1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU): A highly efficient catalyst in glycerol carbonate synthesis

Article abstract of DOI:10.1016/j.molcata.2014.04.016

Transesterification of dimethyl carbonate (DMC) with glycerol (GLY) was investigated using various amines as catalysts. Amidines like 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) were found to be the best catalysts for this reaction. Best results: 98% conversion of GLY with 96% selectivity to GC (TON: 9408), were obtained with DBU as a catalyst. Effect of various reaction conditions on activity and selectivity were investigated using DBU as catalyst. The mechanism of the reaction was investigated with the help of ¹H, ¹³C and ¹⁵N NMR analysis and DFT calculations.

Full text of DOI:10.1016/j.molcata.2014.04.016

Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU): A highly efficient catalyst
in glycerol carbonate synthesis
Mudassir K. Munshi, Swapna M. Gade, Manoj V. Mane, Deepti Mishra, Sourav Pal,
Kumar Vanka∗, Vilas H. Rane, Ashutosh A. Kelkar∗
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Transesterification of dimethyl carbonate (DMC) with glycerol (GLY) was investigated using
various amines as catalysts. Amidines like 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-
diazabicyclo[4.3.0]non-5-ene (DBN) were found to be the best catalysts for this reaction. Best results:
98% conversion of GLY with 96% selectivity to GC (TON: 9408), were obtained with DBU as a catalyst.
Effect of various reaction conditions on activity and selectivity were investigated using DBU as catalyst.
The mechanism of the reaction was investigated with the help of 1H, 13C and 15N NMR analysis and DFT
calculations.
Received 1 April 2014
Accepted 12 April 2014
Available online 24 April 2014
Keywords:
Transesterification
Glycerol
Glycerol carbonate
DBU
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
20–200). Therefore, the development of efficient catalysts for the
conversion of GLY to GC is still a challenge. Herein, we report our
Glycerol (GLY) is one of the renewable resources that is pro-
duced as a waste chemical during the production of fatty acids,
biofuels and biolubricants in quantities greater than the current
demand. Abundant availability of GLY as a waste at low cost has
drawn much attention of academia and industries for its valoriza-
tion [1]. Glycerol carbonate (GC) is one of the products that can be
obtained from GLY. It is a relatively new chemical in the class of
cyclic carbonates, with a wide range of applications in the field
of polymer, fine chemical and pharmaceutical industries. Until
recently, it was produced as a specialty chemical by the stoichio-
metric reaction between GLY and phosgene [1]. However, because
of the toxic properties of phosgene, a significant amount of work
is being carried out on the development of safer routes and is
still being optimized in order to compete with the conventional
methodology [1]. To date, the existing state of the art confirms that
the pathway based on transesterification of dimethyl carbonate
(DMC) with GLY is one of the most popular routes for GC synthesis
and has the potential to replace the conventional route [2]. Vari-
ous kinds of organic and inorganic base catalysts [2] such as metal
oxides, mixed metal oxides [3–5], hydrotalcites [6], metal com-
plexes [7], triethyl amine [8], ionic liquids [9–11] and enzymes [12]
have been used to synthesize GC by this route. However, these cat-
alyst systems suffer from lower activity (Turnover numbers, TONs,
results on the use of amidines as highly efficient catalysts with
very high TONs for GC synthesis. In addition, the reaction mech-
anism is investigated in detail on the basis of 1H, 13C and 15N NMR
spectroscopies, and density functional theory (DFT) calculations.
Moreover, DFT calculations have been carried out to estimate the
gas phase basicity of all the nucleophilic bases used as the catalyst.
2. Experimental
2.1. Materials
Glycerol, Glycidol, glycerol carbonate, 4-dimethylaminopyridi-
ne (DMAP), 1-N,N-dimethylpyrrole, 1,4-diazabicyclo[2.2.2]octane
(DABCO), ethyl amine, butyl amine, hexyl amine, heptyl amine,
octyl amine, nonyl amine, dodecyl amine, diethyl amine, triethyl
amine, tributyl amine, trioctyl amine 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU), 1,5-diazabicyclo[5.4.0]nonene (DBN) were purchased
from Sigma Aldrich. Dimethyl carbonate, dimethyl formamide
and nitromethane were purchased from Spectrochem, India, and
methanol was purchased from Merck, India. All the chemicals were
used as received from suppliers.
2.2. Glycerol carbonate synthesis
The transesterification of GLY was carried out in a 50 ml round
bottom flask equipped with a reflux condenser under vigorous stir-
ring. In a typical run, 0.021 mmol (0.1 mol%) of catalyst with respect
∗
Corresponding authors. Tel.: +91 20 25902544; fax: +91 20 25902621.
E-mail address: aa.kelkar@ncl.res.in (A.A. Kelkar).
http://dx.doi.org/10.1016/j.molcata.2014.04.016
1381-1169/© 2014 Elsevier B.V. All rights reserved.

M.K. Munshi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
145
to GLY was charged to the 50 ml round bottom flask containing GLY
GLY CONV
GD SEL
2 g, (21.73 mmol) and DMC 5.87 g, (65.19 mmol). The reaction was
carried out at a reflux temperature by keeping the oil bath tempera-
ture at 100 ◦C for the selected reaction time. During the course of the
reaction, the temperature decreased from 88 to 71 ◦C as the reaction
progressed. The drop in temperature was because of the formation
of methanol as the reaction progressed. The standard reaction was
carried out for 0.5 h. The reaction mixture was cooled and it was
diluted with N,N-dimethyl formamide, and a sample was taken out
for analysis. The products were analyzed by gas chromatography on
an Agilent 6890N gas chromatograph with HP-Innowax capillary
column (30.0 m × 0.53 mm × 1.00 ␮m film thicknesses). Identifica-
tion of products was done using GC–MS on an Agilent 6890N gas
chromatograph coupled to an Agilent 5973 mass spectrometer
using HP-Innowax capillary column of 30 m × 0.53 mm × 1 ␮m film
thickness. The activity of the catalyst was based on the conver-
sion of the limiting reagent measured under standard conditions of
reaction.
GC SEL
100
80
60
40
20
0
0
20
40
60
80
100
120
Time (min)
Fig. 1. Typical conversion time profile. Reaction conditions: GLY: (2 g, 21.73 mmol),
DMC: (5.87 g, 65.19 mmol), Temp: 100 ◦C (oil bath temperature), Catalyst: 0.1 mol%
(w.r.t GLY).
2.3. NMR analysis
For NMR measurements, Neat sample of DBU, GLY and equimo-
lar mixtures of DBU:GLY (DBU = 1.65 g and GLY = 0.997 g) and
DBU:DMC (DBU = 1.65 g and DMC = 0.976 g) were submitted for
analysis in 5 mm diameter tube. The 1H NMR chemical shifts in
parts per million (ppm) were reported with reference to D2O. And
15N NMR chemical shifts in parts per million (ppm) were reported
with reference to Nitromethane. All the 1H, 13C and 15N spectra
were recorded on a Bruker DRX 500 MHz NMR spectrometer.
the tertiary amine (Table 1, Entry 8, 15 and 16). In all the cases, the
selectivity to GC was very high (83–97%). 1-N,N-dimethylpyrrole
was essentially inactive for the reaction and only trace of product
was observed (Table 1 Entry 7). The order of activity observed was
correlated with their gas phase basicity calculated theoretically
(see Supplementary information) using DFT calculations [14], and
the results obtained show that the activity of amines is positively
dependent on the order of basicity observed for all the amines
except for DMAP (see Table 1).
From the results, it can be seen that very high TONs of 8613 and
9408 were obtained with DBN and DBU as catalyst, respectively, in
7.5 h reaction time with high selectivity to GC (96–99%). To further
confirm the effectiveness of the catalyst system, we carried out
1 mol (GLY) scale reaction with 0.01 mol% DBU (w.r.t GLY), and 88%
GLY conversion with 97.7% selectivity to GC and 2.3% selectivity to
glycidol (GD) was obtained in 7.5 h. This shows that comparable
activity is obtained even after scaling the reaction 46 times (92 g
GLY compared to 2 g GLY as reactant). We have thus demonstrated
that amidines like DBU and DBN are potentially good catalysts
with very high activity for the synthesis of GC. Amidines are well
known organocatalysts for many reactions [15]. However, to the
best of our knowledge, this is the first report on the use of amidines
as catalysts for GC synthesis.
3. Results and discussion
Spurred by recent reports, organocatalysts are now being recog-
nized as powerful tools for GC synthesis by the transesterification
of DMC with GLY [8–11]. Recently, Ochoa-Gómez et al. [8] have
shown that a simple nucleophilic base, triethyl amine, is an effi-
cient catalyst for the synthesis of GC, though high catalyst loading
is required for the observed results (90–98% yield of GC at reflux
temperature using 1:3 molar ratio of GLY:DMC and 10–30 mol%
catalyst loading).
Keeping in mind literature reports,
mercially available amines were screened
a
range of com-
including,
4-dimethylaminopyridine (DMAP), 1,4-diazabicyclo[2.2.2]octane
(DABCO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,5-
diazabicyclo[4.3.0]non-5-ene (DBN), for transesterification of DMC
with GLY and the results are presented in Table 1. The reaction was
carried out using GLY (2 g, 21.73 mmol), DMC (5.87 g, 65.19 mmol),
and 0.01–2 mol% of catalyst relative to GLY. From the results
obtained, it is observed that the structural variation in amines has
a significant influence on its catalytic activity. Very high activity
was obtained with DBU and DBN as catalysts, whereas DABCO
and DMAP showed moderate catalytic activity at 0.1 mol% catalyst
loading (Table 1, Entry 1, 3, 5 and 6) in only 0.5 h reaction time.
Therefore, DBU and DBN were screened at a still lower catalyst
concentration (0.01 mol%) with increasing reaction time and
the results obtained are presented in Table 1. Best results (98%
conversion of GLY with 96% selectivity to GC in 7.5 h, TON: 9408
[13]) were obtained using DBU as the catalyst (Table 1, Entry 2). To
the best of our knowledge, this is the highest TON reported for the
synthesis of GC. In general, all alkyl amines showed lower activity,
and, hence the experiments were carried out using 2 mol% catalyst
loading (w.r.t GLY) for 2 h reaction time keeping other parameters
same. The activity decreased marginally with the increase in the
chain length of primary alkyl amines (Table 1, Entry 8–14), while
the activity increased when going from primary to secondary to
3.1. Effect of reaction conditions on the activity and selectivity
Optimization of reaction conditions was carried out using
21.73 mmol GLY, 65.19 mmol DMC and 0.1 mol% (w.r.t GLY) DBU
as a catalyst and the results are presented below.
Typical conversion-time profile of the reaction is presented in
Fig. 1. From the figure it can be seen that conversion increased with
reaction time and reached 91% in 2 h. Selectivity to GC (90–93%)
and GD (7%) was constant throughout the course of the reaction.
Effect of catalyst loading on the conversion and selectivity was
investigated in a catalyst loading range of 0.1 to 0.4 mol% at a fixed
reaction time of 0.5 h and the results are presented in Fig. 2.
From the results it can be seen that conversion of GLY increased
with increase in catalyst loading and selectivity to GC decreased
marginally with increase in selectivity to GD. The probable rea-
son for increase in GD selectivity could be mainly because of
increase in basicity of the reaction mixture with increase in catalyst
loading; resulting in decarboxylation of GC formed as a product.
Formation of CO2 in these experiments was confirmed by pass-
ing the gas phase through saturated barium hydroxide solution

146
M.K. Munshi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
Table 1
Screening of catalysts for GC synthesis.
Entry
Catalyst
Conversion (%)
Selectivity (%)
GD
Basicitya (kcal/mol)
GC
1b
2c
3d
51
98
44
7
1
0
91
96
98
−251.16
−251.16
−248.68
4e
5f
87
31
1
0
99
93
−248.68
−230.35
6g
5
0
89
−259.75
7
8
T
0
1
T
−192.82
−217.57
53
97
9
51
1
95
−219.49
10
11
48
45
7
5
93
95
−220.08
−220.34
12
39
2
98
−220.31
13
14
15
37
35
95
10
2
83
85
91
−220.37
−220.47
−226.8
0
16
97
0
97
−233.68
17
18
91
76
12
7
88
93
–
–
Reaction conditions: GLY: (2 g, 21.73 mmol), DMC: (5.87 g, 65.19 mmol), temp: 100 ◦C (oil bath temperature), time: 2 h, catalyst: 2 mol%.
a
Gas phase basicity (GPB) calculated using density functional theory (DFT).
Catalyst: 0.1 mol%, time: 0.5 h.
Catalyst: 0.01 mol%, time: 7.5 h.
b,d,f,g
c,e
to obtain white precipitate of barium carbonate. Decarboxylation
of the GC to GD is well known in presence of strong bases [10].
GLY conversion was very high for 1 h reaction time at all catalyst
loadings and additional data is given in Table S1, Supplementary
information.
Effect of temperature was investigated in a temperature range
of 70 ◦C to reflux temperature (100 ◦C oil bath temperature) and the
results are presented in Fig. 3.
Conversion of GLY increased with increase in temperature with
slight drop in selectivity to GC. Thus at 70 ◦C, GLY conversion was
30% with 96% selectivity to GC and 4% selectivity to GD, while at
reflux temperature (oil bath temperature 100 ◦C) GLY conversion
was 91% with 93% selectivity to GC and 7% selectivity to GD.
Transesterification of DMC with GLY is an equilibrium controlled
reaction and hence molar ratio of GLY:DMC used in the reaction is
very important to achieve complete conversion. Effect of GLY:DMC

M.K. Munshi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
147
GLY CONV
GD SEL
GC SEL
80
60
40
20
80
GLY CONV
GD SEL
GC SEL
60
40
20
0.10 0.15 0.20 0.25 0.30 0.35 0.40
Catalyst Amount (mol %)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
DMC/GLY ratio
Fig. 2. Effect of catalyst loading. Reaction conditions: GLY: (2 g, 21.73 mmol), DMC:
(5.87 g, 65.19 mmol), Temp: 100 ◦C (oil bath temperature), time: 0.5 h.
Fig. 4. Effect of GLY to DMC molar ratio. Reaction conditions: time: 1.5 h, catalyst:
0.1 mol% (w.r.t limiting reagent), temp.: 100 ◦C (oil bath temperature).
molar ratio was investigated in a range of 3:1 to 1:3. In this study
the quantity of limiting reactant was fixed at 21.73 mmol. Thus at
GLY:DMC ratio of 1:1 both reactants were used at 21.73 mmol and
in other experiments the quantity of other reactant was increased
accordingly. Conversion reported in this work is for the limiting
reactant. The results obtained at a fixed reaction time of 1.5 h are
presented in Fig. 46.
From the results it can be seen that only 70% conversion was
obtained with GLY:DMC ratio of 1:1. This is expected, since this is
an equilibrium controlled reaction. Selectivity to GC was 94% and
GD formed by decarboxylation of GC was 6%. Conversion of the
limiting reactant increased with increase in ratio indicating that the
activity was dependent on both the reactants. Selectivity pattern
was not affected by a change in molar ratio of GLY:DMC. Thus DBU
was found to be a good catalyst with high selectivity to GC (90–96%)
under the range of reaction conditions investigated. Additional data
is given in Table S1, Supplementary information.
Finally recycle experiment was carried out to check stability
of the catalyst. Initial experiment was carried out using 0.1 mol%
catalyst loading and 1:3 molar ratio of GLY:DMC at reflux tem-
perature for 90 min reaction time. GLY conversion of 92% was
observed in this reaction with 94% selectivity to GC and 5% selec-
tivity to GD. Required amount of fresh GLY and DMC were added
to the reaction mixture to maintain GLY:DMC ratio of 1:3 and
reaction was carried out again. Conversion of GLY was found to
Fig. 5. Structure of DBU.
be lower (69% conversion for recycle experiment compared to 91%
GLY conversion of run 1) during recycle of the catalyst without
affecting selectivity significantly (96% selectivity to GC and 3%
selectivity to GD, respectively). In this experiment significant
dilution of reaction mixture is taking place and also products of
the initial experiment are present in the reaction mixture. Hence
activity cannot be compared with the initial experiment carried
out. However, significant conversion of GLY (69%) was observed
indicating that the catalyst is active in recycle experiment.
3.2. Mechanism of the reaction
From the GC–MS analysis, the formation of glycerol methyl car-
bonate (GMC) in very small quantities was observed at a low DBU
loading (0.01 mol%). This indicates that GMC is formed as an inter-
mediate and its conversion to GC is fast. The formation of GMC
as an intermediate has been reported in the literature earlier [3].
However, the mechanism of GC formation from GLY has not been
studied in detail and it is mentioned that the formation of glyceride
from GLY by abstraction of “H” is the first step [3]. Once glyc-
eride is formed, it attacks on the carbonyl carbon of DMC forming
GMC, which further undergoes a similar reaction cycle and forms
GC [3]. In order to understand the mechanistic features, we have
investigated the role of DBU as a catalyst by NMR spectroscopy.
For this purpose, neat samples of DBU, and equimolar mixtures
of DBU:DMC and DBU:GLY were subjected to 15N NMR analysis
and the results obtained are presented in Table 2. In the 15N NMR
spectrum of free base (DBU); we observed two signals at about
GLY CONV
GD SEL
GC SLE
100
80
60
40
20
0
Table 2
15N NMR data of DBU and equimolar mixture of DBU:DMC and DBU:GLY.
Nuclei
DBU
DBU–DMC
DBU–GLY
70
80
90
100
N1
N5
−293.27
−170.33
−292.26
−173.92
−286.36
−192.60
Temperature (oC)
Note: [1] Chemical shifts are relative to external neat nitromethane. [2] All the three
analysis were carried out without solvent, inorder to avoid complexity in interaction
due to solvent.
Fig. 3. Effect of temperature. Reaction conditions: GLY: (2 g, 21.73 mmol), DMC:
(5.87 g, 65.19 mmol), time: 1.5 h, catalyst: 0.1 mol% (w.r.t GLY).

148
M.K. Munshi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
Fig. 6. The proposed mechanism for transesterification of DMC with GLY for GC synthesis.
Fig. 7. The calculated energy diagram for the reaction of DMC with GLY to GC catalyzed by [DBU] (energies and lengths in kcal/mol and Å, respectively); green, red, black
and blue balls represent carbon, oxygen, hydrogen and nitrogen, atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)

M.K. Munshi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 144–149
149
−293.27 ppm and −170.33 ppm. From the literature [16] it is know
that the amino nitrogen nucleus should be more shielded than the
imino nitrogen by about 100 ppm. Thus we can assign these two
signals to the N1 and N5 nuclei, respectively (Fig. 5).
two base-mediated transformations have barriers of 20.9 kcal/mol
and 3.8 kcal/mol, respectively (see Fig. 7). The calculations there-
fore indicate that the first step, corresponding to the conversion of
the reactants to int.3 is the slowest step of the entire cycle, having
a barrier of 24.5 kcal/mol. The role of the additional GLY molecule
is also shown to be that of a stabilizing agent, via inter-molecular
hydrogen bonding to the oxygen of the carbonyl group of DMC [21].
From the results obtained it can be clearly seen that DBU:DMC
mixture did not show significant changes in chemical shifts of nitro-
gen centers in DBU compared to the chemical shifts of nitrogen
centres of DBU alone. Thus indicating that there is no interaction
between electrophilic carbonyl carbon of DMC and nucleophilic
nitrogen center of DBU. On the other hand, In 15N NMR spectrum
of DBU:GLY mixture it was observed that the signal for N5 nucleus
shifted from −170.33 for only DBU to −192.60 for DBU:GLY mix-
ture indicating more shielding while the signal for N1 shifted from
−292.27 to −286.36 indicating deshielding (see Table 2). Similar
trend of shielding and deshielding in N1 and N5 nuclei of DBU was
observed by the Wiench et al. [16] in their work on detailed study of
DBU and DBN protonation by trifluro acetic acid (TFA) by NMR spec-
troscopy. Observed NMR analysis clearly shows strong interaction
between DBU and GLY.
4. Conclusions
In summary, DBU was found to be the best catalyst for transes-
terification of DMC with GLY. 1H, 13C and 15N NMR analysis clearly
showed strong interaction between GLY and DBU. The mechanism
of the reaction was investigated with the help of DFT calculations
and an additional GLY molecule was found to be involved as a
co-catalyst and was also found to help in stabilizing the key inter-
mediates and transition states. Best results: 98% conversion of GLY
with 96% selectivity to GC (TON: 9408), were obtained with DBU
as a catalyst (0.01 mol% based on GLY) at 100 ◦C in 7.5 h. To the
best of our knowledge, this is the highest activity reported for the
transesterification of DMC with GLY.
Similarly the 1H, 13C NMR analysis for DBU:DMC mixture did
not show significant changes in chemical shifts (see spectra in
Supplementary information for details), indicating that there is no
interaction between DBU and DMC at room temperature. On the
other hand, from the 1H NMR spectrum of DBU:GLY mixture it was
observed that all of the 1H nuclei of GLY showed downfield chemi-
cal shift compare to the 1H nuclei of pure GLY. All of the 1H nuclei of
DBU showed upfield chemical shift compared to 1H nuclei of pure
DBU (see Supplementary information). Thus NMR (1H, 13C and 15N)
clearly showed that there is interaction between acidic OH pro-
tons of GLY and “N5” nitrogen center of DBU and activation of GLY
is the first step of the reaction. Thus from 1H, 13C and 15N NMR
analysis of DBU and equimolar mixture DBU with GLY and DMC it
is confirmed that the activation of GLY can takes place by the base
(DBU) as a first step of this reaction.
Acknowledgments
Authors would like to thank Department of Science and Technol-
ogy (DST), Government of India for financial support to this project.
Mudassir and Swapna would like to thank CSIR, Government of
India for fellowship awarded to them. The analytical help on GC–MS
by Dr. Mrs. S. K. Shingote is greatly appreciated.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.04.016.
To investigate the reaction mechanism in more detail, we have
carried out DFT calculations using the Turbomole 6.4 software [17],
with the TZVP/PBE/B3LYP approach [18–20]. For this purpose cat-
alytic cycle was drawn based on the available literature and results
obtained (Fig. 6). Transesterification is an equilibrium controlled
reaction and, generally, one of the reactants is used in excess. Horn
et al. [21] have proposed the use of an additional molecule of alco-
hol or amine as a co-catalyst in their work on the mechanism of
DBU catalyzed amidation and transesterification of aromatic esters
as a model for depolymerisation of polyethylene terphthalate. In
the present work, transesterification of DMC has been carried out
using GLY as alcohol and very high activity has been obtained using
DBU as the catalyst. It is quite likely that an additional molecule of
glycerol works as a co-catalyst and helps in stabilizing the key inter-
mediates and transition states. The energies of reaction steps were
calculated according to catalytic cycle proposed, and the results are
summarized in Fig. 7. The mechanism for the conversion of GLY and
DMC to the intermediate species GMC shown as “int.3” in Fig. 6 has
been extensively proposed earlier by many researchers [2–4]. This
essentially involves the transfer of a proton (H+) from one of the GLY
molecules to the nitrogen of the base DBU, followed by the transfer
of the proton from the base to one of the methoxide groups to form
methanol. This process goes through two transition states, having
barriers of 24.5 kcal/mol and 3.3 kcal/mol (see Fig. 7). Subsequent
to the formation of int.3, another OH group (sec. hydroxyl group)
of the GLY moiety in GMC can undergo a similar transformation,
losing a proton to the nitrogen of the base and forming a cyclic
species GC, while the proton is then transferred to the remaining
methoxide to provide the second methanol molecule, and yield the
experimentally observed product GC. The transition states for these
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