sp3 Bis-Organometallic Reagents via Catalytic 1,1-Difunctionalization of Unactivated Olefins

Article abstract of DOI:10.1002/anie.202100810

A catalytic 1,1-difunctionalization of unactivated olefins en route to sp³ bis-organometallic B,B(Si)-reagents is described. The protocol is characterized by exceptional reaction rates, mild conditions, wide scope, and exquisite selectivity pattern, constituting a new platform to access sp³ bis-organometallics.

Full text of DOI:10.1002/anie.202100810

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 11740–11744
International Edition:
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doi.org/10.1002/anie.202100810
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Nickel Catalysis Hot Paper
3
sp Bis-Organometallic Reagents via Catalytic 1,1-Difunctionalization
of Unactivated Olefins
Shang-Zheng Sun , Laura Talavera , Philipp Spieß, Craig S. Day, and Ruben Martin*
+
+
3
Abstract: A catalytic 1,1-difunctionalization of unactivated
olefins en route to sp bis-organometallic B,B(Si)-reagents is
contrast, a de novo catalytic technique aimed at accessing sp
bis-organometallics via site-selective olefin 1,1-difunctional-
3
[
6]
described. The protocol is characterized by exceptional
reaction rates, mild conditions, wide scope, and exquisite
ization still remains a desirable scenario. If successful, such
a technology might not only complement existing 1,1-
difunctionalizations that incorporate boron fragments across
3
selectivity pattern, constituting a new platform to access sp bis-
[
7–9]
[9a,c]
organometallics.
olefins,
such as the elegant protocols described by Yin
[
9d]
and Fu,
but also offer an unrecognized opportunity to
rapidly and reliably access versatile 1,2-bis-organometallic
3
M
etal-catalyzed cross-coupling reactions of sp mono-
organometallic reagents have reached remarkable levels of
sophistication as vehicles to rapidly build up sp architec-
tures. Although sp poly-organometallics might a priori
offer an improved versatility and modularity for forging sp
intermediates with an extra carbon.
3
As part of our interest in site-selective olefin functional-
we recently ques-
tioned whether it would be possible to design a Ni-catalyzed
[
1]
3
[10]
ization and ambiphilic a-haloboranes,
3
3
linkages, a limited number of catalytic synthetic routes have
been described for preparing these reagents from simple, yet
1,1-difunctionalization of olefins to generate sp bis-organo-
metallics bearing both B and Si motifs (Scheme 1, bottom).
However, it was unclear whether such a three-component
endeavor could be implemented with an exquisite regio- and
chemoselectivity pattern given the inherent propensity of
transition metals for enabling 1,2-difunctionalization
[
2]
available, precursors. At present, catalytic approaches for
[
3]
their synthesis include olefin 1,2-bis-metallations or 1,2-
[
4,5]
hydrometallation of vinyl metal species (Scheme 1, top). In
[
11]
events. Conceptuality and practicality aside, we recognized
that such a platform might offer a new blueprint for preparing
densely functionalized, yet synthetically versatile, bis-organo-
metallics from simple and readily accessible precursors while
opening new complementary strategies for our ever-growing
[
6–9]
olefin functionalization portfolio.
Herein, we report the
realization of this goal with a platform consisting of a site-
selective [1,2]-Ni migration via the intermediacy of I facili-
tated by the stabilization of the neighboring boron atom, thus
setting the stage for establishing the 1,1-difunctionalized
linkage bearing
a
versatile bis-organometallic B,B(Si)-
[
12,13]
reagent.
Our method is distinguished by its excellent
chemo- and regioselectivity profile, even in the context of
ethylene valorization.
Inspired by Yinꢀs and Brownꢀs work on the catalytic
Scheme 1. Preparation of bis-organometallic reagents.
[
9c,13a]
borylation of olefins with B Pin ,
we began our work by
evaluating the 1,1-difunctionalization of 1a with 2a and
B Pin (3a, Scheme 2). The choice of 2a was not arbitrary, as
2
2
[
+]
[+]
[
*] S.-Z. Sun, L. Talavera, P. Spieß, C. S. Day, Prof. R. Martin
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Av. Paꢀsos Catalans 16, 43007 Tarragona (Spain)
E-mail: rmartinromo@iciq.es
2
2
a-haloboranes are readily accessible on a large scale via
[
14]
Matteson homologation. After some experimentation, the
use of NiBr ·TBA (10 mol%), L1 (10 mol%), t-BuOLi and t-
4
2
BuOH in DME at 308C gave rise to 4a in 73% isolated yield
[
+]
[+]
S.-Z. Sun, L. Talavera, C. S. Day
in 30 min as a single regioisomer, with functionalization
Universitat Rovira i Virgili, Departament de Quꢁmica Analꢁtica
i Quꢁmica Orgꢂnica
c/Marcel lꢁ Domingo, 1, 43007 Tarragona (Spain)
[15]
occurring at the less-hindered site of the olefin. Intrigu-
ingly, 2,2’-bipyridines or 1,10-phenanthrolines—ligands that
are particularly suited for olefin functionalization—failed to
Prof. R. Martin
[16]
provide even traces of 4a. As shown in entries 1–5, only
pyridine ligands possessing a tethered free alcohol delivered
4a, indicating that binding of the latter to the Ni center might
Catalan Institution for Research and Advanced Studies (ICREA)
Passeig Lluꢀs Companys, 23, 08010 Barcelona (Spain)
+
[
] These authors contributed equally to this work.
[9c]
play a non-negligible influence on reactivity. This notion
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100810.
was reinforced by the catalytic competence of Ni(L1) , hence
2
demonstrating the importance of the N,N,O binding mode of
1
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Scheme 2. Optimization of the reaction conditions. 1a (0.20 mmol),
a (0.34 mmol), B Pin (0.34 mol), NiBr ·TBA (10 mol%), L1
2
2
2
4
2
(
10 mol%), t-BuOLi (0.40 mmol), t-BuOH (0.80 mmol), DME (0.5 mL)
at 308C for 0.5 h. [a] GC yields using 1-decane as internal standard. [b]
Isolated yield. B Pin =Bis(pinacolato)diboron.
2
2
[17,18]
L1 (entry 8).
In addition, subtle changes on the nickel
precatalyst, inorganic base or solvent, invariably led to lower
yields of 4a (entries 9–12). Control experiments revealed that
all of the reaction parameters were critical for success
Scheme 3. Scope of 1,1-difunctionalization with a-bromoboranes. As
Scheme 2 (entry 1); isolated yields, average of two independent runs.
a] 5.0 mmol scale. [b] dr=1:1. [c] Isolated as diol upon quenching
with NaBO ·H O. [d] dr=1:1.
[
19]
(
entry 13).
The excellent reactivity encountered for 1a suggested that
[
3
2
our protocol could be equally extended to a wide variety of
olefin counterparts. Gratifyingly, the 1,1-difunctionalization
event showed an excellent chemoselectivity profile, as nitriles
metal bonds, thus setting the stage for promoting site-
3
(
(
4b), ketones (4c), silyl ethers (4d), esters (4j, 4k, 4l), amides
4m) or heterocycles (4n, 4o, 4p) were perfectly accommo-
selective sp cross-coupling reactions at later stages. As
shown by the results compiled in Scheme 4, this turned out
to be the case and the utilization of readily accessible
(bromomethyl)trimethylsilane resulted in a useful entry
point to 1,2-silylboranes by means of catalytic 1,1-difunction-
alization of olefins. In this case, however, the targeted
products were obtained under a Ni/L3 regime with LiCl as
dated (Scheme 3). Notably, organosilanes (4s), alkyl halides
4 f, 4g) and aryl halides (4k, 4l) were all well-tolerated, thus
providing ample opportunities for further derivatization via
(
[
20]
conventional cross-coupling reactions.
Remarkably, the
reaction could be conducted on a gram scale, affording 4a
in 75% yield. In addition, 1,1-disubstituted olefins could be
employed as substrates (4t) whereas the successful prepara-
tion of 4q and 4r indicates that secondary a-haloboranes
might pave the way for accessing diborane fragments at
[15]
additive. These conditions could be employed across a wide
number of unactivated olefins bearing silyl ethers (5b), alkyl
halides (5c, 5d), aryl halides (5g, 5h), heterocycles (5i–5k) or
even epoxides (5l). With a reliable set of conditions in
hand for forging both 1,2-diboranes and 1,2-silylboranes
from unactivated olefins, we turned our attention to apply
these techniques in advanced synthetic intermediates
3
secondary sp alkyl sites. The preparation of 4u is particularly
noteworthy, as it allows for incorporating both a chiral boron
3
entity and two structurally different sp CÀB bonds in
[
22]
a synergistic manner. Note, however, that lower yields and
diastereoselectivities were found in these cases. Unfortu-
nately, internal olefins, styrenes and a-bromoalkyl boronic
esters bearing quaternary carbon centers failed to provide the
targeted products.
(Scheme 5).
As shown, a variety of products bearing
[23]
either a diborane (6a–9a) or silylborane motif (6b–9b)
could easily be accessed from common precursors. These
results are particularly noteworthy, as these intermediates
might be used as vehicles to access libraries of compounds of
interest in drug discovery programs via site-selective CÀB or
CÀSi bond-functionalization.
Encouraged by the results shown in Scheme 3, we
wondered whether our catalytic 1,1-difunctionalization of
olefins could be extended to ambiphilic organometallic
The synthetic value of our site-selective 1,1-difunctional-
ization is further illustrated in Scheme 6. As shown, ethyl-
ene—the largest-volume organic chemical produced in indus-
try -could be employed as substrate, even at atmospheric
[21]
partners other than a-haloboranes. This was particularly
important, as such scenario would give access to 1,2-bis-
3
organometallic reagents with intrinsically different C(sp )-
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Scheme 6. Synthetic applications. path a: furan (0.30 mmol), nBuLi
(
(
(
(
(
(
0.30 mmol), 5a (0.25 mmol), then NBS (0.30 mmol). path b: 5a
0.30 mmol), vinyl bromide (0.60 mmol), LDA (0.60 mmol) then I2
0.66 mmol). path c: 5a (0.40 mmol), vinylMgBr (1.60 mmol) then I2
Scheme 4. Scope of 1,1-difunctionalization with a-bromosilanes. As
Scheme 2 (entry 1), with 2d (0.50 mmol), L3 (10 mol%), tBuOLi
d
1.60 mmol), NaOMe (3.20 mmol). path d: 4a (0.40 mmol), PhBr
0.60 mmol), Pd(OAc) (5 mol%), RuPhos (5 mol%), KOH
(
0.50 mol), LiCl (0.40 mol), DME (0.2 mL); isolated yields, average of
2
1.20 mmol), THF/H O (10:1), 708C. path e: as path d, 1-chloro-2-
two independent runs. [a] 2d (0.60 mmol), 6 h. [b] dr=1:1. [c] starting
alkene with dr=1:1. [d] dr=1.5:1.5:1:1.
2
f
methyl-1-propen (0.60 mmol). path f: as path d, 1,2-dichloroethane
(
1.20 mmol).
pressure, delivering 10a or 10b in an unoptimized 40% and
[
24]
2
8% yield, respectively. More importantly, a site-selective
C-B or CÀSi bond-cleavage platform could be implemented
by employing 4a or 5a as substrates, tacitly indicating that our
technology might be used as a formal linchpin for selectively
incorporating structurally-different carbon linkages into
simple olefin backbones. Interestingly, 14–16 were easily
3
within reach via site-selective sp CÀB bond-cleavage from
[
25]
4
a.
Under the limits of detection, no cross-coupling
3
reaction arising from the cleavage of the secondary sp CÀB
bond was observed in the crude mixtures. Interestingly, the
utilization of 5a instead delivered 11–13 via exclusive C-B
cleavage by using appropriately substituted Grignard
[
26]
[27,28]
reagents or organolithium compounds.
Our available data did not allow us to clarify the mode of
action by which our 1,1-difunctionalization operated. The
following mechanistic scenarios are a priori conceivable: (a)
migratory insertion of in situ generated Ni-Bpin via I,
[
29]
followed by 1,2-[Ni] migration via chain-walking
thus
allowing to locate the Ni center adjacent to a boron atom
due to the valence deficiency of the latter prior to coupling
[30]
with 2 (Scheme 7, path a); (b) single electron transfer from
0
Ni to 2, generating a boron-stabilized radical that adds across
I
the olefin followed by recombination with Ni (II), chain-
walking and coupling with B Pin (path b); (c) formation of
2
2
Scheme 5. Olefin 1,1-difunctionalization in advanced synthetic inter-
mediates. a-bromoborane coupling (left): as Scheme 2 (entry 1); a-
bromosilane coupling (right): as Scheme 4; Isolated yields, average of
two independent runs. [a] dr=1:1. [b] (bromomethyl)trimethylsilane
a vinyl-BPin intermediate III prior to coupling with 2 (path c);
d) addition of two BPin fragments across the olefin in a 1,1-
selective manner (IV) followed by cross-coupling with 2 via
(
[
9d]
(
0.60 mol), 6 h.
C-B cleavage (path d). While the two latter pathways could
1
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agreement 754558. We sincerely thank E. Escudero and J.
Benet for X-Ray crystallographic data.
Conflict of interest
The authors declare no conflict of interest.
Keywords: bis-organometallic · chain-walking · Ni catalysis ·
olefin functionalization
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(
Scheme 1 & Scheme 7, path a).
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therein.
[30] Selected reference for the formation of the alkyl-Ni species
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c) Ref. [9a–d].
11076.
[
[
[
14] D. S. Matteson, R. Ray, J. Am. Chem. Soc. 1980, 102, 7590 – 7591.
15] For details, see Supporting Information.
16] For selected Ni-catalyzed difunctionalization of olefins using
bipyridines or 1,10-phenanthroline ligands, see: a) W. Shu, A.
Garcꢂa-Domꢂnguez, M. T. Quiros, R. Mondal, D. J. Cardenas, C.
Nevado, J. Am. Chem. Soc. 2019, 141, 13812 – 13821; b) A.
Garcꢂa-Domꢂnguez, Z. Li, C. Nevado, J. Am. Chem. Soc. 2017,
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Qing, L. Chu, Nat. Commun. 2018, 9, 3488; d) refs. [9a,b].
[
17] We managed to isolate (L1) NiBr —unambiguously determined
[31] A competitive experiment of 19 and 1-hexene under our
optimized conditions revealed no crossover, with 1,1-difunction-
alization occurred exclusively in the latter and 19 being
recovered unaltered.
[32] These results are consistent with EPR monitoring experiments
performed under the standard catalytic conditions. Under the
limits of detection, no signals corresponding to organic radicals
were observed in the crude mixtures. See ref. [15].
2
2
1
by X-ray diffraction—that also showed a (O-k )-amide binding
mode of L1 with the tethered alcohol positioned away from the
Ni center. The utilization of this precatalyst resulted in 70%
yield of 4a. See ref. [15].
[
18] Deposition Numbers 2046367 ((L1) Ni) and 2046368
2
(
(L1) NiBr ) contain the supplementary crystallographic data
2 2
for this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinforma-
tionszentrum Karlsruhe Access Structures service www.ccdc.
cam.ac.uk/structures.
Manuscript received: January 18, 2021
Accepted manuscript online: February 25, 2021
1
1744 www.angewandte.org
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Angew. Chem. Int. Ed. 2021, 60, 11740 –11744