-butadiene and sub-
sequent cross-coupling.9,10 We next optimized the reaction
conditions using CoCl2 as a catalyst. When the reaction
temperature was decreased to room temperature or 0 °C, no
Previously we reported on the development of the Ni-, Pd-,
and Cu-catalyzed cross-coupling reaction of alkyl (pseudo)-
halides with alkyl Grignard reagents utilizing unsaturated
hydrocarbon additives.4d,e,6 Here we report a Co/LiI/1,3-
butadiene catalytic system as a new route to alkyl−alkyl cross-
coupling reactions,7,8 in which alkyl halides are successfully
Received: April 30, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja404285b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
reaction took place. Both LiI and 1,3-butadiene played
important roles in achieving efficient cross-coupling. For
example, the reduction of n-OctBr took place in the absence
of 1,3-butadiene (entry 11).11 When LiCl or LiBr was used
instead of LiI, the yield of the coupling product decreased to
27% and 31%, respectively (entries 8 and 9), though the
mechanistic roles of LiI in this catalytic system are not clear at
this time.12,13 The use of MgI2 also resulted in an improvement,
but only moderately, with 3a being produced in 60% yield
(entry 10). The addition of N or P based ligands failed to
improve the reaction and sometimes resulted in inhibition.11,13
We conducted further investigations on the Co-catalyzed cross-
coupling using various unsaturated hydrocarbon additives
including substituted dienes, styrene, and phenylpropyne13
and found isoprene to be the best additive (92%, entry 12).
Under optimized reaction conditions, we investigated the
scope and limitations of Grignard reagents (Table 2). The
Table 3. Co-Catalyzed Alkylation of Various Alkyl Halides
Table 2. Co-Catalyzed Cross-Coupling of n-OctBr with
Various Grignard Reagents
a
Reaction conditions: see Table 1, entry 12. Yields were determined
by GC. Isolation yields are in parentheses.
infra). The reaction of ethyl 6-bromohexanoate (7n) gave the
corresponding product in 78% yield without affecting the ethyl
ester group that could react with Grignard reagents (entry 8n).
Thiophene moiety also survived under the reaction conditions
(entry 8o). We then attempted the chemoselective coupling of
dihaloalkanes. An alkyl bromide having a chloro substituent
selectively coupled with t-BuMgCl in 92% yield to afford the
corresponding alkyl chloride 8p. When dibromides were used
as the substrate, site selective coupling was observed at the
primary alkylbromo moiety to give the corresponding
secondary alkyl bromides in 74% and 86% yields as the sole
coupling products (entries 8q,r). The reaction occurred
exclusively at the sp3C−Br bond even in the presence of the
sp2C−Br bond (entries 8s,t).
To estimate the relative reactivity of alkyl halides, we
performed competition reactions using an alkyl fluoride,
bromide, and tosylate and found that n-HepOTs reacted faster
with s-BuMgCl than n-NonBr and n-OctF. From the initial
reaction ratio of each substrate, the relative reactivity was
estimated to be OTs:Br:F = 1:0.3:<0.06.13 Unfortunately, a
similar experiment using an alkyl iodide and tosylate afforded
complicated results due to the OTs-I exchange reaction that
competed with the coupling. We thus estimated the relative
reactivity of iodide and tosylate to be I:OTs = >6:1 by
comparing the independent reactions of n-OctI and n-HepOTs.
Based on these results, the relative reactivities of (pseudo)-
halides were determined to be I > OTs > Br > F ≫ Cl.
Interestingly this is in large contrast to the Ni/1,3-butadiene6c
and Cu/alkyne4e catalytic systems reported by us, where, for
example, alkyl bromides reacted with primary alkyl Grignard
reagents ∼50 times faster than alkyl tosylates in the former
case.6h
a
b
Determined by GC. Isolation yields are in parentheses. n-DecBr
c
instead of n-OctBr. 0.1 mol % of CoCl2 was used.
reaction of n-OctBr with acyclic tertiary alkyl Grignard reagents
afforded the cross-coupling products in high yields. In all cases,
no isomerization was observed (entries 1−4). No reaction took
place when an adamantyl Grignard reagent was used (entry 5).
Acyclic and cyclic secondary alkyl Grignard reagents smoothly
coupled with n-OctBr in 89−93% yields (entries 6, 8, 9). When
the catalyst loading was reduced to 0.1 mol %, 3g was obtained
in 80% yield, where the TON reached 800 (entry 7). Primary
alkyl Grignard reagents were also applicable to the present
catalytic system, giving good yields of products (entries 10, 11),
however the use of MeMgCl and PhMgBr that contain no β-
hydrogens resulted in very poor results (entries 12 and 13, vide
infra). Substituents at the β-position of the Grignard reagent
had a slight effect on the yield (entry 11).
Results for the cross-coupling of various alkyl electrophiles
are shown in Table 3. Alkyl fluorides, which have the strongest
single bond (C−F) in organic compounds,14 coupled with t-
BuMgCl to give 3a in a moderate yield. Although the reaction
of n-OctCl was sluggish and only a 2% yield of coupling
product was obtained, octyl iodide and tosylate reacted with t-
BuMgCl to good yields. Alkyl bromides carrying an arene,
olefin, alkyne, trifluoromethyl, THP, or silyl ether unit coupled
with t-BuMgCl in good to excellent yields (entries 8a-f).
Amide, N-Boc and N-tosyl protecting groups were also well
tolerated (entries 8g−m), where a secondary alkyl Grignard
reagent gave slightly better results than a tertiary one (vide
It is well-known that alkyl radical intermediates generated by
a single electron transfer (SET) from transition metals to alkyl
halides play important roles in Co-catalyzed cross-coupling
reactions of alkyl halides.7,8 We thus examined the reaction
using 6-bromo-1-hexene (9)15 and confirmed that no cyclized
B
dx.doi.org/10.1021/ja404285b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
products 11 were formed in any cases with primary, secondary,
and tertiary alkyl Grignard reagents under optimized conditions
(eq 1). Furthermore, when bromide 12 was used,15 only 3%,
followed by β-hydrogen elimination with the concomitant
formation of a Co−H species, which is known to catalyze the
dimerization of 1,3-butadiene.17 The addition of s-BuMgCl to
the mixture completely suppressed the dimerization of 1,3-
butadiene leading to 17. Based on these results along with the
studies shown in SI,13 the catalytic pathway shown in Scheme 1
Scheme 1. A Plausible Mechanism
<1%, and 2% of the ring-opened products 14 were observed in
the reactions with octyl, 3-heptyl, and 2,5-dimethyl-2-hexyl
Grignard reagents, respectively (eq 2). These results indicate
that radicals are not produced from alkyl bromides as
intermediates in this catalytic reaction.
To gain insights into the stereochemistry of the C−C bond
forming step, we conducted the cross-coupling reaction using a
diastereomerically pure deuterated substrate threo-15 and t-
BuMgCl under optimized conditions and obtained the coupling
is proposed. The reduction of CoCl2 by the alkyl Grignard
reagent gives 0.5 equiv of alkane and alkene and a Co(I)
species, which then undergoes transmetalation and β-hydrogen
elimination to give a Co−H species. The cobalt hydride reacts
with 3 molecules of 1,3-butadiene to form complex A.17
Although complex A has catalytic activity for the dimerization
of 1,3-butadiene to give triene 17, it also readily reacts with
Grignard reagents (R′-MgX) to form an anionic complex B
under the catalytic conditions used. This anionic complex
would possess enhanced nucleophilicity and attacks alkyl
halides (R-X) to give product and complex A directly or
through complex C via an SN2 mechanism. It is noteworthy
that hitherto known Co-catalyzed cross-coupling involves alkyl
radical intermediates generated by facile SET from the Co
center to alkyl halides,7,8 however, this is not the case of the
present catalyst due to the specific features of the anionic
intermediate B.
The evidence that no reaction took place with Grignard
reagents having no β-hydrogen (Table 2, entries 12 and 13)
supports the intermediacy of Co−H species generated at the
initiation step, as shown in Scheme 1. Indeed, the MeMgCl
coupled with n-OctBr to give nonane in 56% yield accompanied
by 21% yield of dodecane when MeMgCl (1.2 mmol) was
added to the reaction mixture of n-OctBr (1.0 mmol) and n-
BuMgCl (0.3 mmol) catalyzed by Co after 30 min.13
1
product 16 in 73% yield. H NMR analysis of the product
revealed that the reaction proceeded with inversion of
stereochemistry at the alkyl halide carbon (erythro:threo =
>95:5). This result clearly suggests that the present reaction
proceeds by an SN2 mechanism, which is consistent with the
evidence for a nonradical mechanism as shown in eqs 1 and 2.
The present reaction worked well with all primary,
secondary, and tertiary alkyl Grignard reagents. We thus
estimated the relative reactivity of these Grignard reagents (eq
4). A competition experiment using an equimolar amount of n-
In conclusion, we report on the development of a Co-
catalyzed cross-coupling reaction of alkyl halides and tertiary
alkyl Grignard reagents that is promoted by 2 mol % of CoCl2
in the presence of 4 mol % LiI and isoprene. This is the first
example of the construction of quaternary carbon centers by
cross-coupling between sp3-carbon centers, except for the case
of Cu catalysts. The present catalytic system also catalyzed the
coupling of secondary and primary alkyl Grignard reagents and
tolerates various functionalities well. The reaction is efficient
and can be run using only 0.1 mol % of CoCl2. In the present
reaction, C−C bond formation proceeded with stereoinversion
via an ionic mechanism without involving the formation of
radical intermediates. Relative reactivities of primary, secondary,
and tertiary alkyl Grignard reagents as well as alkyl halides were
roughly estimated as n-:s-:t-BuMgCl = 1:4:0.6 and I:OTs:Br:F
= >6:1:0.3:<0.06, respectively.
and s-BuMgCl under optimized conditions gave the corre-
sponding coupling products in 14% and 62% yield, respectively.
A similar experiment using a 1:1 mixture of s- and t-BuMgCl
afforded the coupling products in 70% and 10% yields,
respectively. From these experiments, the relative reactivity of
alkyl Grignard reagents was roughly estimated to be n-:s-:t-
BuMgCl = 1:4:0.6.16
To shed some light on the nature of the active catalytic
species, control reactions were conducted. When CoCl2 (0.1
mmol) was treated with n-OctMgCl (0.4 mmol) in the
presence of LiI (0.2 mmol) and 1,3-butadiene (10 mmol) at 50
°C for 1 h, ∼1.5 equiv of octene and 10.8 equiv of 5-methyl-
1,3,6-heptatriene (17), based on the Co used, were obtained.13
The formation of octene can be attributed to the reaction of
CoCl2 with the octyl Grignard reagent via transmetalation
C
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Journal of the American Chemical Society
Communication
(8) Co-catalyzed C−C bond formation between sp3 carbon centers:
(a) Cahiez, G.; Chaboche, C.; Duplais, C.; Giulliani, A.; Moyeux, A.
Adv. Synth. Catal. 2008, 350, 1484. (b) Zhou, W.; Napoline, J. W.;
Thomas, C. M. Eur. J. Inorg. Chem. 2011, 2029. That of allylic and
benzylic nucleophiles, see: (c) Tsuji, T.; Yorimitsu, H.; Oshima, K.
Angew. Chem., Int. Ed. 2002, 41, 4137. (d) Ohmiya, H.; Tsuji, T.;
Yorimitsu, H.; Oshima, K. Chem.Eur. J. 2004, 10, 5640. (e) Someya,
H.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. Org. Lett. 2007, 9, 1565.
(f) Someya, H.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. Tetrahedron
2007, 63, 8609.
(9) To prevent contamination of the reaction mixture by trace
amounts of transition metals, highly pure CoCl2 (99.999%, Aldrich)
and LiI (99.999%, Aldrich) were used for all reported reactions herein.
When reagent grade Co (99.9%) was employed, the formation of 6
predominated probably due to the contaminated Cu.
ASSOCIATED CONTENT
* Supporting Information
Additional experimental results and procedures and character-
ization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
kambe@chem.eng.osaka-u.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(10) Todo, H.; Terao, J.; Watanabe, H.; Kuniyasu, H.; Kambe, N.
Chem. Commun. 2008, 1332.
(11) It is reported that CoCl2/2LiI/4TMEDA system was effective
for alkyl-alkyl coupling; however, the coupling of n-DecBr with s- and
t-BuMgBr resulted in 20% and 0% yields, respectively, and gave decane
and decene as major products; see ref 8a.
We thank the Ministry of Education, Culture, Sports, Science,
and Technology-Japan for financial support, a Grant-in Aid for
Scientific Research and a Grant-in Aid for Young Scientists. T.I.
thanks the NOVARTIS Foundation (Japan), General Sekiyu
Foundation, and the Itoh Chubee Foundation.
(12) Ate complexes play important roles in transition-metal-catalyzed
organic synthesis. CuCl2·2LiCl: (a) Tamura, M.; Kochi, J. Synthesis
1971, 303. Also see, ref 10. MnCl2·2LiCl: (b) Cahiez, G.; Alami, M.
Tetrahedron 1989, 45, 4163. (c) Cahiez, G.; Laboue, B. Tetrahedron
Lett. 1992, 33, 4439 . CoCl2·2LiI: see ref 8a.
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