Iodine(III)-mediated halogenations of acyclic monoterpenoids

Article abstract of DOI:10.3762/bjoc.14.96

Five different halofunctionalizations of acyclic monoterpenoids were performed using a combination of a hypervalent iodine(III) reagent and a halide salt. In this manner, the dibromination, the bromo(trifluoro)acetoxylation, the bromohydroxylation, the iodo(trifluoro)acetoxylation or the ene-type chlorination of the distal trisubstituted double bond occurred with excellent selectivity and moderate to good yields.

Full text of DOI:10.3762/bjoc.14.96

Iodine(III)-mediated halogenations of acyclic monoterpenoids
Laure Peilleron, Tatyana D. Grayfer, Joëlle Dubois, Robert H. Dodd and Kevin Cariou*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 1103–1111.
Institut de Chimie des Substances Naturelles, CNRS UPR 2301,
Université Paris-Sud, Université Paris-Saclay, Avenue de la Terrasse,
91198 Gif-sur-Yvette, France
doi:10.3762/bjoc.14.96
Received: 06 February 2018
Accepted: 26 April 2018
Published: 18 May 2018
Email:
Kevin Cariou* - kevin.cariou@cnrs.fr
This article is part of the Thematic Series "Hypervalent iodine chemistry in
organic synthesis".
* Corresponding author
Keywords:
Guest Editor: T. Wirth
halogenation; hypervalent iodine; monoterpenes
© 2018 Peilleron et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Five different halofunctionalizations of acyclic monoterpenoids were performed using a combination of a hypervalent iodine(III)
reagent and a halide salt. In this manner, the dibromination, the bromo(trifluoro)acetoxylation, the bromohydroxylation, the
iodo(trifluoro)acetoxylation or the ene-type chlorination of the distal trisubstituted double bond occurred with excellent selectivity
and moderate to good yields.
Introduction
In nature, mostly in marine environments, halogenated com- This structural richness has fuelled the development of many
pounds are produced by means of various enzymes that rely on synthetic strategies that take inspiration from these enzymatic
widely available halides as feedstock [1]. These halogenases machineries. A key aspect is to be able to mildly oxidize the
can perform an extremely wide array of electrophilic halogena- halides into halenium equivalents in order to promote halogena-
tions on a myriad of substrates with exquisite chemo-, stereo- tions with increased selectivity. In this regard, hypervalent
and enantioselectivities that remain extremely challenging to iodine reagents [6] have emerged as particularly versatile medi-
rival for the chemist. When applied to terpenic feedstock, they ators [7-10]. We have shown that electrophilic halogenations
give birth to a variety of molecular scaffolds, from the mono- [11-13], or pseudohalogenations [14] can be triggered by com-
chlorinated linear chain of fallachromenoic acid [2], to the bining an iodine(III) derivative with a suitable halide salt. In
pentahalogenated halomon skeleton [3] and encompassing particular, the chemoselectivity of the reaction can be finely
many intricate polycyclic and macrocyclic structures with com- tuned by adjusting several parameters, such as the nature of the
plex vicinal oxygen/halogen patterns, such as bromophycolide halide as well as of the iodine(III) ligands and the halide coun-
B [4] and dichotellide B, which contains both iodine and chlo- terion [15,16]. In the case of polyprenoids, we mostly devoted
rine atoms (Figure 1) [5].
our efforts to achieve the bromocarbocyclization of aryl-geranyl
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
Figure 1: Halogenated terpenoids from natural sources.
derivatives using a combination of iodine(III) oxidant and a iodofunctionalizations of linear terpenoids. For this purpose,
bromide source. In this fashion, the reaction of homogeranyl- five monoterpene derivatives – bearing various functional
benzene with bis(tert-butylcarbonyloxy)iodobenzene and groups (protected and free alcohols, amine, E and Z olefins,
triethylsilyl bromide, followed by acidic treatment led to a diene moiety) aimed at probing potential chemo-, regio- and
tricyclic brominated adduct (Scheme 1, reaction 1). Yet in the stereoselectivity issues – were selected: geranyl acetate (1a),
course of our study we also showed that the reactivity of the key neryl acetate (1b), geraniol (1c), N-tosylgeranylamine (1d) and
bridged bromonium intermediate could also be steered towards myrcene (1e, Figure 2).
non-cyclizing vicinal difunctionalizations using slightly differ-
Results and Discussion
ent combinations of a (diacyloxyiodo)arene and a bromide salt.
Optimizations
Indeed, (diacetoxy)iodobenzene (DIB) and lithium bromide
yield a dibromo adduct (Scheme 1, reaction 2), whereas a
combination of (bis(trifluoroacetoxy)iodo)benzene (PIFA)
and tetra-n-butylammonium bromide (TBAB) gives
bromo(trifluoro)acetoxylated 3a (Scheme 1, reaction 3) [16].
In order to carry out the exploration of the various halogena-
tions that could be performed, geranyl acetate (1a) was chosen
as the model substrate. In our study on terpenoids [16], the reac-
tion conditions for the dibromination of the distal double bond
were easily established from our previous study on the bromina-
We then decided to further explore the synthetic potential of the tion of enamides [11]. Thus, using a slight excess of DIB along
latter two vicinal difunctionalizations of terpenoids by not only with a two-fold amount of lithium bromide at 0 °C in dry
expanding their substrate scope but also by trying to achieve acetonitrile rapidly yielded dibromo adduct 2a in 91% yield
other bromo-oxylations as well as analogous chloro- and/or (Table 1, entry 1). Switching the reaction conditions to
Scheme 1: Previously developed bromo-functionalizations of polyprenoids using iodine(III) reagents.
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Figure 2: Selected monoterpenoids used in this study.
Table 1: Optimization of the reactions conditions.
entry
R (x equiv)
MX (y equiv)
solvent
temp.
time
N°, Y, X (yield %)a
1
2
Ac (1.2)
C(O)CF3 (1.1)
Ac (1.2)
LiBr (2.4)
n-Bu4NBrb (1.2)
LiBr (2.4)
MeCN
MeCN
0 °C
0 °C
rt
5 min
15 minc
5 min
2a, Br, Br (91)
3a, OCOCF3, Br (77)
4a, OH, Br (59)d
4a, OH, Br (65)f
4a, OH, Br (70)
4a’, OEt, Br (68)g
5a, OCOCF3, I (70)
5a, OCOCF3, I (44)
5a, OCOCF3, I (60)j
5a, OCOCF3, I (63)
6a (45)
3
MeCN/H2O
MeCN/H2O
MeCN/H2O
EtOH
4
Ac (1.2)
LiBre (1.3)
−10 °C
−10 °C
−10 °C
0 °C
0 °C
0 °C
0 °C
rt
15 minc
15 minc
105 min
20 min
100 min
20 min
20 min
5 min
5
Ac (1.4)
LiBre (1.6)
6
Ac (1.2)
LiBr (1.2)
7
C(O)CF3 (1.5)h
C(O)CF3 (1.3)i
C(O)CF3 (1.5)
C(O)CF3 (1.5)
Ac (1.2)
KI (2.4)
MeCN
8
n-Bu4NIb (1.5)i
n-Bu4NIb (1.5)
n-Bu4NIb (1.5)
FeCl3 (0.8)
MeCN
9
MeCN
10
11
12
13
CH2Cl2
MeCN
C(O)CF3 (1.2)
C(O)CF3 (1.2)
n-Bu4NClb (1.5)
n-Bu4NClb (1.5)
MeCN
0 °C
0 °C
15 minc
15 minc
6a (36)
CH2Cl2
6a (85)
aIsolated yields; bslow addition of a 0.1 M solution of the TBA salt; c5 min of addition followed by 10 min of stirring; dalong with 6% of 2a;
eslow addition of a 0.1 M aqueous solution of LiBr; ffull conversion was not reached; galong with 25% of 2a; hslow addition of a 0.1 M solution of PIFA;
iinitially 1.1 equiv of PIFA and 1.2 equiv of TBAI, followed by 0.2 equiv of PIFA and 0.3 equiv of TBAI to reach completion; jalong with 20% of 1a.
bromo(trifluoro)acetoxylation requires the use of a PIFA/TBAB 1.3 equivalents. By doing so, both the selectivity and the yield
combination in a 1:1 ratio with slow addition of the latter to the of 4a were improved though full conversion was not attained
reaction mixture thereby preventing the formation of 2a. In this (Table 1, entry 4). Keeping the same procedure, complete reac-
fashion, bromo(trifluoro)acetoxy adduct 3a was obtained in tion was nevertheless achieved by slightly increasing the
77% yield (Table 1, entry 2). The reaction course can also be amount of each reagent thus efficiently giving bromohydroxy-
modified by changing the solvent. For instance, the complemen- lated adduct 4a in 70% yield (Table 1, entry 5). Since the use of
tary hydroxybromination reaction, giving 4a in 59% yield, was water as the sole solvent was not possible because of solubility
achievable if a mixture of acetonitrile and water was used as the issues, the reaction was attempted in ethanol. In this case the
solvent (Table 1, entry 3). In order to suppress the observed for- analogous bromoethoxylated adduct 4a’ could be isolated in
mation of minor amounts of 2a, the reaction temperature was 68% yield, albeit along with 25% of 2a (Table 1, entry 6).
lowered to −10 °C and the amount of lithium bromide, which Turning our attention to iodination, we first used the combina-
was added dropwise as an aqueous solution, was diminished to tion of PIFA and KI that had given the best results with enam-
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
ides [13]. Thus, iodo(trifluoro)acetoxylated adduct 5a was ob- sensitivity of the alcohol and the amine functions, both geraniol
tained in 70% yield (Table 1, entry 7). Interestingly, no traces of (1c) and protected geranylamine 1d gave the dibromo adduct 2c
the diiodo compound were observed even if the hypervalent [16] and 2d in good yields. Finally, the reaction was performed
iodine(III) reagent was slowly added. Transposing the previ- on myrcene (1e), providing 2e in 78% yield and leaving the
ously optimized bromo(trifluoro)acetoxylation conditions but diene moiety untouched.
using TBAI instead of KI did not improve the yield and reac-
tion times and excess reagents were required to reach comple- Bromo(trifluoro)acetoxylation
tion (Table 1, entry 8). Using directly 1.5 equivalents of both The success of the bromo(trifluoro)acetoxylation lies in the
PIFA and TBAI did not suffice to improve the yield, whether in reverse addition protocol whereby the tetrabutylammonium bro-
acetonitrile or in dichloromethane (Table 1, entries 9 and 10). mide is slowly added to the suspension of the terpenoid and
When chlorination was attempted with a combination of DIB PIFA at 0 °C. In this manner, good to excellent yields (57–84%)
and iron(III) chloride in a 1.2:0.8 ratio (i.e., a 1:1 OAc/Cl ratio were obtained for the formation of Z-derivative 3b, alcohol 3c
[12]) in acetonitrile, allylic chloride 6a [17] was obtained with a [16], tosylamine 3d and diene 3e (Scheme 3). Once again, the
moderate 45% yield (Table 1, entry 11). Switching to a combi- selectivity is excellent as no reactions with the other double
nation of PIFA and TBACl (Table 1, entry 12) did not change bonds or with the aryl ring [19] are observed and only one
the course of the reaction towards chloro(trifluoro)acetoxyla- regioisomer is formed in all cases.
tion and, while the yield was moderate in acetonitrile, it was
greatly improved in dichloromethane [18], affording 6a in 85% Bromohydroxylation
yield (Table 1, entry 13).
Although the trifluoroacetoxy group can be readily cleaved
[16], having a direct access to the corresponding bromohydrins
At this stage, we chose to discard the bromoethoxylation reac- would still be desirable especially since selective deprotection
tion which led to mixtures of compounds, leaving five opti- in the presence of another ester would presumably be difficult
mized transformations (dibromination, bromo(trifluoro)acetoxy- to achieve. By using water as the co-solvent, bromohydrins
lation, bromohydroxylation, iodo(trifluoro)acetoxylation and 4b–e were directly prepared from the corresponding starting
ene-type chlorination) which were applied to substrates 1b–e in material, albeit with moderate yields in the case of N-tosylger-
order to explore their scope.
anylamine (1d) and myrcene (1e, Scheme 4).
Bromination
Iodo(trifluoro)acetoxylation
Dibromination
In an analogous fashion to the above-described bromo-
Application of the dibromination protocol to neryl acetate (1b) (trifluoro)acetoxylation, the iodo(trifluoro)acetoxylation
proceeded smoothly and the desired compound 2b was ob- proceeded with both good yields (49–70%) and chemo-selectiv-
tained in nearly quantitative yield without any detectable reac- ities for geranyl and neryl acetates (1a,b), geranylamine 1d and
tion on the Z double bond (Scheme 2). Despite the potential myrcene (1e, Scheme 5). However, the adducts were quite
Scheme 2: Dibromination of acyclic monoterpenoids.
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
Scheme 3: Bromo(trifluoro)acetoxylation of acyclic monoterpenoids.
Scheme 4: Bromohydroxylation of acyclic monoterpenoids.
Scheme 5: Iodo(trifluoro)acetoxylation of acyclic monoterpenoids.
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
sensitive and needed to be manipulated with care (low tempera- (Scheme 7b). A reductive elimination would then generate
ture storage) to avoid rapid decomposition. Nevertheless, this hypohalite R’OX which is presumably the active electrophilic
was particularly the case for alcohol 5c which could only be species. If the reductive elimination only occurred after a
isolated with a 23% yield. This mild protocol offers a comple- second ligand exchange it would then give X2 which is general-
mentary alternative to the use of iodine monoacetate [20] or ly a less efficient electrophilic species than the corresponding
NIS [21] and does not require the use of a strong oxidant such acetoxyhypohalite, especially trifluoroacetoxyhypohalites [26].
as IBX [22]. Compared to the standard procedure for the prepa- Indeed, when we reacted geranyl acetate 1a with bromine we
ration of acetoxyhypohalites that requires the use of expensive did isolate dibromo adduct 2a but only with 47% yield and we
and potentially toxic silver salts [23], our method offers a more also observed the formation of various byproducts such as ene-
practical alternative. It also differs from the more user-friendly bromination adduct 6a’ (Scheme 7b) [27]. Moreover, for the
protocols that rely on Oxone® [20], DIB [24], or PIFA [25] iodination of enamides using the PIFA/KI combination we had
since the iodide source is an iodide salt and not molecular already shown that the observed reactivity is more akin to the
iodine.
one induced by acetoxyhypoiodite [13]. Alternatively, the
involvement of an ammonium-complexed halogen(I) species of
type n-Bu4N[X(O2CCF3)2], as described by Kirschning [28]
Chlorination
In the case of chlorination, we have yet to observe adducts and later Muniz [29] cannot be ruled out although, to the best of
arising from the vicinal difunctionalization of the double bond our knowledge, it has yet to be characterized for X = Cl.
and, in accordance with the literature [13], ene-type products 6 Regardless of the actual halogenation species involved, regiose-
were obtained (Scheme 6). The reaction performed equally well lective halogenation of the terminal double bond of 1 would
with acetates (6a,b), alcohol (6c), amine (6d) and myrcene (6e). then give bridged halonium 9. From there three manifolds can
Once again, excellent chemo- and regioselectivities and good be at play. Pathway a involves the addition of an oxygenated
yields (51–85%) were witnessed.
nucleophile. For X = Br, this is the case when the bromide is the
default reagent because of its slow addition (giving 3) or
because the nucleophile is the co-solvent (water giving 4 and to
a lesser extent EtOH yielding 4’). While the formation of 4 is
Mechanism proposal and control
experiments
Considering the differences and similarities in the outcome of related to the well-established NBS-mediated formation of
the various reaction conditions, a common mechanism with a bromohydrins in aqueous media [30], the formation of 3 is not
central divergence point can be proposed. First, the diace- so straightforward. For instance, submitting 1a to NBS in the
toxyiodobenzene reagent 7 would undergo ligand exchange presence of trifluoroacetic acid led to a complex mixture of
with the halide to give mixed hypervalent iodine(III) 8 products, the major one being allylic bromide 6a’. For X = I,
(Scheme 7a) [12]. We ruled out a possible direct reaction be- the steric bulk would preclude the formation of the vicinal diio-
tween the olefin and the hypervalent iodine(III) reagent as in the dide and only pathway a would thus be operative, yielding 5.
absence of any halide no reaction occurred within 1 h This is not the case for the dibromide 2, which is readily formed
Scheme 6: Chlorination of acyclic monoterpenoids.
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
Scheme 7: General mechanism proposal for the formation of 2–6 and control experiments.
through pathway b when the reaction takes place in the pres- were added. After stirring for 5 min, the reaction mixture was
ence of excess bromide. Finally, if X = Cl, rapid deprotonation diluted with EtOAc, filtered over alumina (EtOAc) and concen-
by the acetate (or the chloride) alpha to the chloronium bridge trated under reduced pressure before purification by flash chro-
(pathway c) explains the formation of allylic chloride 6.
matography.
General procedure B:
Overall, we have further extended the scope of the iodine(III)- bromo(trifluoro)acetoxylation
Conclusion
mediated oxidative halogenation of terpenoids which now To a solution of the geranyl derivative (1.0 equiv) in aceto-
includes dibromination, bromo(trifluoro)acetoxylation, bromo- nitrile (0.07 M) cooled to 0 °C, PIFA (1.1 equiv) was added. A
hydroxylation, iodo(trifluoro)acetoxylation and allylic ene- solution of tetra-n-butylammonium bromide (1.1 equiv) in
chlorination. The conditions are mild and selective and the reac- acetonitrile (0.07 M) was then added dropwise over 5 min.
tions proceed rapidly with generally good yields. Application of After stirring for 10 min, the reaction mixture was diluted with
this tunable strategy towards the rapid constitution of chemical EtOAc, saturated aqueous Na2S2O3 solution was added and the
libraries of biologically active molecules is now actively layers were separated. The aqueous layer was extracted twice
pursued in our laboratory.
with EtOAc. The combined organic extracts were washed with
brine, dried over MgSO4, filtered and concentrated under
reduced pressure before purification by flash chromatography.
Experimental
General procedure A: dibromination
To a solution of the geranyl derivative (1.0 equiv) in aceto- General procedure C: bromohydroxylation
nitrile (0.05 M) at 0 °C, lithium bromide (2.0–2.4 equiv), To a solution of the geranyl derivative (1.0 equiv) in aceto-
4 Å molecular sieves (1.0 mass equiv) and DIB (1.2–1.4 equiv) nitrile (0.1 M) cooled to −10 °C, DIB (1.4 equiv) was added. A
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Beilstein J. Org. Chem. 2018, 14, 1103–1111.
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ORCID® iDs
Tatyana D. Grayfer - https://orcid.org/0000-0003-2844-4861
Robert H. Dodd - https://orcid.org/0000-0001-6477-3609
Kevin Cariou - https://orcid.org/0000-0002-5854-9632
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doi:10.3762/bjoc.14.96
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