Selective Deprotection of tert-Butyldimethylsilyl Ethers Using Nafion-H/Sodium Iodide (or Bromodimethylsulfonium Bromide) in Methanol

Article abstract of DOI:10.1081/SCC-120026345

Catalytic amounts of bromodimethyl sulfonium bromide, or Nafion-H along with NaI (1 equiv.), in methanol cleave a variety of TBDMS ethers readily in high yields. Alkyl TBDMS ethers react more readily and selectively compared to phenolic TBDMS ethers, benzyl, and methyl ethers.

Full text of DOI:10.1081/SCC-120026345

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Selective Deprotection of tert -Butyldimethylsilyl
Ethers Using Nafion-H®/Sodium Iodide (or
Bromodimethylsulfonium Bromide) in Methanol
Shikha Rani a , J. Lokesh Babu a & Yashwant D. Vankar a
a Department of Chemistry , Indian Institute of Technology Kanpur , Kanpur, India
Published online: 16 Aug 2006.
To cite this article: Shikha Rani , J. Lokesh Babu & Yashwant D. Vankar (2003) Selective Deprotection of tert -
Butyldimethylsilyl Ethers Using Nafion-H®/Sodium Iodide (or Bromodimethylsulfonium Bromide) in Methanol, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:23, 4043-4052, DOI:
10.1081/SCC-120026345
To link to this article: http://dx.doi.org/10.1081/SCC-120026345
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 23, pp. 4043–4052, 2003
Selective Deprotection of tert-Butyldimethylsilyl
Ethers Using Nafion-HÕ/Sodium Iodide
(or Bromodimethylsulfonium
Bromide) in Methanol
*
Shikha Rani, J. Lokesh Babu, and Yashwant D. Vankar
Department of Chemistry, Indian Institute of
Technology Kanpur, Kanpur, India
ABSTRACT
Catalytic amounts of bromodimethyl sulfonium bromide, or
Nafion-H along with NaI (1 equiv.), in methanol cleave a variety
of TBDMS ethers readily in high yields. Alkyl TBDMS ethers
react more readily and selectively compared to phenolic TBDMS
ethers, benzyl, and methyl ethers.
Key Words: TBDMS ethers; Selective deprotection; Nafion-H/NaI;
Bromodimethyl sulphonium bromide.
*Correspondence: Yashwant D. Vankar, Department of Chemistry, Indian
Institute of Technology Kanpur, Kanpur 208 016, India; E-mail: vankar@
iitk.ac.in.
4043
DOI: 10.1081/SCC-120026345
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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4044
Rani, Babu, and Vankar
Among various protecting groups for alcohol functionality, tert-
butyldimethyl silyl ether[2] (TBDMS) has been extremely useful mainly
due to its stability under normal acidic and basic conditions and also
because it could be readily deprotected. After the initial discovery[3] of
n-Bu4NþFꢀ as a deprotecting agent for the TBDMS ethers, several new
reagents have been introduced[3] essentially for facilitating work-up,
reducing reaction time, and performing reactions under different
conditions. These can be categorized in six different classes (i) reductive
removal e.g., by using DIBAL,[4a] LiAlH4,[4b] transfer hydrogenation[4c]
;
(ii) oxidative removal with reagents like CAN,[5a] DDQ,[5b,c] NaIO4,[5d]
OxoneÕ[5e]; (iii) fluoride ion based deprotection using NH4F,[6a] LiBF4,[6b]
Zn(BF4)2,[6c] SiF4,[6d] and others; (iv) cleavage under basic conditions
using NaOH,[7a] K2CO3/EtOH[7b]; (v) Lewis (or protic) acid based
removal with reagents like TMSOTf,[8a] BCl3,[8b] ClSiMe3/H2O,[8c]
Ce(OTf )3,[8d] IBr/MeOH,[8e] I2/MeOH,[8f ] BBr3,[8g] n-Bu4NBr/
MeOH,[8h] CeCl3ꢁ7H2O/NaI,[8i] InCl3[8j]; and (vi) hydrolytic cleavage
using H2O/DMSO.[9] However, in some cases the conditions are rather
harsh such as the use of molar equivalent or more of the reagents, heating
at high temperatures etc. Hence the quest continues to find newer
reagents and conditions with improved efficiencies in terms of work-up
and with selective deprotections, for example, between two types of
TBDMS ethers such as alkyl and phenolic TBDMS ethers, or between
a TBDMS ether and other functional groups. Selective deprotection of an
alkyl TBDMS ether in presence of a phenolic TBDMS ether is important
since a number of biologically important natural products such as api-
cularens,[10a] gilvocarcin,[10b] and vancomycin,[10c] contain both phenolic
and alkyl hydroxy groups and in any synthetic endeavour one might need
to selectively deprotect one of these two TBDMS ethers.
In this article we wish to introduce two reagent systems viz.
Nafion-HÕ/NaI or bromodimethylsulfonium bromide (Me2SþBrBrꢀ)
in methanol for easy deprotection of TBDMS ethers in high yields.
Nafion-HÕ is a commercially available solid acid catalyst which has been
found to be a useful reagent[11] in various organic transformations.[12]
Our continued interest[1,13] in exploring the potential of Nafion-H in
organic synthesis led us to find its behavior as an in-situ source of HI
when mixed with NaI. It was found that when a methanol solution of a
TBDMS ether is stirred with NaI and catalytic amount of Nafion-H at
room temperature[14] for 3–15 h, the corresponding alcohols are released
in good to excellent yields (Table 1). Examples include cleavage of a
variety of TBDMS ethers derived from primary, secondary, allylic, and
sugar derived substrates. Besides this, TBDMS ethers are cleaved in the
presence of other functional groups such as an acetate (Entries 4 and 8), a

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Deprotection of tert-Butyldimethylsilyl Ethers
4045
benzoate (Entry 7), a benzyl ether (Entries 3 and 13), and an acetal group
(Entry 13) despite Nafion-H being strongly acidic (acidity is compa-
rable[11] to 96–100% H2SO4, although the pH of the solution under the
present conditions was found to be about 5.5–6). Additionally, phenolic
TBDMS ethers (Entries 9 and 12) also undergo cleavage albeit at 50ꢂC.
This reluctance of phenolic TBDMS ethers to undergo cleavage was
exploited in selective deprotection of an alkyl TBDMS ether in presence
of
a
phenolic TBDMS ether (Entries 16–18). Unfortunately,
however, a THP ether and an acetonide group also get deprotected
under the present conditions (Entries 19 and 20). The reaction could be
visualized to proceed as shown in Sch. 1.
Bromodimethylsulfonium bromide (BDMS)[15] was introduced by
Olah et al.[16] as a source of electrophilic bromine in organic synthesis.
It is a yellow crystalline solid which is readily obtainable from liquid
bromine and dimethyl sulfide. It occurred to us that this reagent
should release HBr upon treatment with methanol (Sch. 2) which, in
turn, should be able to cleave TBDMS ethers in an analogous manner
as does the HI (vide supra). This was found to be the case as a variety of
TBDMS ethers led to the formation of the corresponding alcohols
in good to excellent yields at room temperature (Table 1). As with
CnF2n+1SO3Na
CnF2n+1SO3H +
HI
NaI
+
Me
RO Si t-Bu
Me
Me
MeOH
HI
+
ROH
+
I
Si t-Bu
Me
MeOH
Me
MeO Si t-Bu
Me
Scheme 1.
Br
Me2S OMe
+
MeOH
+
Me2S Br
Br
HBr
+
Scheme 2.

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4046
Rani, Babu, and Vankar
Table 1. Deprotection of TBDMS ethers with Nafion-HÕ (or bromo-
dimethylsulfonium bromide).
With Nafion-H With BDMS
Time Yield Time Yield
S. no.
1
Substrate
Product
(h)
(%)
(h)
(%)
OTBDMS
OH
6
96
4
>99
OTBDMS
7
OH
7
2
3
4
5
8
6
6
8
65
82
81
95
8
6
5
6
70
98
BnO
AcO
OTBDMS
OTBDMS
BnO
AcO
OH
OH
4
4
4
4
84
>99
OTBDMS
OH
6
8
80
8
33
OTBDMS
OH
OBz
OBz
OH
7
8
9
6
7
92
79
4
6
96
88
OTBDMS
OAc
OAc
OTBDMS
OTBDMS
OH
OH
10a
>99
8a >99
OTBMDS
OTBDMS
OH
10
11
8
7
76
76
7
6
87
91
OH
25a
4
>99 22a >99
OH
OTBDMS
12
13
OH
OTBDMS
O
>99
3
>99
O
BnO
BnO
BnO
BnO
OMe
OBn
OMe
OBn
14
15
Cholesterol TBDMS ether Cholesterol
15
7
91
94c
15
6
85.5
87c
OTBDMS
OH
OTBDMS
O
O
OTBDMS
16
4
73d[18]
3
60
OTBDMS
OH
OTBDMS
OTBDMS
17
5
74d[18]
6
81d
OTBDMS
OTBDMS
OTBDMS
OH

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Deprotection of tert-Butyldimethylsilyl Ethers
Table 1. Continued.
With nafion-H
4047
With BDMS
Time
(h)
Yield
(%)
Time
(h)
Yield
(%)
S. no.
18
Substrate
Product
OTBDMS
8b
96 þ>99
5b
>99 þ>99
OH
OTBDMS
OTBDMS
19
20
1
4
98e
99e
1
1.5
97
99
TBDMSO
OTHP
OTBMS
HO
OH
D-mannitol
O
O
O
O
OH
aReaction was carried out at 50ꢀC; 1 equiv. of NaI with wt equivalent of Nafion-
H has been used.
bReaction was carried out at 0ꢀC to room temperature; 1 equiv. of NaI with
catalytic amount of Nafion-H has been used.
cIsolated as its acetate.
dReaction was carried out at 40ꢀC with half wt equivalent of Nafion-H and
1 equiv. of NaI.
eYield based the isolated corresponding acetates.
Nafion-H, BDMS also permitted selective cleavages of alkyl TBDMS
ethers over phenolic TBDMS ethers (Entries 16–18).
In view of the fact that these deprotections are easy to carry out and
there are not too many methods for selective[8c,f,g,17] deprotections of
alkyl TBDMS ethers in presence of phenolic TBDMS ethers, we believe
that the present methods should find application in organic synthesis.
EXPERIMENTAL PROCEDURE
Typical Experimental Procedures Are as Follows
To a stirred solution of a TBDMS ether (1 mmol) in methanol (3 mL)
cooled at 0ꢂC was added Nafion-H (5 mg) and NaI (165 mg, 1.1 mmol).
Stirring was then continued at room temperature till the reaction was
complete (TLC monitoring). The reaction mixture was then filtered and
the solvent evaporated to get the crude product which was purified by
column chromatography.

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4048
Rani, Babu, and Vankar
A stirred solution of a TBDMS ether (1 mmol) in methanol
(3 mL) was cooled to 0ꢂC and then treated with BDMS (5 mg).
The reaction mixture was slowly brought to room temperature and
stirred till the reaction was complete (TLC monitoring). The solvent
was removed under vacuum and the residue treated with water (10 mL)
followed by extraction with CH2Cl2 (3 ꢃ 15 mL), washing with water
and then brine. The organic layer was dried over Na2SO4 and the
solvent evaporated to get crude product which was purified by column
chromatography.
The starting TBDMS ethers as well as the corresponding alcohols
except those corresponding to Entries 16 and 17 are known in the
literature. Hence the recovered alcohols were characterized by spectral
means and compared with the spectral data reported in the literature.
ACKNOWLEDGMENT
We thank the Department of Science and Technology, New Delhi for
financial assistance (Project No. SP/S1/G07/97). One of us (SR) thanks
CSIR for a Senior Research Fellowship. We also thank Profs. G. A. Olah
and G. K. S. Prakash for the gift of Nafion-K.
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G.A.; Vankar, Y.D.; Arvanaghi, M. Synthetic Methods are
Reactions. 73. Conversion of Epoxides and Enamines into a-
Haloketones with Halodimethylsulfonium Halides. Tetrahedron
Lett. 1979, 3653. For other uses see (d) Chow, Y.L.; Bakker, B.H.
Electrophilic Addition of Bromodimethylsulfonium Bromide to
Olefins. Synthesis 1982, 648 and references cited therein; (e)
Majetich, G.; Hicks, R.; Reister, S. Electrophilic Aromatic

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Rani, Babu, and Vankar
Bromination Using Bromodimethylsulfonium Bromide Generated
in situ. J. Org. Chem. 1997, 62, 4321.
17. Lee, A.S.-Y.; Yeh, H.-C.; Shie, J.-J. A Novel, Highly Efficient and
Selective Desilylating Method for Trialkylsilyl Ethers. Tetrahedron
Lett. 1998, 39, 5249.
18. Spectral data of alcohol corresponding to Entry 16. IR (neat) ꢁmax
:
3355 cmꢀ1. H NMR (CDCl3 400 MHz): ꢀ 0.00 (s, 6H, Si(CH3)2),
0.79 (s, 9H, SiC(CH3)3), 3.17 (s, 1H, ArCH2OH), 4.19–4.40 (m, 2H,
ArCH2OH), 6.56–7.07 (m, 4H, C6H4<). 13C NMR (100 MHz): ꢀ
ꢀ4.46, 25.66, 57.70, 65.09, 74.41, 115.26, 120.17, 128.61, 129.69,
133.49, 155.31, 155.58. Analysis calcd. for C13H22O2Si: C, 65.5; H,
9.30%. Found: C, 65.48; H, 9.33%. Spectral data of alcohol corre-
1
1
sponding to Entry 17. IR (neat) ꢁmax: 3311 cmꢀ1. H NMR (CDCl3
400 MHz): ꢀ 0.19 (s, 3H, SiCH3), 0.26 (s, 3H, SiCH3), 0.97 (s, 9H,
SiC(CH3)3), 1.02 (s, 9H, SiC(CH3)3), 3.42 (s, 1H, ArCH2OH), 4.58
(s, 2H, ArCH2OH), 6.33–7.13 (m, 4H, C6H4<). 13C NMR
(100 MHz): ꢀ ꢀ4.47, ꢀ4.42, ꢀ4.22, 18.14, 18.22, 25.66, 25.70,
29.69, 61.67, 69.88, 110.80, 113.05, 124.50, 129.69, 154.36, 156.11,
156.73. Analysis calcd. for C19H36O3Si2: C, 61.90; H, 9.84%.
Found: C, 61.93; H, 9.81%.
Received in India May 23, 2003