Catalytic protodeboronation of pinacol boronic esters: Formal anti-Markovnikov hydromethylation of alkenes

Article abstract of DOI:10.1039/c9sc02067e

Pinacol boronic esters are highly valuable building blocks in organic synthesis. In contrast to the many protocols available on the functionalizing deboronation of alkyl boronic esters, protodeboronation is not well developed. Herein we report catalytic protodeboronation of 1°, 2° and 3° alkyl boronic esters utilizing a radical approach. Paired with a Matteson-CH₂-homologation, our protocol allows for formal anti-Markovnikov alkene hydromethylation, a valuable but unknown transformation. The hydromethylation sequence was applied to methoxy protected (-)-Δ8-THC and cholesterol. The protodeboronation was further used in the formal total synthesis of δ-(R)-coniceine and indolizidine 209B.

Full text of DOI:10.1039/c9sc02067e

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Catalytic protodeboronation of pinacol boronic
esters: formal anti-Markovnikov hydromethylation
of alkenes†
Cite this: DOI: 10.1039/c9sc02067e
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have been paid for by the Royal Society
of Chemistry
*
Florian Clausen, Marvin Kischkewitz, Klaus Bergander and Armido Studer
Pinacol boronic esters are highly valuable building blocks in organic synthesis. In contrast to the many
protocols available on the functionalizing deboronation of alkyl boronic esters, protodeboronation is not
well developed. Herein we report catalytic protodeboronation of 1ꢀ, 2ꢀ and 3ꢀ alkyl boronic esters
utilizing a radical approach. Paired with a Matteson–CH2–homologation, our protocol allows for formal
Received 26th April 2019
Accepted 15th May 2019
anti-Markovnikov alkene hydromethylation,
a
valuable but unknown transformation. The
DOI: 10.1039/c9sc02067e
hydromethylation sequence was applied to methoxy protected (ꢁ)-D8-THC and cholesterol. The
protodeboronation was further used in the formal total synthesis of d-(R)-coniceine and indolizidine 209B.
rsc.li/chemical-science
Organoboron compounds are highly valuable building blocks in boronic esters was recently investigated by Aggarwal and
organic synthesis.1,2 The most important application to be coworkers; however, their method is only applicable to the
named is the Suzuki–Miyaura-coupling.3–5 Moreover, the boron protodeboronation of activated benzylic substrates.38 In 2005,
moiety can be converted into a broad range of functional Renaud and coworkers described an efficient sequence for the
groups.2,6 These transformations include oxidations,7 amina- formal hydrogenation of unactivated alkenes to alkanes using
tions,8–10 halogenations,11,12 and C–C-bond-formations such as a hydroboration-deboronation strategy (Scheme 1a).39 The pro-
alkenylations,13,14 alkynylations15 and arylations.16 To access the todeboronation of in situ generated catechol boronic esters
B-building blocks, various borylation approaches have been proceeds via a radical chain reaction, but this reaction is limited
developed over the years17–19 including the prominent asym- to the more expensive catechol boronic esters and works well for
metric hydroboration reaction reported by H. C. Brown in 2ꢀ alkyl B-esters.40
1961.20
An elegant C–C-coupling/protodeboronation strategy was
However, considering synthetic applications, drawbacks introduced in 2014 by Aggarwal and coworkers (Scheme 1b).41 In
associated with organoboranes are their limited air and mois- this sequence, the protodeboronation of 2ꢀ alkyl pinacol
ture stability.21 Along these lines, the introduction of the more boronic esters was achieved by oxidation of their corresponding
stable boronic ester moiety has signicantly expanded the scope methyl boron ate complexes with a stoichiometric amount of
of boron chemistry. In particular the pinacol boronic esters,
which are usually bench stable,22 easy to purify and oen even
commercially available have played a prominent role. These
features are attractive for chemical transformations, where the
valuable boron moiety remains in the product (homologa-
tions,23–25 conjunctive cross couplings26 or radical-polar cross-
over reactions27–32). However, the increased stability also rises
new challenges, considering the removal of the boron moiety at
the end of a sequence if required. While boranes undergo effi-
cient protodeboronation with propionic acid by proto-
nolysis33–35 or hydrogenation at elevated temperatures,36,37
boronic esters usually do not.21,22 We further noted that the
protodeboronation of unactivated alkyl and especially primary
alkyl boronic esters is underexplored. Reduction of 3ꢀ alkyl
¨
¨
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Corrensstrabe 40,
¨
48149 Munster, Germany. E-mail: studer@uni-muenster.de
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9sc02067e
Scheme 1 Protodeboronation and its application in synthesis.
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manganese(III)acetate. The resulting secondary alkyl radicals
are then reduced with 4-tert-butylcatechol (TBC). In contrast to
the Renaud process, the latter protodeboronation works with
pinacol boronic esters but does not proceed on primary B-esters
(see below) and a large amount of reductant is required.
During a metabolite study, we targeted an anti-Markovnikov-
type hydromethylation of a trisubstituted alkene and noted that
different alkene hydromethylation approaches have been pub-
lished over the years.42 However, all protocols developed so far
provide the Markovnikov product. We therefore envisioned
a
hydroboration/CH2-homologation/protodeboronation
strategy to achieve anti-Markovnikov alkene hydromethylation
(Scheme 1c). In this sequence, the regioselectivity is controlled
in the initial hydroboration step, where suitable methods are
available.43,44 The methylene unit can readily be introduced via
established Matteson-homologation using CH2Br2 as the
methylene source. The real challenge turned out to be the nal
protodeboronation of the intermediate 1ꢀ alkyl boronic ester.
The conditions reported by Aggarwal41 were ineffective for our
systems and also other methods failed. We therefore initiated
a project along those lines and aimed at a catalytic proto-
deboronation using boron ate complexes as substrates. Single
electron oxidation by a redox catalyst should generate the cor-
responding alkyl radicals.45–47 Reduction of these alkyl radicals
to give the targeted products should be achieved by thiols, since
they are known to be efficient H-donors and the resulting thiyl
radicals can be reduced by the photoredox catalyst thereby
regenerating the initial oxidation state of the catalyst.48–51
Hence, an external stoichiometric oxidant is not required
increasing the economy of the protodeboronation.
Scheme
2
Substrate scope of the photoredox catalyzed proto-
deboronation. Conditions: 1a–n (0.2 mmol), PhLi (0.22 mmol), Et2O (2
mL), thiophenol (0.22 mmol), Ir-catalyst PC1 (2.5 mol%) in MeOH/
acetone (1 : 1, 2 mL). Isolated yields, unless otherwise noted. aYield
determined by GC. [Ir] ¼ Ir(dFCF3ppy)2(dtbbpy)PF6 (PC1).
We commenced our investigations by testing different boron
ate complexes derived from boronic ester 1a. In each case, the
ate complex was generated in Et2O. Aer removal of the ether,
the residual ate complex was reacted with 2.5 mol% of the Ir-
catalyst PC1 and thiophenol (1.1 equiv.) in MeOH/acetone at
room temperature under blue light irradiation. B-ate complexes
derived from TBAF,47 DMAP,47 PPh3 (ref. 47) and 3-quinolidi-
nol47 (1.1 equivalent each) did not afford the desired product 2a.
However, with PhLi, the target 2a was obtained in 79% yield
(Scheme 2). Phenyl boronic ester 3 was formed as the stoi-
chiometric byproduct. Notably, in a proof of principle experi-
ment we demonstrated that the cascade also works efficiently by
using catalytic amounts of the thiol (10 mol%) in combination
with diphenyl phosphate (1.1 equiv.) as the external proton
source (95%, see ESI†). However, by using 50 mol% thiophenol
in MeOH/acetone, the yield dropped signicantly, showing the
necessity of the phosphate in the thiol catalytic procedure. Since
thiophenol is cheap, we decided to use the stoichiometric thiol
protocol for the scope studies.
Under optimized conditions, various primary alkyl boronic
esters 1b–n were tested (Scheme 2). Reactions proceeded
smoothly and the products 2b–n were isolated in moderate to
excellent yields (52–93%). Electron-rich arenes are well toler-
ated, as documented by the successful preparation of 2b–c, 2g
and 2k. Furthermore, substrates bearing a triphenyl silane (1i)
and a carbazole moiety (1j) performed well and the desired
products 2i–j were isolated in 72% and 88% yield, respectively.
As expected, the protodeboronation of enantioenriched boronic
ester 1k, readily prepared via stereoselective hydroboration,52
occurs with high stereospecicity (96% es). The reason for the
slight drop in ee might be reversible H-abstraction at the
benzylic position by the thiyl radical. In any case, this sequence
represents a new strategy for the asymmetric synthesis of aryl
dialkyl methanes of type 2k.
The method was also applied to the protodeboronation of
the 2ꢀ alkyl boronic esters 1l and 1m to yield the Boc-protected
pyrrolidine 2l and sulfone 2m in 85 and 93% yield. Further-
more, the 3ꢀ alkyl boronic ester 1n bearing a tert-butylester
group was smoothly protodeboronated (73%). A gram scale
experiment (preparation of 2h) proved the robustness of the
procedure (61%).
In order to verify that the protodeboronation occurs via
a radical pathway, a probe experiment was conducted. The
phenyl boron ate complex derived from the cyclopropylmethyl
substituted boronic ester 1o reacted under standard conditions
to the ring opened alkene 2o (E/Z ¼ 4 : 1, 84%), clearly sup-
porting the radical nature of the cascade (Scheme 3a).53
A
possible mechanism is depicted in Scheme 3b. Photoexcited
PC1 rst oxidizes the boron ate complex A to generate an alkyl
radical Rc along with Ph-Bpin. C-radical Rc then abstracts an H-
atom from thiophenol to form the product 2 and the phenyl
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of the additional C-8 stereocenter via a stereoselective proto-
deboronation. To this end, the 3ꢀ boronic ester 10 was con-
verted into the rigid indolizidine precursor 11 via Boc-
deprotection and lactamization (68%). Protodeboronation by
using the less nucleophilic (3,5-bis(triuoromethyl)phenyl)
lithium for boron ate complex formation in place of PhLi, to
prevent aryl addition to the lactam moiety, afforded the indo-
lizidine 12 in 59% yield and good diastereoselectivity (dr ¼
5 : 1). Final transformation into indolizidine 209B (13) can be
achieved by Grignard-addition to the lactam followed by ster-
eoselective reductive deoxygenation.58
Finally, we focused on the formal alkene hydromethylation
sequence (Scheme 5). Initial studies were conducted with the
boronic esters 14a–d which can be obtained via hydroboration
of the corresponding styrene derivatives (see the ESI†). The
Matteson-CH2-homologation was carried out with in situ
generated CH2BrLi in THF at low temperature. Aer removal
of the solvent, the crude homologated boronic esters 15a–
d were directly subjected to the protodeboronation to provide
the targeted compounds 16a–d in 65–66% overall yields. The
heteroarenes 14e, 14f and a silylated boronic ester were
eligible substrates and provided 16e–g in 51–70% yield. When
the OTs-protected phenol 14h was subjected to the sequence,
the deprotected phenol 16h resulted (48%). The sequence was
also applied to the hydromethylation of more complex
alkenes such as the uorine containing system 14d to obtain
the hydromethylated compound 16d in 66% yield. The hetero
arenes 14e and 14f were tolerated and provided the products
16e and 16f in 51–55% and yields. When the OTs-protected
phenol 14h was subjected to the sequence the deprotected
compound 16h was isolated in 48% yield. The method was
also applied to cyclic olens such as the cyclohexane deriva-
tive 14i to give the product 16i in 61% yield, the methyl pro-
tected (ꢁ)-D8-THC 14j and the cholesterol derivative 14k.
Stereo- and regioselective hydroboration using catecholbor-
ane under neat conditions16 followed by transesterication
with pinacol gave the 2ꢀ alkyl boronic esters 14j and 14k in
55% to 65% yield as single diastereoisomers (see ESI†). CH2-
Scheme 3 Radical probe experiment and proposed mechanism.
thiyl radical, which is reduced by the Ir(II)-complex to regenerate
the starting Ir(III)-complex PC1.
To further document the potential of the protodeboronation,
we applied the novel protocol to the formal total synthesis of
two indolizidine-core containing secondary metabolites, d-(R)-
coniceine (9) and indolizidine 209B (13) (Scheme 4). Starting
with commercial N-Boc-pyrrolidine (4), enantiopure boronic
ester 5 was prepared via a (+)-sparteine mediated asymmetric
lithiation/borylation.54–56 The ester moiety was introduced via
stereospecic radical-induced 1,2-migration27,28 of the corre-
sponding in situ generated vinyl and isopropenyl boron ate
complexes with a-iodo acetates as radical precursors to provide
6 and 10 in 52% and 65% yield. In the coniceine route, 6 was
directly subjected to the protodeboronation to provide the
pyrrolidine 7 in 61% yield. Aer ester cleavage and Boc-
deprotection, the corresponding carboxylic acid was cyclised
with EDC and DMAP to yield the bicyclic lactam 8 in 87% yield,
completing the formal total synthesis57 of d-(R)-coniceine (9). A
crucial step in the indolizidine 209B synthesis was the build-up
Scheme 4 Formal total synthesis of d-(R)-coniceine (9) and indolizidine 209B (13). Reagents and conditions: (a) s-BuLi (1.2 equiv.), (+)-sparteine
(1.2 equiv.) Et2O, ꢁ78 ꢀC, 3 h then isopropyl pinacol borate (1.3 equiv.), ꢁ78 ꢀC, 2 h; (b) vinyllithium (1.3 equiv.), Et2O, ꢁ78 ꢀC to rt, 30 min then tert-
butyl 2-iodoacetate (2.0 equiv.), MeCN, hn, 50 ꢀC, 24 h; (c) PhLi (1.1 equiv.), Et2O, 0 ꢀC to rt, 30 min then PhSH (1.1 equiv.), PC1 (2.5%), blue LED,
MeOH/acetone, 16 h; (d) HCl in dioxane, rt, 8 h then EDC$HCl (2.0 equiv.), DMAP (20 mol%), Hunig's base (2.0 equiv.), CH2Cl2, rt, over night; (e)
¨
see ref. 57; (f) isopropenyllithium (1.3 equiv.), Et2O, ꢁ78 ꢀC to rt, 30 min then ethyl 2-iodoacetate (5.0 equiv.), MeCN, hn, 50 ꢀC, 24 h; (g) TFA (10
eq.), CH2Cl2, 0 ꢀC to rt, 2 h then NEt3 (20 equiv.), CH2Cl2, rt, 12 h; (h) (3,5-bis(trifluoromethyl)phenyl)lithium (1.3 equiv.), Et2O, ꢁ78 ꢀC, 1 h then
PhSH (1.3 equiv.), PC1 (2.5%), blue LED, MeOH/acetone, 16 h; (i) see ref. 58.
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