Trans -Hydroboration-oxidation products in ?5-steroids via a hydroboration- retro -hydroboration mechanism

Article abstract of DOI:10.1039/d0sc01701a

Herein, we report for the first time a trans-hydroboration-oxidation product isolated and characterized under traditional hydroboration-oxidation conditions using cholesterol and diosgenin as substrates. These substrates are excellent starting materials because of the rigidity and different structural environments around the double bond. Further investigations based on experimental evidence, in conjunction with theoretical studies, indicate that the formation of this trans-species occurs via a retro-hydroboration of the major product to generate the corresponding ?6-structure and the subsequent hydroboration by the ?-face. Besides, the corresponding Markovnikov type products have been isolated in synthetically useful yields. The behavior of the reaction under a range of temperatures is also investigated.

Full text of DOI:10.1039/d0sc01701a

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trans-Hydroboration-Oxidation products in Δ5-
steroids via a hydroboration-retro-hydroboration
mechanism
J. Ciciolil Hilario-Martínez,1,2 Fernando Murillo,1 Jair García-Méndez,1 Eugenia Dzib,1 Jesús
Sandoval-Ramírez,2 Miguel Ángel Muñoz-Hernández,3 Sylvain Bernès,4 László Kürti,5
Fernanda Duarte,6 Gabriel Merino,1,* and María A. Fernández-Herrera.1,*
1
Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad Mérida. km 6
Antigua Carretera a Progreso. Apdo. Postal 73, Cordemex, 97310, Merida, Yuc., Mexico.
2 Facultad de Ciencias Químicas, Benemérita Universidad Autónoma de Puebla, Ciudad Universitaria, 72570,
Puebla, Pue., Mexico.
3 Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001,
Cuernavaca, Mor., C. P. 62209, Mexico.
4 Instituto de Física, Benemérita Universidad Autónoma de Puebla. Ciudad Universitaria, 72570, Puebla, Pue.,
Mexico.
5 Department of Chemistry, Rice University, BioScience Research Collaborative, 6500 Main Street, Houston,
Texas 77030, United States.
6 Chemistry Research Laboratory, Oxford University, Mansfield Road, OX1 3TA Oxford, United Kingdom.
*email: mfernandez@cinvestav.mx, gmerino@cinvestav.mx
ABSTRACT
Herein, we report for the first time a “trans-hydroboration-oxidation product” isolated and
characterized under traditional hydroboration-oxidation conditions using cholesterol and
diosgenin as substrates, which are excellent starting materials because of its rigidity and
different structural environments around the double bond. Further investigations based on
experimental evidence, in conjunction with the theoretical studies, indicate that the formation
of this trans-species occurs via a retro-hydroboration of the major product to generate the
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corresponding Δ6-structure and the subsequent hydroboration by the β-face. Besides, the
corresponding Markovnikov type products have been isolated in synthetically useful yields.
The behavior of the reaction under a range of temperatures is also investigated.
INTRODUCTION
The addition of electron-deficient boranes to alkenes is one of the most common reactions to
produce organoboranes.1,2 The resulting alkylboranes are easy-to-handle and highly versatile
intermediates that participate in many synthetically useful transformations. Perhaps the most
common reaction of organoboranes is their oxidation by hydrogen peroxide in an alkaline
medium to furnish the corresponding alcohols regio- and stereospecifically.3 The
hydroboration (HB) of alkenes is a traditional and well-known organic transformation where
the governing stereochemical principle is the addition of hydrogen and BH2 to the same π-
face (syn-addition). This reaction, in principle, proceeds via a four-membered transition state
(Scheme 1).1,2 Most olefins readily undergo HB under the latter conditions, usually giving
the corresponding anti-Markovnikov product, and the subsequent oxidation step with
hydrogen peroxide proceeds with retention of configuration.
H
H
BH3
H
BH2
H
B
syn
addition
four-membered
transition state
Scheme 1. General mechanism for the hydroboration reaction of alkenes.
These observations lead to the generalization that HB takes place via an anti-
Markovnikov syn-addition from the less hindered side of the double bond. Even the HB of
terminal alkynes occurs in an anti-Markovnikov stereospecific fashion, where the syn
addition also results from HB on the same side of the alkyne.
After a plethora of literature reports on olefin HB reactions, the following question
arises: is it possible to obtain a trans-hydroboration product? The answer is yes, but it has
never been reported under traditional HB conditions. To the best of our knowledge, no
2

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examples of olefin trans-HB have been described in the literature. Herein, we report a trans-
hydroboration-oxidation product from a hydroboration-retro-hydroboration pathway
employing cholesterol (1) and diosgenin (2) as substrates. These outcomes violate, in
principle, all HB reported mechanisms so far.
RESULTS AND DISCUSSION
The first application of the hydroboration-oxidation (HBO) procedure to the double
bond in cholesterol (1, Figure 1) was reported in 1959 simultaneously by Wechter4 and
Sondheimer.5,6 Wechter reported that the HB reaction employing BH3·THF yielded a mixture
of dialkyl boranes in 98% of isolated yield. Upon oxidation (H2O2/NaOH), the crude product
was found to consist of 5α-cholestane-3β,6α-diol (1a, 78%), 5β-cholestane-3β,6β-diol (1c,
traces), and a third compound reported as “not characterized” (see Figure 1). In parallel,
Sondheimer reported the use of BH3·OEt2 and cholesterol to deliver, after oxidation using
H2O2 in ethanolic KOH, a mixture of compounds consisting of 1a (68%), 1c (20%), and
recovering some 1 (9%). In general, most of the HBO studies on Δ5-steroids describe the
generation of only one alcohol with the stereochemistry of 1a.7
5-steroids involved in this study
O
O
HO
HO
diosgenin (2)
cholesterol (1)
HO
HO
HO
HO
H
OH
H
OH
OH
a
OH
c
d
b
Figure 1. Structures of four possible products derived from the HBO reaction of substrates 1 and 2 (structures
type a-d).
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We decided to perform the HBO of 1 and 2 (Figure 1).8,9 The reactions were subjected
to a thorough temperature study (from 20 ºC to –20 ºC). In our hands, for both substrates, the
crude contained three main components and ultimately four products were isolated and fully
characterized.
Table 1. Isolated yields from HBO of 1 and 2 at different temperatures.
Isolated yields at different temperatures (%)
Diol type
20 ºC*
10 ºC
0 ºC
–10 ºC
–20 ºC
Diols derived from cholesterol (1)
1a
1b
1c
1e
66
62
59
54
40
9
10
12
14
17
traces
22
traces
24
traces
27
traces
30
traces
38
Diols derived from diosgenin (2)
2a
2b
2c
2e
65
61
55
52
41
8
10
12
15
18
traces
21
traces
23
traces
26
traces
29
traces
36
* Reactions carried out at rt (25 ºC) provided similar yields.
Due to the steric shielding exerted by Me-19, not surprisingly the major products (1a
and 2a) arose via hydroboration of the α-steroidal face.10 It is remarkable that the lowering
of the reaction temperature (+20 °C to –20 °C) led to lower yields of compounds type a (see
Table 1).11-13 Compounds 1b and 2b14 are the Markovnikov addition products. These
compounds have been recently reported employing transition-metal-catalyzed asymmetric
HB reactions.15,16 However, under the typical HB conditions (as in our case) they were
unexpectedly obtained in synthetically meaningful yields (8-18%). It is also notable that their
formation is favored with the decrease of temperature. The isolation of compounds type c
was challenging. However, after several purifications by column chromatography, both 1c
and 2c successfully crystallized from an enriched fraction of c. The full 1D and 2D NMR
characterization data are detailed in the Supplementary Information. Compounds of type d
were not detectable, and the unreacted starting materials were not recoverable from any of
the reactions.
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Unexpectedly, we also found the “trans”-HBO products, 1e and 2e!,17 under classical
HB conditions. The yields of these unexpected stereoisomers are 21-38%, surprisingly high
when considering their nature. At room temperature, yields of products type e are about one-
third of the HBO products type a. The 5α,6β-stereochemistry of 1e and 2e18 are confirmed
by NMR; for further details, see the Supplementary Information.
For structures b, c, and e, the observed stereochemistry in solution is supported by
the single-crystal X-ray crystallography structure determinations of compounds 2b, 1c, 2c,
and 1e, respectively (see SI and CIF files deposited with the CCDC). Of particular interest is
the “trans” product 1e, in which the 6-OH and 19-Me groups are assumed to induce steric
repulsion (Figure 2). Indeed, the O6…C19 separation, 3.029(4) Å, is shortened by 0.19 Å as
compared to the van der Waals distance. However, since the OH group is free to rotate, there
are no occurrences of destabilizing H…H contacts, and both 3 and 6 hydroxyl groups are
engaged in classical intermolecular hydrogen bonds, to form supramolecular chains in the
crystal. The same situation is observed for 1c and 2c.
A
B
HO
H
OH
e
Figure 2. (A) Structure type e (derived from the HBO reaction of 1 and 2). (B) Molecular structure for
compound 1e with displacement ellipsoids at 30% probability level.
To elucidate the formation of the unexpected HBO products type e, we performed a
series of quantum chemical computations at the DLPNO-CCSD(T)19-21/def2-TZVP22 level
by taking the PBE023 structures from a model composed of two rings (a decalin framework,
see Figure 3), and considering the solvent effects (THF) via the SMD24 approximation.
Entropic contributions and thermal corrections were computed at the SMD-PBE0-D325/def2-
TZVP level at 273.15 K. Hydroboration has been theoretically studied by several groups.26-
28
Validation of this methodology and further details are provided in the SI. All these
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computations were done in Gaussian 1629 and Orca 4.0.30 Considering that the alcohol is
formed with retention of configuration in a subsequent oxidation step, we focus our analysis
on the HB mechanism; thus, we label the alkylborane system as 1a’ that produces 1a after
the oxidation.
The Gibbs energy pathways for the suprafacial HB additions of 1 are illustrated in
Figure 3. The smallest barrier (G = 2.7 kcal mol–1) is obtained for one of the α-additions.
It is easily explained due to the presence of Me-19. As is expected, the difference between
anti-Markovnikov and Markovnikov barriers (G = 3.4 kcal mol–1) favors the former
addition. Since the addition involves the bridgehead C-5, these transition states (TSs) are
connected to twisted-chair conformations (INT-1a’ and INT-1b’), which are more stable
than the reactants by 5.4 and 4.5 kcal mol–1, respectively. The barriers to obtain the final
products are relatively low (2.2 and 3.2 kcal mol–1) and correspond to a conformational
arrangement to form the final products with a trans-A/B rings fusion. The formation of both
1a’ (Grxn = 13.5 kcal mol–1) and 1b’ (Grxn = 10.4 kcal mol–1) is exergonic in nature. At
this point, there are no surprises.
The formation of 1c’ and 1d’ involves an additional conformational change to provide
the cis-A/B rings fusion. The barriers for HB additions at the β-face are similar to that
computed for 1b’ (~6 kcal mol–1). The first conformational barriers through TS2-1c’ and
TS2-1d’ are small (1.1 and 2.8 kcal mol–1). However, the barriers for the final step are much
higher (7.2 and 8.5 kcal mol–1) and become the rate-limiting steps. This explains why 1d is
not formed, despite having an HB barrier similar to 1b, and why 1c is only isolated in traces.
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Figure 3. Gibbs energy profiles (in kcal mol-1) of hydroboration of the decalin model of 1 and transition state
structures for the first steps. Bond distances are in Å. In the depicted structures, H atoms are omitted for clarity,
except the H atom involved in the hydroboration.
The pathways discussed above are relevant to understand the HB by both - and -
face, nevertheless, they do not explain the formation of the trans-HB product. We
hypothesize that the formation of this product involves a retro-hydroboration to regenerate
the double bond but at C-6. This leads to the formation of the 6-steroid 3 (5α-cholest-6-en-
3β-ol), followed by typical HB on the -face. Our computations indicate that 1 is more stable
than 3 by 2.6 kcal mol–1. The formation of 3 from 1a’ is concerted via TS1-1e’ (G =18.0
kcal mol–1). The result is the -complex INT-1e’, which is then converted to the trans-
product (1e’) via TS2-1e’ (G = 6.8 kcal mol–1) or to 1f’ via TS2-1f’ (7.1 kcal mol–1). After
all the experimental and computational evidence, these results indicate that even though less
preferred, a retro-hydroboration mechanism is viable under traditional experimental
conditions, explaining the formation of 1e.
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Figure 4. Gibbs energy profiles (in kcal mol-1) for the formation of 1e’ and 1f’ and transition state structures
involved. Bond distances are in Å. In the depicted structures, H atoms are omitted for clarity, except the H atom
involved in the hydroboration.
CONCLUSIONS
In summary, we report the complete characterization of the hydroboration-oxidation
products of cholesterol and diosgenin. Because of the steric effect exerted by Me-19, the
most abundant products are the anti-Markovnikov ones by the α-steroidal face. Surprisingly,
a trans-product is also obtained! This is the first time that such a type of structure is
synthesized and characterized under typical hydroboration-oxidation conditions. The best
way to explain the formation of this “trans-species” is via a retro-hydroboration of the major
product (a-type products) to generate the corresponding Δ6-structure and the subsequent
hydroboration by the -face. We were lucky to select these steroids as substrates because of
its rigidity and different structural environments around the double bond, which allowed us
to isolate and characterize the trans-product.
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EXPERIMENTAL SECTION
General procedure for the hydroboration of Δ5-steroids
To a solution of 1 or 2 (2 mmol) in dry THF (30 mL), NaBH4 (0.38 g, 10 mmol) was added.
The system was kept sealed, under stirring, and under argon atmosphere. The reaction
mixture was set at the temperature specified in Table 1 before the addition of BF3·OEt2 (1.65
mL, 13.4 mmol). After stirring for 2 h, the remaining pressure was released, and the work up
was performed adding brine (10 mL) dropwise. The THF was evaporated under reduced
pressure and the crude product was dissolved in CH2Cl2, washed with brine (1 x 50 mL) and
water (3 x 50 mL), dried over anhydrous Na2SO4 and concentrated in vacuo. The crude
mixture was immediately dissolved in a solution of 2% KOH/MeOH (50 mL) and then 5 mL
of 35% H2O2 were added dropwise. After 1 h of stirring at rt, 200 mL of water were added,
and the resulting precipitate was filtered off and dried under vacuum. The resultant white
solids were purified by flash chromatography on a Combiflash apparatus using a gradient
elution employing hexane/ethyl acetate (see specific details in the Supplementary
Information).
Note: All procedures were carried out employing a commercial 1.0 M solution of BH3·THF
and the results were the same.
ASSOCIATED CONTENT
Supporting Information. Supplementary material has been provided as a separate electronic
file containing detailed experimental procedures, characterization of all the isolated
compounds, computational details, and the cartesian coordinates of all the stationary points.
This material is available free of charge via the Internet.
Data and software availability. The data for the X-ray crystallographic structures of 2b, 1c,
2c, and 1e are available free of charge from the Cambridge Crystallographic Data Center
under accession numbers CCDC: 1921283, 1921284, 1921285, 1921286, respectively.
PRESENT ADDRESSES
† Miguel Ángel Muñoz-Hernández – Department of Chemistry, 310 President's Circle,
Mississippi State University, MS State, Mississippi 39762, United States.
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AUTHOR CONTRIBUTIONS
JCHM, JGM, JSR, and MAFH carried out the experiments. FM, ED, FD, and GM performed
the computational studies. MAMH and SB performed the X-ray determinations. All authors
interpreted the results and LK, FD, GM, and MAFH prepared the manuscript. All authors
have given approval to the final version of the manuscript.
NOTES
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully thank Conacyt (Grants 254742, 254604, 252356, and 268178). JCHM,
FM, and ED acknowledge Conacyt for PhD fellowships, and JGM acknowledges Conacyt
for MSc fellowship.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
H. C. Brown, B. C. S. Rao, J. Am. Chem. Soc. 1956, 78, 5694-5695.
H. Brown, B. C. Rao, J. Org. Chem. 1957, 22, 1135-1136.
P. Kaur, G. L. Khatik, S. K. Nayak, Curr. Org. Synth. 2017, 14, 665-682.
W. J. Wechter, Chem. Ind. (London, U. K.) 1959, 294-295.
S. Wolfe, M. Nussim, Y. Mazur, F. Sondheimer, J. Org. Chem. 1959, 24, 1034-1034.
M. Nussim, Y. Mazur, F. Sondheimer, J. Org. Chem. 1964, 29, 1120-1131.
M. B. Smith, in Organic Synthesis (Third Edition), Academic Press, Oxford, 2010,
pp. 491-540.
[8]
R. Zeferino-Diaz, J. C. Hilario-Martinez, M. Rodriguez-Acosta, A. Carrasco-
Carballo, M. G. Hernandez-Linares, J. Sandoval-Ramirez, M. A. Fernandez-Herrera,
Steroids 2017, 125, 20-26.
[9]
M. A. Fernandez-Herrera, J. Sandoval-Ramirez, H. Lopez-Munoz, L. Sanchez-
Sanchez, Arkivoc 2009, 170-184.
[10] Compound 2a matched an authentic sample of alpha-chlorogenin.
10

Page 11 of 13
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View Article Online
DOI: 10.1039/D0SC01701A
[11] P. K. Agrawal, D. C. Jain, R. K. Gupta, R. S. Thakur, Phytochemistry 1985, 24, 2479-
2496.
[12] S. C. Sharma, O. P. Sati, Phytochemistry 1982, 21, 1820-1821.
[13] M. E. Wall, H. A. Walens, J. Am. Chem. Soc. 1955, 77, 5661-5665.
[14] H. Nawa, M. Uchibayashi, A. Okabori, K. Morita, T. Miki, Chem. Pharm. Bull. 1963,
11, 139-144.
[15] H. Iwamoto, T. Imamoto, H. Ito, Nat. Commun. 2018, 9, 2290.
[16] J. R. Smith, B. S. L. Collins, M. J. Hesse, M. A. Graham, E. L. Myers, V. K.
Aggarwal, J. Am. Chem. Soc. 2017, 139, 9148-9151.
[17] D. G. Chincharadze, A. N. Kel'ginbaev, M. B. Gorovits, L. I. Éristavi, T. T. Gorovits,
N. K. Abubakirov, Chem. Nat. Compd. 1979, 15, 442-446.
[18] Compound 2e matched an authentic sample of beta-chlorogenin.
[19] C. Riplinger and F. Neese, J. Chem. Phys. 2013, 138, 034106.
[20] C. Riplinger, B. Sandhoefer, A. Hansen, F. Neese, J. Chem. Phys. 2013, 139, 134101.
[21] C. Riplinger, P. Pinski, U. Becker, E. F. Valeev, F. Neese, J. Chem. Phys. 2016, 144,
024109.
[22] F. Weigend, Phys. Chem. Chem. Phys. 2006, 8, 1057-1065.
[23] C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158-6170.
[24] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113, 6378-6396.
[25] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010, 132, 154104.
[26] N. J. R. V. E. Hommes and P. v. R. Schleyer, J. Org. Chem. 1991, 56, 12, 4074-4076.
[27] Y. Oyola, D. A. Singleton, J. Am. Chem. Soc. 2009, 131, 3130–3131.
[28] J. O. Bailey, D. A. Singleton, J. Am. Chem. Soc. 2017, 139, 15710–15723.
[29] Frisch, M.J. et al. Gaussian 16, Revision B.01 https://gaussian.com/citation/
(Gaussian, 2016).
[30] F. Neese, F. Wennmohs, U. Becker, C. Riplinger, J. Chem. Phys. 2020, 152, 224108.
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GRAPHICAL ABSTRACT
hidroboration-retro-hydroboration
HO
HO
H
OH
trans-product
5-steroid
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TOC in png format