P(MeNCH2CH2)3N: An efficient catalyst for the desilylation of tert- butyldimethylsilyl ethers

Article abstract of DOI:10.1021/jo991591i

tert-Butyldimethylsilyl (TBDMS) ethers of primary, secondary, and tertiary alcohols and phenolic TBDMS ethers are desilylated to their corresponding alcohols and phenols, respectively, in DMSO, at 80 °C, in 68- 94% yield in the presence of 0.2-0.4 equiv of P(MeNCH₂CH₂)₃N. Using P(i- PrNCH₂CH₂)₃N as the catalyst, 85-97% yields of desilylated alcohols were obtained from TBDMS ethers of 1-octanol, 2-phenoxyethanol, and racemic α- phenyl ethanol. These are the first examples of desilylations of silyl ethers catalyzed by nonionic bases. Both catalysts were much less effective for the desilylation of tert-butyldiphenylsilyl (TBDPS) ethers (22-45% yield) under the same conditions as used for TBDMS ethers. Possible pathways involving nucleophilic attack of the anion of the solvent molecule (generated by the catalyst) at the Si-O bond of silyl ether or a prior activation of the silyl ether by the catalyst via a P-Si interaction followed by nucleophilic attack of the solvent anion are proposed on the basis of ¹H and ³¹P NMR experimental data.

Full text of DOI:10.1021/jo991591i

J . Org. Chem. 2000, 65, 2065-2068
2065
P (MeNCH2CH2)3N: An Efficien t Ca ta lyst for th e Desilyla tion of
ter t-Bu tyld im eth ylsilyl Eth er s
Zhengkun Yu† and J ohn G. Verkade*
Department of Chemistry, Iowa State University, Ames, Iowa 50011
Received October 13, 1999
tert-Butyldimethylsilyl (TBDMS) ethers of primary, secondary, and tertiary alcohols and phenolic
TBDMS ethers are desilylated to their corresponding alcohols and phenols, respectively, in DMSO,
at 80 °C, in 68-94% yield in the presence of 0.2-0.4 equiv of P(MeNCH2CH2)3N. Using P(i-PrNCH2-
CH2)3N as the catalyst, 85-97% yields of desilylated alcohols were obtained from TBDMS ethers
of 1-octanol, 2-phenoxyethanol, and racemic R-phenyl ethanol. These are the first examples of
desilylations of silyl ethers catalyzed by nonionic bases. Both catalysts were much less effective
for the desilylation of tert-butyldiphenylsilyl (TBDPS) ethers (22-45% yield) under the same
conditions as used for TBDMS ethers. Possible pathways involving nucleophilic attack of the anion
of the solvent molecule (generated by the catalyst) at the Si-O bond of silyl ether or a prior activation
of the silyl ether by the catalyst via a P-Si interaction followed by nucleophilic attack of the solvent
1
anion are proposed on the basis of H and 31P NMR experimental data.
In tr od u ction
BF4 ,
- 12 (Me2N)3S[F2SiMe3],13 KO2,14 organotin reagents,15
NBS,16 t-BuOOH/MoO2(acac)2,17 LiAlH4,18 ceric ammo-
nium nitrate,19 LiOH,20 I2,21 and LiCl22 have also been
reported to desilylate TBDMS ethers of alcohols. Al-
though CCl4/MeOH is efficient only for desilylating
TBDMS ethers of primary alcohols,23 CBr4/MeOH is
effective for deprotecting TBDMS, tert-butyldiphenylsilyl
(TBDPS) and triisopropylsilyl (TIPS) ethers of primary
and secondary alcohols and phenols.24 Transition metal-
catalyzed desilylation of TBDMS ethers is effected with
PdCl2(CH3CN)2,25 or Sc(OTf)3,26 and CeCl3‚7H2O/NaI was
Ever since its discovery,1 the tert-butyldimethylsilyl
(TBDMS) group has become one of the most popular
protective groups for OH groups in organic synthesis
because of the ease with which it can be introduced and
removed.2 Methods generally used to cleave Si-O bond
in TBDMSOR ethers for parent alcohol regeneration
include (1) acid-catalyzed Si-O cleavage with HF,3
AcOH,1,4 CF3COOH,5 TsOH,6 HCl,3c,4c,7 H2SO4;8 Lewis
acids such as BF3 and Me2BBr,9b and (2) Lewis base-
9a
catalyzed Si-O cleavage employing fluoride ion.1,3b,e,f,9a,10
Other reagents such as a carboxylic acid resin,11 salts of
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* To whom correspondence should be addressed.
† On leave from Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, P.O. Box 110, Dalian 116023, P. R. China.
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10.1021/jo991591i CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/08/2000

2066 J . Org. Chem., Vol. 65, No. 7, 2000
Yu and Verkade
Sch em e 1
reported to hydrolyze TBDMS, TBDPS, and TIPS ethers
to alcohols in acetonitrile.27 Selective deprotection of a
Et3Si group in the presence of a TBDMS group with a
mesoporous silica MCM-41/MeOH system has also been
reported recently.28 The stability order TBDPS > TBDMS
> TMS for these protective groups in silyl ethers with
respect to hydrolysis has been ascribed to their decreas-
ing steric bulk. Thus TMS ethers can be deprotected
under relatively mild conditions,29 TBDPS ethers are
generally desilylated to their corresponding alcohols
under the same conditions as those used for TBDMS
ethers,2 (e.g., using HF,30 fluoride ion,31 LiOH,32 BF3,33
or LiCl22) although harsher conditions and longer times
are necessary in some cases.
conjugated alcohols (trans,cis-5a and cis,trans-5b) in a
1 to 1 ratio (eq 2).36 Thus we speculated that our nonionic
superbases 1b and 1c might be useful for desilylating
silyl ethers.
Herein we report for the first time an efficient catalytic
procedure for removing the protective TBDMS group
from a variety of TBDMSOR ethers using a nonionic
base, namely, 1b as the catalyst. Some desilylations of
TBDPSOR ethers using a nonionic base, namely 1b, are
also described and several desilylation reactions employ-
ing 1c as the catalyst are also presented. Possible
pathways are proposed for these desilylations.
In the course of our ongoing studies34 on new synthetic
applications of exceedingly strong nonionic bases and
catalysts of type 1 first reported from our laboratories,
we discovered that 1b is an efficient and mild catalyst
for transforming a variety of alcohols to their correspond-
ing TBDMS or TBDPS ethers using TBDMSCl or TB-
DPSCl in the presence of excess Et3N in CH3CN or DMF
(eq 1).35 In a recent investigation of the conjugation of
Resu lts a n d Discu ssion
Proazaphosphatranes 1 are powerful nonionic bases
forming the remarkably stable azaphosphatrane cations
2a -c as their chloride salts. The order of basicity is 1b
< 1c, and 1b has been shown to be capable of deproto-
nating acetonitrile to its corresponding anion -CH2CN,37
1
which initiates a variety of reactions.34 The H and 31P
the TMS ether (4) of linoleyl alcohol catalyzed by 1b in
CH3CN at 40 °C, we discovered that a catalytic amount
of 1b causes quantitative desilylation of 4 to give the
NMR spectra of 1b (or 1c) in DMSO also indicate in the
present work that 1b and 1c deprotonate this solvent.
The nucleophilicity and basicity of 1a -c stems from
transannular bond formation between the phosphorus
atom and the bridgehead nitrogen atom as shown in
cation 2. Partial transannulation in species of type 3
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Tetrahedron Lett. 1987, 28, 6329. (b) Wilson, N. S.; Keay, B. A.
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E. Synlett 1998, 209.
(28) Itoh, A.; Kodama, T.; Masaki, Y. Synlett 1999, 357.
(29) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 2nd ed.; J ohn Wiley and Sons: New York, 1991; pp 68-73.
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J .; Cases, M.; Pardo, D. G. Synlett 1998, 507.
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Chem. Soc., Chem. Commun. 1995, 1735. (b) Amigoni, S.; Schulz, J .;
Martin, L.; Le Floch, Y. Tetrahedron: Asymmetry 1997, 8, 1515. (c)
Haddad, M.; Imogai, H.; Larcheveˆque, M. J . Org. Chem. 1998, 63, 5680.
(32) Derwing, C.; Hoppe, D. Synthesis 1996, 149.
plays an important role in the formation and stability of
these cationic intermediates in catalytic reactions.34o,p,35,38b
Thus it is plausible that anion A [A ) -CH2CN or
-CH2S(O)CH3] generated by 1b or 1c via solvent depro-
tonation nucleophilically attacks the Si-O bond of a silyl
ether to form TBDMSA (or TBDPSA) while liberating
RO- which extracts a solvent proton to form the depro-
tected alcohol (Scheme 1). 31P NMR experiments in CD3S-
(O)CD3 revealed that some 1b (or 1c) and 2b (or 2c) were
present during the entire course of the reaction [for
(33) Gainelli, G.; Galletti, P.; Giacomini, D. Synlett 1998, 611.
(34) (a) Kisanga, P.; D’Sa, B.; Verkade, J . G. J . Org. Chem. 1998,
63, 10057. (b) Wang, Z.; Verkade, J . G. Heteroatom Chem. 1998, 9,
687. (c) Wang, Z.; Verkade, J . G. Tetrahedron Lett. 1998, 39, 9331. (d)
Kisanga, P. B.; McLeod, D.; D’Sa, B. A.; Verkade, J . G. J . Org. Chem.
1999, 64, 3091. (e) Ilankumaran, P.; Verkade, J . G. J . Org. Chem. 1999,
64, 3087. (f) Kisanga, P. B.; Verkade, J . G. J . Org. Chem. 1999, 64,
4298. (g) Liu, X.; Yu, Z.; Verkade, J . G. J . Org. Chem. 1999, 64, 4840,
8058. (h) Wang, Z.; Kisanga, P.; Verkade, J . G. J . Org. Chem. 1999,
64, 6559. (i) D’Sa, B. A.; Kisanga, P.; Verkade, J . G. J . Org. Chem.
1998, 63, 3961. (j) Arumugam, S.; Verkade, J . G. J . Org. Chem. 1997,
62, 4827. (k) Mohan, T.; Arumugam, S.; Wang, T.; J acobson, R. A.;
Verkade, J . G. Heteroatom Chem. 1996, 7, 455. (l) D’Sa, B. A.; Verkade,
J . G. J . Org. Chem. 1996, 61, 2963. (m) Tang, J . S.; Mohan, T.; Verkade,
J . G. J . Org. Chem. 1994, 59, 4931. (n) Tang, J . S.; Verkade, J . G. J .
Org. Chem. 1994, 59, 7793. (o) Verkade, J . G. Coord. Chem. Rev. 1994,
137, 233; Acc. Chem. Res. 1993, 26, 48317. (p) Tang, J . S.; Verkade, J .
G. Angew. Chem., Int. Ed. Engl. 1993, 32, 896. (q) Laramay, M. A. H.;
Verkade, J . G. Z. Anorg. Allg. Chem. 1991, 605, 163.
1
deuterated base, δ ca. -10.6 ppm (t, J PD = 140 Hz)
triplet; for free base, (δ ca. 110-120 ppm (s)]. Interest-
ingly, during the reaction of TBDMS ether 7 [i.e.,
(36) Yu, Z. K.; Verkade, J . G. J . Org. Chem., to be submitted.
(37) Tang, J . S.; Verkade, J . G. J . Am. Chem. Soc. 1993, 115, 1660.
(38) (a) D’Sa, B. A.; Kisanga, P.; McLeod, D.; Verkade, J . G.
Phosphorus, Sulfur Silicon 1997, 124, 223. (b) Wro´blewski, A.; Pinkas,
J .; Verkade, J . G. Main Group Chem. 1995, 1, 69.
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62, 5057.

P(MeNCH2CH2)3N: Catalyst for Desilylation
J . Org. Chem., Vol. 65, No. 7, 2000 2067
Sch em e 2
desilylation of (()-13 catalyzed by 1b was more efficient
in DMSO than in CH3CN (Table 1). It is perhaps not
unreasonable to expect that the strong polarity of DMSO
molecules may favor the formation of alcohols from less-
polar silyl ethers. Thus DMSO was deemed to be the
reaction solvent of choice. That desilylations under the
conditions described here are not proceeding by ordinary
base-promoted silyl cleavage is supported by the failure
of TBDMS ethers such as (()-13 to desilylate in the
presence of 40 mol % 1b in C6D6 at 50 °C over 62 h.
1
Evaporation of the filtrate followed by H NMR analysis
revealed the presence of DMSO, protonated catalyst
(presumably as the A salt) and a TBDMS-containing
compound (presumably TBDMSA). The strong polarity
of the components of this mixture may be responsible for
the poor separation.
Complete desilylation of linoleyl TBDMS ether 6 was
accompanied by the formation of a 1:1 ratio of conjugated
alcohols 5a and 5b in 90% yield (reaction 3 and Table
1). The TMS analogue of 6, namely 4, is also easily
desilylated and isomerized to produce the same alcohols
under relatively milder conditions using CH3CN as the
solvent at 40 °C in the presence of 1b or 1c.36 On the
basis of that study it is suggested that possible interme-
diates in the reaction of 6 and 1b or 1c in DMSO are
those shown in reaction 3. Competitive cleavage of the
CH3(CH2)6CH2OTBDMS] with 1b (40 mol %) in CD3S-
(O)CD3 at room temperature, three sets of triplets were
observed at low field in the 1H NMR spectra of the
reaction mixture. These triplets are assigned to three
different types of -OCH2- groups. Thus the triplet at
3.55 ppm is assigned to the starting silyl ether and the
triplet at 3.35 ppm is associated with the deprotected
alcohol. These assignments were arrived at by comparing
these spectra with those of the starting silyl ether and
the deprotected alcohol, respectively. The third triplet at
3.43 ppm is assigned to a new species formed during the
reaction that may be an intermediate such as 26 in
Scheme 1 or 27 in Scheme 2. As the reaction proceeded
and after more than half the amount of the starting silyl
ether (88.3% conversion of 7 at 71 h) became deprotected,
this new triplet became undetectable due to the low
concentration of the intermediate. For the same reaction
in DMSO-d6 at 80 °C, this triplet was also observed, and
again it disappeared as the reaction reached completion.
Similar results were obtained in the reactions of 8 and
14 with 1b at room temperature or at a higher temper-
ature (e.g., 50 °C) although the signals (a third triplet
peak for 8 and a third quartet peak for 14) were not so
strong as those in the former reaction. Although 31P NMR
spectra of these reactions showed no convincing evidence
for the formation of 27, our previous investigations
revealed the possible formation of analogues of 27, i.e.,
cationic intermediates of type 3.34o,p,35,38a Using the
relatively sterically hindered silyl ether (()-15 as a
substrate, no similar phenomenon was observed in the
1H NMR spectra of the reaction mixture in DMSO-d6
whether the reaction temperature was ambient or 80 °C,
demonstrating that steric factors probably play an im-
portant role in the formation of an intermediate such as
27. It should be noted that the steric hindrance order of
1b and 1c is 1c > 1b. TBDMSA or TBDPSA were
presumably formed although efforts to isolate these
species were not successful. Thus repeated attempts to
separate the mixture by preparative TLC resulted in a
poorly developed brown strip near the starting line that
upon collection and washing with CHCl3 and filtration
was shown by NMR spectroscopy to contain a largely
uninterpretable mixture.
Si-O bond and isomerization of CH2-separated carbon-
carbon double bond lead to the conjugated TBDMS ethers
28a and 28b, and linoleyl alcohol (29). The desilylation
of 28a and 28b and isomerization of 29 then give rise to
the final products 5a and 5b. Another possible interme-
diate is the alkoxyl anion of 29 since superbases 1b and
1c are known to deprotonate alcohols.36,38 For TBDMS
ethers 7-13 derived from primary alcohols, 80-94%
desilylation yields were achieved. Silyl ethers 14-18
derived from secondary alcohols, silyl ethers 19-21
derived from tertiary alcohols, and phenolic ethers 22 and
23 were desilylated to their corresponding products in
moderate to good yields (Table 1). Catalyst 1c was also
efficient for desilylating TBDMS ethers 7, 8, and (()-15
to their corresponding products in 85-97% yield (Table
2). Although 1b and 1c are efficient catalysts for depro-
tecting TBDMS ethers, both of them failed to desilylate
TBDPS ethers efficiently. TBDMS ether (e.g., 22 in Table
1) was much more efficiently desilylated than its TBDPS
ether, i.e., 25, under the same conditions (Table 2). The
desilylation of 24 and 25 provided products in only 22 to
45% yields, respectively (Table 2). The greater steric
hindrance of the TBDPS group compared with that of
the TBDMS group coupled with the bulkiness of 1b and
1c may account for the low yields in these cases.
Reactions carried out in deuterated solvents under the
same reaction conditions showed the order DMSO-d6 >
CD3CN > CD3COCD3 g CD3OD . C6D6 for the relative
rates of desilylation of TBDMS ethers catalyzed by 1b.
It is known that some primary TBDMS ethers (such as
allylic, homoallylic, benzylic, and aryl derivatives) can
be hydrolyzed in DMSO/H2O at 90 °C although all other
primary and secondary TBDMS ethers remain unaffected
under these conditions.39 In addition, it was noted that
Exp er im en ta l Section
Acetonitrile was distilled over P4O10 under nitrogen. The
nonionic superbases 1b34q and 1c38b were prepared according
to our previously published methods although 1b is com-
mercially available.40 TBDMS and TBDPS ethers were pre-
(39) Maiti, G.; Roy, S. C. Tetrahedron Lett. 1997, 38, 495.

2068 J . Org. Chem., Vol. 65, No. 7, 2000
Yu and Verkade
Ta ble 1. Desilyla tion of TBDMS Eth er s to Cor r esp on d in g Alcoh ols Usin g 1b a s a Ca ta lysta
TBDMS
ethersb
equiv of
1b
temp/time
(°C/h)
eluent
alcohol
TBDMS
ethersb
equiv of
1b
temp/time
(°C/h)
eluent
alcohol
ratios (v/v)c
yield (%)d
ratios (v/v)c
yield (%)d
6
7
8
9
0.3
0.2
0.3
0.3
0.3
0.3
0.25
0.2
0.4
0.2
80/36
80/34
80/36
80/36
80/30
80/36
80/36
80/24
80/36
80/19
10:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:2
90e
94
90
84
80
89
83
90
81f
80
(()-15
16
(()-17
(()-18
19
20
21
22
23
0.2
0.3
0.3
0.3
0.4
0.3
0.3
0.3
0.2
80/24
80/36
80/36
80/36
80/27
80/36
80/36
80/24
80/24
5:2
10:1
5:2
5:1
5:2
5:1
5:1
5:1
5:1
89
68
85
89
68
87
81
94
85
10
11
12
(()-13
14
a
No attempt was made to maximize yields by optimizing reaction times, temperatures, and the concentrations of 1b. The solvent is
DMSO except where indicated. bThe preparation of all the ethers was reported in our previous papers.35 cn-Hexane/THF. dIsolated yield
e
was obtained by preparative TLC on silica gel. The ratio of 5a /5b was ca. 1/1 as determined by 1H NMR spectroscopy and GC and by
comparison with such spectra reported in the literature.36 fThe solvent was CH3CN.
Moreover, 1H and 13C NMR spectra of all the isolated products
Ta ble 2. Desilyla tion of TBDP S Eth er s Usin g 1b or 1c a s
except 5a /5b35 were compared with those of the authentic
a Ca ta lyst a n d Desilyla tion of TBDMS eth er s u sin g 1c a s
a Ca ta lysta
samples or those in the literature.41
Gen er a l P r oced u r e for Desilyla tion . DMSO (or CH3CN
as indicated in Table 1) (4 mL) was added under nitrogen to
a 25 mL round-bottom flask charged with catalyst (as indicated
in Tables 1 and 2). Then the silyl ether substrate (1.0 mmol)
was introduced through the rubber septum via a syringe. The
reaction mixture was stirred at 80 °C for the time indicated
in Tables 1 and 2, and after cooling the mixture to room
temperature, it was poured into 150 mL of water followed by
extraction with CHCl3 (5 × 40 mL). The organic layer was
dried overnight with MgSO4 and then filtered. The organic
phase was concentrated to ca. 1.5 mL by rotary evaporation
and subjected to TLC separation on silica gel with the
appropriate eluent as indicated in Tables 1 and 2. The NMR
reactions were performed in a 5 mm NMR tube sealed with a
rubber septum in a manner similar to that described above.
equiv of 1b
or 1c
temp/time
(°C/h)
eluent
ratioc
alcohol
silyl etherb
yield (%)d
24
24
25
25
25
7
1b (0.4)
1c (0.4)
1b (0.3)
1b (0.3)
1c (0.3)
1c (0.2)
1c (0.2)
1c (0.2)
80/36
80/36
80/24
80/36
80/36
80/24
80/36
80/36
5:1
5:1
10:1
10:1
10:1
5:1
22
25
30
39
45
85
95
97
Ack n ow led gm en t. The authors thank the Center
for Advanced Technology Development of Iowa State
University and the Iowa Soybean Promotion Board for
grants in support of this research, and they thank Dr.
P. B. Kisanga for kindly providing a sample of 1c.
8
5:1
5:1
(()-15
a
No attempt was made to maximize yields by optimizing
reaction times, temperatures, and the concentrations of 1b and
b
1c. The solvent was DMSO. The preparations of the ethers were
reported in our previous papers.35 cn-Hexane/THF. dThe isolated
Su p p or tin g In for m a tion Ava ila ble: Compound charac-
terizational data, NMR peak assignments, and copies of 1H
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
yield was obtained by TLC on silica gel.
pared as reported earlier from our laboratories.35 The desily-
lated products werecharacterized bymass and NMRspectroscopies
and were found to be >95% pure by 1H NMR analysis.
J O991591I
(40) Strem Chemical Co.
(41) The Aldrich Library of 13C and 1H NMR Spectra, 1993, Vol. 1.