Benzyltriboronates: Building Blocks for Diastereoselective Carbon-Carbon Bond Formation

Article abstract of DOI:10.1021/jacs.6b12896

A highly diastereoselective carbon-carbon bond-forming reaction involving the tandem coupling of benzyltriboronates, enoates, and alkyl halides is described. This method was enabled by the discovery of α-diimine nickel catalysts that promote the chemosele

Full text of DOI:10.1021/jacs.6b12896

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Communication
Benzyltriboronates: Building Blocks for
Diastereoselective Carbon-Carbon Bond Formation
W. Neil Palmer, Cayetana Zarate, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12896 • Publication Date (Web): 03 Feb 2017
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Benzyltriboronates: Building Blocks for Diastereoselective
Carbon-Carbon Bond Formation
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W. Neil Palmer, Cayetana Zarate, Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, U. S. A
Supporting Information Placeholder
valuable and enable exploration of the full synthetic potential of alkyl
polyboronates.
ABSTRACT: A highly diastereoselective carbon-carbon bond form-
ing reaction involving the tandem coupling of benzyltriboronates,
enoates and alkyl halides is described. This method was enabled by the
discovery of α-diimine nickel catalysts that promote the chemoselec-
tive triborylation of benzylic C(sp3)–H bonds using B2Pin2 (Pin =
pinacolate). The C-H functionalization method is effective with
methylarenes and for the diborylation of secondary benzylic C-H
bonds, providing direct access to polyboron building blocks from readi-
ly available hydrocarbons. Combination of the benzylic perborylation
and conjugate addition-alkylation methods enables a one-pot proce-
dure in which multiple simple precursors are combined to generate
diastereopure products containing quaternary stereocenters.
Scheme 1. Utility of alkyltriboronates in C–C bond-
formation, synthesis of benzylpolyboronates by C–H
borylation, and combination of these methods to pro-
vide a diastereoselective tandem coupling sequence.
Organoboron compounds are versatile reagents for carbon-carbon
and carbon-heteroatom bond formation.1 Arylboronates are the most
widely used nucleophilic partner in Suzuki-Miyaura cross couplings2,3
and most commonly deployed in the pharamaceutical industry due to
their ease of handling, synthetic accessibility and reliability.4 Alkyl-
boronates, typically synthesized by alkene hydroboration5 or Miyaura-
type borylation of alkyl halides,6 are likewise valuable reagents for C-C
bond formation whose utility will continue to grow as methods for
C(sp3)-C(sp3) cross coupling mature.7 Polyboron compounds such as
gem-diboronates are beginning to emerge as useful reagents for C-C
bond formation as well and have been applied to alkylation reactions,8
boron-Wittig olefinations,9 1,2-additions to carbonyls10 and imines,11
addition to pyridine-N-oxides,12 allylic substitution,13 and enantioselec-
tive cross couplings.14 The use of 1,1,1-alkyltriboronates for C–C bond
formation offers similar, if not greater, potential but is less explored and
limited to deborylative boron-Wittig olefination15 or deborylative al-
kylation (Scheme 1A).16
The full synthetic potential of geminal alkylpolyboronates has yet to
be realized principally due to the paucity of straightforward methods
for their preparation. The direct, metal-catalyzed polyborylation of
C(sp3)–H bonds offers a potentially transformative route as readily
available, minimally functionalized hydrocarbons may be used as start-
ing materials. Although C-H borylation is one of the most widely used
C-H functionalization methods, most precious17 and base metal cata-
lysts18 favor C(sp2)-H sites, a result of established kinetic preference for
oxidative addition.19 Metal-catalyzed methods for alkane borylation
have been reported17,20 including recent notable examples of iridium-
and rhodium-catalyzed borylation of methane at high pressures,21 but
chemoselective C(sp3)–H borylation in the presence of C(sp2)–H
bonds is far less common and more challenging. Strategies to overcome
this selectivity bias rely primarily on the use of directing groups,22 while
a handful of non-directed methods selectively functionalize benzylic
C–H bonds.23 In most of these examples, only one C-H bond is con-
verted to a C-B bond.24 New catalytic methods that convert multiple C-
H bonds on the same carbon to boron functional groups would be
Our laboratory previously reported α-diimine cobalt dialkyl and
bis(carboxylate) compounds that are active for the catalytic borylation
of C(sp3)-H bonds of methylarenes with B2Pin2 as the boron source.23d
The 1,1-diboronates were the major products but, with toluene as the
substrate, a small amount (18% yield) of the 1,1,1-triboronate product
was isolated when excess boron reagent and high cobalt loadings of 50
mol% were used. The forcing conditions, low yield and difficulty in
isolation precluded reactivity studies of this interesting benzylpoly-
boronate and inspired development of a synthetically useful catalytic
method. Here we describe α-diimine nickel dialkyl and
bis(carboxylate) complexes that enable the selective preparation of
benzyltriboronates via perborylation of benzylic C–H bonds (Scheme
1B). Combination of the Ni-catalyzed perborylation method with a
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new deborylative conjugate addition protocol (Scheme 1A) provides a
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3
convenient one-pot, tandem procedure for the diastereoselective syn-
thesis of value-added products from simple, abundant precursors
(Scheme 1C).
Inspired by the observation of benzylboronate products by Tobisu,
Chatani and coworkers in Ni-catalysis18f and our own success with N-
alkyl substituted α-diimine (RADI) cobalt dialkyl complexes,23d the
analogous Ni(II) dialkyl complexes were targeted for evaluation in
catalytic C–H borylation. (CyADI)Ni(CH2SiMe3)2 (1) was isolated as
a teal, diamagnetic solid in 79% yield following pyridine displacement
by addition of the free α-diimine to (py)2Ni(CH2SiMe3)2 (py = pyri-
dine).25 The solid-state structure of 1 established a distorted square
planar geometry (See SI, Figure S2). Because both cobalt and nickel
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bis(carboxylate) compounds have proven to be readily synthesized,
Cy-
bench stable precursors for a host of catalytic reactions,18e,23d,26
(
ADI)Ni(OPiv)2 (2) (Piv = pivaloyl) was also prepared by addition of
the free α-diimine to well-defined Ni(II) pivalate precursor,
a
[(NEt3)Ni(OPiv)2]2, a compound first reported by Eremenko and
coworkers.27
With excess (4.0 equiv) B2Pin2 and 25 mol% of 1, the catalytic
borylation of toluene in cyclopentylmethyl ether (CPME) at 80 °C
resulted in isolation of the benzyltriboronate 4 in 84% yield (Scheme
2A). Repeating this procedure with 25 mol% of 2 in combination with
100 mol% of methylaluminoxane (MAO) activator provided a quanti-
tative yield of 4. Notably, the yields of 4 using 1 and 2 were signifi-
cantly higher with lower pre-catalyst loadings and temperatures than
with cobalt, highlighting the advantage of the nickel-catalyzed method.
Nickel sources, α-diimine ligands and catalytic conditions were
evaluated to lower the amount of Ni required (See SI, Table S1). Be-
cause formation of bis(chelate) nickel complexes was believed to be a
deactivation pathway (See SI, Scheme S1), the nickel bis(pivalate)
complex bearing a bulkier diimine ligand was synthesized. Pale green
aIsolated yields. Reactions conducted with 0.55 mmol methylarene or alkylarene
in 0.55 mL of CPME at 80 ºC. See SI for experimental details. b100 mol% MAO
c
as catalyst activator. 4 was isolated with 13% impurity of remaining B2Pin2. 40
mol% MAO activator used.
The development of a direct method for the preparation of 1,1,1-
alkyltriboronates from C(sp3)-H functionalization offers a new oppor-
tunity to explore the utility of these products in C–C bond forming
reactions. Known deborylative boron-Wittig olefination15 and
deborylative alkylation16 protocols were demonstrated (See SI, Scheme
S3). Further efforts focused on development of new C-C bond forming
transformations. Conjugate addition of α-boryl nucleophiles to un-
saturated carbonyls has previously been reported for α-boryl radicals28
and α-boryl-zirconocenes.29 Benzylic α-diboryl anions were postulated
to be sufficiently soft nucleophiles to exhibit similar reactivity. To test
this, a THF solution of 4 and (E)-methylcrotonate was treated with
NaOEt at room temperature, providing 11 (Scheme 3A) in 77% yield
following aqueous workup. Repeating this procedure with methyl
methacrylate and methyl tiglate provided 12 and 13, respectively, in
good yield and, in the case of 13, excellent diastereoselectivity (19:1).
Notably, products resulting from the deborylative conjugate addition
to phenyl-substituted (E)-methyl cinnamate (14) and the α,β,γ,δ-
unsaturated ethyl sorbate (15) were also obtained, albeit at reduced
yields predominantly due to competing 1,2-addition to the more elec-
tron deficient enoates.
(
ipcADI)Ni(OPiv)2 (3, Scheme 2A) was isolated in 89% yield and was
active for the triborylation of toluene. With only 10 mol% of 3 in com-
bination with 40 mol% MAO, quantitative formation of 4 was ob-
tained. This reaction was also successfully scaled; starting with 2.17
mmol of toluene, 1.00 g (99% yield) of 4 was isolated.
With the improved performance of 3, the generality of benzylic tri-
borylation was examined with a series of methylarenes (Scheme 2B).
High yields of the benzyltriboronate products derived from 3-
methylanisole, 4-methylanisole, and m-tolylboronic acid pinacol ester
were obtained. Although these products were isolated following acidic
aqueous workup with no additional purification, the benzyltriboro-
nates underwent protodeborylation to mono- or diboronates upon
prolonged exposure to aqueous conditions or by attempted purifica-
tion by column chromatography. Extended exposure to air also result-
ed in decomposition, likely by autoxidation.
The polyborylation of the benzylic positions of linear alkyl arenes
was also explored with 3. C(sp3)-H borylation of this substrate class
has only been achieved previously using silyl directing groups22d or vast
excess of substrate and long reaction times.23b,e Using 20 mol% of 3 and
Scheme 3. Base-mediated deborylative conjugate addi-
tion using isolated benzyltriboronate 4.a
excess (3.5 equiv)
B2Pin2, the benzylic diborylation of n-
propylbenzene, n-pentylbenzene, and trimethylsilyl-protected 3-
phenylpropanol was accomplished to afford 8, 9, and 10, respectively
(Scheme 2C). Precatalyst 3 was also effective for the synthesis of ben-
zyl diboronates from methylarenes (See SI, Scheme S2) at superior
yields and lower catalyst loadings than previously reported with co-
balt.23d
Scheme 2. Benzylic perborylation of substituted
methylarenes and linear alkylarenes using α-diimine
nickel dialkyl and bis(pivalate) precatalysts.a
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The scope of the alkyl electrophile was also explored. Using toluene,
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B2Pin2, methyltiglate, and quenching with water, 13 was isolated (19:1
dr) in 51% yield (Scheme 4B). Quenching the reaction with iodome-
thane provided the methylated product 23 in 53% yield. Further varia-
tion of the alkyl electrophile might provide access to diastereomerically
enriched products with quaternary stereocenters. This was successfully
demonstrated using allylbromide, 4-fluorobenzylbromide, and propar-
gylbromide to give products 24, 25, and 26, respectively, with high
degrees (>20:1) of diastereoselectivity. The solid state structure of 25
was confirmed by X-ray crystallography (See SI, Figure S5), verifying
the identity of the isolated diastereomer as that predicted by the stere-
ochemical model in Scheme 3B and allowing for assignment of the
diastereomers of 24 and 26 by analogy.
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Scheme 4. Diastereoselective, one-pot tandem tri-
borylation-conjugate addition-alkylation.a
aIsolated yields. Reactions conducted with 0.28 mmol of benzyltriboronate in
5.6 mL THF. See SI for experimental details. Diastereomeric ratios determined
by gas chromatography of crude reaction mixtures and confirmed by 1H NMR
spectroscopy of isolated products. bMajor diastereomer 13 assigned as opposite
of that assigned for 16 (minor diastereomer). cDiastereomer of 16 determined
by X-ray crystallography (see SI, Figure S4).
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Given that the intermediate in the deborylative conjugate addition
of 4 to methyl tiglate proceeds through a sodium enolate, the stere-
ocontrol at the α-position of the ester in 13 likely occurs during pro-
tonation when the reaction is quenched. Intramolecular coordination
of the enolate to a boronate substituent and subsequent axial approach
of an electrophile was hypothesized to explain this phenomenon
(Scheme 3B).30 To test this, the deborylative conjugate addition of 4
and (E)-methylcrotonate was carried out and the reaction was
quenched with iodomethane (Scheme 3A). The resulting product, 16,
was isolated in >20:1 dr as the syn diastereomer opposite that of 13,
consistent with the stereochemical model. These results highlight the
unique ability of the boronate groups to act not only as masking groups
for soft nucleophilic anions but also elements for substrate-controlled
diastereoselectivity.
The possibility of generating these benzyltriboronates in-situ and di-
rectly applying them toward further chemistry was also investigated.
This would not only obviate the need for difficult isolation of these
sensitive reagents, but it would provide a convenient one-pot method
for rapidly building molecular complexity directly from toluene or
other simple substituted methylarenes. The synthesis of 16 was con-
ducted directly from toluene, B2Pin2, (E)-methylcrotonate, and iodo-
methane without isolating 4 (Scheme 4A). Following the Ni-catalyzed
triborylation of toluene, the crude triboronate product was exposed to
the conjugate addition-alkylation conditions to provide 16 in 70%
yield and >20:1 dr. The method was then successfully extended to
other substituted methylarenes. Generation of 5, 6, and 7 in-situ fol-
lowed by conjugate-addition and alkylation yielded products 17, 18,
and 19, respectively, in good to modest yields with high diastereoselec-
tivity (>20:1). The one-pot procedure was also applied using p-
cymene, 4-tolyltrimethylsilane, and N,N-dimethyl-m-toluidine result-
ing in isolation of 20, 21, and 22, respectively, following column
chromatography without the need for purification of the 1,1,1-
triboronate intermediates.
aIsolated yields. See SI for experimental details. Diastereomeric ratios deter-
mined by gas chromatography of crude reaction mixtures and confirmed by 1H
NMR spectroscopy of isolated products. bDiastereomers assigned by analogy to
that of 16. cNMR yield; 22% isolated yield.
In summary, an α-diimine nickel catalyst has been developed that is
effective for the perborylation of benzylic C(sp3)–H bonds of methyl-
and alkylarenes. The unique products are useful building blocks for a
diastereoselective conjugate addition-alkylation sequence, highlighting
the ability of the multiple boronate groups to act simultaneously as soft
carbanion masking groups and intramolecular stereocontrol elements.
Combination of the C(sp3)–H polyborylation with deborylative C–C
bond-formation into a one-pot procedure allowed an effective method
for rapid elaboration of simple and abundant precursors to complex
products poised for further elaboration with remaining C–B bond
functionality.31
ASSOCIATED CONTENT
Supporting Information
Complete experimental details, pre-catalyst optimization studies, and
characterization data of nickel complexes and of boronate products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
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Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hart-
Corresponding Author
pchirik@princeton.edu
wig, J. F. Chem. Rev. 2010, 110, 890.
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L.; Bontemps, S.; Sortais, J.-B.; Sabo-Etienne, S.; Darcel, C. J. Am. Chem. Soc.
2015, 137, 4062. (b) Obligacion, J. V.; Semproni, S.; Chirik, P. J. J. Am. Chem.
Soc. 2014, 136, 4133. (c) Schaefer, B. A.; Margulieux, G. W.; Small, B. L.;
Chirik, P. J. Organometallics 2015, 34, 1307. (d) Obligacion, J. C.; Semproni,
S. P.; Pappas, I.; Chirik, P. J. J. Am. Chem. Soc. 2016, 138, 10645. (e) Léonard,
N. G.; Bezdek, M. J.; Chirik, P. J. Organometallics 2017, 36, 142. (f) Furukawa,
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
WNP acknowledges the National Science Foundation (CHE-
1265988) and the 2016 Eli Lilly Graduate Research Fellowship Award
for financial support. CZ acknowledges MINECO for a FPU fellow-
ship. We thank M. J. Bezdek and I. Pappas for crystallographic studies,
and we also thank Allychem for a generous gift of B2Pin2.
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The geminal diboronate functionality in the conjugate addition
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