Carbon dioxide-based facile synthesis of cyclic carbamates from amino alcohols

Article abstract of DOI:10.1039/c8cc00636a

We report herein a straightforward general method for the synthesis of cyclic carbamates from amino alcohols and carbon dioxide in the presence of an external base and a hydroxyl group activating reagent. Utilizing p-toluenesulfonyl chloride (TsCl), the reaction proceeds under mild conditions, and the approach is fully applicable to the preparation of various high value-added 5- and 6-membered rings as well as bicyclic fused ring carbamates. DFT calculations and experimental results indicate a S_N2-type reaction mechanism with high regio-, chemo-, and stereoselectivity.

Full text of DOI:10.1039/c8cc00636a

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DOI: 10.1039/C8CC00636A
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Carbon dioxide-based facile synthesis of cyclic carbamates from
amino alcohols
Teemu Niemi,a Israel Fernández,*b Bethany Steadman,a Jere K. Mannisto,a and
Received 00th January 20xx,
Accepted 00th January 20xx
Timo Repo*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
between anilines, dihaloalkanes, and carbon dioxide.28 While
certainly more efficient than conventional non-CO2-based carbamate
syntheses, all these methods have one or more drawbacks, namely
the lack of stereoselectivity,29 toxicity, and commercial unavailability
of the starting materials, or the inability to easily access the
substitution patterns required for pharmaceuticals.
We report herein a straightforward general method for the
synthesis of cyclic carbamates from amino alcohols and carbon
dioxide in the presence of an external base and a hydroxyl group
activating reagent. Utilizing p-toluenesulfonyl chloride (TsCl), the
reaction proceeds under mild conditions, and the approach is fully
applicable to the preparation of various high value-added 5- and 6-
membered rings as well as bicyclic fused ring carbamates. DFT
calculations and experimental results indicate a SN2-type reaction
mechanism with high regio-, chemo-, and stereoselectivity.
Amino alcohols (AAs) are often seen as ideal nitrogen sources in
the synthesis of cyclic carbamates from CO2 due to their
inexpensiveness, nontoxicity, and ready accessibility. However, the
cyclization of AAs with carbon dioxide usually requires high
temperatures and pressures even when catalysts, dehydrating
agents, or electrochemical methods are employed.8, 30-40 As a result
of these harsh conditions, side reactions (such as a second
dehydration to yield the corresponding cyclic urea) are
commonplace.30, 32, 34, 35, 41 Furthermore, previous reports are mainly
limited to the synthesis of alkyl substituted oxazolidinones, and
additional functionalization is required to prepare high value-added
chemicals.
Cyclic carbamate core structures are present in a plethora of valuable
chemicals. In particular, 2-oxazolidinones are found in chiral
auxiliaries (with 4-substitution) and in superantibiotics such as
Linezolid and Tedizolid (with 3-aryl,5-alkyl substitution pattern),
whereas aryl-fused 6-membered rings are present in some HIV-
battling antiretrovirals and N-methyl-D-aspartate (NMDA) receptor
antagonists.1-5 These compounds are commonly synthesized by
utilizing hazardous or expensive reagents such as isocyanides and
phosgene.1, 4, 6, 7 As such, the replacement of these starting materials
with the relatively cheap and safer carbon dioxide offers a unique
opportunity to enhance the sustainability and greenness of the
synthesis of these high value-added chemicals.
Several strategies for CO2-based preparation of cyclic
carbamates have been reported, such as CO2’s cycloaddition to
aziridines, oxetanes, or amino epoxides,8-12 and the
transesterification of amines with cyclic organic carbonates, which
are formed in situ in the cycloaddition of carbon dioxide and
epoxides.13, 14 More recently, approaches utilizing the addition of CO2
to acyclic unsaturated compounds such as alkenes, alkynes, and
propargylic amines or alcohols, have garnered attention.15-26
Meanwhile, our group has studied the synthesis of cyclic carbamates
utilizing haloalkylamines,27 and the multicomponent reaction
Herein, we present the synthesis of cyclic carbamates from
carbon dioxide and amino alcohols. Utilizing p-toluenesulfonyl
chloride (TsCl) as an activating reagent, the reaction proceeds under
mild conditions with good yields and high enantiomeric excess, giving
2-oxazolidinones with the 3-aryl-5-alkyl substitution pattern desired
for pharmaceutical applications in a single step. This straightforward
method offers predictable regio-, chemo-, and stereoselectivity, and
the scope is expandable to the syntheses of 6- membered rings and
fused bicyclic compounds, making the process unprecedently
versatile. In addition, the reaction mechanism was elucidated by DFT
calculations.
Our one-pot approach involves the formation of a carbamate salt
from CO2 and AAs in the presence of an external base, followed by
the tosylation of the alcohol group to improve its leaving group
character, and the subsequent ring-closing via intramolecular
substitution (Figure 1). ROH-tosylation has been previously
employed in the synthesis of cyclic carbonates from 1,3-diols42; here,
however, our obvious challenge is the competing N-tosylation
reaction, which also prevents the formation of the carbamate moiety
and subsequently inhibits the cyclization reaction. Thus, we started
our study by finding the optimal reaction conditions in which the
a.T. Niemi, B. Steadman, J. Mannisto, Prof. T. Repo, Department of Chemistry, P.O.
Box 55, 00014 University of Helsinki (Finland), E-mail: timo.repo@helsinki.fi
b.Prof. I. Fernández, Departamento de Quimica Orgánica I and Centro de
Innovación en Química Avanzada (ORFEO-CINQA), Facultad de Ciencias Químicas,
Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid (Spain),
E-mail: israel@quim.ucm.es
Electronic Supplementary Information (ESI) available: Experimental and
computational details, spectroscopic data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Table 1. Reaction condition optimization for maximum selectivity towards 2a.
formation of the N-tosylated product is minimized (Table 1). Using
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DOI: 10.1039/C8CC00636A
the simple, inexpensive and commercially available 2-
(phenylamino)ethan-1-ol 1a as
a model substrate, we first
investigated the reaction conditions we had previously employed
successfully in the synthesis of N-aryl-2-oxazolidinones from anilines
and dibromoalkanes (Table 1, Entry 1)28. However, N-tosylation is
heavily favored under these conditions, as the carbamate species is
too labile to sufficiently protect the nitrogen from a reaction with
TsCl. Indeed, decreasing the temperature and increasing the CO2
pressure to shift the equilibrium towards carbamate formation
(Table 1, Entry 2) leads to a clear increase in selectivity. Lowering the
temperature or raising the pressure any further gives no significant
improvement, so the rest of the experiments were conducted at RT
and 5 bar CO2.
p
(bar)
T
(°C)
Conversion
(%)a
Selectivity
(%)b
Entry
Solvent
MeCN
MeCN
EtOH
Base
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Et3N
1
2
1
5
5
5
5
5
5
5
5
5
60
RT
RT
RT
RT
RT
RT
RT
RT
RT
72
41
25
5
16
65
3
92
4
DCM
70
5
DMF
10
50
18
17
12
27
100
100
47
6
acetone
acetone
acetone
acetone
acetone
7
8
TMG[c]
KOH
100
14
9
10
K2CO3
40
[a] Based on total GC yield of 2a and 3a with mesitylene as external standard.
[b] Defined as Yield(2a)/Yield(2a+3a).
[c] TMG = 1,1,3,3-tetramethylguanidine.
Figure 1. Tosyl-assisted cyclization of carbon dioxide and amino alcohols in the
presence of an external base. The carbamate species must be formed first and
stabilized to avoid the competing N-tosylation reaction, which prevents the
formation of the desired product.
With amino alcohols where the OH group is primary, the reaction
stops at 50% very likely due to the formation of a salt between the
ionic byproducts, the remaining starting material, and possibly
carbon dioxide. Although the exact composition of this salt was not
elucidated, the remaining amino alcohol can be quantitatively
recovered upon dissolving the solid reaction phase into H2O,
indicating that a reversible salt-forming reaction indeed takes place.
Attempts to overcome this plateau by increasing base loading or CO2
pressure were not successful.
Next, we screened different combinations of solvents and bases.
Polar protic solvents (e.g. Table 1, Entry 3) react with TsCl and are
thus not suitable for this reaction, while nonpolar solvents are not
capable of solvating all the reagents and, as a result, generally give
poor conversions (e.g. Table 1, Entry 4). Polar aprotic solvents were
found to offer the highest selectivity towards 2a, with acetone also
significantly enhancing the total conversion (Table 1, Entry 6).
As we turned our attention to other substrates (Figure 2), we
discovered that amino alcohols with secondary hydroxyl groups are
more reactive, as 2b was isolated in 83% yield, which is only slightly
lower than what is achieved with isocyanate-based methods.43, 44
Tertiary hydroxyl groups, meanwhile, are sterically too hindered to
readily undergo cyclization, and a significant decrease in yield is
observed for 2c. In the synthesis of carbamate 6-membered rings
from the corresponding amino propanols, a similar trend can be
seen: the formation of 6-ring 4a from the commercially available 3-
(phenylamino)propan-1-ol stops at 50%, while a more substituted
amino alcohol yields 57% of 4b. Attempts to prepare larger 7- or 8-
membered rings were not successful.
Among the Brønstedt bases screened, Cs2CO3 proved to be the
most efficient. Et3N (Table 1, Entry 7), which is commonly used in
sulfonation reactions shows here low reactivity and poor selectivity.
Like protic solvents, even slightly nucleophilic bases such as 1,1,3,3-
tetramethylguanidine (TMG), and KOH (Table 1, Entries 8-9) favor
the reaction with the sulfonating agent, therefore leading to low
conversion of 1a. Finally, other carbonate bases (Table 1, Entry 10)
are inferior to Cs2CO3 due to their lower solubility in the reaction
medium. Thus, the highest conversion accompanied with good
selectivity is achieved when the reaction is carried out in acetone
under 5 bar CO2 at room temperature, with Cs2CO3 as the base (Table
1, entry 6).
Aromatic ring substituents affect the reaction outcome, as well.
The presence of a weakly activating alkyl group leads to the slightly
enhanced yield observed for 2d, whereas the more strongly
activating alkoxy moiety affords quantitative conversion at the cost
of selectivity, and 2e and the corresponding N-tosylated amino
alcohol are isolated in a ca. 1:1 ratio. This marks the only occasion in
which any N-tosylation was detected under the optimized reaction
2 | J. Name., 2012, 00, 1-3
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conditions. A deactivating halogen substituent, meanwhile, leads to As we envision our method applicable to the ViseywnAtrhticelesiOsnlionef
DOI: 10.1039/C8CC00636A
a decrease in conversion, but full selectivity is maintained in the pharmaceutical compounds and chiral auxiliaries, predictable
synthesis of 2f.
stereochemistry is of utmost importance. To our delight, the reaction
also proceeds with high stereoselectivity, and >99% ee is observed
for most substrates. In fact, only the amino diol 1l undergoes some
racemization, but the corresponding oxazolidinone 2j is still isolated
in a good yield and 90% ee. The products have inverted configuration
at the stereocenter, which hints at a SN2-type mechanism during the
ring-closing step. To shed some further light on the involved reaction
mechanism, the transformation was next studied in silico.
The computed reaction profile for the reaction of the parent
amino alcohol 1a and CO2 in the presence of mesylchloride (MsCl),
as a model of the experimentally used TsCl, is shown in Figure 4,
Figure 2. Conversions of various amino alcohols and the isolated yields (in
parentheses) of the corresponding cyclic carbamates. Reaction conditions: Amino
alcohol 1 (0.5 mmol), TsCl (1.1 equiv.), Cs2CO3 (3 equiv.), CO2 (5 bar), acetone
(5 mL), room temperature, 20 h.
which gathers the corresponding relative Gibbs free energies (∆G298
at 298 K) in acetone computed at the PCM(acetone)-M062X/def2-
TZVPP//PCM(acetone)-M062X/6-31+G(d) Our DFT
level.46
,
calculations indicate that the process begins with the formation of
zwitterionic intermediate INT1 via transition state TS1, a saddle point
associated with the nucleophilic attack of the amino group of 1a to
the electrophilic CO2. The alternative reaction of 1a with the
activating agent MsCl, leading to INT1’ via TS1’, is not competitive in
view of the much higher activation barrier (∆∆G≠ = 12.8 kcal/mol) and
endergonicity (∆∆GR = 11.4 kcal/mol) associated with this process,
which is fully consistent with the complete regioselectivity observed
for the transformation (see above). Although the formation of INT1
is endergonic (∆GR = 17.3 kcal/mol), the subsequent base-mediated
deprotonation of this species is highly exergonic (∆GR = –46.2
kcal/mol) and drives the transformation forward. This step leads to
the formation of the highly stabilized anionic intermediate INT2,
which readily reacts with MsCl to produce, in an exergonic process
(∆GR = –3.2 kcal/mol), the corresponding mesyl derivative INT3. This
step occurs via TS2 (activation barrier of 21.4 kcal/mol) in a typical
SN2 reaction thus releasing HCl, which is then neutralized by the
excess of base. The transformation ends up with the formation of the
experimentally observed carbamate 2a via TS3 in a highly exergonic
process (∆GR = –30.1 kcal/mol) with a reaction barrier of 18.1
kcal/mol (fully compatible with a process occurring at room
temperature). This saddle point is associated with the intermolecular
synchronous displacement of the OMs leaving group by the
nucleophilic (O=)CO– moeity of INT3, thus forming the new C–O bond
of the carbamate. According to the geometry of TS3, this final ring
closure can be viewed as an intramolecular backside SN2 reaction,
which nicely agrees with the inversion of configuration observed in
the processes involving the chiral substrates 1k-p (see Figure 3).
The reaction can also be extended to the synthesis of fused rings
5 from 2-aminobenzyl alcohols with yields up to 80%. Like N-aryl-2-
oxazolidinones,
these
structures
are
omnipresent
in
pharmaceuticals, and can be found, for instance, in Efavirenz, an
antiretroviral used to treat HIV,4 and in NMDA receptor antagonists,
useful for treating stroke, cerebral ischemia, and depression.5 To our
knowledge, the present work represents the first example of a CO2-
based synthesis of these compounds, and is in fact as efficient as the
phosgene-based method.45
To further establish the generality and applicability of this
methodology in synthesis, we probed the chemo- and
stereoselectivity of the reaction by screening difunctionalized and
enantiopure amino alcohols as starting materials (Scheme 3). We
have previously shown that the cyclization of carbon dioxide and 2,3-
dichloropropan-1-amine strongly favors the formation of the more
stable 5-ring (81% 5-ring, 17% 6-ring):27 1i, on the other hand, gives
a mixture of the two possible products in a ca. 5:6 ratio, as the
thermodynamic stability of the 5-ring is offset by the better leaving
group nature of the –Cl. The easily synthesizable amino diol 1j,
meanwhile, expectedly shows preference towards the formation of
the 5-ring 2h, which can be isolated in 85% yield, and intriguingly
with a higher selectivity towards the 5-ring than when the
dihaloalkylamine was employed.
In conclusion, we have developed a facile and general method
for the stereoselective synthesis of cyclic carbamates from carbon
dioxide and amino alcohols. The reaction is applicable to the
preparation of 3-aryl,5-alkyl substituted 2-oxazolidinones found in
Linezolid and other superantibiotics, cyclic fused rings found in
antiretrovirals and NMDA receptor antagonists, as well as 6-
membered rings. Under the optimized reaction conditions, the
process has high regio-, chemo-, and stereoselectivity. Enantiopure
amino alcohols undergo inversion of configuration at the
stereocenter, which hints at a SN2-type reaction mechanism during
the key ring-closing step; this observation was later supported by DFT
calculations.
Figure 3. Chemo- and stereoselectivity of the reaction. All yields are isolated
yields. Reaction conditions: Amino alcohol 1 (0.5 mmol), TsCl (1.1 equiv.),
Cs2CO3 (3 equiv.), CO2 (5 bar), acetone (5 mL), room temperature, 20 h.
This journal is © The Royal Society of Chemistry 20xx
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the PCM(acetone)-M062X/def2-TZVPP//PCM(acetone)-M062X/6-31+G(d) level.
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