Synthesis of N-(dialkylaminoalkyl)alcohols by homogeneously catalyzed hydrogenolysis of cyclic N,O-acetals

Article abstract of DOI:10.1055/s-2002-20039

The homogeneously catalyzed hydrogenation of 1,3-oxazolidines affording unsymmetrically substituted 2-N-(dialkylamino)ethanols is reported showing for the first time that Rh(I) catalysts based on chelating diphosphines can be advantageous for this reaction.

Full text of DOI:10.1055/s-2002-20039

PAPER
375
Synthesis of N-(Dialkylaminoalkyl)alcohols by Homogeneously Catalyzed
Hydrogenolysis of Cyclic N,O-Acetals
Synthesis of
N
-
(Di
i
alkylam
t
inoalk
a
yl)alcoholsli I. Tararov,*a Renat Kadyrov,b Thomas H. Riermeier,b Armin Börner*a
a
Institut für Organische Katalyseforschung an der Universität Rostock e.V., Buchbinderstr. 5/6, 18055 Rostock, Germany
Fax +49(381)4669324; E-mail: vitali.tararov@ifok.uni-rostock.de; E-mail: armin.boerner@ifok.uni-rostock.de
b
Degussa AG, Projekthaus Katalyse, Geschäftsbereich Creavis, Industriepark Hoechst, Gebäude G 830, 65926 Frankfurt/Main, Germany
Received 6 November 2001; revised 14 December 2001
amines by the assistance of homogeneous Rh(I) catalysts
bearing diphosphines as ancillary ligands.4
Abstract: The homogeneously catalyzed hydrogenation of 1,3-ox-
azolidines affording unsymmetrically substituted 2-N-(dialkylami-
no)ethanols is reported showing for the first time that Rh(I)
catalysts based on chelating diphosphines can be advantageous for
this reaction.
Herein, we report that this finding is of considerable syn-
thetic value. Thus, 1,3-oxazolidines can also be cleaved
under these conditions affording N-trisubstituted 2-amino
ethanol derivatives in excellent yields. 1,3-Oxazolidines
of the general structure 1 serving as starting materials are
easily prepared from a broad variety of commercially
available 1,2-aminoalcohols and the corresponding alde-
hydes and ketones, respectively (Scheme 2).5
Key words: hydrogenation, transition metals, catalysis, amino al-
cohols
The selective cleavage of cyclic N,O-acetals under the
formation of substituted -aminoalkyl alcohols has great
synthetic potential since it offers an interesting alternative
for the alkylation of secondary amines with functionalized
alkylhalogenides (Scheme 1). In addition, the facile prep-
aration of cyclic N,O-acetals which serve as substrates in
this reaction makes them attractive precursors for unsym-
metrically substituted tertiary amines.
Scheme 2
As listed in Table 1 these 1,3-oxazolidines are well suited
substrates for the reductive cleavage with [Rh(dp-
pb)COD]BF4
[dppb = 1,4-bis(diphenylphosphino)bu-
tane] as homogeneous precatalysts in methanol as solvent.
Under mild conditions (50 bar H2 pressure, ambient tem-
perature) 1,3-oxazolidines were quantitatively converted
into the desired 2-dialkylamino ethanols (2).
Scheme 1
In general, the chemoselective cleavage of cyclic N,O-ac-
etals can be achieved by means of hydride reagents.1 Re-
cently, the use of alkali metals was suggested for this
reaction.2 However, the scope of this transformation is
limited to 2-aryl substituted cyclic N,O-acetals. In con-
trast, less is known about the chemoselective reductive
cleavage of cyclic N,O-acetals with molecular hydrogen,
although this methodology represents an ecologically be-
nign approach for the synthesis of -hydroxyalkylamines.
Hitherto, the only example described in the literature is
concerned with the reduction of cyclic N,O-acetals under
the conditions of a heterogeneous catalysis.3
The generality of this hydrogenation reaction is shown by
employing different types of 1,3-oxazolidines (1a j) as
substrates. Thus, relevant 1,3-oxazolidines can be derived
from N-alkylaminoethanols and formaldehyde (1a), aro-
matic aldehydes (1b e), aliphatic aldehydes (1f g) and
acetophenone (1h). In all trials fast transformations were
observed and only amines 2a,c h were produced. Partic-
ular noteworthy is the clean reaction of a substrate with a
phenolic group (1d). The nitro group in oxazolidine (1i)
did not survive under these conditions. Unfortunately, the
1,3-oxazolidine (1j) derived from substituted aniline was
not cleaved.
In the course of our mechanistic studies on the reductive
amination of aldehydes and ketones with secondary
amines, we found that acyclic N,O-acetals can be cleanly
and quantitatively transformed into the corresponding
The hydrogenation can be even conducted under solvent
free conditions as shown using substrates 1b and 1f as ex-
amples. In the presence of 0.1 mol% of the precatalyst, H2
uptake ceased after 10 and 20 hours, respectively. After
distillation, analytically pure amines 2a and 2f were ob-
tained in 86% and 92% yield (For NMR data see Tables 4
and 5).
Synthesis 2002, No. 3, 18 02 2002. Article Identifier:
1437-210X,E;2002,0,03,0375,0380,ftx,en;T10501SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

376
V. I. Tararov et al.
PAPER
Table 1 Hydrogenolytic Cleavage of 1,3-Oxazolidinesa
Substrate
1a
R1
R2
R3
H
H
H
H
H
H
H
Me
H
H
H
H
Product
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
–
Time (h)b
PhCH2
Me
Me
Me
Me
Me
Me
Me
Me
Ph
H
2a
3
0.3
0.5
1.5
2
1b
Ph
2a
1c
2-MeC6H4
4-HOC6H4
2-furyl
Ph(Me)CH
C7H15
Ph
2c
1d
2d
1e
2e
1f
2f
0.4
0.2
4
1g
2g
1h
2h
1i
4-NO2C6H4
Ph
mixture
20
24
10
20
1j
no reduction
1b
Me
Me
Ph
2a
2f
1f
Ph(Me)CH
–
a Reaction conditions: substrate (5 mmol), [Rh(dppp)COD]BF4 (0.01 mmol), MeOH (10 ml), 50 52 bar initial H2 pressure, r.t.
b Time required for quantitative conversion.
Diastereomeric oxazolidines such as 3 and 5 can also be neous Pd based catalysts since benzylic group in the prod-
successfully subjected to hydrogenation (Scheme 3). The ucts remained untouched.
examples depicted emphasize the importance of the new
method for the selective N-benzylation of aminoalcohols,
All solvents and liquids used in hydrogenations were distilled and
which can be useful for N-protection strategies in multi-
step syntheses. In each case the conversion proceeded
quantitatively and both diastereomers were reduced af-
fording a single stereoisomer.
kept under Ar. Other commercial reagents were used without addi-
tional purification. NMR spectra were recorded with a Bruker ARX
400 spectrometer. Chemical shifts ( , in ppm) are given for 1H rel-
ative to TMS as internal standard and for 13C relative to the residual
CDCl3 peak (77.36 ppm). Spin-spin coupling constants (J) are given
in Hz. The optical rotation was measured on a ‘gyromat-HP’ instru-
ment (Fa. Dr. Kernchen). Melting points are corrected.
It should be noted that the usage of our Rh (I) phosphine
catalyst proved superior to the application of heteroge-
3-Benzyl-1,3-oxazolidine (1a)
Prepared according to the protocol given in ref.6; bp 89 °C/0.05
mbar. NMR data are given in Tables 2 and 3.
1,3-Oxazolidines (1b j, 3, 5)
Prepared by azeotropic removal of H2O using a Dean Stark trap by
refluxing a 1:1 mixture of amine and carbonyl compound in ben-
zene as solvent in the presence of cat. amounts of p-TsOH H2O. In
case of 1h, toluene was used as solvent. When separation of H2O
was complete, the solvent was evaporated and the residue was dis-
tilled in vacuum. 1,3-Oxazolidines had the following bp: 1b, 96
97 °C/10 mbar (Lit.7135 °C, 14 torr); 1c, 103 104 °C, 5 mbar; 1e,
70 71 °C, 5 mbar; 1f, 60 62 °C, 0.3 mbar; 1g, 99 100 °C, 10
mbar; 1h, 89 90 °C, 5 mbar (Lit.8 99 °C, 3 torr); 1i, 118 119 °C,
0.06 mbar (solidified after several days). Solid substrates were re-
crystallised and had the following mp: 1d, 108 110 °C, 1j, 85
86 °C (MeOH) [Lit.9 84 85 °C (MeOH)]. NMR data for substrates
1a j are given in Tables 2 and 3.
Scheme 3
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Synthesis of N-(Dialkylaminoalkyl)alcohols
377
a
Table 2 1H NMR Characterization of 1,3-Oxazolidines 1a j in CDCl3
Com-
pound
1,3-oxazolidine ring position
R1
R2
R3
C-2
C-4
C-5
1a
1b
1c
4.28 (s, 2 H)
2.94 (t, 3 H, J = 6.8) 3.78 (t, 3 H, J = 6.8) 3.68 (s, 2 H), 7.2
7.4 (m, 5 Harom
)
4.56 (s, 1 H)
4.82 (s, 1 H)
2.55 2.63 (m, 1 H);
3.18 3.25 (m, 1 H)
3.59 4.04 (m, 2 H) 2.19 (s, 3 H)
7.18 7.42 (m, 5 Hrom)
2.54 2.63 (m, 1 H),
3.11 3.19 (m, 1 H)
3.80 3.96(m, 2 H)
2.14 (s, 3 H)
2.31 (s, 3 H), 6.98 7.10
(m, 3 Harom), 7.38 7.44
(m, 2 Harom
)
1d
1e
4.46 (s, 1 H)
4.80 (s, 1 H)
2.54 268 (m, 1 H),
3.18 3.29 (m, 1 H)
3.87 4.03 (m, 2 H) 2.15 (s, 3 H)
3.87 4.00 (m, 2 H) 2.28 (s, 3 H)
6.71 (d, 2 Harom, J = 8.3
Hz), 7.19 (d, 2 Harom, J =
8.3 Hz)
2.60 2.68 (m, 1 H),
3.18 3.26 (m, 1 H)
6.23 6.27 (m, 1 Harom),
6.31 6.35 (m, 1 Harom),
7.32 7.35 (m, 1 Harom
)
1fb
3.88 (d, 1 H,
J = 4.2)
2.14 (s, 3 H)
2.23 (s, 3 H)
1.22 (d, 3 H, J = 7.1)
1.26 (d, 3 H, J = 7.1)
4.01 (d, 1 H,
J = 4.9)
1g
3.77 3.90
(m, 1 H)
2.54 2.63 (m, 1 H),
3.13 3.22 (m, 1 H)
3.77 3.90 (m, 2 H) 2.34 (s, 3 H)
0.86 (t, 3 H, J = 7.0),
1.18 1.67 (m, 12 H, 6
CH2)
1h
1i
2.72 2.88 (m, 2 H)
3.59 3.68 (m, 1 H), 2.29 (s, 3 H)
3.82 3.90 (m, 1 H)
7.09 7.26 (m, 3 Harom),
1.45
(s, 3 H)
7.38 7.46 (m, 2 Harom
)
4.72 (s, 1 H)
5.87 (s, 1 H)
2.61 2.71 (m, 1 H),
3.12 3.21 (m, 1 H)
3.90 3.98 (m, 2 H) 2.23 (s, 3 H)
7.54 (d, 2 H, J = 8.6),
8.09 (d, 2 H, J = 8.6)
1j
3.49 3.59 (m, 1 H),
3.71 3.78 (m, 1 H)
4.05 4.16 (m, 2 H) 6.47 6.53 (m, 2 Harom), 6.69 6.76 (m, 1 Harom),
7.12 7.20 (m, 2 Harom), 7.28 7.38 (m, 3 Harom),
7.42 7.48 (m, 2 Harom
)
a Coupling constants J in Hz.
b Mixture of 2 diastereomers in 1.5:1 ratio; only selected resonances are given. Anal. Calcd for C12H17NO: C, 75.35; H 8.96; N, 7.32. Found:
C, 75.15; H, 8.99; N, 7.44.
(2rac,4S)-2-Phenyl-3-oxa-1-azabicyclo[3.3.0]octane (3)
Mixture of the two diastereomers in ca. 3:1 ratio. Bp 116 117 °C,
5 mbar.
1H NMR (CDCl3): = 0.77 (d, 3 H, J = 6.3, Me C), 2.17 (s, 3 H,
Me N), 2.95 (dq, 1 H, J = 6.3, 8.2, CH Me), 4.68 (s, 1 H, N CH
O), 5.13 (d, 1 H, J = 8.2, O CH Ph), 7.2 7.7 (m, 10 Harom).
1H NMR (CDCl3): = 5.44 (s, O CH N, major isomer), 5.51 (s,
O CH N, minor isomer). Other resonances could not be assigned.
13C NMR (CDCl3): major isomer, = 25.6 (CH2), 30.4 (CH2), 54.0
(CH2N), 71.5 (CH2O), 98.9 (OCHN), 126.7 (CH), 128.0 (CH),
128.4 (CH), 141.5 (C); minor isomer, = 25.8 (CH2), 31.3 (CH2),
47.5 (CH2N), 64.2 (CHN), 71.3 (CH2O), 94.9 (OCHN), 126.9 (CH),
128.1 (CH), 128.3 (CH), 137.4 (C).
13C NMR (CDCl3): = 15.3 CH3 C), 36.0 (CH3 N), 64.3 (CH N),
82.7 (CH O), 99.1 (N CH O), 127.9 (CH), 128.2 (CH), 128.3
(CH), 128.7 (CH), 128.8 (CH), 129.5 (CH).
Hydrogenation of 1,3-Oxazolidines; General procedure
A glass beaker the precatalyst [Rh(dppb)COD]BF4 (7.2 mg, 0.01
mmol) and a stirring bar was placed in a standard stainless autoclave
(25 ml inner volume) equipped with the valve and connected with a
vacuum pump, Ar and H2 lines. The air was evacuated with a pump
and the autoclave was filled with Ar. This cycle was repeated 2 3
times. In the flow of Ar through the open valve MeOH (10 mL) and
liquid substrate (5 mmol) were added with a syringe (solid sub-
(2rac,4S,5R)-3,4-Dimethyl-2.5-diphenyl-1,3-oxazolidine (5)
Mixture of two distereomers in ca. 12:1 ratio. Mp.72 73 °C (EtOH)
(Lit.10 73 74 °C). Chemical shifts are given only for major isomer.
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378
V. I. Tararov et al.
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Table 3 13C NMR Characterization of 1,3-Oxazolidines 1a j in CDCl3
Compound 1,3-oxozolidine ring position
R1
R2
R3
C-2
C-4
C-5
1a
86.7 (CH2)
52.1 (CH2)
58.1 (CH2)
63.34 (CH2),
127.3(CH), 128.5
(CH), 128.8
(CH), 139.1 (C)
1b
1c
98.5 (CH)
96.0 (CH)
54.9 (CH2)
54.7 (CH2)
65.5 (CH2)
64.8 (CH2)
38.4 (CH3)
38.7 (CH3)
127.8 (CH), 128.5 (CH),
128.9 (CH), 139.3 (C)
19.1 (CH3), 125.8 (CH),
127.4 (CH), 128.4 (CH),
130.6 (CH), 136.5 C), 137.2
(C)
1d
1e
98.1 (CH)
91.5 (CH)
54.4 (CH2)
53.9 (CH2)
64.9 (CH2)
64.9 (CH2)
37.5 (CH3)
38.8 (CH3)
115.2 (CH), 128.5 (C), 128.9
(CH) 157.8 (C)
108.9 (CH), 110.0 (CH),
142.9 (CH), 151.6 (C)
1fa
101.2, 102.0 (CH) 55.0, 55.3 (CH2) 64.6, 65.1 (CH2)
40.2, 41.2 (CH3) 15.5, 17.5 (CH3), 43.3, 43.4
(CH); 126.4, 126.6, 128.2,
128.3, 128.5, 1288.6 (CH);
143.3, 144.2 (C)
1g
97.5 (CH)
54.9 (CH2)
64.1 (CH2)
39.0 (CH3)
14.2 (CH3), 22.8 (CH2), 25.1
(CH2), 29.4 (CH2), 29.9
(CH2), 31.9 (CH2), 33,4
(CH2)
1h
1i
98.8 (C)
53.7 (CH2)
54.5 (CH2)
48.1 (CH2)
62.9 (CH2)
65.6 (CH2)
65.1 (CH2)
37.8 (CH3)
38.8 (CH3)
126.0 (CH), 127.4 (CH),
128.1 (CH); 145.2 (C)
24.2 (CH3)
97.1 (CH)
91.7 (CH)
123.5 (CH), 128.6 (CH),
147.1 (C), 148.3 (C)
1j
113.1 (CH), 117.6 (CH), 127.0 (CH), 128.6 (CH),
128.8 (CH), 129.1 (CH), 139.8 (C), 145.7 (C)
a Mixture of 2 diastereoisomers in 1.5:1 ratio.
strates 1d,j and 3 were placed in the reaction vessel together with
the catalyst). The valve was closed and the autoclave was pressur-
ized with H2. The contents of the autoclave were stirred with a mag-
netic stirrer. The consumption of H2 was monitored by the decrease
of pressure with a pressure detector. When the hydrogen consump-
tion ceased the autoclave was opened, the solution evaporated in
vacuum and the residue analyzed by NMR spectroscopy. The data
for amines 2a,c h are given in Tables 4 and 5.
1H NMR (CDCl3): = 0.98 (d, 3 H, J = 6.7, CH3 C), 2.17 (s, 3 H,
CH3N), 2.91 (dq, 1 H, J = 6.7, 4.9, CH N), 3.57 (d, 1 H, J = 13.5,
PhCHHa), 3.62 (d, 1 H, J = 13.5, PhCHHb), 4.85 (d, J = 4.9, CH
O), 7.18 7.35 (m, 10 Harom).
13C NMR (CDCl3): = 10.2 (CH3 C), 38.9 (CH3 N), 59.4 CH2Ph),
63.7 (CH N), 73.9 (CH O), 121.7 (CH), 126.5 (CH), 128.3 (CH),
128.5 (CH), 128.9 (CH), 139.7 (C), 142.8 (C).
(S)-N-Benzylprolinol (4)
[ ]D
Acknowledgment
23
25
58 (c 2, CHCl3) {Lit.11 [ ]D
59.9 (c 2, CHCl3)}.
Financial support from Degussa AG, Project House Catalysis
(Frankfurt, Germany) and the Fonds der Chemischen Industrie is
gratefully acknowledged.
1H NMR (CDCl3): = 1.62 1.77 (m, 2 H), 1.77 2.0 (m, 2 H),
2.24 2.35 (m, 1 H), 2.68 2.80 (m, 1 H), 2.93 3.04 (m, 1 H), 3.36
(d, 1 H, J = 13.1, PhCHa), 3.41 3.51 (m, 1 H), 3.61 3.69 (m, 1 H),
3.98 (d, 1 H, J = 13.1, PhCHb), 7.2 7.4 (m, 5 Harom).
13C NMR (CDCl3): = 23.7 (CH2), 28.0 (CH2), 54.7 (CH2), 58.8
(CH2), 62.1 (CH2), 64.6 (CH), 127.4 (CH), 128.6 (CH), 127.4 (CH),
139.3 (C).
References
(1) (a) Alberola, A.; Alvarez, M. A.; Andres, C.; Gonzalez, A.;
Pedros, R. Synthesis 1990, 156. (b) Page, P. C. B.; Heaney,
H.; Rassias, G. A.; Reignier, S.; Sampler, E. P.; Talib, S.
Synlett 2000, 104; and references cited therein.
(1R, 2S)-N-Benzylephedrine (6)
23
Mp 48 49 °C (hexane) (Lit.12 49 50 °C). [ ]D
29.6 (c 2.4,
CHCl3) {Lit.11 [ ]D
29.5 (c 2.35, CHCl3)}.
23
Synthesis 2002, No. 3, 375–380 ISSN 0039-7881 © Thieme Stuttgart · New York

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Synthesis of N-(Dialkylaminoalkyl)alcohols
379
Table 4 1H NMR Characterization of 2-(dialkylamino)ethanols, R1R4NCH2CH2OH (R4 = R2R3CH), 2a,c h (positions of OH resonance are
a
not indicated) in CDCl3
Compound
R1 = Me
R4
NCH2
CH2O
2ab
2c
2.21 (s, 3 H)
2.24 (s, 3 H)
PhCH2; 3.55 (s, 2 H), 7.21 7.34 (m, 5 Harom
)
2.57 (t, 2 H, J = 5.5)
2.47 (t, 2 H, J = 5.5)
3.61 (t, 2 H, J = 5.5)
3.49 (t, 2 H, J = 5.5)
2-MeC6H4CH2; 2.08 (s, 3 H), 3.39 (s, 2 H),
6.98 7.16 (m, 4 H)
2d
2e
2.12 (s, 3 H)
2.20 (s, 3 H)
4-HOC6H4CH2; 3.37 (s, 2 H), 6.61 (d, 2 H, J =
8.5), 6.98 (d, 2 H, J = 8.5)
2.48 (t, 2 H, J = 5.5)
3.56 (t, 2 H, J = 5.5)
3.54 (t, 2 H, J = 5.5)
2-furylmethyl; 3.54 (s, 2 H), 6.12 (dd, 1 H, J = 2.50 (t, 2 H, J = 5.5)
0.8 and 3.2), 6.24 (dd, 1 H, J = 2.0 and 3.2), 7.29
(dd, 1 H, J = 0.8 and 2.0)
2fc
2g
2h
2.15 (s, 3 H)
2.17 (s, 3 H)
2.08 (s, 3 H)
Me(Ph)CHCH2; 1.16 (d, 3 H, J = 6.9), 2.34
2.52 (m, 2 H), 2.77 2.89 (m, 1 H), 7.06 7.24
2.34 2.52 (m, 2 H)
3.32 3.41 (m, 2 H)
3.51 (t, 2 H, J = 5.4)
3.38 3.51 (m, 2 H)
(m, 5 Harom
)
C8H18; 0.81 (t, 3 H, J = 7.0, CH3), 1.15 1.27 (m, 2.44 (t, 2 H, J = 5.4)
10 H, 5 CH2), 1.34 1.45 (m, 2 H, CH2), 2.31 (t,
2 H, J = 7.6, CH2N)
PhMeCH; 1.28 (d, 3 H, J = 6.8), 3.57 (q, 1 H, J 2.32 2.52 (m, 2 H)
= 6.8), 7.1 7.2 (m, 5 Harom
)
a Coupling constants J in [Hz]. b Bp 111 °C, 5 mbar (Lit.2 103 °C, 1 torr). Anal. Calcd for C10H15NO: C, 72.69; H 9.15; N, 8.48. Found: C,
72.43; H, 9.17; N, 8.49. c Bp 115 °C, 3 mbar. Anal. Calcd for C12H19NO: C, 74.57; H 9.90; N, 7.25. Found: C, 74.68; H, 9.88; N, 7.32.
Table 5 13C NMR characterization of 2-(dialkylamino)ethanols, R1R4NCH2CH2OH (R4 = R2R3CH), 2a,c h in CDCl3
Compound
R1 = Me
41.8
R4
NCH2
58.6
CH2O
58.7
2a
PhCH2; 62.5 (CH2), 127.4 (CH), 128.6 (CH),
129.3(CH), 138.7 (C)
2c
42.1
2-MeC6H4CH2; 61.0 (CH2), 125.7 (CH), 127.4
(CH), 130.1 (CH), 130.5 (CH), 136.7 (C), 137.2
(C)
58.8
59.0
2d
2e
2f
41.7
42.0
38.6
4-HOC6H4CH2; 61.9 (CH2), 115.8 (CH), 128.3
(C), 131.0 (CH), 156.3 (C)
58.2
58.4
58.7
58.7
59.1
59.7
2-furylmethyl; 54.2 (CH2), 109.2 (CH), 110.5
(CH), 142.5 (CH), 152.3 (C)
Me(Ph)CHCH2; 20.3 (CH3), 42.3 (CH), 65.6
(CH2), 126.7 (CH), 127.5 (CH), 128.8 (CH),
146.2 (C)
2g
2h
42.2
37.9
C8H18; 14.5 (CH3), 23.1 (CH2), 27.7 (CH2), 27.9
(CH2), 29.8 (CH2), 30.0 (CH2), 32.3 (CH2), 58.4
CH2N)
58.9
55.5
59.5
58.8
PhMeCH; 17.8 (CH3), 63.8 (CH), 127.4 (CH),
128.1 (CH), 128.6 (CH), 143.1 (C)
(2) Azzena, U.; Melloni, G.; Nigra, C. J. Org. Chem. 1993, 58,
6707.
(3) Pohland, A.; Sullivan, H. R.; McMahon, R. E. J. Am. Chem.
Soc. 1957, 66, 1442.
(4) Tararov, V. I.; Kadyrov, R.; Riermeier, T. H.;, Börner, A.
Adv. Synth. Catal., in press.
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