Bromodimethyl sulfonium bromide mediated rapid and facile protection of amines

Article abstract of DOI:10.1080/00397911.2010.497592

A new clean protocol for protection of aryl and aliphatic amines with t-butoxycarbonyl (t-BOC) and benzyloxycarbonyl (Cbz) catalyzed by simple (bromodimethyl) sulfonium bromide has been developed. Rapid protection of amines in excellent yields in totally solvent-free conditions has been achieved. Copyright

Full text of DOI:10.1080/00397911.2010.497592

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(Bromodimethyl)sulfonium Bromide
Mediated Rapid and Facile Protection of
Amines
M. Shailaja a , A. Manjula a & B. Vittal Rao a
a Organic Chemistry Division II, Indian Institute of Chemical
Technology, Hyderabad, India
Published online: 20 May 2011.
To cite this article: M. Shailaja , A. Manjula & B. Vittal Rao (2011) (Bromodimethyl)sulfonium
Bromide Mediated Rapid and Facile Protection of Amines, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:14, 2073-2080, DOI:
10.1080/00397911.2010.497592
To link to this article: http://dx.doi.org/10.1080/00397911.2010.497592
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Synthetic Communications1, 41: 2073–2080, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.497592
(BROMODIMETHYL)SULFONIUM BROMIDE
MEDIATED RAPID AND FACILE PROTECTION
OF AMINES
M. Shailaja, A. Manjula, and B. Vittal Rao
Organic Chemistry Division II, Indian Institute of Chemical Technology,
Hyderabad, India
GRAPHICAL ABSTRACT
Abstract
A new clean protocol for protection of aryl and aliphatic amines with
t-butoxycarbonyl (t-BOC) and benzyloxycarbonyl (Cbz) catalyzed by simple (bromodi-
methyl)sulfonium bromide has been developed. Rapid protection of amines in excellent yields
in totally solvent-free conditions has been achieved.
Keywords Amine; benzyloxycarbonyl chloride; (bromodimethyl)sulfonium bromide;
di-tert-butyldicarbonate; protection
INTRODUCTION
Protecting groups play vital roles in carrying out reactions selectively at one
reactive site in a multifunctional compound. The significance of protecting groups
can be judged by the numerous literature reports appearing on new groups as well
as many new methods of introduction or removal of them.[1] Efficient protection of
amines is a key step in organic synthesis, more so because it has become an essential
tool in contemporary peptide[2] and organic chemistry.[3] t-Butoxycarbonyl (t-BOC)
and benzyloxycarbonyl (Cbz) are among the favorite protecting groups for protection
of amines.[4] The acid labile nature of the t-BOC has both advantages and disadvan-
tages as it can be used as a deprotection method. Cbz, on the other hand, is stable to
basic and most aqueous acidic media, cleaved by either photolysis or hydrogenolysis.
The protection of amines by the Cbz group has been achieved in a variety of
base-mediated reaction conditions[5] whereas very few reports exist for the protection
Received December 14, 2009.
Address correspondence to B. Vittal Rao, Organic Chemistry Division II, Indian Institute of
Chemical Technology, Hyderabad 500 607, India. E-mail: vittalrao@iict.res.in; raobommena@gmail.com
2073

2074
M. SHAILAJA, A. MANJULA, AND B. V. RAO
of amines in an acid medium.[6] In pursuit of our endeavor to develop novel and
environmentally friendly methods, we herein report an operationally facile, rapid,
efficient, and high-yielding protocol for protection of amines by the Cbz group using
catalytic amounts of (bromodimethyl)sulfonium bromide[7] (Me2SBr2) reagent.
(Bromodimethyl)sulfonium bromide, which is easy to prepare, has several attractive
features as a catalyst, such as clean reaction conditions, versatility, mildness, and
ease of handling, added to the fact that the reagent has not been fully exploited
yet. Our earlier experience with it proved its versatility for development of cleaner
synthetic methods including solventless approaches.
RESULTS AND DISCUSSION
Benzylamine and benzyloxycarbonyl chloride (CbzCl, 50% solution in toluene)
were treated with 10 mol% of Me2SBr2 at room temperature to afford the corre-
sponding Cbz protected amine in good yield (Table 1) in 10 min. Further reactions
were carried out to test the generality of the method using a variety of substrates
and substitutions. In each case, the reaction proceeded efficiently at ambient tem-
perature and afforded the corresponding Cbz-protected amine. Protection is equally
effective with secondary amines such as piperidine and morpholine (entries 5 and 6).
The interesting feature was the facile protection of the amino group of a-amino acid
ester (entry 10) as well as b-amino acid esters derived from sugars, which could find
applications in peptide chemistry. Additionally, the method is highly chemoselective,
as only amine functionality is protected even in the presence of alcohol functionality
(entry 7). All products were characterized by 1H NMR, infrared (IR), and mass spec-
trometry data. Reaction rate and conversion yields were dependent on the nature of
the amines. In general, primary amines react faster and give better yields.
In an additional investigation and application of the catalyst, the amine protection
was also attempted with (Boc)2O by employing Me2SBr2 under similar conditions. The
reaction was attempted at room temperature with di-tert-butyldicarbonate, amine, and
10mol% Me2SBr2. The process proceeds smoothly to generate N-Boc amines in
good yields almost instantaneously. All the products were characterized by the
1H NMR, IR, and mass spectrometry, and the data were in full agreement with the
literature data. The protocol, N-boc protection of amines, is of general applicability
with a variety of amines protected in a facile manner (Table 2). In addition, the Boc pro-
tection was equally effective in the case of amino acid esters (Table 2, entry 7) as well as
chemoselective (Table 2, entry 5). The yields were greater for Boc protection, and reac-
tion times are shorter when compared to the Cbz protection. Compared to the other
Scheme 1. Me2SBr2-catalyzed Cbz protection of amines.

RAPID AND FACILE PROTECTION OF AMINES
2075
Table 1. Me2SBr2-catalysed Cbz protection of amines
Entry
1.
Amine
Product
Time (min)
Yield (%)
12
10
15
89
84[13]
85
2.
3.
10
10
83[6a]
74[12]
4.
15
90[13]
5.
6.
7.
10
15
72[13]
76[14]
12
18
87[13]
88
8.
9.
10
20
15
86[6a]
83[6a]
20[13]
10.
11.
15
88
(Continued )

2076
M. SHAILAJA, A. MANJULA, AND B. V. RAO
Table 1. Continued
Entry
Amine
Product
Time (min)
Yield (%)
12.
20
75
Note: The superscript number in yield column is the reference number for reported compounds.
Scheme 2. Me2SBr2-catalyzed Boc protection of amines.
reagents and methods known for Boc protection of amines such as base catalyzed,[8]
neutral,[9] yittria–zirconia[10] mediated, or Lewis acid catalyzed,[11] the Me2SBr2-cata-
lyzed protocol has distinct advantages such as simplicity, short reaction times, environ-
mentally friendly, clean approach.
The role of the catalyst, (bromodimethyl)sulfonium bromide, is somewhat like
that of Lewis acid: an electrophilic activation of benzyloxycarbonyl chloride and
di-tert-butyldicarbonate.
In conclusion, the Me2SBr2-catalyzed direct protection of amines by both CbZ
and Boc groups is an operationally facile, rapid, mild, and environmentally friendly
clean synthetic method. The protocol is general and applicable to a variety of alkyl,
and substituted alkyl, aryl, substituted aryl amines, and amino acid esters.
EXPERIMENTAL
To 1 mmol of an amine, 1 mmol of the protecting group [CBzCl, 50% solution
in toluene or (Boc)2O] was added dropwise, followed by 10 mol% of Me2SBr2
catalyst. The reaction mixture stirred at room temperature, and the progress of
the reaction was monitored by thin-layer chromatography (TLC). After completion
of the reaction as indicated by the disappearance of the amine, the reaction mixture
was quenched with water and extracted with ethyl acetate. The organic layer washed
with water and brine, dried over anhydrous sodium sulfate, filtered, and concen-
trated under reduced pressure to yield the product.
Physical and spectroscopic data of compounds are in full agreement with those
reported in literature; citations have been included in both Tables 1 and 2. Spectral
data for 1b, 1d, 11, and 12, which are new, and a few other important compounds are
given below.

RAPID AND FACILE PROTECTION OF AMINES
2077
Table 2. Me2SBr2-catalyzed Boc protection of amines
Time
(min)
Yield
(%)
Entry
Amine
Product
1
5
6
8
85[9]
77[9]
92[9]
2
3
4
5
5
8
81[11]
82[11]
6
10
80[9]
7
8
10
7
80[9]
80[9]
9
10
75[11]
Note: The superscript number in yield column is the reference number for reported compounds.

2078
M. SHAILAJA, A. MANJULA, AND B. V. RAO
Compounds in Table 1
Benzyl 2-methoxybenzylcarbamate 1b. Oil; IR (n cmꢀ1): 3335, 3030, 2925,
1726; 1H NMR (CDCl3, 200 MHz, d): 7.40–7.20 (m, 5H, Ar), 6.90 (m, 3H, Ar), 5.02
(s, 2H, Bn), 4.30 (d, 2H, CH2, J ¼ 5.76 Hz), 4.20 (brs, 1H, NH), 3.80 (s, 3H, OCH3).
13C NMR (CDCl3 þ DMSO, 75 MHz, d): 45.22, 59.60, 110.15, 119.86, 125.08,
126.73, 127.89, 128.74, 130.43, 132.45, 132.15, 137.83, 158.50; EI mass (m=z): 272
(M þ 1). CHN analysis: calcd. C, 62.13; H, 4.56; N, 4.53. Found: C, 61.59; H,
4.45; N, 4.38.
Benzyl [4-(triflouromethyl)benzyl]carbamate 1c. Mp 43 ꢁC; IR (n cmꢀ1):
3300, 1720; 1H NMR (CDCl3 þ DMSO, 300 MHz, d): 7.56 (d, 2H, Ar, J ¼ 8.29 Hz),
7.42 (d, 2H, Ar, J ¼ 8.29 Hz), 7.30 (m, 5H, Ar), 5.02 (s, 2H, Bn), 4.80 (brs, 1H, NH),
4.30 (d, 2H, CH2, J ¼ 6.03 Hz); 13C NMR (CDCl3 þ DMSO, 75 MHz, d): 44.72,
66.64, 125.39, 126.56, 127.60, 128.56, 128.60, 129.81, 136.62, 143.38, 156.68; EI mass
(m=z): 332 (M þ Na). CHN Analysis: calcd. C, 70.80; H, 6.31; N, 5.16. Found: C,
70.01; H, 6.42; N, 5.09.
Benzyl (2-phenylethyl)carbamate 2. Mp 55 ꢁC; IR (n cmꢀ1): 3303, 3033,
1
1735; H NMR (CDCl3 þ DMSO, 300 MHz, d): 7.34 (m, 10H, Ar), 5.03 (s, 2H,
Bn), 4.25 (brs, 1H, NH), 3.40 (t, 2H, CH2, J ¼ 13.03 Hz), 2.80 (t, 2H, CH2,
J ¼ 6.99 Hz); EI mass (m=z): 241 (Mþ). CHN Analysis: calcd. C, 75.30; H, 6.66;
N, 5.49. Found: C, 74.01; H, 6.62; N, 5.43.
Benzyl (1-phenylethyl)carbamate 4. Mp 46 ꢁC; IR (n cmꢀ1): 3327, 3029,
2929, 1688; 1H NMR (CDCl3 þ DMSO, 300 MHz, d): 7.30 (m, 10H, Ar), 5.10
(s, 2H, Bn), 4.60 (brs, 1H, NH), 4.38 (q, 1H, CH), 2.40 (d, 3H, CH3, J ¼ 4.80 Hz);
EI mass (m=z): 278 (M þ Na).
Benzyl morpholine-1-carboxylate 5. Oil; IR (n cmꢀ1): 3440, 2910, 2852,
1
1697; H NMR (CDCl3, 200 MHz, d): 7.30 (m, 5H, Ar), 5.10 (s, 2H, Bn), 3.60
(br, 4H, N-CH2), 3.40 (t, 4H, J ¼ 4.91 Hz, CH2); EI mass (m=z): 221 (Mþ).
Benzyl 4-methylpiperazine-1-carboxylate 6. Oil; IR (n cmꢀ1): 3440, 2923,
2853, 1700; 1H NMR (CDCl3, 300 MHz, d): 7.30 (m, 5H, Ar), 5.09 (s, 2H, Bn), 4.14
(t, 2H, CH2, J ¼ 4.72 Hz), 2.68 (t, 2H, CH2, J ¼ 4.76 Hz), 1.60 (m, 2H, CH2), 1.10
(t, 2H, CH2, J ¼ 4.56 Hz), 0.96 (S, 3H, CH3, J ¼ 6.60 Hz); EI mass (m=z): 234 (Mþ).
Benzyl (2-methoxyethyl)-1-carbamate 7b. Oil; IR (n cmꢀ1): 3337, 3033,
1
2925, 1708; H NMR (CDCl3, 200 MHz, d): 7.50 (m, 5H, Ar), 5.02 (s, 2H, Bn),
4.20 (brs, 1H, NH), 3.40 (t, 2H, CH2, J ¼ 5.29 Hz), 3.30 (t, 2H, CH2, J ¼ 5.29 Hz,),
3.22 (s, 3H, OCH3); EI mass (m=z): 208 (M ꢀ 1), 167 (M-OCH3).
4a-Methyl-2-phenyl-7-chloroindeno[1,2-d][1,2,3]oxadiazine-2,4a(4H,5H)-
1
dicarboxylate 11. Mp 122–125 ꢁC; IR (n cmꢀ1): 3454, 3032, 2951, 1746; H NMR
(CDCl3 þ DMSO, 300 MHz, d): 7.64 (d, 2H, J ¼ 8.12 Hz Ar), 7.50 (S, 1H, Ar),
7.40–7.20 (m, 5H, Ar), 5.50 (d, 1H, J ¼ 10.0 Hz, OCH), 5.30 (s, 2H, Bn), 5.10 (d,
1H, J ¼ 10.0 Hz, OCH), 3.63 (s, 3H, OCH3), 3.42 (d, 1H, J ¼ 18 Hz, CH), 3.20 (d,
1H, J ¼ 18 Hz, CH); FAB mass: 401 (M þ 1)þ. CHN analysis: calcd. C, 59.90; H,
4.27; N, 6.96. Found: C, 58.68; H, 4.22; N, 6.82.

RAPID AND FACILE PROTECTION OF AMINES
2079
Benzyl (S)-1-((3aR,6S,6aR)-6-methoxy-2,2-dimethyltetrahydrofuro[3,2-d]
[1,3]dioxol-5-yl)-3-oxopentylcarbamate 12. 1H NMR (CDCl3 þ DMSO,
200 MHz, d): 7.35–7.20 (m, 5H, Ar), 5.85 (d, 1H, J ¼ 3.30 Hz), 5.35 (s, 1H), 5.21 (s,
2H, OCH2 Bn), 4.50 (d, 1H, J ¼ 3.33 Hz), 4.30 (brs, 1H, NH), 4.23–4.05 (q, 2H,
J ¼ 6.66, 13.32 Hz, OCH2), 3.62 (m, 1H), 3.42 (s, 3H, OCH3), 2.70–2.52 (m, 2H,
CH2), 1.45 (s, 3H, CH3), 1.30 (s, 3H, CH3), 1.28 (t, 3H, J ¼ 6.65 Hz OCH2CH3);
ESI mass: 421 (M þ 1þ). CHN analysis: calcd. C, 60.01; H, 6.24; N, 3.33. Found:
C, 59.39; H, 6.05; N, 3.28.
Compounds in Table 2
tert-Butyl (1-phenylethyl)carbamate 15. Mp 65 ꢁC; IR (n, cmꢀ1): 3388,
3031, 2932, 1684; 1H NMR (CDCl3 þ DMSO, 200 MHz, d): 7.30 (m, 5H, Ar),
5.40 (br, 1H, CH), 4.70 (br, 1H, NH), 2.58 (s, 3H, CH3), 1.40 (s, 9H, t-Bu). CHN
Analysis: calcd. C, 65.30; H, 7.70; N, 5.90. Found: C, 66.64; H, 7.74; N, 5.96.
tert-Butyl morpholine-4-carboxylate 20. Mp 54 ꢁC; IR (n, cmꢀ1): 3443,
2929, 2866, 1697; 1H NMR (CDCl3 þ DMSO, 200 MHz, d): 3.60 (t, 4H, J ¼ 5.28 Hz,
N-CH2), 3.38 (t, 4H, J ¼ 5.32 Hz, CH2), 1.49 (s, 9H, t-Bu). CHN Analysis: calcd. C,
56.01; H, 8.71; N, 7.05. Found: C, 56.97; H, 8.75; N, 7.11.
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