Tertiary and Quaternary Carbon Formation via Gallium-Catalyzed Nucleophilic Addition of Organoboronates to Cyclopropanes

Article abstract of DOI:10.1021/acs.orglett.7b03349

GaCl₃ and (IPr)GaCl₃/AgSbF₆ formed γ-tertiary and γ-quaternary carbons via homoconjugate addition of organoboron nucleophiles to diester- and ketone-functionalized cyclopropanes. Electron donor group cyclopropane substituents were not needed, allowing electron-deficient aryl, alkenyl, alkyl, and hydrogen-substituted cyclopropanes to be used. The catalytic conditions were compatible with alkenyl, alkynyl, and aryl nucleophiles, including ortho-substituted aromatics, to synthesize highly hindered quaternary carbons. Alkynyl nucleophiles formed substituted cyclopentenes. A control experiment supports an intermediate carbocation in quaternary carbon center formation.

Full text of DOI:10.1021/acs.orglett.7b03349

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tertiary and Quaternary Carbon Formation via Gallium-Catalyzed
Nucleophilic Addition of Organoboronates to Cyclopropanes
Truong N. Nguyen and Jeremy A. May*
Department of Chemistry, University of Houston, 3585 Cullen Blvd, Fleming Building Rm 112, Houston, Texas 77204-5003, United
States
S
* Supporting Information
ABSTRACT: GaCl3 and (IPr)GaCl3/AgSbF6 formed γ-
tertiary and γ-quaternary carbons via homoconjugate addition
of organoboron nucleophiles to diester- and ketone-function-
alized cyclopropanes. Electron donor group cyclopropane
substituents were not needed, allowing electron-deficient aryl,
alkenyl, alkyl, and hydrogen-substituted cyclopropanes to be
used. The catalytic conditions were compatible with alkenyl, alkynyl, and aryl nucleophiles, including ortho-substituted aromatics,
to synthesize highly hindered quaternary carbons. Alkynyl nucleophiles formed substituted cyclopentenes. A control experiment
supports an intermediate carbocation in quaternary carbon center formation.
significant synthetic challenge in organic chemistry is the
quaternary centers via Lewis acid catalysis (Scheme 1C). This
Acontrolled formation of quaternary carbon centers, plan was predicated on access to stabilized tertiary cationic
especially via robust reactions from easily obtained starting
materials.1 To form these hindered carbons in a fixed relationship
to versatile functional group handles such as carbonyls is key to
generating synthetic building blocks for molecular targets. In
general, the formation of carbonyl compounds with α-2 and β-
quaternary carbons3 has seen greater success than the formation
ofall-carbonquaternarycentersatthecarbonylγ-position.4 Given
recent successes in β-5 and γ-C−C bond forming reactions6 using
neutral organoboronate π-nucleophiles (Scheme 1A,B, respec-
tively), where the boron substituent controls the site of bond
formation instead of Friedel−Crafts or Markovnikov consid-
erations, we saw an opportunity for the controlled formation of γ-
intermediates formed from opening the disubstituted donor/
acceptor cyclopropane 5.
The addition of carbon nucleophiles to donor/acceptor
cyclopropanes has been well established, with examples dating
back over a century.7 However, only recently has the use of
nucleophilic organoboronates been reported.6 Historically, a
productive reaction has required strong electron-withdrawing
(“acceptor”) cyclopropyl substituents. Only the acceptor group is
sufficient for C−C bond formation with strongly nucleophilic
carbanions. The additional presence of a cyclopropyl electron
donor group is necessary for the use of neutral carbon
nucleophiles. Substrates bearing only one electron deficient
group usually react poorly.7 Examples of the formation of
quaternary carbon centers via nucleophilic cyclopropane opening
are rare.8,9 Rearrangements are more common for generating
quaternary centers from cyclopropanes.10
Scheme 1. Reactions with Organoboron Nucleophiles
Having recently reported an approach to tertiary carbon
formation using the cheap Brønsted acid catalyst (n-
6b
Bu)4NHSO4 and found that it was not suitable for the
formation of quaternary carbon centers, we looked to Lewis
acid catalysts to promote the nucleophilic ring opening of donor/
acceptor cyclopropane 7a with organoboronate nucleophile 8a
(Table 1). For preliminary studies, the formation of tertiary
carbon centers was attempted, though the use of the sterically
encumbered o,o-disubstituted aryl 8a was employed to ensure
that hindered C−C bonds could be formed. Unfortunately,
experiments with manycommon Lewisacids failed toprovideany
product (entry 1). The only initial encouraging results were the
use of Bi(OTf)3 or Sc(OTf)3 (entries 2 and 3). Gallium11 has
been shown to catalyze transformations utilizing organo-
Received: November 9, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03349
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Probing the Reaction Components
Scheme 2. Tertiary Carbons: Cyclopropyl Substituents
temp
(°C)
time
(h)
yield
a
entry catalyst (equiv) additive
solvent
(%)
b
c
1
2
3
Lewis acid
(0.20)
3 Å MS
3 Å MS
3 Å MS
DCE
40
24
24
24
<2
Bi(OTf)3
(0.20)
DCE
DCE
40
40
7
Sc(OTf)3
(0.20)
30
4
5
6
GaCl3 (1.0)
3 Å MS
MgSO4
AgSbF6
CH2Cl2
CH2Cl2
CH2Cl2
23
0.25
1
45
91
99
d
GaCl3 (0.60)
0−23
23
(IPr)GaCl3
(0.10)
5
a
Yield determined by 1H NMR integration relative to an internal
b
standard. Lewis acid = Yb(OTf)3, Sn(OTf)2, Cu(OTf)2, InCl3, SnCl4,
ZnCl2, EtAlCl2, FeCl2, La(OTf)3, and Gd(OTf)3. DCE = Cl-
(CH2)2Cl. MgSO4 did not catalyze the reaction in the absence of
c
d
a
b
Isolated yields (average of ≥2 trials). Five millimoles of 7a,
c
GaCl3. Cyclopropane added dropwise for 30 min.
IPrGaCl3 (1 mol %), AgSbF6 (2 mol %), 48 h, 91% yield. Product was
d
a single diastereomer. Cyclopropane added dropwise at −10 °C.
e
GaCl3 (1 equiv), −10−23 °C gave 65% yield of 9h.
stannanes12 andorganoboronates;13 therefore, itwasemployedin
this screen. GaCl3 turned out to promote the reaction at lower
temperature and much more rapidly than Bi(OTf)3 or Sc(OTf)3
(15 min vs 24 h), though initial trials used a full equivalent (entry
4). Analysis of reaction temperature, catalyst equivalents, and
additive led to a highly productive transformation using GaCl3
(0.6 equiv) in CH2Cl2 in the presence of MgSO4 while warming
from 0 to 23 °C (entry 5). Slow addition of the cyclopropane to a
solution of GaCl3 and the boronic acid significantly increased the
yield by preventing substrate dimerization. Exploring the use of a
cationic N-heterocyclic carbene (NHC)-ligated gallium catalyst,
generated from (IPr)GaCl3 and AgSbF6, also proved to be highly
amenable to forming the γ-substituted product 9a (entry 6).
(IPr)GaCl3 offers many benefits in its ease of handling and
stability, so this was the preferred catalyst where possible.
a
Scheme 3. Tertiary Carbons: Nucleophile Diversity
Reaction scope exploration focused on substrates that reacted
poorly for our Brønsted acid-catalyzed addition to cyclo-
propanes:6b substrates bearing neutral or electron poor groups
at the site of bond formation. (IPr)GaCl3 catalyzed the reaction
well for nearly all of these substrates. These new conditions
accommodated an unsubstituted phenyl substituent, and 9a was
obtained in excellent isolated yield even when the reaction was
performed on larger scale and the catalyst loading was reduced to
1 mol % (Scheme 2; footnote b). In general, diester-function-
alized substrates were examined because of the ease of
cyclopropane synthesis, but bis-activation was not required for a
successful reaction. The acceptor group could also be a ketone
(9b) as was the case with a Brønsted acid.6b Remarkably,
substrates bearing electron-deficient aromatics at the point of
attack performed as well as those with a relatively neutral phenyl
ring(9cand9d). Anadditionalringfusedtothecyclopropanehad
little impact on the reaction, and complete diastereoselectivity
was observed for trans-indane 9e. Products 9a−e exhibited
impeded rotation about the o,o-disubstituted aryl−C bond as
evidenced bythe symmetry of the dimethoxyphenyl beingbroken
a
Isolated yields (average of ≥2 trials). Cyclopropane added dropwise
b
over 30 min. Reaction run at −40 °C.
remarkable substrates gave 9g and 9h, which bore only an alkyl
chain or hydrogens as substituents at the reactive carbon. Only a
few existing cationic methods react with the latter substrate (i.e., 7
h) to form C−C bonds from cyclopropanes.8a,14
There was significant flexibility in the unsaturated nucleophiles
that were useful. Aryl rings with or without ortho substituents
worked well with (IPr)GaCl3 (Scheme 3). Thus, aryl rings with
electron-donating groups were excellent nucleophiles (9i−9k),
and a styrene boronic acid also reacted well (9f). The use of
alkenylboronic acids provides an orthogonal approach to starting
from vinyl cyclopropanes like that which gave 9f. While
alkynylboronic acids are inherently unstable, the alkynyl
trifluoroborate salt could be used successfully. The γ-substituted
adduct apparently undergoes an additional 5-endo-dig cycliza-
tion, however, to form cyclopentene 10a.15 In general, organo-
trifluoroborate congeners of 8 were productive nucleophiles, but
did not function as well as the boronic acids (see Supporting
Information for examples).
1
in the H NMR. Complete regioselectivity was observed in
generating the γ-addition product 9f, though there was the
potential for isomers to be formed from a putative allylic cationic
intermediate generated from a styrenyl cyclopropane. Two
B
DOI: 10.1021/acs.orglett.7b03349
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
yield. More sterically hindered quaternary centers, resulting from
replacing the methyl with a phenyl or benzyl group, afforded 12i
or 12j, respectively. The yields were somewhat lower for these
highly hindered carbon centers, but the products were again
obtained in useful quantities. If a larger i-propyl group was
incorporated, though, little of the product 12l was obtained.
As was the case for the tertiary centers, several additional
nucleophiles were used with the gallium catalyst to form
quaternary carbons. Thus, the conditions identified in Table 2
could give the mono-ortho methyl or isopropyl aryl ether adduct
12m or 12n, respectively (Scheme 5). Interestingly, the
Table 2. Optimization for Quaternary Carbon Formation
catalyst
(equiv)
time yield
a
entry
1
BXn
additive
AgSbF6
temp (°C)
(h) (%)
B(OH)2 (IPr)GaCl3
(0.10)
23
5
30
2
3
4
5
B(OH)2 GaCl3 (0.60)
MgSO4
MgSO4
MgSO4
MgSO4
0−23
0−23
−40−23
−78−23
1
1
3
5
35
42
60
94
BF3K
BF3K
BF3K
GaCl3 (0.60)
GaCl3 (0.60)
GaCl3 (0.60)
a
Scheme 5. Quaternary Carbons: Nucleophile Diversity
a
Yield determined by 1H NMR integration relative to an internal
standard. Cyclopropane added dropwise over 30 min.
One of the most significant challenges in organic synthesis is
the controlled formation of quaternary carbons.1 Examples of
cyclopropanes asprecursors forquaternary carbonsare veryrare,9
despite the ease with which these building blocks may be
synthesized. Yet when the tetrasubstituted cyclopropane 11a was
examined, it was found that the high reactivity of Ga as a catalyst
extendedtothatofahighlysubstituted cyclopropane, forming the
γ-quaternary carbon center in diester 12a in an efficient manner
(Table 2). Both (IPr)GaCl3 and GaCl3 initially produced
identifiable amounts of 12a, though the latter was more reactive
(entries 1 and 2). The trifluoroborate salts produced more
product than the boronic acids below room temperature (entry
3), and starting the reaction at lower temperatures improved the
outcome even more (entries 4 and 5).
a
Isolated yields (average of ≥2 trials). Cyclopropane added dropwise
b
over 30 min. Used GaBr3 (60 mol %) at −40−23 °C.
A β-methyl-β-phenyl-substituted cyclopropane provided the γ-
functionalized 12a in 92% yield after 5 h (Scheme 4). In general, a
variety of electron-donating and electron-withdrawing aryl
variations were viable in the reaction (12b−12g). Both aryl
groups in product 12b bear ortho substitution, and so formation
of that highly hindered C−C bond occurred in a lower but useful
nucleophile with the larger isopropyl substituent gave a higher
yield of product. The use of a styrenyl boronate was still tolerated
to give the allylic quaternary carbon in 12p. Lastly, an alkynyl
trifluoroborate generated the γ-substituted product of cyclo-
propaneopening, butagainunderwenta5-endo-digcyclizationto
cyclopentene 10b having two quaternary carbons.
For cyclopropanes with substituents, products derived from
nucleophilicattack attheunsubstitutedcarbonwerenotdetected.
This observation suggests that the stabilization of positive charge
controls regioselectivity in the reaction, although nucleophilic
attack can take place in the absence of cyclopropane substitution
(see 9h, Scheme 2). Our current hypothesis is that the degree of
intermediate cationic character at the electrophilic carbon varies
according to cyclopropane substitution. For 9h, little charge
buildup occurs, and the reaction is likely SN2-like (13, Scheme
6A). At the other end of the spectrum are the quaternary carbon-
forming substrates, which can form a carbocation/enolate 1,3-
dipole (14) that is typically invoked in cycloadditions to donor/
acceptor cyclopropanes (Scheme 6B).16 The latter supposition is
supported by a control experiment using the enantioenriched
cyclopropane 11e, which gave racemic γ-substituted 12e in 75%
yield (Scheme 6C).
a
Scheme 4. Quaternary Carbons: Cyclopropyl Substituents
In conclusion, the use of GaCl3 and (IPr)GaCl3/AgSbF6
facilitated the formation of γ-tertiary and γ-quaternary carbon
center formation via homoconjugate addition of organoboron
nucleophiles to diester- and ketone-functionalized cyclopro-
panes. Unlike previous homoconjugate additions, electron donor
group-substituents were not needed on the cyclopropanes,
allowing electron-deficient aryl, alkyl, alkenyl, and hydrogen-
substituted cyclopropanes to be viable. The catalytic conditions
were compatible with alkenyl, alkynyl, and aryl nucleophiles,
a
Isolated yields (average of ≥2 trials). Cyclopropane added dropwise
b
1
over 30 min. Product observed by H NMR, but not isolated.
C
DOI: 10.1021/acs.orglett.7b03349
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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̈
ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
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S
Complete experimental procedures and compound
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jmay@uh.edu.
ORCID
Jeremy A. May: 0000-0003-3319-0077
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Welch Foundation (grant E-1744) for generous
financial support.
■
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