Boron difluoride complexes of carbamoyl Meldrum's acids

Article abstract of DOI:10.1016/j.jfluchem.2012.07.004

5-[Hydroxy(aryl/alkylamino)methylene]-2,2-dimethyl-1,3-dioxane-4,6-diones react with BF₃·Et₂O in mild conditions leading to the formation of boron difluoride complexes of carbamoyl Meldrum's acids. The X-ray structure has been obtain

Full text of DOI:10.1016/j.jfluchem.2012.07.004

Journal of Fluorine Chemistry 144 (2012) 65–68
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Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Boron difluoride complexes of carbamoyl Meldrum’s acids
Natalia Pawelska a,1, Łukasz Ponikiewski b,1, Sławomir Makowiec a,
*
a Department of Organic Chemistry, Faculty of Chemistry, Gdan´sk University of Technology, Narutowicza 11/12, 80-233 Gdan´sk, Poland
b Department of Inorganic Chemistry, Faculty of Chemistry, Gdan´sk University of Technology, Narutowicza 11/12, 80-233 Gdan´sk, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
5-[Hydroxy(aryl/alkylamino)methylene]-2,2-dimethyl-1,3-dioxane-4,6-diones react with BF3ÁEt2O in
mild conditions leading to the formation of boron difluoride complexes of carbamoyl Meldrum’s acids.
The X-ray structure has been obtained for one representative complex. The obtained new compounds are
air and moisture stable at standard ambient conditions and easily isolable.
Received 30 March 2012
Received in revised form 9 July 2012
Accepted 10 July 2012
Available online 7 August 2012
ß 2012 Elsevier B.V. All rights reserved.
Keywords:
Meldrum’s acid
Lewis acids
Boron
Complexes
1. Introduction
importantly, during our previous researches we observed that
these ketenes can undergo cycloaddition to aldimines to form 3-
Acyl derivatives of Meldrum acids have a broad scope of
applications in organic synthesis [1]. Their use in organic synthesis
is mainly due to the ability to create ketenes in the course of
thermal decomposition [2–4]. These ketenes as strongly acylating
agents can react with a wide range of nucleophiles and as a result
form various useful compounds such as, for example: 3-
carbamoyl-b-lactams in the modified Staudinger reaction. The
initial success in the field of synthesis of -lactams encouraged us
to try to develop a method of synthesis 4-unsubstituted-3-
carbamoyl- -lactams. Preparation 4-unsubstituted- -lactams in
the typical or modified Staudinger reaction carries difficulties
associated with unstability of monomeric formaldehyde aldimines
or necessity to use surogates of formyl aldimines. In order to obtain
b
b
b
substituted-b-lactams [5,6] isooxazolols [7], pilicides [8], and
derivatives of tetramic acid [9]. However, the formation of ketenes
is not the only useful reaction of derivatives of Meldrum’s acids.
The addition of metaloorganic species or reduction of the
conjugated double bond [10,11], also may take place. The least
explored area is action of the Meldrum’s acid derivatives as the
nucleophilic reagents, only in the case of thiocarbamoyl Meldrum’s
acid the appropriate anion was used as nucleophilic agent [12]
whereas in the case of the oxygene analog none example could be
find.
4-unsubstituted-b-lactams we performed several unsuccessful
experiments with ketenes generated from 1 and surogates of
formaldehyde aldimines described in literature, such as: dithio-
carbamates [15], formaldehyde N,N-dialkylhydrazones [16], and
glyoxal imines [17]. Eventually we ran a reaction of 1 equiv. of 1 in
boiling toluene with 2 equiv. of N-methylene-tert-butylamine as a
one of most stable formaldehyde aldimines. To ensure depolimer-
isation of imine we added 6 equiv. of boron trifluoride etherate as a
well known agent for depolimerisation of hexahydro-1,3,5-
triazines [18]. From the reaction mixture, beside a large amount
of tar, we isolated a small amount of a new compound with the 1H
NMR spectra almost identical with that of the starting material
except for the lack of acidic proton; on the other hand TLC
chromatography also showed that the new compound is not as
acidic as 5-[hydroxy(aryl/alkylamino)methylene]-2,2-dimethyl-
1,3-dioxane-4,6-diones. A similar experiment conducted with
HCl or SnCl4 instead of BF3ÁEt2O did not result in the formation of
the new compound. Moreover, other experiments carried out
between 1 and tripyrolidine in the presence of BF3ÁEt2O in boiling
toluene also demonstrated the formation of the same new
compound (entry 1, Table 1). These aforementioned experiments
strongly indicate that 1 and BF3ÁEt2O are necessary for formation of
2. Results and discussion
Recently we focused on the reactivity of carbamoyl ketenes
generated from 5-[hydroxy(aryl/alkylamino)methylene]-2,2-di-
methyl-1,3-dioxane-4,6-diones 1. Ketenes formed during thermal
decomposition of 1 can acylate amines [13], alcohols and thiols
[14] leading to the formation of malonamide derivatives, but, more
* Corresponding author. Fax: +48 58 3472694.
E-mail address: mak@pg.gda.pl (S. Makowiec).
1
Fax: +48 58 3472694.
0022-1139/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2012.07.004

66
N. Pawelska et al. / Journal of Fluorine Chemistry 144 (2012) 65–68
Table 1
starting materials and leaving all other parameters of the reaction
unchanged. After the disappearance of the starting material, which
takes 3.5 h from the reaction mixtures, we isolated anhydrides 2a
and 2b with high yield (entries 2 and 6, Table 1). We also ran the
reaction of 1a with 6 equiv. of BF3ÁEt2O in boiling DCE; however, as
in the case of toluene we observed the formation of a large amount
of byproducts.
Synthesis of boron difluoride complexes of carbamoyl Meldrum’s acids.
In subsequent studies we carried out a two series of reactions:
first where 1a–f were heated in boiling DCM only in the presence of
6 equiv. of BF3ÁEt2O (entries 3, 7, 9, 11, 13 and 15, Table 1) and the
second series where 1a–f were heated in boiling DCM with 6 equiv.
of BF3ÁEt2O in the presence of 2 equiv. of triethyl amine (entries 8,
10, 12, 14 and 16, Table 1). In the case of the reactions carried out in
the presence of tertiary amine yields of the 2 were even two to four
fold higher that in the reaction without base. This fact clearly
demonstrates that formation of anionic form of the carbamoyl
Meldrum’s acid strongly facilitates ligand substitution on the
boron atom. Additionally we run the reaction of 1a with in the
strongly acidic solution of HBF4 in diethyl ether but in this case we
observed formation only a small amount of 2a (entries 4 and 5,
Table 1).
The last aspect we decided to test was the problem if other acyl
derivative of Meldrum’s acid are also able to react in such a way
and form chelates with boron trifluoride. We ran two reactions of
5-[hydroxy(phenyl)methylene]-2,2-dimethyl-1,3-dioxane-4,6-
diones with 6 equiv. of BF3ÁEt2O, first was performed in boiling
DCM and the second at room temperature.
In both cases the reaction led to consumption of the starting
material what took respectively 5 and 24 h, and in both cases we
isolated only acetophenone as the main product. We already
observed that decomposition to ketene catalysed by acid is faster
for usual acyl Meldrum’s acids than in the case of carbamoyl
derivatives; however it is surprising that even at room tempera-
ture this reaction path is overwhelming.
From the other hand, selective ability of carbamoyl Meldrum’s
acids for formation of difluoroboron complexes might be explained
taking into account the presence of nitrogen atom with the lone
electron pair at the end of the conjugated system which is capable
for stabilizing such a complex.
.
Entry
1,2
R
Time (h)
Yield (%)
1a
2b
a
a
a
a
a
b
b
b
c
c
d
d
e
e
f
Ph
4
27
62
60
15
15
71
45
66
43
56
36
79
48
63
14
55
Ph
5
3
4c
5d
6b
Ph
3
Ph
4
Ph
3.5
5
3-ClC6H4
3-ClC6H4
3-ClC6H4
Et
7
8e
3.5
4.5
3
9
10e
11
12e
13
14e
15
16e
Et
3
Cyclo-hexyl
Cyclo-hexyl
Naphtyl
Naphtyl
4-NO2C6H4
4-NO2C6H4
2.5
2.5
5
5
4
f
4.5
a
0.33 equiv. of tripyrolidine was used, and toluene as a solvent.
2 equiv. of N-methylene-tert-butylamine was used.
3 equiv. of HBF4ÁEt2O was used.
b
c
d
e
6 equiv. of HBF4ÁEt2O was used.
2 equiv. of triethylamine was used.
the new compound. Structure elucidation with 1H, 13C and
elemental analysis strongly suggested that the new formed
compound might be a 5-[difluoroboroxy(phenylamino)methy-
lene]-2,2-dimethyl-1,3-dioxane-4,6-dione 2a. Which should be
considered as a carbamoyl Meldrum’s acid difluoroboric acid
mixed anhydride.
Fortunately 2a gave crystals suitable for X-ray crystallography.
X-ray date has confirmed our previous suppositions, the acidic
proton of the 2a is replaced with BF2 moiety which also coordinates
to the adjacent carbonyl oxygen (Fig. 1).
To the best of our knowledge such a derivative of Meldrum’s
acids was not described up to now in chemical literature.
At this point we decided to check whether it is possible to
increase the yield of 2 and whether the observed process is a
general phenomenon or limited only to this model of reactants. At
the beginning we decreased the temperature of the process as the
observed reaction should not require as a high temperature as
decomposition of Meldrum’s acid derivative to ketene. We also
anticipate that in boiling toluene part of the initially formed 2 may
subsequently decompose to an ketene. Therefore, we performed
two experiments in boiling dichloromethane using 1a and 1b as
In summary, the obtained difluoroboron complexes of carba-
moyl Meldrum’s can be considered as far analogies to difluor-
oboron complexes of 1,4-dihydro-4-oxoquinoline-3-carboxylic
acid [19].
3. Conclusion
A new reaction of 5-[hydroxy(aryl/alkylamino)methylene]-2,2-
dimethyl-1,3-dioxane-4,6-diones 1 with BF3ÁEt2O in the presence
of tertiary amine is reported. We obtain carbamoyl Meldrum’s acid
difluoroboric acid mixed anhydrides in mild conditions with high
yield. The new product 2 is easily isolable and stable at room
temperature.
4. Experimental
4.1. General experimental procedures
Reagents were purchased from Sigma–Aldrich. Toluene were
distilled from potassium under argon. Dichloromethane and 1,2-
dichloroethane was distilled from K2CO3 Analytical TLC was
performed on aluminium sheets of silica gel UV-254 Merck. Flash
chromatography was performed using 40–63
mm of Zeochem
silica gel and with aluminium oxide neutral Grade I POCH. The 1H,
13C were recorded on Varian Gemini 200 and Varian Unity Plus
500, chemical shifts (
d) in ppm rel. to internal Me4Si; coupling
Fig. 1. X-ray crystal structure representation of the complex 2a [22].
constants J in Hz. Elemental analysis were performed on Vario El

N. Pawelska et al. / Journal of Fluorine Chemistry 144 (2012) 65–68
67
Cube CHNS Elementar. High-resolution (HRMS) was recorded on
MicroMas Quattro LCT mass spectrometer. Melting points were
determined with Warsztat Elektromechaniczny W-wa apparatus
and are not corrected. Commercially unavailable reagents were
prepared using literature procedures as follows: 5-[hydroxy(phe-
nylamino)methylene]-2,2-dimethyl-1,3-dioxane-4,6-dione (1a)
[20], 5-{[hydroxy(3-chlorophenylamino)methylene]-2,2-dimeth-
yl-1,3-dioxane-4,6-dione (1b) [6], 5-[Hydroxy(ethylamino)methy-
5-[Difluoroboroxy(ethylamino)methylene]-2,2-dimethyl-1,3-
dioxane-4,6-dione (2c). Purification by flash column chromatog-
raphy on neutral aluminium oxide (EtOAc-toluene, 1:5); mp 145–
147 8C. 1H NMR (500 MHz, CDCl3):
d
= 1.31 (t, J = 7.3 Hz, 6 H), 1.81
(s, 12 H), 3.55–3.61 (m, 4 H), 9.13 (brs, 2 H). 13C NMR (50 MHz,
CDCl3):
= 14.6, 26.4, 36.7, 74.8, 109.2, 161.9, 165.9, 171.0. 19F
NMR (470 MHz, CDCl3)
d
d
= À144.62. Anal. Calcd for C9H12BF2NO5:
C, 41.10; H, 4.60; N, 5.33; Found: C, 39.67; H, 5.26; N, 5.73.
5-[Difluoroboroxy(cyclohexylamino)methylene]-2,2-dimeth-
yl-1,3-dioxane-4,6-dione (2d). Purification by flash column
chromatography on neutral aluminium oxide (EtOAc-toluene,
lene]-2,2-dimethyl-1,3-dioxane-4,6-dione
(1c)
[6],
5-
[Hydroxy(cyclohexylamino)methylene]-2,2-dimethyl-1,3-diox-
ane-4,6-dione (1d) [6], N-methylene-tert-butylamine [21].
1:5); mp 137–140 8C. 1H NMR (200 MHz, CDCl3):
12 H), 1.62–1.75 (m, 4 H), 1.81 (s, 12 H), 1.96–2,01 (m, 4 H), 3.91–
4.09 (m, 2 H), 9.12 (brs, 2 H). 13C NMR (50 MHz, CDCl3):
= 24.7,
25.5, 26.4, 32.7, 51.0, 74.3, 109.2, 161.9, 164.5, 171.0. 19F NMR
(470 MHz, CDCl3)
49.24; H, 5.72; N, 4.42; Found: C, 49.37; H, 5.84; N, 4.66.
5-[Difluoroboroxy(naphtylamino)methylene]-2,2-dimethyl-
1,3-dioxane-4,6-dione (2e). Purification by flash column chro-
matography on neutral aluminium oxide (EtOAc-toluene, 1:5);
d = 1.34–1.42 (m,
4.2. Syntheses of carbamoylo Meldrums’s acids (1e–f)
d
5-[Hydroxy(1-naphthylamino)methylene]-2,2-dimethyl-1,3-
dioxane-4,6-dione (1e). Following the typical literature proce-
dure [6,20] for 1a–b using Meldrum’s acid (0.72 g, 5 mmol)
anhyd. DMF (5 ml) Et3N (1.4 ml, 10 mmol); naphtylisocyanate
(0.845 g, 5 mmol,) yield 1.345 g (86%); Mp 119–120 8C. 1H NMR
d
= À144.67. Anal. Calcd for C13H18BF2NO5: C,
(200 MHz, CDCl3):
8.00 (m, 4 H), 11.60 (brs, 1 H), 13.62 (brs, 1 H). 13C NMR
(50 MHz, CDCl3): = 26.9, 74.3, 105.7, 121.7, 122.2, 125.8, 127.2,
d = 1.82 (s, 6 H), 7.48–7.62 (m, 3 H), 7.63–
mp 248–251 8C. 1H NMR (200 MHz, CDCl3):
7.49–7.60 (m, 6 H), 7.63–7.95 (m, 6 H), 11.43 (brs, 2 H). 13C NMR
(50 MHz, CDCl3): = 26.5, 75.3, 109.8, 121.0, 122.9, 125.9, 127.2,
127.4, 128.2, 128.9, 129.1, 129.4, 134.4, 162.5, 165.5, 171.9. 19F
NMR (470 MHz, CDCl3)
= À143.77. Anal. Calcd for
17H14BF2NO5: C, 56.54; H, 3.91; N, 3.88; Found: C, 56.55; H,
3.96; N, 3.92.
d = 1.90 (s, 12 H),
d
127.7, 128.2, 128.7, 129.2, 130.4, 134.6, 165.1, 168.4, 171.2.
HRMS (ESI-): m/z [MÀH] calcd for C13H11N2O7: 307.0566; found:
307.0563.
5-[Hydroxy(4-nitrophenylamino)methylene]-2,2-dimethyl-
1,3-dioxane-4,6-dione (1f). Following the typical literature proce-
dure [6,20] for 1a–b using Meldrum’s acid (0.72 g, 5 mmol) anhyd.
DMF (5 ml) Et3N (1.4 ml, 10 mmol); p-nitro-phenylisocyanate
(0.820 g, 5 mmol,) yield 1.243 g (80%); crystallized from CH2Cl2,
d
d
C
5-[Difluoroboroxy(4-nitrophenylamino)methylene]-2,2-di-
methyl-1,3-dioxane-4,6-dione (2f). Purification by flash column
chromatography on neutral aluminium oxide (EtOAc-toluene,
mp 210–215 8C dec. 1H NMR (500 MHz, CDCl3):
7.67 (d, J = 9.1 Hz, 2 H), 8.27 (d, J = 9.1 Hz, 2 H), 11.45 (brs, 1 H),
16.50 (brs, 1 H). 13C NMR (125 MHz, CDCl3):
= 26.5, 74.9, 106.2,
d
= 1.78 (s, 6 H),
1:7); mp 154–156 8C. 1H NMR (200 MHz, CDCl3):
d
1.89 (s, 12 H),
7.69–7.74 (d, J = 9.1 Hz, 4 H), 8.30–8.35 (d, J = 9.1 Hz, 4 H), 11.32
(brs, 2 H). 13C NMR (50 MHz, acetone-d6):
26.1, 76.3, 111.1, 125.3,
126.1, 141.0, 147.4, 162.2, 166.8, 173.0. 19F NMR (470 MHz, CDCl3)
d
d
121.8, 125.3, 141.1, 145.2, 164.3, 170.0, 171.6. HRMS (ESI-): m/z
[MÀH] calcd for C17H14NO5: 312.0872; found: 312.0877.
d
= À142.77. Anal. Calcd for C13H11BF2N2O7: C, 43.85; H, 3.11; N,
7.87; Found: C, 44.15; H, 3.07; N, 8.16.
4.3. Syntheses of 5-[Difluoroboroxy(aryl/alkylamino)methylene]-2,2-
dimethyl-1,3-dioxane-4,6-diones (2a–f)
Acknowledgment
4.3.1. General procedure
Scientific work financed from funds for science in 2010–2011 as
a research project N N204 088338.
To a solution of 1 (1 mmol) in anhd. CH2Cl2 15 ml, 6 mmol
BF3ÁEt2O was added, triethyl amine or N-methylene-tert-butyla-
mine (2 mmol) was added if specified in Table 1. The resulting
mixture was stirred and heated to reflux for the time specified in
Table 1. After disappearance of starting material, reaction mixture
was washed with sat. aq NaHCO3 (5 ml) and if amine was added
with 2 M aq HCl (5 ml). The organic solution was dried with
MgSO4, filtered and solvents was removed under reduced pressure,
and the residue was purified as follow:
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b = 5.547(8),
c = 16.278(3) A,
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3
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