Unveiling the role of boroxines in metal-free carbon-carbon homologations using diazo compounds and boronic acids

Article abstract of DOI:10.1039/c7sc02264f

By means of computational and experimental mechanistic studies the fundamental role of boroxines in the reaction between diazo compounds and boronic acids was elucidated. Consequently, a selective metal-free carbon-carbon homologation of aryl and vinyl boroxines using TMSCHN₂, giving access to TMS-pinacol boronic ester products, was developed.

Full text of DOI:10.1039/c7sc02264f

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Unveiling the role of boroxines in metal-free carbon-carbon
homologations using diazo compounds and boronic acids
Claudio Bomio, Mikhail A. Kabeshov, Arthur R. Lit, Shing-Hing Lau, Janna Ehlert, Claudio
Battilocchio and Steven V. Ley*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
By means of computational and experimental mechanistic studies the fundamental role of boroxines in the reaction between
diazo compounds and boronic acids was elucidated. Consequently, a selective metal-free carbon-carbon homologation of
aryl and vinyl boroxines using TMSCHN2, giving access to TMS-pinacol boronic ester products, was developed.
www.rsc.org/chemicalscience
In this work, we report a detailed computational and
experimental study unveiling the crucial role of the boronic acid
anhydrides – boroxines in these coupling reactions with
diazocompounds and demonstrate how this knowledge can be
applied to improve the scope of the method.
Introduction
Carbon-carbon bond formation reactions have been at the
centre of organic synthesis for decades allowing efficient and
quick assembly of complex molecular structures.1–6 Among
many others, organoboron compounds are often the reagents
of choice as they promote C-C bond formation reactions with
high chemo- and stereoselectivity without the need for
Results and discussion
Initially, the intriguing difference between the behaviour of the
expensive, sometimes unstable and toxic transition metal
7,8
p-methoxyphenyl boronic acid (
methoxyphenylboroxine ( ) in their reactions with TMS
diazomethane (TMSCHN2, ) was noteworthy (Scheme 1).
1) and the respective p-
catalysts.
A large number of highly efficient methods have
4
2
been described where a boron atom plays initially a role of the
Lewis acid followed by a σ-bond migration thus allowing a
metal-free coupling reaction between an electrophile
(organoborane, organoboronic ester, dihaloborane)9–13 and a
nucleophile (organolithium intermediates, sulfur ylides,
diazocompounds).9,14–23
Coupling between boronic acids and diazo compounds has
attracted considerable interest in the synthetic community as
boronic acids are readily available, usually stable and more
atom efficient reagents when compared to their ester
analogues.16,24–26 Additionally, the homologation using TMS-
diazomethane (TMSCHN2) and organoboron compounds is
useful to install a trimethylsilyl and boron functionality in a
metal-free fashion.27–32 These doubly functionalised carbon
substrates are of interest, as they may be used to generate
sequential or orthogonal functionalization at the same carbon
atom.28,29 To date, the homologation with TMSCHN2 has only
been performed in combination with boronic acids to access the
TMS-free homologation products.26,33 While several proposed
mechanisms have been reported, the role of a boronic acid
anhydride (boroxine) intermediate in these reactions remains
unclear. 16,26,33
Scheme 1: Reactions of the boronic acid 1 and boroxine 4 with TMS diazomethane 2.
When mixing the boronic acid
1 with TMSCHN2 2 at ambient
temperature, nearly instant decolourisation of the reaction
mixture (originating from TMSCHN2) and strong gas evolution
was observed. As a result, the formation of a roughly 1:1
mixture of homologation product
starting material was isolated. By contrast, when using
boroxine instead of boronic acid under the same conditions,
no gas evolution and only very slow decolourisation was
observed. As expected, the TMS-homologated product was
3 and Bpin ester of the
6
4
1
a.Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge
CB21EW,UK. E-mail:svl1000@cam.ac.uk; Web:http://www.leygroup.ch.cam.ac.u
k
5
† Additional data is available from the University of Cambridge Data Repository
isolated as the main product in 75% yield after 29h of stirring at
RT (Scheme 1).
website:
https://doi.org/XX.XXXXX/XXX.XXXX
.
Electronic
Supplementary
Information (ESI) available. See DOI: xx.xxxx/xxxxxxxxxx
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The decomposition of TMSCHN2 (
2
) to diazomethane is known TMSCHN2 (
2
) and react smoothly with boroxines (Gact = 18.5 kcal
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DOI: 10.1039/C7SC02264F
-1
to occur under acidic conditions and is due to the formation of mol ). However, the reaction between diazomethane and
H3O+ through an equilibrium between boronic acids or boronic acids might be possible, but would require significantly
boroxines and their anionic tetrahedral species in the presence higher activation energy (Gact = 29.2 kcal mol-1). Additionally,
of water. Thereby this decomposition can rationalise the only mono addition of TMSCHN2 (2) to boroxine is expected due
difference in the products obtained (Scheme 1).34,35 In order to to the significantly higher activation barrier (27.2 kcal mol-1,
further understand the role of boroxines in the coupling of lower diastereomeric transition state) for the second insertion
diazocompounds with boronic acids, quantum chemistry into the benzylic boroxine intermediate (Figure 1, E).
calculations at Density Functional Theory
pVTZ// B97xD/cc-pVDZ with SMD solvation model (solvent = performed (Table 1). As already previously described, when
dichloromethane, 2)
(B97xD/cc- To verify the computed results, control experiments were


=8.93) as implemented in Gaussian 09) level using boronic acid
1
, a fast decomposition of TMSCHN2 (
occurred with formation of the homologated product and only
traces of the TMS homologation product being observed. The
conversion of the starting material into using 1.03 eq of
TMSCHN2 ( ) gave only a 52% yield (entry 1). In contrast, when
using boroxine ( ) and 3.10 eq of TMSCHN2 ( ) (1.03 eq per
boron) the exclusive formation of the TMS homologation
product was observed (90% yield, entry 2). Addition of DIPEA
to stabilise TMSCHN2 ( ) led to much slower generation of
were therefore performed (Figure 1).36–42
3
5
1
5
2
4
2
5
2
diazomethane but had no effect on the final product
distribution (entries 3 and 5). When water was added to the
reaction (in order to shift the equilibrium towards the boronic
acid
1) lower conversion towards homologation products 3 and
5
was observed, regardless of the presence of DIPEA. Moreover,
it can be observed that water has the effect to increase the rate
of decomposition of TMSCHN2 (entries 4, 6 and 7). Finally, the
addition of large excess of base using boroxine
decreases the reaction rate (entry 8). This also can explain why
only small amount of TMS homologation product is observed
5 significantly
5
when boronic acids and DIPEA are added at room temperature
(entries 3 and 5).
Table 1: Experimental homologation studies with boronic acid
using TMSCHN2 ( ).
1 and boroxine 4
2
Figure 1. The computed reaction pathway for the coupling between boronic species and
TMS diazomethane (R = Ph).
It was found that both reactions, using boronic acid or boroxine
in combination with TMSCHN2 (
2
), proceed via a two-step
Entry
R-BX2
TMSCHN2
Timeb
1h
DIPEA
-
H2O
-
Yielda (
5:3:6)
mechanism involving a sequence of a coordination transition
state (point B, Figure 1), unstable intermediate (point C, Figure
1) and the highest energy point on the coordinate for both
examples, which corresponds to the transition state for
migratory insertion (point D, Figure 1). In contrast to other
reactions involving boronic acids and boroxines, where similar
reactivity for both is usually observed,34 the striking difference
of 13.9 kcal mol-1 (36.4 vs 22.5, respectively; Figure 1) for the
activation Gibbs energies was computed. According to these
computational results, only the boroxine, not the boronic acid,
1
2
3
4
5
6
7
8
RB(OH)2
(RBO)3
1%:43%:48%
75%:0%:10%
2%:40%:51%
0%:14%:83%
2%:42%:47%
0%:16%:80%
0%:19%:78%
80%:0%:17%
1.03 eq
3.10 eq
1.03 eq
1.03 eq
1.03 eq
1.03 eq
1.03 eq
29h
15h
10h
65h
23h
1h
-
-
-
RB(OH)2
RB(OH)2
RB(OH)2
RB(OH)2
RB(OH)2
(RBO)3
2.1 eq
2.1 eq
6.0 eq
6.0 eq
-
2.0 eq
-
2.0 eq
2.0 eq
-
is expected to be reactive towards TMSCHN2 (2) (the barrier of
36.4 kcal mol-1 cannot be achieved under conventional
conditions of organic reactions in solution).43,44
67hc
6.0 eq
3.10 eq
a) NMR yields. R = 4-MeOPh. b) Time required for full disappearance of TMSCHN2. c)
TMSCHN2 not fully consumed.
In order to check whether similar differences in reactivity are
expected for reactions of diazomethane with boroxines and
boronic acids, additional calculations using the same
computational model, were performed. It can be concluded
from the analysis that diazomethane should behave similarly to
To rank organoboron reagents by their reactivity towards
TMSCHN2 (2), further computational studies were performed
and the results were subsequently compared with reactivity
trends previously reported in the literature (Figure 2). From all
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the computed examples, organoborane 14 showed the highest As already described in scheme 1, the desired product
5 could
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DOI: 10.1039/C7SC02264F
reactivity
difluoroorganoborane 13, boroxine 12 and catecholborane
Organoboranes are known to react fast with TMSCHN2 ( ) at - homologation reactions with TMSCHN2 (
78°C while catecholboranes need heating (60°C) over an excess of 0.033 equiv. of TMSCHN2 ( ) per boron atom was
towards
TMSCHN2,
followed
by be obtained in 75% yield after stirring at rt for 29h. To our
8
.
delight, it was found that, unlike in other reported
), using just a slight
2
2
2
extended time period (12h).27,45 Both the reported reactivities sufficient to reach full conversion.26,33 To speed up the process,
as well as the experimentally observed behaviour of boroxines the reaction was performed at 60°C in toluene, to give 78% of
are in good correlation with our calculations. To prove that both the desired product (
the MIDA and pinacol boranes are not reactive, the product, the corresponding aldehyde was observed
substrates were treated with TMSCHN2 ( ) at reflux in toluene (approximately 10%), which is likely to arise from the oxidation
for 3d. In all cases, no homologation products were observed. of the TMS-boroxine intermediate. It was possible to suppress
5) after just 3h (Table 2, Entry 2). As a side
7
8
2
Again, this is in agreement with our calculations.
the formation of this byproduct by adding DIPEA to the reaction
mixture, which resulted in an improved yield of 88% for
compound
5 (Table 2, Entry 3). Increasing the temperature to
85°C led to full conversion after 1h without affecting the yield.
By increasing the amount of DIPEA from 3.00 equiv. to 3.60
equiv. the yield of the homologation could be further improved
to 93%. Further attempts to increase the yield by adding more
DIPEA did not lead to any improvements. Finally, applying the
optimised conditions to the boronic acid
of the desired TMS homologated product
1
led to the formation
in moderate yield
5
(50%) which can be attributed to the very poor solubility of
boronic acid and water in toluene and easier formation of the
boroxine at elevated temperature (entry 6).
Figure 2. Computed reactivities of organoboron reagents towards TMS diazomethane
To conclude therefore, it was determined that boroxines are
ideal reagents for reactions with diazo compounds, as they
allow excellent reactivity and reasonable atom economy.
Additionally, these results corroborate the fact that boroxines
are most likely the reactive species in the coupling between
diazo compounds and boronic acids.26
Our studies continued with the optimisation of reaction
conditions by using boroxines, obtained via dehydration of
boronic acids using a Dean-Stark apparatus (Table 2).
Table 2: Optimisation of the TMS homologation with boroxines.
Entry
solvent
T
time
29h
3h
DIPEA
-
yield
75%a,b
78%b
88%
1
2
3
4
5
6
24°C
60°C
60°C
85°C
85°C
85°C
CH2Cl2/THF (4:1)
toluene
-
3h
3.0 eq
3.0 eq
toluene
toluene
1h
87%
toluene
1h
93%
3.6 eq
3.6 eq
toluene
1h
50%a,c
1.03 eq. TMSCHN2 per boron atom. a) NMR yield, b) Aldehyde formation observed, c)
Reaction performed with boronic acid 1.
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Figure 3: Yields of scaled up reactions (0.1 mol). a) Reaction performed without DIPEA.
In addition to the scope, the homologation was performed on a
gram-scale with three boronic acids (5, 18, 30) in order to
demonstrate the practical usefulness of this reaction. As shown
in Scheme 3 the homologation proceeded smoothly on scale by
applying identical reaction conditions.
Conclusions
We have shown experimentally and by means of DFT
computations that boroxines of the corresponding boronic
acids are likely to be the reactive intermediates in the
homologation reaction with diazo compounds. Consequently,
we have developed a metal-free, robust and scalable approach
towards bench stable benzyl and allyl TMS-Bpin products with
high functional group tolerance using TMSCHN2 and boroxines.
Current investigations are directed towards the selective
functionalisation of the TMS-Bpin products
Experimental
General Procedure for Preparation of TMS-Bpin products: The
reaction was carried out in dry conditions under an atmosphere of
argon. To a mixture of boroxine (0.15 mmol, 1.0 equiv.) and N,N-
diisopropylethylamine (0.094 mL, 0.54 mmol, 3.6 equiv.) in toluene
(0.75 mL) was added (trimethylsilyl)diazomethane (0.23 mL, 0.465
mmol, 2 M in hexanes, 3.1 equiv.). The reaction mixture was stirred
at 85 ˚C for 1 h and allowed to cool down to room temperature.
Pinacol (70.9 mg, 0.60 mmol, 4.0 equiv.) was added and the reaction
mixture was stirred at room temperature for 2 h. The reaction was
quenched with a saturated aqueous solution of NH4Cl and the
aqueous phase was extracted with EtOAc. The combined organic
extracts were washed with brine, dried (MgSO4) and concentrated in
vacuo. The crude residue was purified by silica gel flash column
chromatography to afford the desired TMS-Bpin product.
Scheme 2: Scope table of the homologation reaction using TMSCHN2 and boroxines. a)
Reaction performed without DIPEA. General comment: TMS-boroxine intermediates are
prone to protodeboration and oxidation and can therefore not be isolated.
Acknowledgements
Having the optimal conditions in hand, the scope was expanded
to other boronic acids (see table 2). The reaction showed broad
functional group tolerance (-NO2, halogen, ester, amide,
thiophene). The conditions could also be applied to vinyl
boronic acid precursors (31 to 34). Significant difference in
reactivity could be observed with substrates bearing strongly
electron withdrawing groups in para position (29 and 30), where
the use of DIPEA led to the protodeborated byproducts. In order
to suppress protodeboration, the reaction with electron poor
substrates had to be performed without the presence of base,
with unavoidable 10-15% aldehyde formation as a side product.
We are grateful to the Swiss National Science Foundation (C.
Bo.), the Croucher Foundation (S. H. L), Erasmus+ (J.E.), and the
Engineering and Physical Sciences Research Council (M. K.,
C.Ba., and S. V. L. grant no. EP/K009494/1, EP/M004120/1 and
EP/K039520/1) for financial support.
Notes and references
1
2
A. Suzuki, Angew. Chemie - Int. Ed., 2011, 50, 6723–6733.
D.-H. Lee, K.-H. Kwon and C. S. Yi, Science, 2011, 333,
1613–1616.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 6
Chemical Science
Please do not adjust margins
Journal Name
ARTICLE
3
4
B. G. Hashiguchi, S. M. Bischof, M. M. Konnick and R. A.
Periana, Acc. Chem. Res., 2012, 45, 885–898.
N. Rodríguez and L. J. Goossen, Chem. Soc. Rev., 2011, 40,
5030–5048.
Z. Shao and H. Zhang, Chem. Soc. Rev., 2012, 41, 560–572.
R. Kumar and E. V Van der Eycken, Chem. Soc. Rev., 2013,
42, 1121–46.
32
33
34
C. H. Burgos, E. Canales, K. Matos and J. A. SoVdieewrqAurtiicslte,OJ.nline
Am. Chem. Soc., 2005, 127, 8044–8049.
C. Wu, G. Wu, Y. Zhang and J. Wang, Org. Chem. Front.,
2016, 3, 817–822.
DOI: 10.1039/C7SC02264F
5
6
D. G. Hall, Ed., Boronic Acids: Preparation and Applications
in Organic Synthesis, Medicine and Materials, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim, Germany, 2011.
E. Kühnel, D. D. P. Laffan, G. C. Lloyd-Jones, T. Martínez Del
Campo, I. R. Shepperson and J. L. Slaughter, Angew.
Chemie - Int. Ed., 2007, 46, 7075–7078.
Y. S. Lin, G. De Li, S. P. Mao and J. Da Chai, J. Chem. Theory
Comput., 2013, 9, 263–272.
T. H. Dunning Jr, J. Chem. Phys., 1989, 90, 1007.
R. a Kendall, T. H. Dunning Jr. and R. J. Harrison, J. Chem.
Phys., 1992, 96, 6796.
A. V. Marenich, C. J. Cramer and D. G. Truhlar, J. Phys.
Chem. B, 2009, 113, 6378–6396.
M. A. Kabeshov, O. Kysilka, L. Rulíšek, Y. V. Suleimanov, M.
Bella, A. V. Malkov and P. Kočovský, Chem. - A Eur. J., 2015,
21, 12026–12033.
S. Schenker, C. Schneider, S. B. Tsogoeva and T. Clark, J.
Chem. Theory Comput., 2011, 7, 3586–3595.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmami, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, T. Keith, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E.
Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jamarmillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J.
Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz and J. Cioslowski, Gaussian 09,
Revision D.01, Gaussian, Inc., Wallingford CT, 2013.
Y. Xia, A. S. Dudnik, Y. Li and V. Gevorgyan, Org. Lett., 2010,
12, 5538–5541.
7
8
K. S. Egorova and V. P. Ananikov, Angew. Chemie - Int. Ed.,
2016, 55, 12150–12162.
35
36
M. L. Crawley and B. M. Trost, Applications of Transition
Metal Catalysis in Drug Discovery and Development: An
Industrial Perspective, Wiley, Hoboken, NJ, USA, 2012.
D. S. Matteson, Tetrahedron, 1998, 54, 10555–10606.
D. S. Matteson, J. Org. Chem., 2013, 78, 10009–10023.
H. C. Brown, Organic Synthesis via Boranes, Wiley-
Interscience, New York, 1975.
O. A. Argintaru, D. Ryu, I. Aron and G. a. Molander, Angew.
Chemie - Int. Ed., 2013, 52, 13656–13660.
G. A. Molander and D. W. Ryu, Angew. Chemie - Int. Ed.,
2014, 53, 14181–14185.
M. Burns, S. Essafi, J. R. Bame, S. P. Bull, M. P. Webster, S.
Balieu, J. W. Dale, C. P. Butts, J. N. Harvey and V. K.
Aggarwal, Nature, 2014, 4–9.
A. Bonet, M. Odachowski, D. Leonori, S. Essafi and V. K.
Aggarwal, Nat. Chem., 2014, 6, 584–9.
J. Barluenga, M. Tomás-Gamasa, F. Aznar and C. Valdés,
Nat. Chem., 2009, 1, 494–499.
V. K. Aggarwal, Y. F. Guang and A. T. Schmidt, J. Am. Chem.
Soc., 2005, 127, 1642–1643.
J. Hooz and S. Linke, J. Am. Chem. Soc., 1968, 90, 6891–
6892.
J. Hooz and S. Linke, 1968, 90, 5936–5937.
D. J. Pasto and P. W. Wojkowski, J. Org. Chem., 1971, 36,
1790–1792.
9
10
11
37
38
39
40
12
13
14
41
42
15
16
17
18
19
20
21
22
23
24
25
C. Sandford and V. K. Aggarwal, Chem. Commun., 2017, 53,
5481–5494.
C. Peng, W. Zhang, G. Yan and J. Wang, Org. Lett., 2009, 11,
1667–1670.
H. Li, Y. Zhang and J. Wang, Synthesis (Stuttg)., 2013, 45,
3090–3098.
D. N. Tran, C. Battilocchio, S.-B. Lou, J. M. Hawkins and S. V. 43
Ley, Chem. Sci., 2015, 6, 1120–1125.
C. Battilocchio, F. Feist, A. Hafner, M. Simon, D. N. Tran, D.
M. Allwood, D. C. Blakemore and S. V. Ley, Nat. Chem.,
2016, 8, 360–367.
J.-S. Poh, S.-H. Lau, I. G. Dykes, D. N. Tran, C. Battilocchio
and S. V. Ley, Chem. Sci., 2016, 7, 6803–6807.
J.-P. Goddard, T. Le Gall and C. Mioskowski, Org. Lett.,
2000, 2, 1455–1456.
M. G. Civit, J. Royes, C. M. Vogels, S. A. Westcott, A. B.
Cuenca and E. Fernández, Org. Lett., 2016, 18, 3830–3833.
E. La Cascia, A. B. Cuenca and E. Fernández, Chem. - A Eur.
J., 2016, 22, 18737–18741.
D. S. Matteson and D. Majumdar, Organometallics, 1983,
2, 230–236.
44
45
J. K. Lam, H. V. Pham, K. N. Houk and C. D. Vanderwal, J.
Am. Chem. Soc., 2013, 135, 17585–17594.
R. C. Neu and D. W. Stephan, Organometallics, 2012, 31,
46–49.
26
27
28
29
30
31
D. J. S. Tsai and D. S. Matteson, Organometallics, 1983, 2,
236–241.
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