Hexamethyldisilazane in the presence of n,n′,n″n?- tetramethyltetra-2,3-pyridinoporphyrazinato copper (II) is a new, mild and highly efficient reagent for silylation of alcohols and phenols

Article abstract of DOI:10.1080/10426500490474987

Alcohols and phenols are protected with hexamethyldisilazane in the presence of N,N′,N″,N?-tetramethyletra-2,3- pyridinoporphyrazinato copper (II) in good-to-excellent yields at room temperature.

Full text of DOI:10.1080/10426500490474987

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Phosphorus, Sulfur, and Silicon and the Related
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HEXAMETHYLDISILAZANE IN THE PRESENCE
OF N,N′,N″,N‴-TETRAMETHYLTETRA-2,3-
PYRIDINOPORPHYRAZINATO COPPER (II) IS A NEW, MILD
AND HIGHLY EFFICIENT REAGENT FOR SILYLATION OF
ALCOHOLS AND PHENOLS
Batool Akhlaghinia a , Mozafar Asadi b , Elham Safaee b & Mozhgan Heydarpoor a
a Department of Chemistry, Damghan University of Basic Sciences , Damghan, Iran
b Department of Chemistry, Faculty of Sciences, Shiraz University , Shiraz, Iran
Published online: 16 Aug 2010.
To cite this article: Batool Akhlaghinia , Mozafar Asadi , Elham Safaee & Mozhgan Heydarpoor (2004) HEXAMETHYLDISILAZANE
IN THE PRESENCE OF N,N′,N″,N‴-TETRAMETHYLTETRA-2,3-PYRIDINOPORPHYRAZINATO COPPER (II) IS A NEW, MILD AND HIGHLY
EFFICIENT REAGENT FOR SILYLATION OF ALCOHOLS AND PHENOLS, Phosphorus, Sulfur, and Silicon and the Related Elements,
179:10, 2099-2104, DOI: 10.1080/10426500490474987
To link to this article: http://dx.doi.org/10.1080/10426500490474987
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Phosphorus, Sulfur, and Silicon, 179:2099–2104, 2004
ꢀC
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490474987
HEXAMETHYLDISILAZANE IN THE PRESENCE OF
N,Nꢀ,Nꢀꢀ,Nꢀꢀꢀ-TETRAMETHYLTETRA-2,3-
PYRIDINOPORPHYRAZINATO COPPER (II) IS A
NEW, MILD AND HIGHLY EFFICIENT REAGENT
FOR SILYLATION OF ALCOHOLS AND PHENOLS
Batool Akhlaghinia,a Mozafar Asadi,b Elham Safaee,b
and Mozhgan Heydarpoora
Department of Chemistry, Damghan University of Basic
Sciences, Damghan, Irana; and Department of Chemistry,
Faculty of Sciences, Shiraz University, Shiraz, Iranb
(Received February 5, 2004; accepted March 23, 2004)
Alcohols and phenols are protected with hexamethyldisilazane in the
presence of N,Nꢁ,Nꢁꢁ,Nꢁꢁꢁ-tetramethyletra-2,3-pyridinoporphyrazinato
copper (II) in good-to-excellent yields at room temperature.
Keywords: Alcohols; hexamethyldisilazane (HMDS); N,Nꢁ,Nꢁꢁ,Nꢁꢁꢁ,-
tetramethyltetra-2,3-pyridinoporphyrazinato copper (II); phenols;
silylation
INTRODUCTION
The role of silyl groups has been recognized of late as an important
part of organic chemistry from both analytical and synthetic points
of view, especially as a protecting group in many syntheses of rea-
sonable complexity.1 Generally the formation of silyl ethers is carried
out by treatment of alcohols with silyl chlorides or silyl triflates in
the presence of a base,2 Li2S,3 and sometimes a nonionic super base
catalyst.4 However, some of these methods frequently suffer from draw-
backs such as lack of reactivity or the difficulty in removal of amine
salts derived from the reaction of by-product acid and cobases during
the silylation reaction. 1,1,1,3,3,3-Hexamethyldisilazane(HMDS) is a
We gratefully acknowledge the partial support of this study by Damghan University
Research Council.
Address correspondence to Batool Akhlaghinia, Department of Chemistry, Damghan
University of Basic Sciences, Damghan, Iran. E-mail: b-akhlaghinia@dubs.ac.ir
2099

2100
B. Akhlaghinia et al.
stable, commercially available, and cheap reagent for trimethylsilyla-
tion of hydrogen-labile substrates,1a giving ammonia as the only by-
product. On the other hand, silylation using this silazan-type reagent
is nearly neutral and does not need special precautions. However, the
low silylating power of HMDS is a main drawback for its application,
which needs forceful conditions and long reaction times in many in-
stances. A variety of catalysts, such as (CH3)3SiCl,5 ZnCl2,6 nitrogen lig-
and complexes of metalchlorides,7 zirconium sulfophenyl phosphonate,8
K-10 montmorilonite,9 and special types of catalysts having the gen-
eral formula X1-NH-X2 in which at least one of the groups X1,X2 is
electron-withdrawing such as an ester or amide group, has also been
reported for the silylation of a wide range of functional groups using
HMDS.10
In this research we studied the catalytic effect of N,Nꢁ,Nꢁꢁ,Nꢁꢁꢁ,-
tetramethyltetra-2,3-pyridinoporphyrazinato copper (II) ([Cu(2,3-
tmtppa)]4+) (I) in trimethylsilylation of alcohols and phenols (Figure 1).
The tetramethymetalloporphyrazine complexes are phthalocyanine
derivatives in which the outer benzene rings are replaced with electron-
withdrawing pyridine rings. Phthalocyanines closely resemble por-
phyrins in many of their physico-chemical properties.
FIGURE 1

Silylation
2101
Two kinds of related compounds, namely the 2,3-pyridinopor-
phyrazine and 3,4-pyridinoporphyrazine, have been reported. The
metal complexes of these compounds are synthesized by a template
procedure analoguous to that of phthalocyanidine. The most com-
mon methods use dicyanopyridine or pyridinedicarboxylic acids. 2,3-
pyridinoporphyrazine complexes are synthesized from 2,3-dicyano-
pyridine or 2,3-pyridinedicarboxlic acids. The synthetic methods for
3,4-pyridinoporphyrazines are essentially similar to those of the 2,3
congeners. The solubility of these complexes in common organic sol-
vents is considerably low. The N,Nꢁ,Nꢁꢁ,Nꢁꢁꢁ-tetramethylated quaternized
forms of tetrapyridinoporphyrazines are tetra-positively charged and
hence water soluble. The water soluble form of mentioned complexes
are prepared by quaternization of the pyridine nitrogen with dimethyl
sulfate at 120◦C in dimethyl formamid.11,12
Unlike phthalocyanines, the chemical and physical properties of
tetrapyridinoporphyrazine complexes have received little attention.
However, their uses as catalysts in thiol and cumen autoxidation have
been described.13,14
RESULTS AND DISCUSSION
We are especially interested in exploring the potential use of neutral or
nearly neutral catalyst. Herein, we wish to describe a new protocol for
the mild and rapid trimethylsilylation of a wide variety of alcohols and
phenols using HMDS and a catalytic amount of [Cu(2,3-tmtppa)]4+
.
At first, the protection of benzyl alcohol under catalytic effect of
[Cu(2,3-tmtppa)]4+ in acetonitrile at room temperature was studied.
Benzyl trimethylsilyl ether was produced after 10 min in quantitative
yield. We then applied these conditions for the protection of structurally
different alcohols and phenols. (Scheme 1).
SCHEME 1
In order to optimize the reaction conditions, we first examined the
effect of different ratios of ROH/DHP/catalyst. Employing the 1/0.8/0.01
ratio gave the best results and produced trimethylsilyl ethers in quan-
titative yields (Table I).

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B. Akhlaghinia et al.
TABLE I Trimethylsilylation of Alcohols and Phenols Catalyzed by
[Cu(2,3tmtppa)]4+
Entry
ROH
Time (min)
Conversion (%)a
Isolated Yield (%)
1
2
10
15
100
100
92
90
3
4
20
100
100
93
85
4.5 (h)
5
6
7
8
5
5
5
5
100
100
100
100
98
96
98
97
9
5
100
100
91
93
11
20
12
30
100
95
13
14
4 (h)
3 (h)
100
100
95
93
15
16
3 (h)
75
100
100
90
94
aGC yield using internal standard.

Silylation
2103
Compounds that were trimethylsilylated in this way are primary,
secondary, tertiary, and benzylic alcohols. Generally, in the cases of pri-
mary and secondary alcohols the reactions were completed within less
than 30 min in acetonitrile at room temperature accompanied by evo-
lution of NH3 gas from the reaction mixture.
The mechanism for these transformations is unclear. One idea may
be that [Cu(2,3-tmtppa)]4+ acts as a Lewis acid, which may polarize
the Si-N bond in HMDS to produce the silylating agent 1 (Scheme 2).
A rapid reaction with alcohol then ensues, leading to 2 and the con-
comitant release of the corresponding silyl ether. This is followed by
the formation of the ammonia-CuII complex 3. Cleavage of 3 leads to
the fast evolution of NH3 and release of catalyst, which re-enters the
catalytic cycle (Scheme 2). Nevertheless, at this time there is no experi-
mental evidence for this feature of I, and the actual role of this reagent
should be studied further in detail.
SCHEME 2
In this article we not only investigated one of the catalytic effect
of [Cu(tmtppa)]4+ in organic synthesis but also introduced an efficient
solid acid catalyst for protection of hydroxyl groups with HMDS. This
complex is an interesting catalyst with high reusable capacity and that
separated from the reaction mixture easily. Also, workup and isolation
of the product in this method is very easy.

2104
B. Akhlaghinia et al.
EXPERIMENTAL
The products were purified by column chromatography, and the pu-
rity determinations of the products were accomplished by GLC on
a Shimadzu model GC-10A instrument or by TLC on silica-gel poly-
gram STL G/UV 254 plates. Fourier transform infrared (FTIR) spectra
were recorded on a Perkin Elmer RXI spectrometer. NMR Spectra were
recorded on a Bruker Avance DPX 250 MHz instrument.
The catalyst was prepared and purified by the method described in
the literature.11
Typical Procedure for the Protection of Benzyl Alcohol
To benzyl alcohol (l mmol, 0.108 g) and HMDS (0.8 mmol, 0.128 g) in ace-
tonitrile (5 ml), [Cu(2,3-tmtppa)]4+ (0.01 mmol, 0.0111 g) was added and
the mixture stirred at room temperature. GC analysis showed that the
reaction was completed after 5 min. The reaction mixture was filtered.
Column chromatography of the filtrate on silica gel using n-hexane as
eluent gave benzyl trimethylsilyl ether in 98% yield.
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