A metal-free iodine-mediated conversion of hydroxamates to esters

Article abstract of DOI:10.1080/00397911.2020.1737130

A metal-, oxidant-, and additive-free conversion of hydroxamates to esters have been achieved using molecular iodine as the reagent using a novel but not-so-explored heron-type rearrangement. The reaction proceeds with almost equal facility with substrates having either electron-donating or electron-withdrawing substituent. Similarly, α,?-unsaturated, and sterically hindered ortho-substituted hydroxamates also undergo the desired transformation smoothly.

Full text of DOI:10.1080/00397911.2020.1737130

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
A metal-free iodine-mediated conversion of
hydroxamates to esters
Subhankar Ghosh, Jeet Banerjee, Rajat Ghosh & Shital K. Chattopadhyay
To cite this article: Subhankar Ghosh, Jeet Banerjee, Rajat Ghosh & Shital K. Chattopadhyay
(2020): A metal-free iodine-mediated conversion of hydroxamates to esters, Synthetic
Communications, DOI: 10.1080/00397911.2020.1737130
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https://doi.org/10.1080/00397911.2020.1737130
A metal-free iodine-mediated conversion of hydroxamates
to esters
Subhankar Ghosh, Jeet Banerjee, Rajat Ghosh, and Shital K. Chattopadhyay
Department of Chemistry, University of Kalyani, Kalyani, India
ABSTRACT
ARTICLE HISTORY
Received 24 November 2019
A metal-, oxidant-, and additive-free conversion of hydroxamates to
esters have been achieved using molecular iodine as the reagent
using a novel but not-so-explored heron-type rearrangement. The
reaction proceeds with almost equal facility with substrates having
either electron-donating or electron-withdrawing substituent.
Similarly, a,ß-unsaturated, and sterically hindered ortho-substituted
hydroxamates also undergo the desired transformation smoothly.
KEYWORDS
Amides; metal-free;
molecular iodine; radical;
synthetic methodology
GRAPHICAL ABSTRACT
O
O
I2/NaHCO3/reflux
OR
OR
N
H
18 examples, up to 82% yield
Metal free
External oxidant free
Additive free
Broad scope
Introduction
Synthesis of esters from suitable precursors is a fundamental transformation in organic
chemistry because of the high profile use of esters in pharmaceutical, cosmetics, and fra-
grance industries.[1] By far, the carboxylic acids are the most used precursors and a pleth-
ora of methodologies have been developed over the decades.[2–8] Nonetheless,
continuous improvements over the succinct difficulties in the esterification of acids
continue to be reported, in particular for sterically hindered and electron-rich sub-
strates.[9–11] Conversion of acids to amides usually does not suffer from many of these
drawbacks of the esterification of acids and hence an indirect approach of conversion of
amides to esters has also gained popularity. In this regard, hydroxamates offer significant
opportunities. Direct conversion of hydroxamates to esters has been reported using metals
such as silver oxide,[12] nickel peroxide,[13] lead acetate,[14,15] copper bromide,[16] copper
triflate,[17] nickel acetylacetonate,[18] etc., and oxidizing agents such as m-CPBA,[16] and
CONTACT Shital K. Chattopadhyay
skchatto@yahoo.com
Department of Chemistry, University of Kalyani, Kalyani
741235, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
S. GHOSH ET AL.
O
O
I2/conditions
NHOBn
OBn
1
2
Scheme 1. Conversion of the hydroxamate 1 to the ester 2.
Table 1. Optimization of conditions for the conversion of 1 to 2.
Entry
Reagents (equiv.)
I2 (1)/NaHCO3 (3)
Conditions
Percentage of 2a
1
MeCN, rt, 24 h
10
2
3
4
5
6
7
8
9
10
11
12
13
14
15
I2 (1)/NaHCO3 (3)
I2 (2)/NaHCO3 (3)
I2 (3.5)/NaHCO3 (3)
I2 (3.5)/NaHCO3 (3)
I2 (3.5)
I2 (3.5)/NaHCO3 (3)
I2 (3.5)/NaHCO3 (3)
I2 (3.5)/K2CO3 (3)
NIS (3.5)/NaHCO3 (3)
nBu4NþIꢀ (3.5)/NaHCO3 (3)
I2 (3.5)/KOH (3)
MeCN, reflux, 16 h
MeCN, reflux, 16 h
MeCN, reflux, 5 h
MeCN, reflux, 16 h
MeCN, reflux, 16 h
THF, reflux, 16 h
Toluene, reflux,16 h
MeCN, reflux, 5 h
MeCN, reflux, 5 h
MeCN, reflux, 5 h
MeCN, reflux, 5 h
MeCN, reflux, 5 h
MeCN, reflux, 5 h
24
42
72
58
No reaction
33
21
63
55
Trace
32
I2 (3.5)/Et3N (3)
I2 (3.5)/Pyridine (3)
I2 (3.5)/NaHCO3 (3)
No reaction
No reaction
Trace
MeCN, ultrasound, 6 h
Optimized values are represented in bold.
PhI(OAc)2,[19] and ceric ammonium nitrate.[13] Three metal-free approaches have also
[21]
been reported which employ NBS,[20] tBuOCl/NaN3,
and Bu4NþIꢀ/electrochemical
oxidation[22] for the desired conversion of hydroxamates to esters.
Results and discussion
In recent years, molecular iodine mediated transformations are gaining popularity as
metal-free means of organic transformation.[23–28] Iodine reagents have also been
used for the esterification of carboxylic acids.[29–34] We have recently reported[35] an
iodine-mediated method for intramolecular C–H amination of hydroxamates as a
metal-free approach to quinolone ring system. Herein, we describe the utility of
molecular iodine in the conversion of hydroxamates to esters.[36,37]
Thus, when a solution of N-benzyloxybenzamide (1, Scheme 1) in acetonitrile was
stirred in the presence of 1 equiv. of iodine and 3 equiv. of sodium bicarbonate at room
temperature a slow conversion to the benzyl ester 2 in very poor yield was noticed
(entry 1, Table 1). Putting to prolonged reflux marginally improved the yield (entry 2).
But when the equivalent of iodine was increased from one to two (entry 3), a significant
change in yield (42%) was observed. Further, the increase in the amount of iodine to
3.5 equiv. increased the yield to an acceptable 72% within 5 h. However, attempts to fur-
ther increase, the yield by prolonging the reaction and/or increasing the iodine concen-
tration did not prove to be successful. The use of three and a half equivalents of
NaHCO3 also proved to be mandatory as no reaction took place without it (entry 6).
It was observed that potassium carbonate can also be used as a base (entry 9)
but with a somewhat drop in the yield. On the other hand, the replacement of iodine

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SYNTHETIC COMMUNICATIONSV
3
by N-iodosuccinimide somewhat lowered the yield (55%); but the use of n-tetrabutylam-
monium iodide as replacement of iodine was entire of no avail (entry 11). When
sodium bicarbonate was replaced with potassium hydroxide (entry 12), the reaction pro-
ceeded sluggishly and the yield was lower. However, no reaction took place when
organic bases like triethylamine (entry 13) or pyridine (entry 14) were used. Similarly,
the use of ultrasound (entry 15) as the source of energy did not prove to be efficacious.
Thus, the optimum condition (entry 4) was used for the study of the substrate scope of
the reaction.
The reaction proceeded well in the presence of an electron-donating substituent such
as methyl or methoxy (substrates 1b, 1c, Fig. 1). The halogen-substituted substrates 1d
O
O
I2 (3.5 equiv),
NaHCO3 (3 equiv)
NHOBn
OBn
MeCN, reflux, 5 h
2 a-p
1a-p
O
O
X
O
O
X
X
X
MeO
Cl
1c, X = NHOBn
1d, X = NHOBn
1a, X = NHOBn
1b, X = NHOBn
2c, X = OBn (71%)
2d, X = OBn(73%)
2a, X = OBn(72%)
2b, X = OBn(74%)
O
O
O
O
X
O2N
O2N
X
X
X
O2N
I
NO2
1f, X = NHOBn
1e, X = NHOBn
1g, X = NHOBn
1h, X = NHOBn
2f, X = OBn (80%)
2e, X = OBn(69%)
2g, X = OBn(82%)
2h, X = OBn(77%)
O
X
O
X
X
S
X
O
O
N
1l, X = NHOBn
1k, X = NHOBn
1i, X = NHOBn
1j, X = NHOBn
2l, X = OBn (61%)
2k, X = OBn(65%)
2i, X = OBn (67%)
2j, X = OBn(66%)
O
X
O
X
OMe O
X
Ph
X
Ph
O
OMe
1p, X = NHOMe
1o, X = NHOMe
1n, X = NHOBn
1m, X = NHOBn
2p, X = OMe (71%)
2o, X = OMe (71%)
2n, X = OBn(70%)
2m, X = OBn(68%)
O
X
X
N
N
O
1q, X = NHOBn
1r, X = NHOBn
2q, X = OBn(59%)
2r, X = OBn(52%)
Figure 1. Substrate scope of the developed methodology.

4
S. GHOSH ET AL.
O
O
I
NH
OR
N
I2
heat
NaHCO3
OR
3
1
4
O
O
O
N
N
N
OR OR
OR
5
O
OR
+ N2
2
2
Scheme 2. Probable reaction mechanism for the transformation of 1 to 2.
O
O
NHOBn
OBn
OMe
I2/NaHCO3/
MeO
MeO
MeCN,reflux
5h
1c
2c (70%)
+
+
O
O
NHOMe
1o
2O (72%)
Scheme 3. Experimental support for the proposed mechanism.
and 1e reacted similarly, but the reaction was somewhat better with the nitro-substi-
tuted substrates 1f–h. The a,ß-unsaturated aromatic and heteroaromatic hydroxamates
1i–k also underwent the desired transformation without much complication and the
products were obtained with similar yields. Similarly, the non-conjugated substrates 1l
and 1m could similarly be converted to the respective esters 2l and 2m. The ortho-
disubstituted hydroxamate 1n, fortunately, posed little steric resistance to deliver the
ester 2n in good yield. The reaction was found to be also applicable to the N-methoxy
derivatives 1o and 1p which proceeded with very similar yields. The methodology could
also be extended to the heteroaryl derivatives 1q and 1r although the corresponding
products 1o and 1p formed with slightly lower yields.
In view of the above results and based on precedence, we believe that the reaction
may proceed through the N-iodo derivative 3 (Scheme 2) which fragments to generate
the N-centered radical 4. It subsequently dimerizes to the hydrazine derivative 5 which
undergoes Heron-type rearrangement[38] to form the product 2. To gain some

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SYNTHETIC COMMUNICATIONSV
5
experimental support, the reaction was conducted in the presence of a known radical
inhibitor TEMPO. Although an adduct could not be isolated, it considerably decreased
the yield (40%). However, when 2,6-di-tert-butyl-4-methylphenol (BHT) was used as a
radical inhibitor, the reaction was completely suppressed.
The intra-molecular nature of the reaction was supported by the following cross-over
experiment (Scheme 3). Thus, when a mixture of N-benzyloxy-4-methoxybenzamide
(1c) and N-benzyloxy-4-methyl- benzamide (1o) was subjected to the optimized reac-
tion conditions, the mixture of the corresponding esters 2c (70%) and 2o (72%) was
obtained with undetected cross-over.
Conclusion
In conclusion, we have developed a metal-, additive-, and external oxidant-free proced-
ure for the conversion of hydroxamates to esters using readily available and environ-
mentally friendly molecular iodine and sodium bicarbonate as a reagent. A range of
substrates containing electron-withdrawing or electron-donating groups, ortho disubsti-
tution, and active unsaturation were found to be effective for the conversion. Thus, the
present methodology may complement the existing procedures and hence may find an
application.
Experimental
General procedure for conversion of N-alkoxyamides (1) to carboxylic esters (2)
Sodium bicarbonate (56 mg, 0.66 mmol) and iodine (195 mg, 0.77 mmol) were added to
N-(benzyloxy)benzamide (50 mg, 0.22 mmol) in anhydrous MeCN (3 mL). The mixture
was heated to reflux for 5 h. It was then allowed to cool to rt, diluted with H2O (10 mL)
and then extracted with EtOAc (2 ꢁ 10 mL). The combined organic extract was washed
with saturated aqueous solution of Na2S2O3 (2 ꢁ 5 mL), H2O (10 mL), brine (10 mL),
and then dried (MgSO4). It was then filtered and the filtrate was concentrated under
reduced pressure to leave a residue which was purified by column chromatography on
silica gel (petroleum ether:ethyl acetate ꢀ98:02) to provide the benzyl benzoate (2a)[12]
as a colorless viscous liquid (Yield: 34 mg, 72%). IR (neat): 3034, 2927, 1716, 1602 cmꢀ1
.
1H NMR (400 MHz, CDCl3): d ¼ 8.00 (1H, d, J ¼ 8 Hz), 7.46 (1H, m), 7.37–7.15 (7H,
m), 5.28 (2H, s). 13C NMR (100 MHz, CDCl3) d ¼ 166.5, 136.1, 133.1, 130.1, 129.7,
128.6, 128.4, 128.3, 128.2, 66.7.
General procedure for synthesis of N-alkoxyamides 1a–p
EDC.HCl (240 mg, 1.26 mmol) was added portion-wise to a stirred solution of benzoic
acid (92 mg, 0.75 mmol) in dry DCM (5 mL) at 0 ꢂC during 15 min. Then a solution of
O-benzylhydroxylamine hydrochloride (100 mg, 0.63 mmol) and N-methylmorpholine
(69 mL, 0.63 mmol) in dry DCM (5 mL) was added dropwise over 10 min to the solution
of the acid at the same temperature. The reaction mixture was allowed to come to rt
and stirred for 16 h. It was then diluted with DCM (10 mL) and the combined organic
solution was washed successively with saturated aqueous solution of NaHCO3

6
S. GHOSH ET AL.
(2 ꢁ 7.5 mL), HCl (2N, 2 ꢁ 5 mL), H2O (1 ꢁ 10 mL), brine (1 ꢁ 10 mL), and then dried
(Na2SO4). It was then concentrated under reduced pressure and the residue was purified
by column chromatography on silica gel using a mixture of petroleum ether:ethyl acet-
ate (80:20) as eluent to provide the product 1a[12] (83%) colorless solid; IR (neat): 3239,
1
2875, 1647, 1577 cmꢀ1; H NMR (400 MHz, CDCl3) d 9.31 (1H, brs), 7.58 (2H, d,
J¼ 7.6 Hz), 7.37 (1H, t, J¼ 8 Hz), 7.30–7.24 (7H, m), 4.88 (2H, s); 13C NMR (100 MHz,
CDCl3) d 166.5, 135.3, 132. 0, 131.9, 129.3, 128.7, 128.6, 128.6, 127.2, 78.3.
1
Analytical data of all new compounds as well as copies of their H and 13C NMR
spectra are provided in the supporting material.
Funding
This work was financially supported by Department of Science and Technology, Govt. of India
[EMR/2017/001336], and University of Kalyani.
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