Strategy towards olefin carbohydroxylation: Transmetalation of 2-rhodaoxetanes with organoboron nucleophiles

Article abstract of DOI:10.1002/anie.201003348

Taking the first step: A 2-rhodaoxetane undergoes efficient transmetalation with a variety of functionalized aryl- and alkenyl boronic acids (see scheme). This process exhibits excellent functional-group compatibility and constitutes the first step in a proposed catalytic oxidative olefin functionalization. Copyright

Full text of DOI:10.1002/anie.201003348

Angewandte
Chemie
DOI: 10.1002/anie.201003348
Olefin Functionalization
Strategy Towards Olefin Carbohydroxylation: Transmetalation of 2-
Rhodaoxetanes with Organoboron Nucleophiles**
Alexander Dauth and Jennifer A. Love*
Catalytic olefin functionalization provides a means to convert
readily available petrochemical feedstocks into valuable
synthetic building blocks. A conventional method for con-
verting olefins into functionalized alcohols involves sequen-
tial olefin epoxidation and nucleophilic ring opening. This
strategy suffers from the requirement of harsh conditions for
ring opening, which generally require strong nucleophiles.
Moreover, conversion of terminal aliphatic olefins into chiral
products is particularly problematic, owing to the low
enantioselectivity of terminal aliphatic olefin epoxidation,
despite recent progress.[1] Indeed, the typical strategy to
obtain chiral, aliphatic epoxides derived from terminal olefins
is by resolution of the enantiomeric mixture, which results in
loss of half of the material. A second strategy involves dipolar
epoxidations[12] and other olefin oxidation reactions[13] as well
À
as some C C bond-forming reactions as the Tebbe olefination
or the nickel-catalyzed reductive coupling of alkynes and
epoxides.[14] A number of different early,[15] mid,[16] and
late[12,13,17] transition metallaoxetanes have been prepared
and isolated; many of their structural features have been
investigated.
We envisioned carbohydroxylation could involve the
following steps: 1) oxidation of an olefin-coordinated metal
complex to the 2-metallaoxetane; 2) transmetalation, result-
ing in ring opening of the metallaoxetane; 3) reductive
elimination and subsequent olefin coordination to regenerate
the catalyst (Scheme 2). Upon workup, a functionalized
À
cycloaddition of nitriles or nitrenes with subsequent N O
bond scission.[2] Alternatively, sequential Paterno–Bꢀchi and
oxetane hydrogenation provides the corresponding carboxy-
droxylated product.[3] Both of these procedures involve more
than one step and have significant substrate limitations. In
comparison, a one-step carbohydroxylation reaction would be
more efficient. Nevertheless, only a few one-step protocols
have been disclosed; these, too, suffer from inefficiency or
limited scope.[4–8] Clearly, a need exists for more efficient
carbohydroxylation strategies with broader substrate scope.[9]
In response to this need, we propose that 2-metallaoxetanes
can serve as reactive intermediates in a novel catalytic
carbohydroxylation. Eventually, we anticipate that this meth-
odology will be applicable to the enantioselec-
tive synthesis of functionalized alcohol building
blocks from petrochemical feedstocks.
Scheme 2. Proposed catalytic cycle for carbohydroxylation of olefins.
2-Metallaoxetanes (Scheme 1) have been
invoked as intermediates in various transfor-
mations, most prominently olefin oxidation
reactions.[10] While their possible intermediacy
triggered controversial debates at times,[11]
recent experimental and theoretical data
clearly establishes their involvement in selected
alcohol product would be produced. The proposed carbohy-
droxylation would constitute a mild, one-step equivalent to
olefin epoxidation/nucleophilic ring opening, with different
stereochemical implications. With substituted olefins, this
method would provide complementary stereochemistry to
simple expoxidation/ring-opening if reductive elimination
proceeds with retention of configuration. The proposed
catalytic process can also be considered the equivalent of
Scheme 1.
Generic
structure of
metallaoxe-
tanes.
[*] A. Dauth, Prof. J. A. Love
Chemistry Department, University of British Columbia
2036 Main Mall, Vancouver, British Columbia, V6T 1Z1 (Canada)
Fax: (+1)604-822-2847
E-mail: jenlove@chem.ubc.ca
Homepage: http://www.chem.ubc.ca/personnel/faculty/love/
index.shtml
À
olefin insertion into C O bonds.
As a starting point, we sought to explore the reactivity of
2-metallaoxetanes with transmetalating reagents. Of partic-
ular interest was the report by de Bruin et al., demonstrating
the conversion of a TPA-RhI-ethylene complex 2 (TPA =
trispyridylmethylamine) into the corresponding rhodaoxe-
tane 3 by aqueous hydrogen peroxide solution (Scheme 3).[17a]
This would constitute the first step in a potential catalytic
oxidative olefin functionalization reaction (Scheme 2). The
mechanism of oxidation has been studied computationally.[18]
[**] We thank the following for support of this research: NSERC
(Discovery Grant, Research Tools and Instrumentation Grants),
University of British Columbia (UGF, Laird and 4YF to A.D.), and
Mountain Equipment Coop (A.D.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003348.
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1H NMR spectroscopy (Figure 1). Based on comparison to
1,3,5-trimethoxybenzene as an internal standard, 4a is
produced in 71% yield. Upon isolation, a 42% yield of 4a
is obtained.
Characterization of the product was conducted as follows.
The rhodaoxetane moiety of 3 has two signals (d = 4.99 ppm,
t, 3J(H,H) = 7.5 Hz, O-CH2-CH2 and 2.40 ppm, dt, 3J(H,H) =
7.6 Hz, 2J(Rh,H) = 2.5 Hz, CH2-CH2-Rh) that disappear
during the course of the reaction while the new product
shows two new signals (H1 = 3.92 ppm, dd, 3J(H,H) = 5.5 Hz,
J = 5.3 Hz and H2 = 3.55–3.47 ppm, m). Likewise, the signals
for the protons in the methylene bridges of the TPA ligand are
shifted to lower field (d = 5.37 ppm, d[AB], 2J(H,H) =
15.3 Hz in 3; 5.91 ppm, d[AB], 2J(H,H) = 15.1 Hz, H3 in
4a). Finally, the signals ascribed to the protons ortho to the
pyridine N atom also shift. Rhodaoxetane 3 has signals at d =
9.04 ppm, d, 3J(H,H) = 5.3 Hz and d = 8.71 ppm, d, 3J(H,H) =
6.1 Hz, whereas 4a has signals at 8.67, 3J(H,H) = 5.5 Hz (H8)
3
and d = 8.85 ppm, d, J(H,H) = 5.9 Hz (H8’). Two additional
new signals can be observed in the aromatic region of the
1H NMR spectrum: Two [AB] type doublets arise at d =
7.52 ppm and 7.25 ppm both integrating to 2H. Their
correlation is affirmed through the clear roofing effect as
well as cross-peaks observed in the COSY spectrum of the
compound. Similar results were obtained with (E)-styrylbor-
onic acid as a substrate which gave rise to compound 4b
(Scheme 5).
Scheme 3. Reported synthesis of 2-rhodaoxetane 3.
Inspired by the elegant simplicity of this report, we
decided to use rhodaoxetane 3 for preliminary stoichiometric
investigations. De Bruin et al. had reported reactivity of the
rhodaoxetane species with electrophiles, but nucleophiles
were found to be ineffective.[17d] Nevertheless, we anticipated
the transmetalation could be achieved under suitable con-
ditions. Thus, our first objective was to explore the reactivity
of rhodaoxetane 3 with various organometallic reagents.
Gratifyingly, 3 reacted readily with various aryl- and
alkenylboronic acids to generate 4, without any evidence of b-
hydride elimination (Scheme 4). It is noteworthy that 4 can be
Scheme 5. Reaction product with (E)-styrylboronic acid.
The chemical shifts are consistent with the observations of
de Bruin et al. regarding acid-mediated rhodaoxetane ring
3
opening. The smaller J coupling constants for the signal at
Scheme 4. Transmetalation of 3 with arylboronic acids.
d = 3.92 ppm (H1) of 5.5 Hz and 5.3 Hz could also indicate a
ring expansion through transesterification of the organo-
boronic acid to a six-membered oxametallacycle rather than
ring opening[17d] to give a compound of the general structure
of 5 (Scheme 6). Low-resolution ESI mass spectra of the
reaction products show a single signal at M+ reduced by 18
mass units. This can be best explained by dehydration of the
boronic acid moiety of the transmetalated products in the
spectrometer. It should be noted that the MS results are also
consistent with transesterification of the boronic acid. High
resolution mass spectra confirm the empirical formula.
To differentiate between these options and in the absence
of X-ray quality crystals, we performed further spectroscopic
generated directly from the olefin complex 2 with H2O2 and
arylboronic acid being added simultaneously.[19] Moreover,
although Kuwano and co-workers recently proposed trans-
metalation of organoboron nucleophiles to a RhIII center as a
possible catalytic pathway,[20] to the best of our knowledge our
work presents the first experimental evidence for transmeta-
lation involving a RhIII O bond.
À
For example, when rhodaoxetane 3 is treated with one
equivalent of p-BrC6H4B(OH)2 in methanol at room temper-
ature over 8 h one new product (4a) is formed as indicated by
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Figure 1. 1H NMR spectrum comparing 3 (top) to 4a (bottom).
in any of the other signals in the spectrum which suggests that
the observation is valid.[24]
In addition, the 2D NOESY spectrum of 4a shows a clear
NOE contact between the axial TPA–methylene bridge
proton signal at d = 5.91 ppm (H3) and the H10 signal at
d = 7.52 ppm (Figure 1). In the (E)-styrylboronic acid product
4b a similar contact is observed between H10 and H3 (axial).
There are also contacts between H9 and H3 (axial) as well as
H10 and H1. It is questionable whether these correlations
would be possible in the six-membered ring structures 5a or
5b.
Scheme 6. Possible alternative structures.
When rhodaoxetane 3 was exposed to alkylboronic acids
under standard conditions no conversion of starting material
could be detected (Table 1, entries 13–15). This can be
interpreted as a greater reluctance of sp3-hybridized carbon
centers to perform transmetalation which is commonly
observed. In comparison, we would expect that alkylboronic
acids should undergo the ring expansion (transesterification)
in the same manner as their aryl and alkenyl counterparts.
Furthermore, organoboronic esters reacted successfully
and the expected products were formed in comparable yields.
Yet, the reaction times were notably longer when compared
to their parent boronic acid (compare Table 1, entry 4 and
Table 2 entries 4 and 5). Employment of the sterically more
demanding pinacol ester led to a greater increase in reaction
time. The spectra of 4a, 4s, and 4t, referenced to an internal
standard, show different chemical shifts for most signals
which would not be expected in the case of transesterification
as the same product should be formed. These results are
consistent with transmetalation (which would form a different
product in each case), rather than transesterification of the
investigations. The multiplicity of the 13C signal of C9 should
elucidate this issue. A doublet would appear in the case of
coupling to Rh (I = 1/2) while a quartet should indicate
connectivity to boron (I = 3/2). 13C NMR data of the isolated
product complexes was inconclusive as the quarternary signal
of interest (C9) could not be detected directly. Literature
[22]
À
precedents exist for both cases—non-observable C B as
well as C Rh[23] signals.
À
In 4a indirect detection of the quarternary carbon atom
through HMBC (heteronuclear multiple bond coherence)
contacts to H10 and H11 was possible, and in compound 4b an
HSQC (heteronuclear single quantum coherence) experi-
ment allowed for indirect detection of C9. The shape of the
HSQC signal indicates the multiplicity of a doublet for the
carbon signal with a coupling constant 2J of ca. 6 Hz.
Although the digital resolution of the experiment (7.4 Hz) is
on the same scale as the observed coupling constant, the offset
of the HSQC signal was reproducible and it was not detected
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Communications
Table 1: Scope of transmetalation.
standard. In some cases, the product was isolated; yields are
given in parentheses. 1H NMR spectra, as well as HRMS,
have been obtained for all products, 13C NMR spectra could
be acquired for the isolated products. The transmetalation
reaction can also be carried out successfully in acetone or
dichloromethane although limited solubility of the boronic
acids in the latter requires slightly longer reaction times.
One equivalent of boronic acid was sufficient to com-
pletely consume the starting rhodaoxetane and obtain
moderate to excellent yields. As expected, reaction rates
increased with an increase of concentration of the organo-
boron nucleophiles. It is noteworthy that up to an excess of
150 equivalents, the reaction rate still seemed to be dependent
on the concentration of the boronic acid and no saturation
was reached. While the reaction of 3 with 1 equivalent of 4-
bromophenylboronic acid took just over 7 h to come to
completion at room temperature, the employment of
10 equivalents reduced the reaction time to 280 min and
upon use of 100 equivalents the reaction was completed after
Entry[a]
R
X
Product
t
Yield[b]
1
2
3
4-X-C6H4
(E)-styryl
4-X-C6H4
Br
4a
4b
4c
4d
4e
4 f
4g
4h
4i
280 min 71% (42%)
420 min 63% (56%)
250 min 71%
200 min 92%
560 min 94% (75%)
490 min 69%
270 min 42%
600 min 87% (48%)
n/a
380 min 73%
180 min 77%
400 min 53%
14 days
14 days
14 days
Cl
4
F
5
H
6
7
CH3
Ph
8
9
OCH3
OH
N(CH3)2
C(O)CH3 4k
SCH3
0%
10
11
12
13[c]
14[c]
15[c]
4j
1
90 min (reaction times monitored by H NMR spectroscopy
in 10–30 min intervals).
4l
CH3
CH3CH2
CH3(CH2)3
4m
4n
4o
0%
0%
0%
Overall, the reaction showed excellent functional-group
compatibility. While an unprotected alcohol functionality is
not tolerated by the protocol (Table 1, entry 8), arylboronic
acids with halide (Table 1, entries 1–3), ether (Table 1,
entry 7), tertiary amine (Table 1, entry 9), ketone (Table 1,
entry 10), and thioether (Table 1, entry 11) substituents read-
ily generated the corresponding ring-opened products in 42–
94% NMR yields. In addition, the reaction yield is remark-
ably insensitive to electronic effects, as arylboronic acids with
electron-withdrawing, electron-donating, and neutral sub-
stituents reacted cleanly.
[a] Reaction conditions: 5 mg 3 (0.1 mL of a 0.086m standard solution),
10 equiv boronic acid, 0.6 mL CD3OD, RT. [b] NMR yield referenced to an
internal standard of 1,3,5-trimethoxybenzene; isolated yield after
purification in parentheses. [c] No conversion of starting material was
observed after 14 days.
Table 2: Product-based mechanistic studies.
De Bruin et al. suggested a mechanism for acid-mediated
ring opening of rhodaoxetane 3; protonation of the rhodaox-
etane oxygen atom activates the structure which then opens
À
under Rh O bond cleavage in the presence of a coordinating
ligand.[17d] In transmetalation, coordination of the rhodaox-
etane oxygen would generate an -ate complex. In the present
studies, no intermediates were observed spectroscopically
([D4]methanol, RT). The data in Table 1 show that arylbor-
onic acids bearing electron-withdrawing groups reacted the
fastest, although there is no apparent linear correlation.
Typically, formation of borate complexes during transmeta-
lation is rate limiting and sensitive to electronic effects,[21]
which is consistent with both of our observations. Accord-
ingly, we propose the following mechanism for transmetala-
tion: In a first step the rhodaoxetane oxygen coordinates to
the boron center to give the -ate complex. This activates both,
the oxetane and the boronic acid for the next step, the actual
transmetalation from boron to rhodium (Scheme 7).
Entry[a]
X
Y
Product
t
Yield[b]
1
2
3
4
5
6
2-Br
3-Br
B(OH)2
B(OH)2
4p
4q
4r
270 min 75%
300 min 71%
14 days
440 min 62%
350 min 68%
21 days
2,6-(CH3)2 B(OH)2
31%
4-Br
4-Br
4-Br
pinacol ester
neopentyl ester 4t
BF3K 4u
4s
0%
[a] Reaction conditions: 5 mg 3 (0.1 mL of a 0.086m standard solution),
10 equiv boronic acid, 0.6 mL CD3OD, RT. [b] NMR spectroscopic yield
referenced to an internal standard of 1,3,5-trimethoxybenzene.
aryl boronic acid or esters, which would show the same
product in each case.
To further test our mechanistic hypothesis, we examined
other organoboron nucleophiles. Increased steric bulk around
the boron center (as with ortho substitution or boronic esters)
should impede coordination of the oxetane and thus slow the
reaction. In addition, if an -ate complex is indeed formed,
trifluoroborates—in which the boron center is already
tetravalent—should react slowly, if at all. If no pre-coordina-
tion of the oxetane is necessary, trifluoroborates should be
reactive.
Overall, this data is more supportive of transmetalation of
4-bromophenylboronic acid than transesterification/ring
expansion. Having established that transmetalation was
feasible, we sought to explore the reactivity of a series of
para-substituted arylboronic acids (Table 1, entries 1 and 3–
11). In most cases, the yields reported are based on 1H NMR
spectroscopy, using 1,3,5-trimethoxybenzene as an internal
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As evident in Table 2, ortho or meta monosubstituted
boronic acids did not affect the reaction rate or the yield
significantly (Table 2, entries 1 and 2). In the case of two ortho
substituents (Table 2, entry 3) though, both reaction rate and
yield are decreased substantially. The reactivity of aryl
boronic esters and lack of reactivity of aryl trifluoroborates
supports the postulated mechanism of involving pre-coordi-
nation of boron to the rhodaoxetane oxygen, although other
possibilities cannot be rigorously excluded at present. Further
mechanistic studies are in progress.
In summary, we have reported the transmetalation of a
wide variety of organoboron nucleophiles to rhodaoxetane 3
as a key step towards a novel carbohydroxylation protocol.
This report presents the first experimental evidence for
transmetalation from boronic acids to a RhIII center, as well as
the first transmetalation in general involving a RhIII O bond.
À
Preliminary mechanistic studies suggest an interesting
double-activation of the employed reagents through oxetane
oxygen–boron pre-coordination. Investigations regarding
reductive elimination from this species are ongoing and will
be reported in due course.
Received: June 2, 2010
Revised: September 7, 2010
Published online: October 22, 2010
Keywords: boronic acid · olefin functionalization ·
.
rhodaoxetane · rhodium · transmetalation
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experimental parameters and sample spectra see the supporting
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Angew. Chem. Int. Ed. 2010, 49, 9219 –9224