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Mild Oxidative Removal of the p-Methoxybenzyl Ether Protecting Group by Homogeneous Electron Transfer.

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C 0 M MU N I CAT1 0N S
and regenerated from the amine during the course of the
reaction (method 2). In the latter case only catalytic amounts
of the electron transfer agent are necessary.
Both methods usually give good to excellent yields and
the alcohol is obtained in high purity. In contrast to the
reductive deprotection procedures, double bonds are not
attacked. Numerous oxidation-labile functions should tolerate
the new procedure because the redox potential of the mediator
is very low. Direct anodic deprotection, however, needs an
anode potential of at least f1.6V us. SCE and fouling of
the electrode is often encounteredC6!
Table 1 . Oxidative cleavage of the p-methoxybenzyl ether protecting group
by homogeneous electron transfer.
RO-CH2-ChH40CH3
R
1-Octyl
1 -Methylheptyl
E-4-Hepten-I -yl
Mild Oxidative Removal of the p-Methoxybenzyl Ether
Protecting Group by Homogeneous Electron Transfer[
By Werner Schmidt and Eberhard Steckhan[*]
The benzyl ether function is one of the mqst common
protecting groups for alcohols. It is usually removed by catalytic hydrogenation or less often by sodium in liquid ammonia[21.
If, however, the substrate contains additional reducible functions, this procedure is unsatisfactory.
We now report a simple and mild method for the removal
of the p-methoxybenzyl ether protecting group by homogeneous electron transfer under oxidative conditions. The
stable cation radical of tris(p-bromopheny1)amine is employed
as electron transfer agent (mediator). Bond cleavage takes
place in moist acetonitrile according to the following reaction
scheme:
(Br<6H4)3N'@
+
RO-CH2-R'
G=
(Br-C6H4)3N + [ROi:Hz-R']'@
(21
(31
(1)
0
( I ) + (5) +RO<H-R'+
(a)
(41
(2)
(C)
16)
(6)
f
HzO
-+
ROH
+
R'CHO
+
H@
(dl
R'=
The reversible potential of the redox couple ( 1 ) / ( 2 )
(Eo= + 1.05 V us. SCE) is about 550mV more negative than
the irreversible oxidation potentials of the ethers (3)[31. The
driving force of the reaction is the deprotonation step (b).
By addition of 2,6-dimethylpyridine the bond cleavage can
be substantially accelerated[41.
The cation radical (I) can either be used in stoichiometric
amounts as its readily obtainabler5]stable hexachloroantimonate (method I), or it can be electrochemically generated
[*] Priv.-Doz. Dr. E. Steckhan, DipL-Chem. W. Schmidt
Organisch-chemisches Institut der Universitat
Orleans-Ring 23, D-4400 Munster (Germany)
Anyew. Chem. Jnr. Ed. Engl. 17 ( 1 9 7 8 ) No. 9
(3)
Yield of R-OH
Method I
86
81
75
[%,I
[a]
Method 2
95
87
83
[a] Material yield
General procedure
Method 1 :To a solution of 2 mmol of the respective p-methoxybenzyl ether (3) (2 mmol) and 2,6-dimethylpyridine
(0.212g, 2mmol) in CH3CN[71/CH2C12
(5: 1)a solution of tris(p-bromopheny1)ammoniumyl hexachloroantimonate (4.08g,
5mmol) in CH3CN/CH2CI2(5: 1) is added dropwise until
the blue colour of the cation radical (I) persists. For product
workup, water is added to the reaction solution which is
subsequently reduced, saturated with KC1, and extracted with
ether. After extraction with NaHS03 solution the ether phase
is dried over MgS04. The alcohols are identified by gas chromatography and mass spectrometry on comparison with authentic samples and isolated by bulb-to-bulb distillation.
Method 2: In a divided beaker-type glass cell (20°C, Pt
anode, Pt cathode) a solution of tris(p-bromopheny1)amine
(0.964 g, 2 mmol) in CH3CN/CH2C12(5 : 1 ; 0.2 M LiC104) is
used as anolyte. The catholyte consists of CH,CN/I.OM
LiC104. After application of an anode potential of + 1.2 V
(us. Ag/AgCl) the blue color of ( 1 ) appears. When the respective p-methoxybenzyl ether (3) ( 5 mmol) and 2,6-dimethylpyridine (0.535g, 5 mmol) are added the solution is decolorized.
After reappearance of the blue color of the electron transfer
agent (consumption of about 0.013 F) the electrolysis is
stopped. The workup procedure is the same as in method 1.
Received: May 29, 1978 [Z 24 IE]
German version: Angew. Chem. 80, 71 7 (1978)
CAS Registry numbers:
( I ) , 37881-41-7; ( 1 ) hexachloroantimonate, 24964-91-8; (2), 4316-58-9; (3)
(R=octyl), 54384-75-7; ( 3 ) ( R = I-methylheptyl), 54384-76-8; (3) ( R = E - 4 hepten-1 -yl), 67523-82-4
[ l ] Indirect Electrochemical Processes, Part 4. This work was supported
by the Deutsche Forschungsgemeinschaft.-Part 3 : W Schmidr, E . Steckhan, .I.Electroanal. Chem. 89, 215 (1978).
[2] J . F. W McOniie: Protective Groups in Organic Chemistry. Plenum
Press, London 1973, p. 98.
[3] The peak potential for the oxidation of p-methoxybenzyl I-octyl ether
was found to be E , = + 1.6OV, us. SCE, using cyclic voltammetry. The
oxidation potentials for the remaining ethers deviate only slightly from
this value.
[4] Homogeneous electron transfer is proved by electrochemical methods.
W Schmidr, E. Steckhan, to be published.
[5] F. A . Bell, A . Ledwirh, D. C. Sherrinyton, J. Chem. SOC.C 1969, 2719.
673
[6] L. L. Miller, J . F. WoK E. A. Mayeda, J. Am. Chem. Soc. 93, 3306
(1971); E . A . Mayeda, L. L. Miller, J . F . WOK ibid. 94, 6811 (1972);
S . M. Weinreb, G . A . Epling, R. Cumi, M . Reitano, J. Org. Chem. 40,
1356 (1975).
[7] Acetonitrile (Merck), water content 0.5 %.
Influence of Cyclopropyl Substituents on the Relative
Rate of Dichlorocarbene Addition to Olefinsp'l
By Eckehard T.: Dehmlow and Axel Eulenberger[*]
Enormous rate enhancements caused by cyclopropyl rings
adjacent to developing positive charges are well known. However, the influence of cyclopropane on additions to double
bonds is less clear: In hydratation and bromination, cyclopropane accelerates relative to n-butyl by about lo4['], and in
cycloadditions with tetracyanoethylene and arylsulfonyl isocyanates accelerations of lo3 are found[']. Only small differences are observed, however, when arylsulfenyl chlorides are
added'']. In the ene-reaction with singlet oxygen, the electronic
effect of cyclopropane is comparable with that of methyl[31.
We subjected a number of cyclopropylalkenes to reaction
with dichlorocarbene generated by phase-transfer catalysis[4]
and characterized the products. The same olefins were subsequently employed in competition experiments with less than
equivalent amounts of dichlorocarbene. Analysis was performed by gas chromatography after calibration for each of
the products determined in the preliminary experiments. Table
1 shows the reactivities (f5 %) relative to 3,3-dimethyl-lbutene ( 1 4 ) . Steric and electronic effects seem to counteract
.Table 1 . Relative reactivities of various alkenes towards dichlorocarbene.
Alkene
Re].
reactivity
Alkene
each other. A cyclopropyl ring exerts a powerful accelerating
effect compared to its open-chain analog [ ( / 0 ) / ( 1 2),
(1 1 ) / ( I 4), ( 5 ) / ( 1 3 ) ] but-depending on the steric situationthis effect is somewhat more [ ( 2 ) / ( 4 ) ] or somewhat less
pronounced [ ( 3 ) / ( 1 ) , ( 5 ) / ( 4 ) ] than that of methyl. Furthermore, the effect of a double bond is roughly comparable
to that of a three-membered ring. I-Methylcyclopropyl compounds [(6)/(1 I )] are less reactive than the unsubstituted
compounds [(2)/(10)] for steric reasons.
Thus it is apparent that no significant cyclopropyl effects
occur in an unequivocally concerted cycloaddition with little
charge separation in the transition state. Finally, the selectivities of dibromo- and chlorofluorocarbene towards the
cyclopropylalkene pair [ ( 2 ) / ( 5 ) ] were compared. As expected
CClF (relative reactivity R, = 1.82) was more selective than
CClz ( R ,= 1.55),and CBrz (R,= 1.29) was less selective.
Received: June 5, 1978 [Z 23 IE]
German version: Angew. Chem YO. 716 (1978)
CAS Registry numbers.
( I ) , 18738-69-7; ( 2 ) , 4663-22-3; ( 3 ) , 23603-63-6; (4), 115-11-7, (5). 822-93-5;
(6), 78-79-5; (7), 3422-07-9; ( S ) , 110-83-8; ( Y ) , 106-99-0; ( l o ) , 693-86-7;
( J l ) , 16906-27-7; ( J Z ) , 563-45-1; ( J 3 ) , 16746-02-4; ( / 4 ) , 558-37-2;
dichlorcarbene, 1605-72-7
[ I ] D. G . Garratt, A . Modro, K. Oyama, G . H . Schmidt, T T Tidwell, K .
Yates, J. Am. Chem. SOC.96, 5295 (1974).
[2] F. E&berger, 0. Gerlach, Chem. Ber. 107, 278 ( I 974).
[3] A . A . Frimer, D. Rot, M . Spreeher, Tetrahedron Lett. 1977, 1927.
[4] E . V Dehmlow, Angew. Chem. 86, 187 (1974); 8Y, 521 (1977); Angew.
Chem. Int. Ed. Engl. 13, 170 (1974); 16, 493 (1977).
Rel.
reactivity
Dimethylsilanediyliron Complexes[l ]
2035
(8)
200
1415
(9)
140
970
(JO)
100
950
(JJ)
76
910
(12)
12
830
(13)
9
By Hideki Sakurai, Yoshijiasu Kamiyama, and Yasuhiro Nakadaira"]
Schmid et al!'] recently reported the preparation of base-stabilized silanediyliron complexes. We now wish to communicate
our observations on the first stable mononuclear dimethylsilanediyliron complexes.
When a yellow-orange suspension of FeZ(C0)9 (l.OOg,
2.75 mmol) and pentamethyldisilane (I ) (498 mg, 3.77 mmol)
in benzene (10ml) was stirred at room temperature for 1Sh
under argon, a yellow-brown homogeneous solution was
obtained, from which a
yellow-orange
complex
tricarbonylhydrido(dimethylsi1anediyl)trimethylsily~iron( 3 )
(523 mg, 1.92 mmol) was isolated in 70 % yield as an oil, b. p.
30 oC/O.OOS torr.
Fe2(CO),
+
H-SiMez-SiMezR
( I ) , R = Me
(2), R = H
744
(141
1
--+
r
Me,Si=Fe(CO)s
I
SiMezR
+
...
( 3 ) , R = Me
(4), R = H
~
[a] Addition to methyl-substituted double bond
[b] Monoaddition.
[*] Prof. Dr. E. V. Dehmlow, Dipl.-Ing. A. Eulenberger
Institut fur Organische Chemie der Technischen Universitat
Strasse des 17. Juni 135, D-1000 Berlin 12 (Germany)
[**I Support by the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie is gratefully acknowledged.
674
The complex ( 3 ) is extremely air-sensitive and decomposes
instantaneously on exposure to air, but correct elemental analyses are obtained on careful handling. The 'H-NMR (CSz)
spectrum of ( 3 ) shows, in addition to the FeH signal at
[*] Prof. Dr. H. Sakurai, Dr. Y . Kamiyama, Dr. Y. Nakadaira
Department of Chemistry, Faculty of Science, Tohoku University
Sendai 980 (Japan)
Angew. Chem. Int. Ed. Engl. 17 ( 1 9 7 8 ) N o . 9
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removal, mild, ethers, group, protection, homogeneous, transfer, oxidative, methoxybenzyl, electro
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