Oxidant controlled regio- and stereodivergent azidohydroxylation of alkenes via I2 catalysis

Article abstract of DOI:10.1039/c5cc02374b

A novel, I₂ catalyzed regio- and stereodivergent vicinal azidohydroxylation of alkenes leading to 1,2-azidoalcohols in high yields (up to 92%) and excellent dr (up to 98%) has been developed. This unprecedented transformation employs NaN₃ and DMF as N- and O-nucleophiles respectively. The role of DMF as the O-source in the reaction has been unequivocally proven by ¹⁸O labelling studies.

Full text of DOI:10.1039/c5cc02374b

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Oxidant Controlled Regio- and Stereodivergent
Azidohydroxylation of Alkenes via I2 Catalysis
P. K. Prasad, R. N. Reddi and A. Sudalai*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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catalytic electrophilic iodination with stoichiometric co-
oxidants is rare. We reasoned that, 1,2-additions of two
nucleophiles across -C=C- bond could be possible using
electrophilic iodine source owing to its ability to form
electrophilic iodonium ion. In this context, Komatsu and co-
workers have first reported I2-catalyzed aziridination of
alkenes using chloramine-T as N-nucleophile.16 Thereafter,
Yoshimura et al. reported the I2-mediated cyclopropanation of
alkenes using malononitrile as C-nucleophile.17 More recently,
we have demonstrated the dihydroxylation of alkenes using
benzoic acid and DMSO as O-nucleophiles.18 In hetero-
difunctionalization (e.g. azidohydroxylation), switching to one
of the possible isomers with required regio- and
diastereoselectivity is challenging due to little difference in the
relative reactivity of O- and N-nucleophiles under different
conditions.
Abstract: A novel, I2 catalyzed regio- and stereodivergent vicinal
azidohydroxylation of alkenes leading to 1,2-azidoalcohols in high
yields (up to 92%) and excellent dr (up to 98%) has been developed.
This unprecedented transformation employs NaN3 and DMF as N-
and O- nucleophiles respectively. The role of DMF as O- source in
the reaction has been unequivocally proven by 18O labelling studies.
Vicinal difunctionalization of alkenes is an attractive strategy
for the assembly of heteroꢀfunctionalized organic compounds.
In this context, dioxygenation,1 dinitrogenation2 and oxyꢀ
nitrogenation3 across the alkenes are well studied. However, the
direct vicinal azidohydroxylation across unsymmetrical alkenes
in requisite regioꢀ and diastereoselective fashion is rare.3d,3e The
resulting structural units comprising vicinal azidohydroxy
functional groups are recurrent in drugs, natural products and
synthetic materials4 and find tremendous utility in the synthesis
of biologically active amino sugars,5 nucleosides,6 lactams,7
triazoles,8 oxazolines9
and
in
the
chemistry
of
peptidomimetics10 and pseudopeptides11. Conventionally, 1,2ꢀ
azidoalcohols are prepared from ring opening of epoxides12 or
displacement of halohydrin13 with various azide sources under
acidic/basic conditions as well as chemoselective reduction of
αꢀazido ketones.14 Nevertheless, these reports suffer from
disadvantages such as use of prefunctionalized starting
materials leading to reduced atom economy, harsh reaction
conditions accompanied with low functional group tolerance
and often poor regioꢀ and diastereoselectivity. Thus, a mild
synthetic procedure that employs readily available starting
materials and overcomes the above difficulties is desirable for
regioꢀ and stereodivergent synthesis of 1,2ꢀazidoalcohols.
Scheme 1: Azidohydroxylation of alkenes
In 2013, Studer et al. have reported the direct azidooxygenation
of alkenes giving azidohydroxy derivative
amount of N3ꢀI(III) reagent and TEMPONa as
nucleophiles respectively, thus requiring additional steps to
achieve 1,2ꢀazidoalcohols.19 N,NꢀDimethylformamide is
popular formylating agent;20 however, its role in nucleophilic
substitution reactions is quite rare21 and opens up a new area of
research yet to be established. For the first time, we report, a
controlled synthesis of either regioisomers of 1,2ꢀazidoalcohols
4
using excess
N
ꢀ and
Oꢀ
I2 catalysis has been increasingly explored for C-C, C-O and
C-N bond formation as environmentally benign and
inexpensive oxidation reagent in place of rare or toxic heavy
metal oxidants.15 However, olefin functionalization using
a
with high diastereomeric ratios using DMF as
and NaN3 as ꢀnucleophile (Scheme 1).
Oꢀnucleophile
a.aChemical Engineering and Process Development Division, CSIR-National Chemical
Laboratory, Pashan Road, Pune, 411008, India.
N
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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In order to determine its scope, a variety of olefins were
evaluated under the optimized reaction conditions and the
results are summarized in Table 2. When styrenes, with –CH3
Table 1: I2ꢀcatalyzed regiodivergent azidohydroxylation of styrene:
optimization studies[a]
(
1b), ꢀOH (1c), ꢀBr (1d), ꢀNO2 (1e) groups on aromatic ring
Table
2:
I2ꢀcatalyzed
regioꢀ
and
stereodivergent
azidohydroxylation of alkenes: substrate scope
no.
halogen
(10 mol %)
oxidant
base
solvent
yield of
2a[b]
53
10
48
1
2
3
4
5
6
7
8
9
10
11
12
13[d]
I2
I2
I2
I2
I2
I2
I2
I2
I2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
KtOBu
NaH
DBU
Et3N
Et3N
Et3N
Et3N
DMF
THF+DMF
CH3CN+DMF
CH2Cl2+DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
DMSO+ DMF
Products[a]
substrates
Products[b,c]
(3a–n)
18
(2a–n)
(1a-n)
90,38[c]
18
44
32
65
5
11
14
82,78[e]
2a (90%)
2b (88%)
2c (76%)
1a R = H
1b R = CH3
1c R = OH
3a (82%) (12:1)
3b (89%) (9:1)
---
nBu4NI
NaI
KI
I2
50%
aq.H2O2
2d 82%
2e 77%
1d
1e
3d (86%) (10:1)
3e (76%) (10:1)
[a]Reaction conditions: styrene (1 mmol), NaN3 (2 mmol), halogen source
(10 mol %), base (1 mmol), oxidant (2 mmol); 8 mL of solvent (1:1), 25
°C, 8 h. [b]Isolated yields after column chromatographic purification. [c]
5
mol % of I2 was used. [d] 3a was formed as major product. [e] 30% aq. H2O2
was used.
2f (78%)
2g (79%)
2h (84%)
2i (74%)
1f
1g
1h
1i
3f (83%) (9:1)
3g (83%) (9:1)
3h(74%) (8:1)
3i (78%) (5:1)
Initially, when styrene (1a) (1 mmol) was treated with a
mixture of I2 (10 mol %) and TBHP (5ꢀ6 M solution in decane
contains <4 % water, 2 mmol) followed by addition of Et3N (1
mmol) and sodium azide (2 mmol) at 0 °C in DMF, to our
delight we obtained 2ꢀazidoꢀ1ꢀphenylethanꢀ1ꢀol
(2a) as
exclusive regioisomer albeit in moderate yield 53% (Table 1).
Encouraged by this result, it was of interest to optimize the
reaction conditions in order to obtain improved yield without
affecting its regioselectivity. Thus, other solvents like THF,
CH3CN and CH2Cl2 were screened and found to be unsuitable
for this reaction. Hence, we found DMF was crucial in
obtaining desired azidoalcohol 2a. Surprisingly, out of solvent
combinations screened (entries 2ꢀ5), DMSO+DMF (v/v = 1/1)
mixture resulted in excellent yield (90%) of the desired product
2a. Decrease in I2 catalyst loading to 5 mol % however had a
deleterious effect on yield (38%) (entry 5). Further
modification, either in iodine source or base did not show any
significant improvement in the product yield (entries 6ꢀ12). On
the contrary, when 50% aq. H2O2 was used as oxidant instead
of TBHP, a complete reversal in product regioselectivity was
observed affording 2ꢀazidoꢀ2ꢀphenylethanꢀ1ꢀol (3a) in 82%
yield (entry 13). Even, 30% aq. H2O2 can be employed that
afforded 3a in good yield (78%). Furthermore, with 70% aq.
TBHP, it gave a 3:1 mixture of 2a and 3a in 78% combined
yield. Other oxidants such as cumene hydroperoxide and oxone
2j (87%) dr = 95:5
2k(87%) dr = 98:2
1j
3j (92%) dr = 95:5[d]
3k (82%) dr = 92:8
1k
2l (88%) dr = 93:7
2m (80%) dr = 92:8
2n (82%) dr = 96:4
1l R = H, R1=H
1m R= OH R1=H
1n R = OH,R1=OMe
3l (86%) dr = 97:3
3m (78%) dr = 94:6
3n (80%) dr = 98:2
[a]Reaction condition A: styrene (1 mmol), I2 (10 mol %), Et3N (1 mmol), 5ꢀ6
M TBHP in decane (2 mmol), NaN3 (2 mmol), DMSO+DMF (4 mL each), 0
°C to 25 °C, 8 h; [b] condition B: styrene (1 mmol), I2 (10 mol %), Et3N (1
mmol), 50% aq. H2O2 (2 mmol), NaN3 (2 mmol) DMSO+DMF (4 mL each)
⁰
0 C to 25 °C, 8 h; [c] ratio of 3:2; [d] diastereomeric ratios were determined
afforded 2a in 26 and 14% yields respectively. Thus, the using 1H NMR and HPLC analysis.
preliminary studies established an oxidantꢀdirected switch to
either regioisomers 2a or 3a in excellent yields directly from
styrene in a single step.
were treated with I2 (10 mol %), TBHP (2 equiv), NaN3 (2
equiv) and Et3N (1 equiv) in DMF:DMSO (1:1) at 25 °C, the
corresponding 1,2ꢀazido alcohols 2a were obtained in 76ꢀ
–
e
2 | J. Name., 2012, 00, 1-3
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90% yields with excellent regioselectivity (>95%). In the case reaction pathway does not involve the formation of hypervalent
of disubstituted αꢀmethyl styrene (1f), the azidohydroxylation iodine species to yield
product 2f was obtained in high regioselectivity consistent with
2 or 3 and TBHP/H2O2 is solely used for
Table 3: Stoichiometric control experiments for azidohydroxylation of
our initial findings. Also, 1ꢀoctene (1g) gave the desired
products 2g in high yield (79%) with high regioselectivity
(>19:1). Interestingly, sterically congested and abundantly
available isoprenol and prenol derivatives 1h and 1i too
afforded 2h and 2i respectively in high yields with excellent
styrene[a]
regioselectivity. Cyclohexene
(1j) and indene (1k) also
participated in azidohydroxylation reaction affording the
respective products 2j and 2k in predicted regioselectivity
accompanied with high diastereoselectivity (Table 2) in favour
of syn isomer. Interestingly, other open chain internal alkene 1l
gave the corresponding 1,2ꢀazido alcohol 2l with excellent
diastereomeric ratio which is also a key intermediate for the
syntheses of the psychostimulant drug, norpseudoephedrine,22
as well as the antidepressant drug, cathinone.22 Other allylic
alcohol 1m and 1n were also tested and the diastereoselectivity
obtained in each case remained high. Specifically, substrate
with free hydroxy group was also found tolerent under this mild
protocol.
On the contrary, when 50% aqueous H2O2 was used as coꢀ
oxidant in the reaction, azidoalcohols with complementary
regioselectivity were obtained. Here again, substituted styrenes
1a-f afforded 3a-f in high yields (76ꢀ89%) and
regioselectivities (88ꢀ95%). Also, electronically unbiased
aliphatic alkenes such as octene 1g, terminal disubstituted
alkene 1h and trisubstituted alkene 1i smoothly underwent this
transformation affording the respective azido alcohols 3g, 3h
and 3i in high yields (74ꢀ84%) albeit in moderate
regioselectivity (5:1). Remarkably, when symmetrical and
unsymmetrical disubstituted alkenes 1j and 1k were tested for
their diastereoselectivity under the present conditions, 1,2ꢀ
azidoalcohols 3j and 3k were obtained with major anti isomer
this time. No discrepancy in anti-diastereoselectivity was
observed in substrates 1l, 1m and 1n thus affording 3l, 3m and
3n in high yields (78ꢀ 86%). However, electron deficient
no.
1
solvent
DMF
base oxidant azide products [b]
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
5(72%)
2
DMF
Et3N
Et3N
5 (38%) + 6 42%)
3
DMF + DMSO
anhyd.
TBHP
5 (34%) + 6 (22%)
+ 7 (11%)
4
DMF + DMSO
DMF + DMSO
Et3N aq.50%
H2O2
ꢀꢀꢀ
5 (32%) + 6 (26%)
+ 7 (18%)
5
6
Et3N
ꢀꢀꢀ
NaN3 8 (22%) + 6 (18%)
+ 2a (48%)
DMF + DMSO
+ H2O
Et3N
ꢀꢀꢀ
NaN3 5 (35%) + 3a
(48%)
[a] Reaction condition: styrene (1 mmol), I2 (1 mmol), oxidant (2 mmol),
NaN3 (2 mmol), base (2 mmol), solvent, 25 °C, 3 h; [b] Isolated yields
after column chromatographic purification
I2 (10 mol %)
50% aq. H2O2 (2 equiv)
NaN3 (2 equiv)
N3
18OH
18OH
N3
Ph
Ph
Ph
Et3N (1 equiv)
DMF-18O:DMSO (1:1)
25 °C, 8 h
2a-18O (6%)
3a-18O (78%)
1a
Scheme 2. Isotopic labelling experiment using DMFꢀ18O
regeneration of I2. (iv) Account for reversal in regio and
stereoselectivity of the product: However, when the above
reaction was performed under aqueous conditions (DMSO:
DMF:H2O = 1:1:0.4), iodoformate
ted (entry 6). Further reaction of iodo formate
equiv) and Et3N (1 equiv) in DMF:DMSO:H2O (1:1:0.4) gave
3a in 54% yield suggesting the formation of cyclic intermediate
III, which can account for the reversal in regioꢀ and stereoseleꢀ
ctivity of azidohydroxylation under the aqueous conditions.
5
and product 3a were isolaꢀ
with NaN3 (2
5
conjugated
alkenes
failed
to
undergo
catalytic
azidohydroxylation under either condition.
The results of experiments for the mechanistic understanding of
the reaction are summarized in Table 3: (i) DMF as oxygen
source and role of Et3N in hydrolysis: When styrene was treated
condition B
condition A
Me2N
2
O
R
R1
I
I
with I2 (1 equiv) in DMF, the corresponding iodoformate
isolated in 72% yield. In the presence of Et3N, iodoalcohol
was obtained along with which suggest the role of Et3N in
hydrolysis of formate (entries 1 and 2). (ii) Co-oxidants do
not compete with DMF as O-nucleophile: In the presence of coꢀ
oxidants (entries 3 and 4), iodoformate was still formed.
5 was
NaN3
R1
O
6
N
H
R
O
N
5
O
R
O
R
R1
I
R1
I2
5
N3
oxidant
5
II
I
H2
O
base
5
Further, when 18O labelled DMF was used in the reaction,
styrene gave 6% yield of 2aꢀ18O and 78% of 3aꢀ18O under
condition B (Scheme 2). This clearly establishes that DMF
serves as a source of oxygen and thus proves it to be more
HO
R
R1
N3
N3
R1
NaN3
base
O
O
R
OH
R
R1
H2O
2
3
III
Scheme 3: Plausible mechanism for I2-catalyzed azidohydroxylation
of alkenes
Based on the above experiments and literature precedence,18,21
the above mechanism has been proposed (Scheme 3). Initially,
alkene reacts with iodine to form iodonium ion which
potent
Oꢀnucleophile as compared to TBHP, H2O2, H2O or
DMSO under present reaction condition. (iii) Possibility of
formation of hypervalent iodine is ruled out: Further, on adding
NaN3 in the absence of coꢀoxidants (entry 5), azido alcohol
product 2a was indeed obtained in 48% yield. Therefore,
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3
(a) T. J. Donohoe, C. K. Callens, A. Flores, A. R. Lacy and A. H.
Rathi, Chem. Eur. J., 2011, 17, 58-76. (b) V. A. Schmidt and E.
J. Alexanian, J. Am. Chem. Soc. 2011, 133, 11402-11405. (c)
D. F. Lu, C. L. Zhu, Z. X. Jia and H. Xu, J. Am. Chem. Soc., 2014,
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Tetrahedron Lett.1999, 40, 5073-5074. (e) X-F. Xia, Z. Gu, W.
Liu, H. Wang, Y. Xia, H. Gao, X. Liu and Y-M. Liang. J. Org.
Chem. 2015, 80, 290-295.
undergoes regioselective ring opening with DMF to give the
corresponding iodo intermediate
stereoselective displacement with azide ion to form species II.
This on hydrolysis affords syn azido alcohols . On the other
hand, under aq. H2O2 condition, the iodo intermediate is
hydrolyzed in situ to form iodoformate . The proposed species
III formed from by the anchimeric assistance shown by the
formate group, reacts with azide anion in a regioselective
manner to give anti azido alcohol with the liberation of iodide
I followed by subsequent
2
I
5
5
4
5
S. J. Gorzynski, Synthesis, 1984, 629-656.
11 E. F. V. Scriven and K. Turnbull, Chem. Rev. 1988, 88
,
3
297- 368.
B. T. Smith, V. Gracias and J. Aube, J. Org. Chem. 2000, 65
ion, which is then reoxidized with TBHP/H2O2 to regenerate I2
in the catalytic cycle.
6
7
,
3771-3774.
K. Hang, H. Haiyang, V. Stepanenko, M. De Jesus, C.
Torruellas, W. Correa and M. Ortiz-Marciales, J. Org. Chem.
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2487.
G. Sabitha, R. S. Babu, M. Rajkumar and J. S. Yadav, Org.
The application of this novel method is amply illustrated in
the concise total synthesis of (±)ꢀchloramphenicol (9
),23 an
antibiotic drug and (±)ꢀcytoxazone (11),24 a cytokine modulator.
1,2ꢀsyn azido alcohol 2m, prepared by the present protocol was
subjected to simple reductionꢀprotection sequence
8
9
Lett., 2002, 4, 343-345.
10 R. Martin, G. Islas, A. Moyano, A. Pericas, A. Miquel and A.
Riera, Tetrahedron 2001, 57, 6367-6374.
11 S. Brase, C. Gil, K. Knepper and V. Zimmermann, Angew.
Chem. Int. Ed., 2005, 44, 5188-5240.
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Vaccaro, Green Chemistry 2013, 15, 2394-2400. (b) J. S.
Yadav, B. V. S. Reddy, B. Jyothirmai, M. S. R. Murty,
Tetrahedron Lett., 2005, 46, 6559-6562. (c) S. W. Chen, S. S.
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Scheme 4. Total Synthesis of (±)-chloramphenicol and (±)-cytoxazone
followed by regioselective nitration gave (±)ꢀchloramphenicol
(9) in 71% yield. Also, azidodiol 3n, prepared from 1n using
condition B, on reductionꢀBoc protection sequence, gave
aminodiol 10 in 76% yield, which under basic condition underꢀ
went cyclization to give (±)ꢀcytoxazone (11) in 90% yield
13 (a) G. Iacazio and M. Réglier, Tetrahedron Asymmetry 2005,
16, 3633-3639. (b) R. W. Draper, J. Chem. Soc. Perkin Trans I
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Chem., 2008, 73, 6433-6436. (b) M. Watanabe, K. Murata
and T. Ikariya, J. Org. Chem. 2002, 67, 1712-1715. (c) M. A.
Reddy, N. Bhanumathi and K. R. Rao, Chem. Commun. 2001,
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Synlett, 2000, 1049-1051. (e) A. V. R. Rao, M. K. Gurjar and V.
(
Scheme 4).
To summarize, we have developed, for the first time, regioꢀ and
diastereoselective azidohydroxylation of alkenes to give vicinal
azidoalcohols in high yields. The regioꢀ and stereodivergence
observed in the process is driven by the oxidants chosen in
combination with catalytic I2. Mechanistic study revealed that
Kaiwar, Tetrahedron Asymmetry, 1992, 3, 859.
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and Y. Hirai, Synthesis, 2007, 3286-3289.
16 T. Ando, D. Kano, S. Minakata, I. Ryu and M. Komatsu,
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DMF is crucial for regiodivergence and acts as an
Oꢀsource in
this azidohydroxylation reaction. This methodology has
successfully been applied to the concise syntheses of (±)ꢀ
chloramphenicol and (±)ꢀcytoxazone. We believe that this mild,
environmentally benign and operationally simple method would
find tremendous application in streamlining the synthesis of
various drugs and synthetically useful intermediates. Chiral
version of this azidohydroxylation process is currently
underway in our laboratory and will be reported in due course.
17 (a) A. Yoshimura, S. R. Koski, B. J. Kastern, J. M. Fuchs, T. N.
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Chem. Eur. J., 2014, 20, 5895-5898. (b) A. Yoshimura, T. N.
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