Palladium-catalyzed addition of R2NH to double bonds. Synthesis of α-amino tetrahydrofuran and pyran rings

Article abstract of DOI:10.1016/S0040-4020(01)00446-X

Efficient addition of R¹R²NH to unsaturated heterocycles has been achieved using palladium catalysts. The methodology was applied to the synthesis of a number of α-amino tetrahydrofurans and pyrans. Nature of amine, ring size and cat

Full text of DOI:10.1016/S0040-4020(01)00446-X

TETRAHEDRON
Pergamon
Tetrahedron 57 ꢀ2001) 5445±5450
Palladium-catalyzed addition of R2NH to double bonds.
Synthesis of a-amino tetrahydrofuran and pyran rings
Xiaohui Cheng and King Kuok ꢀMimi) Hiip
Department of Chemistry, King's College London, Strand, London WC2R 2LS, UK
Received 29 January 2001; revised 5 April 2001; accepted 20 April 2001
AbstractÐEf®cient addition of R1R2NH to unsaturated heterocycles has been achieved using palladium catalysts. The methodology was
applied to the synthesis of a number of a-amino tetrahydrofurans and pyrans. Nature of amine, ring size and catalyst were found to affect the
reaction. Mechanistic aspects of the reactions were explored by deuterium labelling experiments. q 2001 Elsevier Science Ltd. All rights
reserved.
1. Introduction
2. Results and discussion
The formation of a carbon±nitrogen bond through the direct
addition of an N±H bond across an unsaturated bond has
long been a challenge to synthetic chemists.1,2 In our quest
to ®nd ef®cient air and moisture stable metal catalysts for
hydroamination reactions under pH neutral conditions, we
were particularly attracted by a report in which bisꢀtri-
phenylphosphite)palladiumꢀII) dithiocyanate catalyzed the
hydroamination of morpholine to 2,3-dihydrofuran in 45
turnovers.3
Anumber of palladiumꢀII) precursors were screened for
the reaction between morpholine and 2,3-dihydrofuran
ꢀTable 1). We found that the addition of phosphorus ligands
is unnecessary and that K2PdꢀSCN)4 is a good catalyst ꢀentry
4); the reaction occurred cleanly at ambient temperature and
yielded the desired product regiospeci®cally. The combina-
tion of palladium and thiocyanate seems to imbue special
activity, as yields dropped quite dramatically when thio-
cyanate was substituted by other anions ꢀentries 5±7).
Entries 1 and 8 con®rm de®nite involvement of palladium
in the process. The thiocyanate anion is envisaged to have
an important role, even though its precise coordination
mode and function is not known. The addition of excess
KSCN to the reaction mixture improves the catalytic turn-
over, which suggests that this rather unusual anion acts, at
least, to stabilize important reaction intermediates.
The synthesis of a-amino tetrahydrofuran and pyran
rings are particularly interesting as 5- and 6-membered
functionalised heteroatom rings are found in many nucleo-
sides4±6 and glycolipids7 that have interesting biological
activities. These molecules are prepared generally from
one of two routes: Substitution of a halide or hydroxyl
group by an amine, or the hydroamination of a double
bond in the presence of an acid or base.8,9 These methods
are limited by the nucleophilicity of the amine, and involve
equilibrium processes that lead to modest yields. They also
often require fairly harsh reaction conditions, making them
unsuitable for the synthesis of fragile molecules.
Table 1. Effect of different catalysts
The idea of employing a palladium catalyst to effect the
formation of these molecules is particularly interesting, as
the methodology offers hydroamination reactions at pH
neutral conditions. As far as we are aware, there is no
work reported in the area, so we decided to initiate a
study to explore the scope and limitations of the reaction.
Entry
Catalyst
Time ꢀh) Temperature ꢀ8C) Yielda ꢀ%)
1
2
3
4
5
6
7
8
None
12
12
12
12
16
16
14
24
80
20
20
20
20
20
20
20
0
Pd{PꢀOPh)3}2ꢀSCN)2
PdꢀPPh3)2ꢀSCN)2
K2PdꢀSCN)4
PdꢀNCMe)2Cl2
PdꢀOAc)2
86
90
91
26
46
29
0
PdI2
KSCN
Keywords: C±N bond formation; homogeneous catalysis; palladium;
amination.
Corresponding author. Tel.: 144-020-78481183; fax: 144-020-
78482810; e-mail: mimih@ch.kcl.ac.uk
Reaction conditions: 2.9 mmol of amine to 2.5 mL of dihydrofuran,
substrate/catalystˆ45.
p
a
Isolated yields.
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020ꢀ01)00446-X

5446
X. Cheng, K. K. Hii /Tetrahedron 57 $2001) 5445±5450
Table 2. Hydroamination of 2,3-dihydrofuran
Reaction conditions: as before. Reaction times are unoptimised ꢀduration of an overnight experiment).
Isolated yields of compounds estimated to be .95% pure by 1H NMR.
a
b
Substrate/catalystˆ11.25.
The reaction is equally applicable to other secondary alkyl
amines, which required no more than gentle or moderate
heating ꢀTable 2). The reactions were again regiospeci®c,
giving a-amino substituted tetrahydrofuran as the only
observable product. The yields are generally high compared
to previously reported methods. For example, under basic
conditions, 2,3-dihydrofuran reacts with morpholine and
pyrrolidine to give 2 and 3, in 60 and 45% yields, respec-
tively.10 However, the reaction only seems to work with
secondary alkyl amines, as the use of aryl amines ꢀaniline
and N-methyl aniline) leads to a mixture of unidenti®able
products.
products. The reaction with aniline gave 10 as the major
product, whereas the formation of the methyl substituted
11 was slower and required higher temperature, suggesting
that the steric demand imposed by the amine is an important
factor.
2.1. Mechanism of the catalytic reaction
The reactions did not appear to be acid-catalysed as sulfuric
and p-toluenesulfonic acids failed to induce any reaction
between morpholine and 2,3-dihydropyran, even after heat-
ing at 608C for 24 h.
In contrast, the corresponding hydroamination reactions of
2,3-dihydropyran take longer, and occur only at elevated
temperatures ꢀTable 3). The yields of these reactions are
mostly very high, and seem to be limited by the volatility
of the amine ꢀentries 3 and 4). It is interesting to note that
while the phosphine-ligated [PdꢀPPh3)2ꢀSCN)2] complex
failed to induce any reaction between morpholine and
dihydropyran, the introduction of primary and secondary
arylamines to the unsaturated pyran ring is possible ꢀentries
5 and 6). For the latter reactions, the addition of triphenyl-
phosphine ligand was necessary to stabilize the catalyst over
the prolonged reaction period and also led to cleaner
Although transition metal catalysed hydroamination reac-
tions via double bond activation has been proposed,11 we
did not observe any interaction between K2PdꢀSCN)4 and
dihydropyran. However, successive addition of measured
quantities of morpholine to a solution of K2PdꢀSCN)4
indicated, tentatively, by 1H NMR spectroscopy, the forma-
tion of a ¯uxional and equilibrating mixture of complexes
corresponding to a bisꢀmorpholine) palladium dithiocyanate
stoichiometryÐthis seems to agree with the result of a
previous study, where a palladium complex with the same
stoichiometry has also been proposed as the resting state of
an intramolecular hydroamination reaction.12 We thus

X. Cheng, K. K. Hii /Tetrahedron 57 $2001) 5445±5450
5447
Table 3. Hydroamination of 3,4-dihydro-2H-pyran
Reaction conditions as before.
Catalyst A: K2PdꢀSCN)4; B: PdꢀPPh3)2ꢀSCN)2.
a
b Isolated yields.
Scheme 1. Proposed mechanism.

5448
X. Cheng, K. K. Hii /Tetrahedron 57 $2001) 5445±5450
Figure 1. Isotopic-labeling experiment. 13C±{1H} NMR spectrum showing the resonance due to C3: X6 ꢀsinglet); pD-6 ꢀtriplet); V D2-6 ꢀquintet, partially
obscured). The 13C±{1H} NMR spectrum was acquired with suitably long relaxation delays to minimize noe effects.
Scheme 2. Ring opening and closure via an enamine intermediate.
suggest a mechanism that involves transient hydrido-
palladium complexes ꢀScheme 1). The proposal is supported
by the known regiochemistry of Pd±H insertion to
unsaturated furan rings.13
mutarotation process ꢀanomeric effect) of tetrahydropyran
derivatives,14,15 and is a fairly common phenomenon for
glycosylamines and amides.7 In this case the tetrahydro-
pyran ring opens and closes via an enamine intermediate,
leading to the scrambling of deuterium labels at C-3. Note
that in the reaction mixture this process will be occurring in
the presence of an excess of dihydropyran. As no deuterium
label was lost from the product, it rules out the involvement
of an oxonium intermediate.
To investigate the mode of N±H addition, catalytic hydro-
amination reaction of 2,3-dihydropyran was performed with
1-deuterio-morpholine in 10 molar excess of the pyran.
1
Integration of the corresponding H signals of the resultant
product revealed that there is an apparent absence of exactly
one proton at the C3 position. This observation strengthens
our belief that an oxonium intermediate is not involved in
the reaction, as this will invariably lead to deuterium being
incorporated into dihydropyran, which was present in
excess.
3. Conclusion
In conclusion, a range of a-alkylamine tetrahydrofurans, as
well as a-alkyl and a-arylamino-tetrahydropyrans can be
synthesized under pH neutral conditions by a hydroamina-
tion reaction catalysed by palladium thiocyanate complexes.
The a-aminotetrahydropyran rings are found to undergo
rapid reversible ring opening in solution. Further applica-
tions of the reaction are currently being investigated.
On closer inspection, the 13C NMR spectrum of the same
sample revealed the presence of the mono-deuterio
compound, as well as di-hydro and di-deuterio isomers of
6 ꢀFig. 1). This was con®rmed by the analysis of the parent
peaks of the three isomers by mass spectrometry, which
revealed an equal distribution of all three isomers: m/z
ꢀFAB): 171 ꢀno deuterium, 77.5%), 172 ꢀone deuterium,
100%) and 173 ꢀtwo deuteriums, 83.9%), which is roughly
1:1:1 ꢀtaking into account 13C isotopic distribution). This led
us to suspect that the product undergoes an intermolecular
exchange process, which leads to the scrambling of the
deuterium isotope. Indeed, when a solution of 6 was heated
to 608C in methanol-d4, we observed a slow incorporation of
deuterium label onto C-3 as the intensity of the H-3 signal
decreased.
4. Experimental
4.1. General
Proton and carbon NMR spectra were recorded for solutions
in deuteriochloroform, using Bruker AM360 or AMX400
spectrometers. H NMR data of known compounds are in
agreement with reported values, which were quoted
accordingly. The chemical shifts are in parts per million
ꢀd ppm), unless otherwise stated, referenced to TMS ꢀd
0). The coupling constants are in Hertz ꢀJ Hz). The
following abbreviations are used: sÐsinglet, tÐtriplet,
1
Thus we propose the operation of a ring-opening process
as depicted in Scheme 2. The process is akin to the

X. Cheng, K. K. Hii /Tetrahedron 57 $2001) 5445±5450
5449
qÐquartet, mÐmultiplet, dÐdoublet, brÐbroad. Melting
points were determined on an Electrothermal Gallenhamp
melting point apparatus and are uncorrected. Mass
spectra ꢀMS) and high-resolution mass spectra ꢀHRMS)
were recorded using the FAB technique by the Mass
Spectrometry Service within The University of London's
Intercollegiate Research Services ꢀULIRS). Mass data are
reported in mass units ꢀm/z), and values in brackets show the
relative intensity from the base peak ꢀas 100%).
temperature ꢀ334 K). 1H NMR ꢀ400 MHz, CDCl3) d:
1.72±1.88 ꢀm, 4H), 2.17 ꢀs, 3H), 2.20±2.40 ꢀm, 4H),
2.41±2.60 ꢀm, 2H), 2.65±2.71 ꢀm, 2H), 3.65 ꢀdd, Jˆ6,
13 Hz, 1H), 3.76 ꢀdd, Jˆ7, 13 Hz, 1H), 4.48 ꢀt, Jˆ5 Hz,
1H). 13C NMR ꢀ100.6 MHz, CDCl3, 298 K) d: 25.9, 28.7,
46.4, 48.0 ꢀbr.), 55.5, 68.3, 96.0. HRMS calculated for
C9H18N2O 170.1419, observed 170.1431.
4.2.5. 1,4-Bis--tetrahydrofuran-2-yl)-piperazine, 5. Colour-
less crystalline solid. mp 77±788C. nmax ꢀNujol) 2902, 1461,
4.1.1. Potassium tetrakis-thiocyanate)palladium-II).16
The compound was prepared by a modi®ed procedure:
PdCl2ꢀNCMe)2 ꢀ0.26 g, 1.0 mmol) was added to a solution
of potassium thiocyanate ꢀ0.39 g, 4.0 mmol) in acetone
ꢀ15 mL), and the reaction mixture was re¯uxed for
30 min. The product crystallized as a red solid upon cooling.
It was collected by ®ltration, and washed with acetone
ꢀ2£5 mL). Asecond crop of product was obtained by
slow evaporation of the ®ltrate. Yield 0.41 g ꢀ99%). nmax
1
1376 cm21. H NMR ꢀ360 MHz, CDCl3) d: 1.73±1.96 ꢀm,
8H), 2.48±2.57 ꢀm, 4H), 2.67±2.79 ꢀm, 4H), 3.66±3.73 ꢀm,
2H), 3.76±3.83 ꢀm, 2H), 4.50±4.56 ꢀm, 2H). 13C NMR
ꢀ90.5 MHz, CDCl3) d: 25.6, 28.4, 47.7, 68.0, 95.7. Found:
C, 63.65; H, 9.80; N, 12,25. C12H22N2O2 requires C, 63.70;
H, 9.80; N, 12.40%. HRMS calculated for C12H22N2O2
226.1682, observed 226.1680.
4.2.6. 4--Tetrahydropyran-2-yl)-morpholine, 6. Clear oil.
bp 58±608C/0.8 Torr ꢀlit.17 111.58C/12 Torr). nmax ꢀliquid
ꢀKBr disc)18 nꢀCxN) 2122, 2093 cm21
.
1
®lm) 2943, 2849, 1453, 1008 cm21. H NMR ꢀ360 MHz,
CDCl3) d: 1.40±1.70 ꢀm, 5H), 1.81±1.95 ꢀm, 1H), 2.58
ꢀm, 2H), 2.86 ꢀm, 2H), 3.43 ꢀm, 1H), 3.62±3.74 ꢀm, 4H),
3.77 ꢀm, 1H), 3.98 ꢀdd, Jˆ3, 11 Hz, 1H). 13C NMR
ꢀ90.5 MHz, CDCl3, 298 K) d: 23.4, 25.7, 28.3, 48.1, 67.2,
67.4, 93.6.
4.2. Typical experimental procedure -Table 1, entry 3)
To a mixture of 2,3-dihydrofuran ꢀ2.5 mL, 33 mmol) and
morpholine ꢀ250 mg, 2.9 mmol) was added potassium
tetrakisꢀthiocyanate)palladiumꢀII) ꢀ26 mg, 0.06 mmol).
The reaction mixture was stirred at 208C overnight. The
volatile reagents were then removed under reduced pres-
sure, and petroleum ether ꢀ40±60, 30 mL) was added to
the residue. The precipitate was removed by ®ltration, and
the ®ltrate concentrated under reduced pressure. The crude
product was puri®ed by distillation to give 1 ꢀ410 mg, 91%
yield).
4.2.7. 1-Methyl-4--tetrahydropyran-2-yl)-piperazine, 7.
Clear oil. bp 76±788C/1 Torr nmax ꢀliquid ®lm) 2936,
1
1456, 1373, 1009 cm21. H NMR ꢀ400 MHz, CDCl3) d:
1.33±1.51 ꢀm, 5H), 1.77 ꢀm, 1H), 2.16 ꢀs, 3H), 2.20±2.42
ꢀm, 4H), 2.46±2.57 ꢀm, 2H), 2.71±2.83 ꢀm, 2H), 3.31 ꢀm,
1H), 3.68 ꢀm, 1H), 3.86 ꢀm, 1H). 13C NMR ꢀ100.6 MHz,
CDCl3) d: 23.8, 26.2, 28.9, 46.5, 47.9, 55.8, 67.8, 93.8.
HRMS calculated for C10H20N2O 184.1575, observed
184.1582.
4.2.1. 4--Tetrahydrofuran-2-yl)-morpholine, 1. Colour-
less oil. bp 568C/0.6 Torr ꢀlit.10 76±778C/4 Torr). nmax
ꢀliquid ®lm) 2954, 1454, 1261, 1040 cm21 1H NMR
.
4.2.8. 1--Tetrahydropyran-2-yl)-pyrrolidine, 8. Clear oil.
bp 58±598C/2.2 Torr ꢀlit.17 85.58C/9.5 Torr). nmax ꢀliquid
ꢀ360 MHz, CDCl3) d: 1.75±2.00 ꢀm, 4H), 2.50 ꢀm, 2H),
2.70 ꢀm, 2H), 3.65 ꢀm, 4H), 3.70 ꢀm, 1H), 3.83 ꢀm, 1H),
4.50 ꢀbr. m, 1H). 13C NMR ꢀ90.5 MHz, CDCl3) d: 25.9,
28.4, 48.5, 67.4, 68.3, 96.3.
1
®lm) 2042, 1448, 1290, 1072 cm21. H NMR ꢀ400 MHz,
CDCl3) d: 1.40±1.51 ꢀm, 4H), 1.61±1.80 ꢀm, 6H), 2.63±
2.69 ꢀm, 2H), 2.75±2.81 ꢀm, 2H), 3.33 ꢀm, 1H), 3.85±3.90
ꢀm, 2H). 13C NMR ꢀ100.6 MHz, CDCl3) d: 23.4, 24.0, 25.8,
31.0, 47.2, 66.8, 90.5.
4.2.2. 1--Tetrahydrofuran-2-yl)-pyrrolidine, 2. Colourless
oil. bp 65±678C/4.5 Torr ꢀlit.10 90±938C/34 Torr). nmax
1
ꢀliquid ®lm) 2061, 1467, 1352, 1003 cm21. H NMR10
4.2.9. Dibutyl--tetrahydropyran-2-yl)-amine, 9. Clear oil.
bp 65±668C/0.4 Torr ꢀlit.17 119.5±1208C/10.5 Torr). nmax
ꢀ360 MHz, CDCl3) d: 1.68±1.98 ꢀm, 8H), 2.63±2.74 ꢀm,
4H), 3.73±3.85 ꢀm, 2H), 4.69 ꢀt, Jˆ5.8 Hz, 1H). 13C NMR
ꢀ90.5 MHz, CDCl3) d: 23.8, 25.4, 29.9, 47.8, 67.8, 93.4.
ꢀliquid ®lm) 2933, 1465, 1376, 1076 cm21 1H NMR
.
ꢀ360 MHz, CDCl3) d: 0.89 ꢀt, Jˆ7 Hz, 6H), 1.21±1.70
ꢀm, 13H), 1.81±1.90 ꢀm, 1H), 2.50±2.60 ꢀm, 2H), 2.62±
2.74 ꢀm, 2H), 3.37 ꢀm, 1H), 3.95 ꢀm, 2H). 13C NMR
ꢀ90.5 MHz, CDCl3) d: 14.0, 20.5, 24.3, 26.5, 30.5, 31.8,
50.1, 67.7, 92.5.
4.2.3. Dibutyl--tetrahydrofuran-2-yl)-amine, 3. Colour-
less oil. bp 67±698C/0.7 Torr ꢀlit.10 112±1158C/12 Torr).
nmax ꢀliquid ®lm) 2953, 1465, 1376 cm21 1H NMR10
.
ꢀ400 MHz, CDCl3) d: 0.87 ꢀt, Jˆ7 Hz, 6H), 1.23±1.46
ꢀm, 8H), 1.62±1.94 ꢀm, 4H), 2.54 ꢀm, 4H), 3.60±3.89
ꢀ2H, m), 4.81 ꢀt, Jˆ6.5 Hz, 1H). 13C NMR ꢀ100.6 MHz,
CDCl3) d: 14.4, 21.0, 25.9, 29.4, 31.6, 50.0, 67.4, 95.6.
4.2.10. Phenyl--tetrahydropyran-2-yl)-amine, 10. Colour-
less crystalline solid. mp 73±748C, bp 112±1148C/5 Torr
ꢀlit.17 1148C/5 Torr). nmax ꢀNujol) 3350, 2970, 1604,
1
1376 cm21. H NMR18 ꢀ360 MHz, CDCl3) d: 1.45±1.72
4.2.4. 1-Methyl-4--tetrahydrofuran-2-yl)-piperazine, 4.
Colourless oil. bp 75±778C/1.8 Torr. nmax ꢀliquid ®lm)
2936, 1455, 1371, 1286 cm21. At ambient temperature the
methine proton and carbon signals were very broad in the
NMR spectra, probably due to conformational ring ¯ip of
the nitrogen heterocycle. The signals sharpened at higher
ꢀm, 4H), 1.86±1.95 ꢀm, 2H), 3.56 ꢀm, 1H), 3.99 ꢀm, 1H),
4.28±4.30 ꢀbr. d, 1H), 4.69 ꢀdd, Jˆ2, 9 Hz, 1H), 6.72±6.80
ꢀm, 3H), 7.19 ꢀdd, Jˆ7, 8 Hz, 2H). 13C NMR ꢀ90.5 MHz,
CDCl3) d: 23.0, 25.3, 31.8, 65.8, 82.4, 113.9, 118.7, 129.2,
145.5.

5450
X. Cheng, K. K. Hii /Tetrahedron 57 $2001) 5445±5450
4. Fregnan, G. B.; Prada, M.; Torsello, A. L.; Vidali, M.
Methyl-phenyl--tetrahydropyran-2-yl)-amine,
4.2.11.
11. Pale yellow oil. bp 94±988C/0.4 Torr ꢀlit.19 1018C/
J. Pharm. Pharmacol. 1981, 33, 106±108.
5. Takeuchi, Y.; Chang, M.-R.; Hashigaki, K.; Yamato, M.
Chem. Pharm. Bull. 1991, 39, 1629±1631.
0.5 Torr). nmax ꢀliquid ®lm) 2941, 1599, 1503 cm21. H
1
NMR ꢀ360 MHz, CDCl3) d: 1.53±2.07 ꢀm, 6H), 3.03 ꢀs,
3H), 3.65 ꢀm, 1H), 4.12 ꢀm, 1H), 4.81 ꢀm, 1H), 6.89 ꢀt,
Jˆ7 Hz, 1H), 6.98 ꢀd, Jˆ9 Hz, 2H), 7.32 ꢀdd, Jˆ7, 9 Hz,
2H). 13C NMR ꢀ90.5 MHz, CDCl3) d: 24.2, 25.7, 30.0, 32.5,
67.7, 89.0, 115.4, 118.8, 129.0, 149.9.
6. Kunz, K. R.; Iyengar, B. S.; Dorr, R. T.; Alberts, D. S.;
Remers, W. A. J. Med. Chem. 1991, 34, 2281±2286.
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Acknowledgements
È
11. Senn, H. M.; Blochl, P. E.; Togni, A. J. Am. Chem. Soc. 2000,
122, 4098±4107.
The authors thank King's College London, K. C. Wong
Education Foundation and the Foreign and Commonwealth
Of®ce for a studentship ꢀX. H. C.), and Johnson Matthey plc
for the generous loan of palladium salts. We also thank Jon
Cobb and Jane Hawkes for help with NMR experiments and
interpretations.
12. Vogels, C. M.; Hayes, P. G.; Shaver, M. P.; Westcott, S. A.
Chem. Commun. 2000, 51±52.
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