Ni-Catalyzed Carboxylation of Unactivated Alkyl Chlorides with CO2

Article abstract of DOI:10.1021/jacs.6b04088

A catalytic carboxylation of unactivated primary, secondary, and tertiary alkyl chlorides with CO₂ at atmospheric pressure is described. This protocol represents the first intermolecular cross-electrophile coupling of unactivated alkyl chlorides, thus leading to new knowledge in the cross-coupling arena.

Full text of DOI:10.1021/jacs.6b04088

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Communication
Ni-catalyzed Carboxylation of Unactivated Alkyl Chlorides with CO2
Marino Börjesson, Toni Moragas, and Ruben Martin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04088 • Publication Date (Web): 07 Jun 2016
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Ni-catalyzed Carboxylation of Unactivated Alkyl Chlorides with CO2
Marino Börjesson,† Toni Moragas† and Ruben Martin†§*
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†Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Cataꢀ
lans 16, 43007 Tarragona, Spain
§ Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23, 08010, Barcelona, Spain
Supporting Information Placeholder
In recent years, carbon dioxide (CO2) has gained considꢀ
erable momentum as C1 synthon in catalytic endeavors,
holding promise to create new paradigms in synthetic
sequences.7 Among these, the ability to prepare carboxꢀ
ylic acids from CO2 is particularly appealing due to the
ubiquity of these recurrent motifs in a myriad of biologiꢀ
callyꢀrelevant molecules.8 Prompted by the work of
Osakada,9 we10 and others11 have described catalytic
carboxylation techniques of organic (pseudo)halides.
Despite the advances realized, (a) the carboxylation of
unactivated secondary or tertiary organic (pseuꢀ
do)halides still constitutes a daunting, yet unsolved,
challenge and (b) unactivated alkyl chlorides cannot be
employed as coupling partners, neither in carboxylation
events nor in intermolecular crossꢀelectrophile coupling
reactions.4 Herein, we describe the realization of all theꢀ
se challenges by designing a catalytic carboxylation that
allows for the coupling of unactivated primary, secondꢀ
ary and even tertiary alkyl chlorides (Scheme 1, bottom).
This protocol operates with an exquisite chemoselectiviꢀ
ty profile at atmospheric pressure of CO2 while obviatꢀ
ing the need for stoichiometric organometallic reagents.
ABSTRACT: A catalytic carboxylation of unactivated
primary, secondary & tertiary alkyl chlorides with CO2
at atmospheric pressure is described. This protocol repꢀ
resents the first intermolecular crossꢀelectrophile couꢀ
pling of unactivated alkyl chlorides, thus leading to new
knowledge in the crossꢀcoupling arena.
Catalytic crossꢀelectrophile coupling processes of organꢀ
ic (pseudo)halides have gained considerable momenꢀ
tum,1 representing straightforward alternatives to classiꢀ
cal nucleophilic/electrophilic regimes based on wellꢀ
defined, and in many instances, airꢀsensitive organomeꢀ
tallic reagents (Scheme 1, path b vs path a).2 While notoꢀ
rious difficult transformations have been developed, the
formidable high activation energy required for effecting
C(sp3)–Cl cleavage and the inherent proclivity of alkyl
metal species towards parasitic βꢀhydride elimination or
homodimerization pathways3 has contributed to the preꢀ
vailing perception that unactivated alkyl chlorides canꢀ
not be utilized in intermolecular crossꢀelectrophile
events.4,5 If successful, however, such a largely void
terrain would not only lead to new knowledge in retroꢀ
synthetic analysis, but also would set the stage for iteraꢀ
Table 1. Optimization of the Reaction Conditions.a
tive techniques with polyhalogenated backbones,6
a
highly rewarding scenario that would dramatically imꢀ
prove our everꢀexpanding synthetic portfolio.
Scheme 1. Unactivated Alkyl Chlorides in Cross-Coupling.
"Classical" nucleophile-
electrophile regimes
"Unconventional" cross-
electrophile regimes
-
+
δ
δ
M
+
δ
nucleophile
electrophile
X
catalyst
path a
catalyst
path b
H
H
H
M=MgX,ZnX,..
X = Cl << Br < I
no R-M required
unprecedented (X=Cl)
well-defined R-M
ample precedents
Rate oxidative
addition
This work: catalytic reductive carboxylation of unactivated alkyl-Cl
catalytic reversal
of polarity
R1 R2
+
R1 R2
R1 R2
CO2H
CO2
electrophile
–
δ
Cl
δ
Ni
Ni
I
II
H
H
H
1o,2o & 3o
alkyl-Cl
mild & user-friendly
iterative scenarios
chemoselective
latent
nucleophile
electrophile
a 1a (0.20 mmol), NiBr2·glyme (10 mol%), L4 (24 mol%),
Mn (0.60 mmol), TBAB (0.20 mmol) in DMF (0.16 M) at
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b
60 ºC under CO2 (1 atm). HPLC yields using anisole as
internal standard. c Isolated yield. d NiBr2·glyme (5 mol%).
a As Table 1, entry 1. b Isolated yields, average of two indeꢀ
pendent runs. c 1q (1.5 mmol). d NiBr2·diglyme (10 mol%),
L6 (24 mol%), LiCl (1 equiv), 90 ºC. e 50 ºC. f 70 ºC. g Usꢀ
ing Zn (3 equiv) and TBAB (2 equiv) in DMA at 80 ºC.
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8
Our study began by evaluating the reaction of UVꢀactive
1a with CO2 (1 atm). Traces of 2a, if any, were detected
under previously reported carboxylation events,10,11 reinꢀ
forcing the notion that the carboxylation of 1a would be
far from trivial. After some experimentation,12 a combiꢀ
nation of NiBr2·glyme, L4, TBAB, Mn in DMF at 60 ºC
provided the best results, delivering 2a in 85% yield.13
As expected, the nature of the ligand backbone exerted a
profound influence on the reaction outcome. Indeed,
rigid phenanthroline backbones possessing ortho-
substituents were particularly suited for our purposes,
minimizing βꢀhydride elimination or homodimerization
pathways (entries 4ꢀ6). As shown in entries 7 and 8, the
use of other precatalyts or solvents had a deleterious
effect.12 While one might argue that TBAB might be
triggering a Br/Cl exchange en route to alkylꢀNi(II)Br
species,14 the carboxylation with TBAC or LiCl suggests
otherwise (entries 9ꢀ10).15 Notably, 5ꢀ10% yield of 2a
was obtained with 1aꢀBr or 1aꢀI, resulting predominantꢀ
ly in βꢀhydride elimination or homodimerization. As
expected, control experiments revealed that all variables
were critical for success (entries 11ꢀ12).16
Encouraged by these results, we turned our attention to
explore the generality of our protocol with a host of unꢀ
activated alkyl chlorides (Table 2). In line with our exꢀ
pectations, the reaction was rather general and distinꢀ
guished by an exquisite chemoselectivity profile, as aceꢀ
tals (2b), esters (2c, 2d, 2e and 2j), aryl fluorides (2c),
heterocycles (2d, 2e and 2j), amides (2f), aldehydes
(2h), ketones (2g), silyl ethers (2p), nitriles (2t), carbaꢀ
mates (2o) or alkenes (2u) could all be perfectly acꢀ
commodated. Interestingly, free aliphatic alcohols (2m
and 2v) or even their most acidic phenol congeners do
not interfere (2q), thus illustrating the potential of our
methodology in protectingꢀgroupꢀfree strategies. While
C–O electrophiles have been utilized in Niꢀcatalyzed
crossꢀcoupling reactions,17,18 we found exclusive forꢀ
mation of 2n and 2r, thus providing an additional handle
for further functionalization. Although the available litꢀ
erature data suggested that the carboxylation of unactiꢀ
vated secondary or tertiary alkyl halides would be a
chimera, we found that 2w-2y were within reach under a
Ni/L6 regime using LiCl as additive, even as single diaꢀ
stereoisomers as univocally shown by Xꢀray crystallogꢀ
raphy (2x). Albeit in lower yields, the preparation of 2z19
constitutes a rare example of a crossꢀelectrophile couꢀ
pling of unactivated tertiary alkyl halides,20 showcasing
the full potential of our catalytic protocol. With these
conditions in hand, we wondered whether unactivated
alkyl chlorides containing alkyne motifs on the side
chain could trigger a CO2 insertion at distal reaction
sites. As shown in Table 3, this was indeed the case, and
carbocyclic skeletons possessing a carboxylic acid withꢀ
in a rather elusive tetrasubstituted olefin framework
were all obtained in good yields (4a-4f) at 3 mol% cataꢀ
lyst loadings, even at 3 mmol scale (4c) using L5. Parꢀ
ticularly noteworthy was the observation that a formal
anti-carbometalation motion21 was favored when utilizꢀ
ing secondary alkyl chlorides based on a Ni/L6 couple
(4e and 4f).22
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Table 2. Carboxylation of Unactivated Alkyl Chlorides.a,b
Table 3. Cascade Cyclization/Carboxylation Events.a,b
a As Table 1 (entry 1), but using 3 mol% loadings and L5. b
c
Isolated yields, average of at least two independent runs.
3c (3 mmol). d Using L6 at 70 ºC. e 90 ºC.
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Scheme 3. Mechanistic Experiments.
The feasibility of promoting an intermolecular crossꢀ
electrophile coupling reaction with unactivated alkyl
1
2
chlorides suggested that our methodology could open up
possibilities in iterative crossꢀcoupling scenarios of polꢀ
yhalogenated backbones.6 As illustrated in Scheme 2, 6
could be selectively prepared from densely halogenated
5 in a catalytic crossꢀelectrophile coupling with tert-
butyl bromide.20a Subsequently, SuzukiꢀMiyaura reacꢀ
tion using a Buchwald protocol under a Pd/XPhos reꢀ
gime23 resulted in 7, which ultimately generated 8 upon
simple exposure to our optimized carboxylation condiꢀ
tions based on L4. Taken together, the results of Tables
2ꢀ3 and Scheme 2 tacitly illustrates the prospective imꢀ
pact of this methodology in both crossꢀelectrophile couꢀ
plings and catalytic carboxylation processes.
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Scheme 2. Iterative Coupling with Polyhalogenated Motifs.
In summary, we have documented an unconventional
intermolecular crossꢀelectrophile coupling of unactivat-
ed primary, secondary or even tertiary alkyl chlorides
with CO2 at atmospheric pressure. The salient features
of this novel transformation are the exquisite chemoseꢀ
lectivity profile, mild conditions and ease of execution,
allowing for cascade processes or iterative scenarios.
Further extensions to other intermolecular crossꢀ
electrophile processes are currently underway.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
spectral data. This material is available free of charge via
the Internet at http://pubs.acs.org.
Next, we decided to gather indirect evidence about the
mechanism by studying the stereochemical course of 9a
and 9b (Scheme 3). As shown, an erosion of stereoꢀ
chemical integrity was observed regardless of the subꢀ
strate utilized, suggesting that singleꢀelectron transfer
processes (SET) and Ni(I) species might come into
play.24,25 Taking this into consideration, we turned our
attention to study the reactivity of the putative
Ni(0)(L4)2 (11) and Ni(I)(L4)2 (12) species, both of
which could easily be prepared from Ni(COD)2.10a As
expected, 11 was found to be catalytically competent as
reaction intermediates, delivering 2a in 80% yield. Imꢀ
portantly, 2a was obtained in comparable yields when
using stoichiometric amounts of 11 or 12, either in the
absence or in the presence of TBAB, thus confirming
that TBAB was not essential for the reaction to occur
and leaving a reasonable doubt about the involvement of
in situ generated alkylꢀNi(II)Br species.26, Athough we
cannot rule out other conceivable pathways,27 at present
we propose a catalytic scenario consisting of the initial
formation of alkylꢀNi(II)Cl species followed by comꢀ
proportionation with Ni(0)Ln en route to putative alkylꢀ
Ni(I) species28 that might rapidly insert CO2 into the
C(sp3)–Ni bond prior SET mediated by Mn,29 ultimately
recovering back the propagating Ni(0)Ln species.30
AUTHOR INFORMATION
Corresponding Author
* rmartinromo@iciq.es
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
We thank ICIQ, the European Research Council (ERCꢀ
277883), MINECO (CTQ2015ꢀ65496ꢀR & Severo Ochoa
Excellence Accreditation 2014ꢀ2018, SEVꢀ2013ꢀ0319) and
Cellex Foundation for support. Johnson Matthey, Umicore
and Nippon Chemical Industrial are acknowledged for a
gift of metal & ligand sources. M. B thank MINECO for a
FPU fellowship. We sincerely thank E. Escudero and E.
Martin for XꢀRay crystallographic data.
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(19) Nonꢀcaged tertiary alkyl chlorides provided traces of carꢀ
boxylation products.
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Soc. 2015, 137, 11562. (b) Zhao, C.; Jia, X.; Wang, X.;
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(21) The anti-motion is tentatively ascribed to the intermediacy
of radical intermediates. See ref. 10c.
(22) For selected antiꢀcarbometalation techniques of organomeꢀ
tallic species, see: (a) Fressigne, C.; Girard, A. –L.; Duranꢀ
detti, M.; Maddaluno, J. Angew. Chem. Int. Ed. 2008, 47,
891. (b) Simaan, S.; Marek, I. Org. Lett. 2007, 9, 2569, and
references therein.
(23) Kinzel, T.; Zhang, Y.; Buchwald, S. L. J. Am. Chem. Soc.
2010, 132, 14073.
(24) In line with this notion, significant inhibition was observed
with radical scavengers such as TEMPO or BHT. The presꢀ
ence of radical species gains credence by observing 5ꢀexoꢀ
trig cyclization at different Ni/L4 loadings when employing
6ꢀchloroꢀ1ꢀhexene as substrate, suggesting that a radicalꢀ
escape rebound mechanism might be occurring.
(25) For Ni(I) species generated upon SET, see refs. 10aꢀe, 11f
and the following references: a) Laskowski, C. A.; Bungum,
D. J.; Baldwin, S. M.; Del Ciello, S. A.; Iluc, V. M.; Hillꢀ
house, G. L. J. Am. Chem. Soc. 2013, 135, 18272; b)
Breitenfeld, J.; Ruiz, J.; Wodrich, M. D.; Hu, X. J. Am.
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Am. Chem. Soc. 2013, 135, 16192, and references therein.
(26) Importantly, a stoichiometric reaction of 1a with 11 in the
absence of Mn revealed that CO2 insertion into the C(sp3)–
Ni bond occurred. This observation is noteworthy, as all
previous carboxylations of unactivated alkyl electrophiles
required the presence of Mn, even with stoichiometric
amounts of Ni complexes (refs. 10c and 10e), arguably indiꢀ
cating that a different mechanism takes place with alkyl
chloride counterparts. See ref. 12
(27) At present, direct CO2 insertion into the C(sp3)–Ni(II)Cl
bond or in situ formation of alkyl–Ni(I) species via SET
mediated by Mn cannot be excluded.
(28) For selected comproportionation events en route to Ni(I)
species, see: a) Cornella, J.; GomezꢀBengoa, E.; Martin, R.
J. Am. Chem. Soc. 2013, 135, 1997. b) Velian, A.; Lin, S.;
Miller, A. J. M.; Day, M. W.; Agapie, T. J. Am. Chem. Soc.
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Iglesias, M.; Cardenas, D. J. Angew. Chem., Int. Ed. 2007,
46, 8790. d) Jones, G. D.; Martin, J. L.; McFarland, C.; Alꢀ
len, O. R.; Hall, R. E.; Haley, A. D.; Brandon, R. J.;
Konovalova, T.; Desrochers, P. J.; Pulay, P.; Vicic, D. A. J.
Am. Chem. Soc. 2006, 128, 13175.
(12) See Supporting Information for details.
(13) Traces of βꢀhydride elimination were observed in the crude
mixtures under our optimized reaction conditions.
(14) Under the limits of detection, no alkyl bromide was detected
through the course of the reaction of 1a with TBAB by
HPLCꢀmonitoring in the absence or presence of CO2.
(15) Ammonium halides have been proposed to speed up elecꢀ
tronꢀtransfer processes from Mn to the metal center: (a)
Iyoda, M.; Otsuka, H.; Sato, K.; Nisato, N.; Oda, M. Bull.
Chem. Soc. Jpn. 1990, 63, 80, and refs. 10d and 11f.
(16) The use of Zn as reductant with either 1a or 1w resulted in a
significant erosion in yield (68% & 30%, respectively).
(29) Recently, Ni(I) species have shown to rapidly react with
CO2: Menges, F. S.; Craig, S. M.; Tötsch, N.; Bloomfield,
A.; Ghosh, S.; Krüger, H. –J.; Johnson, M. A. Angew.
Chem., Int. Ed. 2016, 55, 1282.
(30) See ref. 12 for a proposed mechanistic rationale.
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