Selective carbon-carbon bond formation: Terpenylations of amines involving hydrogen transfers

Article abstract of DOI:10.1039/c3gc36982j

The well-defined ruthenium(ii) complex A featuring a phosphine sulfonate chelate promotes the introduction of terpene moieties onto cyclic saturated amines through hydrogen auto transfers without side alkene reduction. These eco-friendly transformations enable the production of diverse N- and C-terpenoid alkaloids with only water and carbon dioxide as benign side products.

Full text of DOI:10.1039/c3gc36982j

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ARTICLE TYPE
Selective Carbon-Carbon Bond Formation : Terpenylations of Amines
Involving Hydrogen Transfers
Zeyneb Sahli, Basker Sundararaju, Mathieu Achard and Christian Bruneaua*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
wastes are still unknown. Recently, we demonstrated that wellꢀ
50 defined ruthenium(II) and iridium(III) containing phosphine
sulfonate as chelating ligand allowed the C(3)ꢀH
functionalization of amines with aldehydes via an oxidant free
dehydrogenation.16 This reactivity involving hydrogen
autotransfers took advantage of the ability of the in situ generated
55 metal hydride species resulting from the dehydrogenation of
amines to reduce the iminium intermediates formed upon
condensation with aldehydes. Herein, we report on the
preparation of various Nꢀterpenylated amines and their
application in catalytic hydrogen autotransfer reactions for the
60 access to diverse amines containing two or three terpene moieties
via CꢀH functionalization.
The well-defined ruthenium(II) complex
A featuring a
phosphine sulfonate chelate promotes the introduction of
terpene motives onto cyclic saturated amines through
hydrogen “auto”transfers without side alkene reduction.
10 These eco-friendly transformations enable the production of
diverse N- and C- terpenoid alkaloids with only water and
carbon dioxide as benign side products.
Introduction
The broad family of terpenes and their oxygenated terpenoid
15 partners resulting from the assembly of isoprene units represent a
class of secondary metabolites arising from biomass and isolated
from plants and fungi.1 Besides their historical application in
perfume, fragrances,1a,2 or medicines,3 these compounds have
also attracted considerable interest as raw materials for fine
20 chemicals,2,4 agrochemicals,5 and as renewable ressource for
rubber and polymerization chemistry.6 Arising from the
attachment of terpene moieties to various non terpenic organic
molecules such as indoles, coumarins, hybrid isoprenoids are a
special class of molecules which revealed high biological
25 activity, presumably due to their increased affinity to biological
membranes.7 Similarly, terpene alkaloids ranging from prenylated
R2
geraniol, phytol....
+
N
R3
H
R
OH
citronellal, myrtenal,
perillaldehyde...
HCO2H (1.2 equiv.)
cat. A (1.25 mol%)
toluene, 140 °C
route b
CO2, H2O
R1
O
HCO2H (1.2 equiv.)
neat, 90°C
Ph
Ph
H
H
R2
N
Ru
O
P
Leuckart-Wallach
Cl
+
R1
R2
R3
S
O
O
N
R3
CO2, H2O
cat. A
cat. A (1.25 mol%)
toluene, 140 °C
H2O
route c
route a
R2
R1
OH
to steroidal alkaloids, have
a broad range of biological
+
N
H
R3
properties.8,9 Another interest of « azaꢀterpenes » results in the
quaternization of the amines allowing the access of diverse
30 amphiphiles with potential industrial properties as surfactants or
ionic liquids.10 Synthetic preparation of Nꢀterpenylated amines
with simple terpenes usually involved telomerization,
hydroamination and related reactions.4b,11,12 Geranylation and
prenylation of amines with allylic alcohol derivatives such as
35 prenol, linalool, geraniol via catalytic nucleophilic substitution
are also documented.13 Reductive amination of terpenaldehydes
represent another important approach.14 However, even if the
preparation of C(2)ꢀ and C(3)ꢀprenylated indole derivatives to
access isoprenoids are reported,15 straightforward accesses of
40 novel Cꢀterpenylated saturated amines minimizing the steps and
citronellol...
Scheme 1 Preparation of tertiary amines via Nꢀterpenylation.
Results and Discussion
65 Initially, the preparation of the Nꢀterpenylated alkaloids 3 (Figure
1) was easily achieved by using three different pathways. Starting
from aldehydes such as citronellal, myrtenal, safranal with
secondary cyclic amines 2, Nꢀterpenylation occurred under neat
reaction condition using the soꢀcalled LeuckartꢀWallach reaction
70 in up to 95% isolated yield (Scheme 1 – route a).17,18 From allylic
alcohols such as geraniol and phytol, Nꢀalkylated amines 3 were
obtained via
a tandem isomerizationꢀreductive amination
sequence in the presence of cat. A and formic acid acting as
hydrogen donor through hydrogen transfer processes by using our
75 previously described methodology (Scheme 1 – route b).19
Finally the borrowing hydrogen methodology starting from
alcohol such as citronellol led to the formation of Nꢀgeranylated
amines in up to 63% isolated yield (Scheme 1 – route c).20
Overview of the Nꢀterpenylated amines 3 is outlined in Figure 1.
80 With the freshly prepared Nꢀterpenylated amines 3 in hand, we
a UMR 6226 CNRS – Université de Rennes 1, Institut des Sciences
chimiques de Rennes, Organometallics : Materials and Catalysis,
Campus de Beaulieu, 35042 Rennes Cedex, France. Fax: +33
223236939; Tel: +33 223236602; E-mail: christian.bruneau@univ-
45 rennes1.fr
† Electronic Supplementary Information (ESI) available: Experimental
procedures and characterization data for cat. D and compounds 3, 4, 5.
See DOI: 10.1039/b000000x/
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O
N
N
N
Ph
N
N
Ph
But
Ph
Ru
O
Ir
P
Cl
P
Ru
O
Cl
N
Cl
O
S
O
S
O
O
O
3c (68% [a],
61% [b], 63% [c])
3d (79% [a]) 3e (62% [a],
3a (81% [a],
57% [b], 58% [c])
3b (65% [a])
cat. D
cat. C
cat. B
54% [c])
Figure 2 Evaluated precatalysts.
N
N
N
cat. C, the expected product 4a was formed but large amounts of
35 side products resulting from the over reduction of the C=C
insaturations via hydrogen transfer processes were observed
(entry 3). Attempts with lower amount of formic acid and
reduced reaction temperature were unsuccessful highlighting the
low chemoselectivity for this transformation (entry 4). On the
40 other hand, the application of the ruthenium(II) cat. D was more
fruitful affording the expected digeranylated piperidines 4a in a
67% isolated yield (entry 5). The transformation in the presence
of the ruthenium cat. B occurred with a better chemoselectivity
and led to 62% isolated yield (entry 1). Finally, the best result
45 was obtained by using 1.5 equiv. of formic acid and a reduced
temperature of 130 °C at the last stage yielding 80% of the
isolated digeranylated piperidine 4a (entry 2). With the
Nꢀterpenylated cyclic amines prepared from Figure 1 and under
our best reaction conditions, the substrate diversity toward
50 rutheniumꢀcatalyzed Cꢀterpenylation was explored with various
biosourced terpenaldehydes (Table 2). In most cases the reaction
went smoothly affording the expected diterpenylated amines in
moderate to good yields. Reaction of piperidine 3a,
tetrahydroisoquinoline 3b and pyrrolidine 3e derivatives with
55 racemic citronellal 1a led to the formation of stereoisomeric
mixtures of digeranylated amines in 62ꢀ80% isolated yield
(entries 1ꢀ3). NꢀGeranylated morpholine 3d was also compatible
affording 4d in 69% isolated yield (entry 4). The presence of
stereoisomers in these previous compounds prevented the
60 accurate determination of the diastereoselectivity during CꢀH
functionalization and in order to tackle this issue we decided to
start from the achiral amine 3f derived from safranal. It was noted
3i (49% [a])
N
3h (62% [a])
3g (86% [a])
3f (73% [a])
N
N
N
N
3j (57% [a])
3k (52% [b])
Figure 1 Overview of prepared Nꢀterpenylated amines 3 via
hydrogen transfers (preparation route indicated in brackets).
next explored the reactivity of the racemic Nꢀgeranylated
piperidine 3a with citronellal 1a towards βꢀgeranylation in the
presence of the ruthenium complex cat. B, the iridium precatalyst
5
cat.
containing
[Ru(pꢀcymene)Cl2]2 with the deprotonated hydroxyquinoline (see
C
featuring phosphine sulfonate chelate and cat.
D
a
pyridone chelate obtained by treatment of
10 S I) (Figure 2). Thus, alkylated piperidine 3a was first reacted
with citronellal 1a in the presence of a catalyst and CSA as
additive at 150 °C for 16 h, and after cooling formic acid was
added as a reducing agent at the last stage of the procedure to
ensure complete reduction of the unsaturated intermediates at 150
15 °C for 2 h (Table 1). Under these reaction conditions, the amines
act both as nucleophile through the formation of enamine
intermediates and as hydrogen donor for the generation of metal
hydride species enabling the reduction of the iminium species.
Noteworthy, piperidines 4a were obtained as a mixture of eight
20 stereoisomers due to the presence of three stereogenic carbon
centers in the final molecule including the formation of one of
them during the βꢀalkylation step. When the βꢀgeranylation was
attempted under our initial reaction conditions with iridium
that
a good 72% isolated yield was obtained but most
significantly even at these high temperatures a promising 72%
65 diastereoisomeric excess was observed (entry 5). Noteworthy,
NMR analyses revealed that no C=C isomerization occurred on
the dienic structure. α,βꢀUnsaturated aldehydes have never been
examined during βꢀalkylation of amines and interestingly, during
the transformation of the enantiopure amine 3g with (ꢀ)ꢀmyrtenal,
70 a moderate diastereoisomeric excess of 62% was obtained but
importantly using 1.5 equivalent of formic acid highlighted that
the former conjugated double bond was not hydrogenated
affording amine 4h in 57% isolated yield (entry 8). The same
trend was observed with perillaldehyde and both C=C bonds
75 remained intact giving amines 4g, 4i and 4j in up to 71% isolated
yield (entries 7, 9 and 10). In contrast, the reaction of 3a with an
Table 1 C(3)ꢀTerpenylation of piperidine 3a with citronellal 1aa
O
N
1/ Cat. B, C or D
2/ HCO2H
N
N
+
+
Toluene
1a
3a
4a
undesired overeduction
products
25
HCO2H
CSA
Conversion
(Yield)c
Entry
catalyst
Selectivityb
(n equiv.)
(mol %)
1
2d
3
Cat. B
Cat. B
Cat. C
Cat. C
Cat. D
2.0
1.5
1.5
1.0
1.5
7
4
4
4
4
73
90
58
65
90
92(62)
91(80)
89
4e
92
N
O
1/ cat. B (3 mol%)
CSA (4 mol%)
5
83(67)
N
a All reactions were carried out at 0.135 M concentration in toluene at 150
°C for 16 h, 3 h under an inert atmosphere of argon with 1a/3a/[Cat] in
1/1.1/0.03 molar ratio and CSA as additive.b Selectivity calculated by GC
analysis toward 4a and side products arising from overꢀreduction.
30 c Conversion based on GC analysis. Number in parenthesis is isolated
yield. d Second step at 130 °C. e Second step at 100 °C.
2/ HCO2H (2 equiv.)
toluene
150 °C, 18 h, 3 h
76%
3a
4a
Scheme 2 Geranylation with citrals
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Table 2 Rutheniumꢀcatalyzed βꢀterpenylation of amines 3a
1/ cat. B (3 mol%)
CSA (4 mol%)
2/ HCO2H ( 1,5 equiv.)
O
R1
+
R1
H
+ H2O
N
R2
3a-h
N
n
toluene
150 °C, 130 °C
16 h, 3 h
n
R2
4a-j
1a-c
Entry
1
Terpene 1
Amine 2
Product 3
d.ec.
Yieldd
80
O
H
N
N
ꢀb
1a
3a
4a
N
N
2
3
ꢀb
68
62
4b
3b
N
N
ꢀb
3e
4c
O
N
O
N
4
ꢀb
69
3d
4d
N
N
5
6
72
ꢀb
72
56
3f
4e
N
N
4f
3g
N
N
7
8
9
ꢀb
62
5
60
57
71
3h
4g
O
H
N
N
1b
4h
3g
O
H
N
N
1c
3h
4i
N
ꢀb
67
4j
10
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a All reactions were carried out at 0.135 M concentration at 150 °C for 16 h under an inert atmosphere of argon with 1/3/[Ru] in 1/1.1/0.03 molar ratio. b
stereoisomeric mixture. c diastereomeric excess. d isolated yield.
directly taken for GC analysis and purified by column
chromatography (Et2O/PE/Et3N) to afford the βCꢀalkylated
50 amines 4.
1/ cat. A (3 mol%)
CSA (15 mol%)
2/ HCO2H (3 equiv.)
N
O
N
3-(3,7-dimethyloct-6-en-1-yl)-1-((2,6,6-trimethylcyclohexa-
1,3-dien-1-yl)methyl)piperidine 4e
+
toluene
150 °C, 18 h, 3 h
53 %
5a
1a
( 2 equiv.)
3a
Compound 4e was prepared according to the general procedure
for the preparation of amines 4 after purification through column
55 chromatography (Et2O/PE/Et3N: 1/9/0.025) in 72% yield with a
1
72% diastereoisomeric excess. H NMR (400 MHz, CDCl3): δ
5
Scheme 3 Trigeranylated piperidines 5a
5.71ꢀ5.64 (m, 2H), 5.02 (t, 1H, J= 7.0 Hz), 2.86 (s, 2H), 2.68ꢀ2.63
(m, 2H), 1.92ꢀ1.74 ( m, 6H), 1.69 (s, 3H), 1.61(s, 3H), 1.53 (s,
3H), 1.36ꢀ1.20 (m, 8H), 1.13ꢀ1.02 (m, 4H), 0.98 (s, 6H), 0.77 (d,
60 3H, J= 6.3 Hz); 13C NMR (75 MHz, CDCl3): δ 134.6, 129.9,
128.8, 126.5, 124.1, 124.0, 59.4, 59.2, 55.5, 52.8, 39.6, 36.0,
36.0, 35.5, 33.1, 32.5, 31.6, 30.8, 30.4, 30.2, 29.9, 28.6, 25.1,
24.7, 24.5, 18.5, 17.4, 16.6, HRMS calculated for C25H44N+:
[M+H]+ 358.34738 , found [M+H]+ 358.3474.
acyclic α,βꢀunsaturated aldehyde such as citrals led to the major
formation of the diterpenylated amines 4a resulting from the
reduction of the former conjugated C=C enal bond whereas the
distal unsaturation remained intact (Scheme 2). At this stage,
10 piperazine derivatives 3i and 3j didn’t react likely due to the
difficult deprotonation of the iminium intermediates preventing
enamine formation. Finally, another approach to highlight the
interest of our methodology in synthesis consisted in the
formation of triterpenylated amines arising from functionalization
15 at the two C(3)ꢀposition of the Nꢀterpenylated amine 3a. We
demonstrated that the chemoselective formation of the
trigeranylated piperidine 5a was possible in the presence of 2
equivalents of aldehyde 1a simply by increasing the amount of
Brønsted acid (CSA) from 4 to 15 mol % leading to the formation
20 of the lipophilic amine 5a in 53% isolated yield (Scheme 3).
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functionalization of Nꢀterpenylated cyclic amines was
successfully performed with various terpenaldehydes without side
25 alkene reduction in the presence arene ruthenium(II) catalyst
containing
a chelating phosphine sulfonate. Starting from
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40 Procedure for the preparation of amines 4:
To a stirred solution of amine 3 (1.1 equiv.) in 1.5 mL of toluene
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