Reductive Difunctionalization of Aryl Alkenes with Sodium Metal and Reduction-Resistant Alkoxy-Substituted Electrophiles

Article abstract of DOI:10.1021/acs.orglett.0c00490

A general method for alkali-metal-promoted reductive difunctionalization of alkenes has been developed by means of reduction-resistant alkoxy-substituted electrophiles. A series of 1,2-diboration and 1,2-dicarbofunctionalization products can be synthesize

Full text of DOI:10.1021/acs.orglett.0c00490

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Letter
Reductive Difunctionalization of Aryl Alkenes with Sodium Metal
and Reduction-Resistant Alkoxy-Substituted Electrophiles
Mizuki Fukazawa, Fumiya Takahashi, Keisuke Nogi,* Takahiro Sasamori, and Hideki Yorimitsu*
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ABSTRACT: A general method for alkali-metal-promoted reductive difunc-
tionalization of alkenes has been developed by means of reduction-resistant
alkoxy-substituted electrophiles. A series of 1,2-diboration and 1,2-dicarbo-
functionalization products can be synthesized by employing trimethoxyborane
and strained cyclic ethers such as oxirane and oxetane. In addition,
unsymmetrical carbo- or thioborations have been accomplished via sequential treatment with trimethoxyborane and carbon or
sulfur electrophiles.
Scheme 1. Reductive Difunctionalization of Alkenes with
Alkali Metals
lkali-metal-promoted reduction of unsaturated hydro-
carbons represented by trans-hydrogenation of alkynes
A
has been regarded as a classical yet powerful method in organic
synthesis.1 As a pioneering discovery, in 1914, Schlenk found
that C−C double bonds of alkenes can be reduced by sodium
metal.2 Although many follow-up works have been developed,
those transformations generally focus on introduction of
hydrogen onto alkenes, the accessible products thus being
simple alkanes (Scheme 1a, left).1 Difunctionalization of
alkenes by means of aprotic electrophiles has been far less
developed despite its latent synthetic utility (Scheme 1a, right).
To achieve such reductive difunctionalization, two intrinsic
concerns should be addressed: (1) decomposition of labile
anionic species and (2) preferential reduction of aprotic
electrophiles instead of alkenes. Radical anions generated via
single-electron reduction (SER) of alkenes are often labile and
decompose via oligomerization3 or protonation (Scheme 1b-
1). Electrophiles should be thus placed together with alkenes
in the same reaction vessel before the SER event. However,
typical aprotic electrophiles including organic halides or
carbonyl compounds are preferentially reduced over alkenes,
which results in homocoupling of the halides or carbonyls with
recovery of alkenes (Scheme 1b-2). Probably due to these
difficulties, only two examples, disilylation and dialkylation
with methylchlorosilanes and dialkyl ketones, have been
reported.4 Even worse, these reactions suffer from restricted
substrate scope: only styrene and α-methylstyrene are
applicable. Development of general strategy for alkali-metal-
promoted reductive difunctionalization is thus of high value.5,6
From this perspective, we focused on alkoxy-substituted
electrophiles as competent electrophiles. The resonance effect
of their oxygen atoms renders alkoxy-substituted electrophiles
resistant to reduction; that is, alkoxy-substituted electrophiles
can be compatible with SER of alkenes with alkali metals
(Scheme 1c, step a). As a consequence, the labile radical
anions generated can be instantly trapped by the coexisting
alkoxy-substituted electrophiles, which ends up with reductive
difuctionalization of alkenes (Scheme 1c, step b). To
demonstrate the feasibility and utility of the strategy, we first
exploited reductive diboration of alkenes by using trialkox-
Received: February 7, 2020
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© XXXX American Chemical Society
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A

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yboranes as reduction-resistant boron electrophiles for the
synthesis of 1,2-bis(boronate)s of high synthetic versatility.7−9
We started optimization of the diboration by employing (E)-
4-methoxystilbene ((E)-1a) and B(OMe)3. A Na dispersion10
(2.5 equiv) was added to a THF solution of (E)-1a, B(OMe)3
(6 equiv), and LiI (2.5 equiv) at 0 °C (Table 1). Owing to the
and B(OMe)3. In the absence of B(OMe)3, treatment of 1a
with Na dispersion caused rapid decomposition of anionic
species within 1 min, and subsequent addition of B(OMe)3
afforded only a 17% yield of 2a (entry 12). Several byproducts
including 1-(4-methoxyphenyl)-2-phenylethane were observed.
Under the optimized conditions (Table 1, entry 11), 2a was
isolated in 67% yield from (E)-1a with high threo selectivity
(Table 2, entry 1). Interestingly, the reaction of (Z)-1a also
Table 1. Optimization Study for Reductive Diboration of
(E)-1a
Table 2. Reaction Scope with Respect to Stilbene
Derivatives
NMR yield (%)
entry
1
2
deviations from above conditions
(dr)
none
68 (90:10)
64 (88:12)
5 (31:69)
66 (69:31)
36 (90:10)
40 (91:9)
12 (93:7)
68 (87:13)
48 (72:28)
73 (46:54)
72 (93:7)
17 (80:20)
a
MeOBpin instead of B(OMe)3
B(OiPr)3 instead of B(OMe)3
without LiI
Li powder instead of Na dispersion
Li powder instead of Na dispersion without LiI
Li granule instead of Na dispersion without LiI
DME instead of THF
Et2O instead of THF
1,4-dioxane instead of THF
THF/TMEDA (v/v = 9/1)
3
4
5
6
7
8
9
b
10
11
12
c
THF/TMEDA (v/v = 9/1)
a
b
c
Without addition of pinacol. At room temperature. B(OMe)3 was
added 1 min after completion of the addition of Na dispersion.
large surface area of Na dispersion (average particle size: < 10
μm), the reaction was complete within 0.5 h. After protection
of the installed boron moieties with pinacol, a 68% yield of
diboration product 2a was obtained in a good diastereomeric
ratio (dr) (entry 1). The alkoxy substituents on the boron
electrophiles have an important effect. Although the reaction
proceeded with MeOBpin with slightly lower product yield,
the employment of B(OiPr)3 significantly decreased the yield
of 2a (entries 2 and 3). The steric hindrance around the boron
atom of B(OiPr)3 would hamper the C−B bond formation
(see Scheme 2 for mechanistic discussions). The addition of
LiI is effective for high threo selectivity; the dr was decreased to
69:31 in the absence of LiI (entry 4). The diboration
proceeded with Li powder (particle size: 10−200 μm) instead
of Na dispersion with high stereoselectivity, whereas the
conversion of 1a was insufficient (entries 5 and 6). We
presume that the electron transfer from the Li powder would
be slower than that from the Na dispersion due to the smaller
surface area of the Li powder. The particle size of reductant is
indeed important; only a 12% yield of 2a was obtained with
granular Li (particle size: ca. 3 mm) (entry 7). Instead of THF,
the reaction also proceeded in 1,2-dimethoxyethane (DME)
while the use of Et2O fairly decreased the yield and
diastereoselectivity (entries 8 and 9). Although 2a was
obtained in 73% yield by utilizing 1,4-dioxane as the solvent,
the dr was almost 1:1 (entry 10). Eventually, the yield of 2a
increased to 72% by employing THF/TMEDA (v/v = 9/1)
cosolvent system (entry 11).
a
b
5.0 mmol scale. Isolated as the corresponding 1,2-diol after
c
oxidation with H2O2. Determined by 1H NMR and GC−MS
analyses of the crude product.
afforded threo-2a in a similar yield with a similar
diastereoselectivity (entry 2). Moreover, threo-2a was obtained
in high dr (93:7) even from an isomeric mixture of 1a (E/Z =
51:49) (entry 3). In general, catalytic and noncatalytic
diboration of alkenes proceeds in a syn fashion: the reactions
of (E)-alkenes mainly provide threo products, while those of
Notably, the order of adding the reagents was critical for the
diboration; Na dispersion must be added to a mixture of 1a
B
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(Z)-alkenes provide the erythro products.7,8 In sharp contrast,
our diboration affords threo-1,2-diborylalkanes regardless of the
stereochemistry of alkenes, which allows us to avoid laborious
purification of stereoisomeric mixtures of alkene.
Table 3. Reaction Scope with Respect to Styrene Derivatives
The stereoconvergent diboration also proceeded with (E)-
and (Z)-stilbenes (1b) to afford threo-2b in good yields
(entries 4−6). The dimethylamino group of 1c did not hamper
the diboration (entry 7). Owing to the instability of the
corresponding 1,2-diborylalkane 2c, the product was isolated
as 1,2-diol 3c after oxidation by H2O2. The reaction took place
with methylsulfanyl-substituted 1d to provide the correspond-
ing diborylalkane 2d in 45% yield accompanied by small
amounts of the corresponding monoborylation products and
remaining 1d (entry 8). 4-Fluorostilbene (1e) was hardly
applicable to the reaction; only a 29% yield of diboration
product 2e was obtained, whereas substrate 1e was fully
consumed (entry 9).11 On the other hand, the reaction with 3-
fluorostilbene (1f) successfully proceeded and subsequent
H2O2 oxidation afforded 1,2-diol 3f in 69% yield in high dr
(entry 10). The ortho-substituents of 1g and 1h did not disturb
the reaction resulting in the formations of 1,2-diborylalkanes
2g and 2h in 77% and 74% yields, respectively (entries 11 and
12).
Instead of stilbene derivatives, both (E)- and (Z)-β-
methylstyrene (1i) provided 2i in moderate yields in the
same dr (Table 3, entries 1 and 2). Although β-cyclo-
hexylstyrene 1j provided the product in 36% yield, the bulkier
tert-butyl substituent of 1k was detrimental to the reaction
(entries 3 and 4). Particularly in the latter case, borylation
products were hardly observed while a large part of 1k was
consumed. These suggests that bulky alkyl groups would
hamper the reaction of radical anions A (See Scheme 2) with
B(OMe)3. The reaction with β-cyclopropylstyrene 1l afforded
2l in 53% yield (entry 5). Notably, no ring-opened products
were observed. Because the ring opening of cyclopropylmethyl
anion (t1/2 > 1800 s, at −70 °C)12 is much slower than that of
cyclopropylmethyl radical (t1/2 = 5.3 × 10−7 s, at −70 °C),13
the formation of 1l-A having benzylic radical would be more
reasonable than the formation of 1l-A′ (eq 1).
a
b
1
With 10 equiv of B(OMe)3. Determined by H NMR analysis of
the crude product after H2O2 oxidation.
As described in Tables 2 and 3, threo-diboration products
were selectively obtained regardless of the stereochemistry of
alkenes. Based on this stereoconvergence, we propose a
reaction mechanism as shown in Scheme 2. First, SER of
alkene with Na dispersion generates the corresponding anionic
A having benzylic radical with the loss of the stereochemistry
of the C−C double bond. Radical anion A would be instantly
trapped with B(OMe)3 to generate borate B. The second SER
with Na dispersion would convert B to borylated carbanions C
and C′ that are stabilized by the intramolecular coordination of
the methoxy group to the alkali metal. Although C and C′
would interconvert to each other via inversion of the benzylic
anion center,14 the latter would be more stable due to the
steric repulsion between the substituents Ar and R. Finally, C−
B bond formation between C′ and B(OMe)3 would proceed
with retention of the stereochemistry to afford threo-diboration
product.15 We presume that the addition of LiI would
The present diboration selectively occurs at conjugated C−
C double bonds; the reaction of 1-phenyl-1,11-dodecadiene
(1m) furnished diboration product 2m in 53% with the
terminal olefin moiety being intact (Table 3, entry 6). The
reaction accommodated cyclic 1,2-dihydronaphthalene (1n) to
afford trans-2n in 44% yield as a sole product (entry 7). In
place of aryl and alkyl substituents, the pinacolatoboryl group
at the β position of 1o did not prevent the reaction to furnish
triborylalkane 2o in 62% yield (entry 8). Alkenes 1 should
have at least one aromatic ring on the olefinic carbons, and no
reaction took place with 4-octene because SER with Na
dispersion did not occur. As another conjugated alkene, we
tried the reductive diboration of 1,3-cyclohexadiene (1p). The
reaction with B(OMe)3 before H2O2 oxidation afforded the
corresponding 1,2-diol 3p and 1,4-diol 3p′ albeit with low
yields (entry 9).
C
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modulate the reactivity of carbanions C and C′ by Na-to-Li
transmetalation. Owing to the higher reactivity of a C−Na
bond than that of a C−Li bond, in the absence of LiI, C can
also react with B(OMe)3 before isomerization into C′.
Attempts to isolate borylated carbanion intermediates C and
C′ resulted in the unexpected formation of six-membered 1,4-
diboratinane like C′2 (confirmed by single-crystal X-ray
diffraction analysis16) generated through dimerization of C′.
In light of the proposed mechanism in Scheme 2, we
envisioned that trapping of borylated carbanion C’ with other
Scheme 4. Bis(hydroxyalkylation) with Oxirane and
Oxetane
Scheme 2. Possible Reaction Mechanism
a
1
Determined by H NMR analysis of the crude products.
instance, isobutylene oxide was involved in the reaction with
alkenes to afford the corresponding 1,6-diols 5a and 5b with
high erythro selectivity. The two stereoisomers of the products
could be separated by column chromatography on silica gel;
isomerically pure 5a and 5b were isolated in 67% and 65%
yields, respectively. The stereochemistry of erythro-5a was
confirmed by X-ray crystallographic analysis.16 Instead of
isobutylene oxide, oxetane also underwent the reaction with 1b
to provide 1,8-diol 5c in good yield albeit with lower
diastereoselectivity. The two diastereomers were separated by
column chromatography on silica gel, and 31% and 48% yields
of threo- and erythro-5c were obtained, respectively.
electrophiles would accomplish unsymmetrical difunctionaliza-
tion (Scheme 3). Stilbene (1b) reacted with 1 equiv of
Scheme 3. Reductive Carbo- and Thioboration
In conclusion, we have developed a general strategy for
alkali-metal-promoted reductive difunctionalization of alkenes.
The use of reduction-resistant alkoxy-substituted electrophiles
allowed selective SER of alkenes, which ended up with
introduction of carbon, boron, and even sulfur atoms onto
alkenes. The combined use of alkali metals and alkoxy-
substituted electrophiles should be a prospective strategy for
reductive functionalizations of various unsaturated hydro-
carbons.
ASSOCIATED CONTENT
* Supporting Information
■
sı
a
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00490.
Stereochemistry was not determined.
Experimental procedures, X-ray crystallographic analysis,
and spectral data (PDF)
B(OMe)3 at −78 °C with the aid of Na dispersion, and 3.0
equiv of MeI were subsequently added. Gratifyingly, desired
methylboration product 4a was successfully obtained in 69%
yield. Notably, in contrast to the diboration, erythro-4a was
obtained as a major product with high stereoselectivity. We
speculate that the C−C bond formation between C′ and MeI
proceeds with inversion of the stereochemistry of the
nucleophilic carbon center.15 The use of butyl iodide and
allyl chloride also furnished erythro-4b and -4c, respectively.
Instead of carbon electrophiles, the reaction with dimethyl
disulfide provided thioboration product 4d.
As shown above, alkoxy-substituted boron electrophiles,
especially B(OMe)3, have proved to be suitable for the alkali-
metal-promoted difunctionalization. To further validate the
generality of this strategy, we attempted to employ alkoxy-
substituted carbon electrophiles. To our delight, strained cyclic
ethers were applicable to the present system (Scheme 4). For
Accession Codes
CCDC 1978131−1978132 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Keisuke Nogi − Department of Chemistry, Graduate School of
Science, Kyoto University, Kyoto 606-8502, Japan;
orcid.org/0000-0001-8478-1227; Email: knogi@
kuchem.kyoto-u.ac.jp
D
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Ishino, Y.; Tsumagari, Y.; Nishiguchi, I. J. Org. Chem. 1995, 60,
458. (b) Ohno, T.; Sakai, M.; Ishino, Y.; Shibata, T.; Maekawa, H.;
Nishiguchi, I. Org. Lett. 2001, 3, 3439. (c) Liu, Y.; Li, Y.; Qi, Y.; Wan,
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H.; Honda, J.; Akaba, R. Tetrahedron Lett. 2012, 53, 6519. For
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Noda, K.; Kuramochi, K.; Zhang, T. Org. Lett. 2018, 20, 1953.
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bis(boronate)s: (a) Takaya, J.; Iwasawa, N. ACS Catal. 2012, 2,
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Hideki Yorimitsu − Department of Chemistry, Graduate School
of Science, Kyoto University, Kyoto 606-8502, Japan;
orcid.org/0000-0002-0153-1888; Email: yori@
kuchem.kyoto-u.ac.jp
Authors
Mizuki Fukazawa − Department of Chemistry, Graduate School
of Science, Kyoto University, Kyoto 606-8502, Japan
Fumiya Takahashi − Department of Chemistry, Graduate
School of Science, Kyoto University, Kyoto 606-8502, Japan
Takahiro Sasamori − Graduate School of Natural Sciences,
Nagoya City University, Nagoya, Aichi 467-8501, Japan;
orcid.org/0000-0001-5410-8488
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00490
́
Shishido, R.; Ito, H.; Fernandez, E. Chem. Soc. Rev. 2017, 46, 415.
(8) Selected recent examples of diboration of alkenes: (a) Miralles,
́
́
N.; Cid, J.; Cuenca, A. B.; Carbo, J. J.; Fernandez, E. Chem. Commun.
2015, 51, 1693. (b) Farre, A.; Soares, K.; Briggs, R. A.; Balanta, A.;
Benoit, D. M.; Bonet, A. Chem. - Eur. J. 2016, 22, 17552. (c) Fang, L.;
Yan, L.; Haeffner, F.; Morken, J. P. J. Am. Chem. Soc. 2016, 138, 2508.
(d) Yan, L.; Meng, Y.; Haeffner, F.; Leon, R. M.; Crockett, M. P.;
Morken, J. P. J. Am. Chem. Soc. 2018, 140, 3663.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(9) Our recent report on borylation chemistry with alkali metals and
bis(pinacolato)diboron (B2pin2): Takahashi, F.; Nogi, K.; Sasamori,
T.; Yorimitsu, H. Org. Lett. 2019, 21, 4739.
This work was supported by JSPS KAKENHI Grant Numbers
JP16H04109, JP18H04254, JP18H04409, JP19H00895, and
JP18K14212 as well as JST CREST Grant Number
JPMJCR19R4. We thank KOBELCO ECO-Solutions Co.,
Ltd., for providing Na dispersion. We also thank Prof. Shohei
Saito and Mr. Takuya Yamakado (Kyoto University) for
measurement of the particle size of Li powder.
(10) For recent examples of reductive transformations by means of
Na dispersion as a highly reactive yet easy-to-handle reducing agent,
see ref 9 and (a) An, J.; Work, D. N.; Kenyon, C.; Procter, D. J. J. Org.
Chem. 2014, 79, 6743. (b) Han, M.; Ma, X.; Yao, S.; Ding, Y.; Yan, Z.;
Adijiang, A.; Wu, Y.; Li, H.; Zhang, Y.; Lei, P.; Ling, Y.; An, J. J. Org.
Chem. 2017, 82, 1285. (c) Han, M.; Ding, Y.; Yan, Y.; Li, H.; Luo, S.;
Adijiang, A.; Ling, Y.; An, J. Org. Lett. 2018, 20, 3010. (d) Lei, P.;
Ding, Y.; Zhang, X.; Adijiang, A.; Li, H.; Ling, Y.; An, J. Org. Lett.
2018, 20, 3439. (e) Zhang, B.; Li, H.; Ding, Y.; Yan, Y.; An, J. J. Org.
Chem. 2018, 83, 6006. (f) Ding, Y.; Luo, S.; Adijiang, A.; Zhao, H.;
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Y.; Luo, S.; Ma, L.; An, J. J. Org. Chem. 2019, 84, 15827.
(11) 4-Fluorostilbene would undergo defluorination. Possible
defluorination pathways are shown in Scheme S1. Although we
tested the reaction of 4-chlorostilbene, dechlorination products such
as stilbene and 1,2-diphenylethane were obtained accompanied by the
remaining substrate.
(12) Lansbury, P. T.; Pattison, V. A.; Clement, W. A.; Sidler, J. D. J.
Am. Chem. Soc. 1964, 86, 2247.
(13) Calculated value based on the literature: Maillard, B.; Forrest,
D.; Ingold, K. U. J. Am. Chem. Soc. 1976, 98, 7024.
(14) (a) The Chemistry of Organolithium Compounds; Rappoport, Z.,
Marek, I., Eds.; Wiley: New York, 2003. (b) Hoppe, D.; Hense, T.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2282.
(15) Organolithium compounds can undergo electrophilic sub-
stitution with either retention or inversion of their stereochemistry of
the nucleophilic carbon center. The reaction pattern (inversion vs
retention) is often switched by changing electrophiles; see ref 14 and
(a) Gawley, R. E. Tetrahedron Lett. 1999, 40, 4297. (b) Basu, A.;
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E
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