Solvent-free transesterification of methyl levulinate and esterification of levulinic acid catalyzed by a homogeneous iron(III) dimer complex

Article abstract of DOI:10.1016/j.mcat.2020.110777

Levulinic acid esters MeC(O)CH₂CH₂CO₂R (LAE) are emerging bio-based chemicals used as solvents, additives and plasticizers. In this work a variety of levulinates (R= n-butyl, n-hexyl, n-octyl, 2-ethylhexyl, geranyl, 2-ethoxyethyl, benzyl, 2-octyl, cyclohexyl, menthyl) is obtained from the solvent-free transesterification of methyl levulinate (ML) and esterification of levulinic acid (LA), catalyzed by a dimeric complex of iron(III). The results are competitive with the few related reports of literature mainly based on heterogeneous catalysis. This first systematic study based on a homogeneous catalytic system therefore represents a significant extension within the field of biomass valorization.

Full text of DOI:10.1016/j.mcat.2020.110777

MolecularCatalysis483(2020)110777
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Solvent-free transesterification of methyl levulinate and esterification of
levulinic acid catalyzed by a homogeneous iron(III) dimer complex
Massimo Melchiorrea, Raffaele Amendolab, Vincenzo Benesserea, Maria E. Cucciolitob,c
,
Francesco Ruffob,c,*, Roberto Espositob,c,
*
a ISUSCHEM srl, Piazza Carità, 32, 80134, Napoli, Italy
b Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, via Cintia 21, 80126, Napoli, Italy
c Consorzio Interuniversitario di Reattività Chimica e Catalisi, Via Celso Ulpiani 27, 70126, Bari, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
Levulinic acid esters MeC(O)CH2CH2CO2R (LAE) are emerging bio-based chemicals used as solvents, additives
and plasticizers. In this work a variety of levulinates (R= n-butyl, n-hexyl, n-octyl, 2-ethylhexyl, geranyl, 2-
ethoxyethyl, benzyl, 2-octyl, cyclohexyl, menthyl) is obtained from the solvent-free transesterification of methyl
levulinate (ML) and esterification of levulinic acid (LA), catalyzed by a dimeric complex of iron(III). The results
are competitive with the few related reports of literature mainly based on heterogeneous catalysis. This first
systematic study based on a homogeneous catalytic system therefore represents a significant extension within
the field of biomass valorization.
Levulinic acid
Methyl levulinate
Esterification
Transesterification
Iron(III)
1. Introduction
direct esterification of acids b), because methyl esters are more man-
ageable, in terms of stability and solubility [7]. Sometimes the use of
The synthesis of chemicals must be re-formulated to fully embrace
the circular economy, and especially those of great industrial interest
should be derived from renewable resources or, better, from waste. In
this contest, biomass is a powerful resource [1]. One of its most
abundant fraction is the lignocellulosic portion (LCB) that is becoming a
main source of raw material, due to its abundance, availability and
variety of chemical compounds derivable from. Among these, levulinic
acid (LA) and its derivatives are raising scientific and industrial interest
as renewable chemical platforms [2]. In particular, levulinate esters
(LAE in Scheme 1) already cover many roles in industry [3], as solvents,
additives, plasticizers and other. The presence of the additional car-
bonyl group allows further transformation and derivatizations for the
synthesis of different libraries of compounds [4].
acids as a starting compounds may be preferable, because they are
immediately available and less volatile, so they raise fewer safety is-
sues.
Literature offers rare hints of transesterification of methyl levulinate
(in Table 1) and, besides numerous examples of direct esterification of
levulinic acid with ethanol or butanol [8], also esterification with
heavier alcohols is poorly represented (Table 2 lists the examples re-
levant to this study). These few reports generally refer to heterogeneous
acid catalysis [9], frequently limited by incomplete conversions, high
alcohol : LA ratios, significant catalyst loadings, use of solvents or
auxiliaries.
Recently, our research group became active in the conversion of
biomass into solvents and chemicals promoted by homogeneous tran-
sition metal complexes [19]. In the course of these studies, we came
across an iron(III) dinuclear complex (c1 in Fig. 1) that effectively
promote benchmark transesterification reactions. [20]
Both LA and its methyl ester ML, that can be directly derived from
biomass [5], are convenient starting materials for the synthesis of LAE,
according to reactions a) and b) of Scheme 1.
The reactions are equilibria, and the relative constants often favor
the reactants [6]. Therefore, their driving force consists in the pro-
gressive removal of water or methanol. Moreover, the presence of a
catalyst is necessary to have acceptable reaction rates. Transesterifica-
tion a) is a powerful method, often advantageous compared to the
On these bases, we have extended our interests to the solvent-free
synthesis of levulinate esters of industrial utility using the dinuclear
complex c1 as homogeneous catalyst. Transesterification of ML (reac-
tion a of Scheme 1) proceeds with high conversions even with sec-
ondary alcohols, and the comparison with the corresponding
⁎ Corresponding authors at: Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, via Cintia
21, 80126, Napoli, Italy.
E-mail addresses: ruffo@unina.it (F. Ruffo), roberto.esposito@unina.it (R. Esposito).
https://doi.org/10.1016/j.mcat.2020.110777
Received 27 November 2019; Received in revised form 13 January 2020; Accepted 14 January 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Fig. 1. Structure of complexes c1 and c2 used as catalysts.
Scheme 1. Transesterification of methyl levulinate (a) and esterification of
levulinic acid (b).
were performed without solvents or auxiliary substances at 100 °C in an
open system connected to a cold trap.
mononuclear catalyst c2 confirms the importance of its binuclearity to
enhance performance. The same catalyst c1 promotes the direct ester-
ification of LA with primary heavy alcohols according to reaction b of
Scheme 1. As far as we know, this is the first systematic study based on
a homogeneous [21] catalytic system for these transformations, and
therefore it represents a significant extension of this emerging sector of
biomass transformation.
Yields were evaluated by integrating appropriate regions in the
NMR spectra of the reaction mixtures (e.g. Fig. 2 for entry 3). The in-
tegral of the ester protons -C(O)OCHx- (IE) in the product was compared
with that of one of the two methylenes of the levulinic structure (IL),
which are practically coincident in the substrate and in the product:
I
E
x
yield =
× 100.
I
The dLa2ta show that c1 is very active even at low loadings (0.05 %
mol/mol), while the corresponding mononuclear c2 promotes very low
yield (entry 4 vs entry 7). These results confirm that the dimeric nature
of the iron(III) complexes is a decisive issue, as already disclosed by
Ohshima [20].
2. Results and discussion
2.1. Transesterification of methyl levulinate (ML)
For both catalyst loadings, increasing the molar ratio 1a: ML from
1.1 : 1–1.5 : 1 improves the yields (entry 1 vs. entry 3, entry 2 vs. entry
4 of Table 3), whereas a higher value (2 : 1) has no significant beneficial
A screening of reaction conditions was carried out in the transes-
terification of ML with n-butanol (1a) promoted by c1 and c2 (Table 3),
by varying the ratio 1a : ML and the catalyst loading. The reactions
Table 1
Literature reports on transesterification of methyl levulinate (ML) with heavy alcohols according to Scheme 1a.
Entry (ref)
Product
Catalyst
Conditions
Yield, %
45
1 [10]
Sulphated SnO2 (10% w/w)
Toluene, 110 °C, 10 h MLA : ROH 1 : 1
2 [11]
3 [12]
4 [13]
Amberlist 15 (10 % w/w)
Toluene, 110 °C, 10 h MLA : ROH 1 : 1
Toluene 80 °C, 24 h MLA : ROH 1 : 1.5
110 °C, 8h MLA : ROH 1 : 1.5
54
98
87
Pentafluoro-phenylammonium triflate (1 % mol/mol)
p-toluenesulfonate resin (10% w/w)
Table 2
Literature reports on esterification of levulinic acid (LA) with heavy alcohols according to Scheme 1b.
Entry (ref)
Product
Catalyst
Conditions
Yield, %
93
1 [8e]
Stannosilicate (10 % w/w)
120 °C, 4 h LA : ROH 1 : 3
2 [8d]
3 [14]
4 [15]
5 [16]
6 [17]
7 [12]
MIM-HSO4 (25% mol)
90 °C, 90 min LA : ROH 1 : 2
120 °C, 6 h LA : ROH 1 : 1.5
100 °C, 10 h LA : ROH 1 : 1.5
Toluene, 110 °C, 90 min LA : ROH 1 : 1.5
120 °C, 6 h LA : ROH 1 : 27
Toluene
88
92
99
95
88
96
H3PW12O40 (10% w/w)
HPW/Al-MCM-41 (25 % w/w)
SO3H@CGO (3% w/w)
GO
Pentafluoro-phenylammonium triflate (1 % mol/mol)
80 °C, 2h
LA : ROH 1 : 1
8 [13]
9 [18]
p-toluenesulfonate resin (10% w/w)
110 °C, 4 h LA : ROH 1 : 1
93
45
HClO4/SiO2 (10% w/w)
100 °C, 10 h LA : ROH 1 : 5
10 [16]
SO3H@CGO (3% w/w)
Toluene, 110 °C, 90 min LA : ROH 1 : 1.5
97
2

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Table 3
Screening of catalysts c1 and c2 in transesterification of ML with n-butanol
(1a).
Entry
Catalyst
Cat. loading, % mol/mol
1a : ML
Yielda (at 18 h) a, %
1
2
3
4
5
6
7
c1
c1
c1
c1
c1
c1
c2
0.05
0.5
1.1 : 1
1.1 : 1
1.5 : 1
1.5 : 1
2 : 1
71
83
80
92
80
93
5
2
2
0.05
0.5
1 (95
1 (97
1
1)
1)
0.05
0.5
2 : 1
2
0.5
1.5 : 1
1
a
Through 1H NMR spectroscopy, reaction time=4 h; T =100 °C.
Fig. 3. Transesterification of ML with 1a in the conditions of entry 4. Yield vs.
time plot.
effect (entry 3 vs. entry 5; entry 4 vs. entry 6). At the optimized ratio
1.5 : 1, the catalyst loading of 0.5 % mol/mol gives better yield than
0.05 % mol/mol within 4 h (entry 3 vs entry 4), while high and com-
parable yields were obtained prolonging the reaction time to 18 h
(entries 3 and 4 in brackets). These results improve by far those re-
ported in literature for the same substrate (entries 1 and 2 of Table 1).
The trend of yield vs. time in the conditions of entry 4 is reported in
Fig. 3, where the activity of the catalyst can be appreciated even at high
reaction time. Progress of the reaction was esteemed from the evolution
of the 1H NMR spectra of the reaction mixture, where the progressive
disappearing of the reagent signals was accompanied by growth of
those pertaining to the product (Fig. 4).
loading (0.5 %) was adopted with the more challenging secondary al-
cohols: 2-octanol, cyclohexanol and menthol were esterified with good
to excellent yields (entries 8–10).
The efficiency of the method was also verified through the simple
isolation of n-butyl levulinate on a larger scale. After the reaction, the
mixture was filtered on a thin layer of Florisil, which was extracted with
a small aliquot of ethyl acetate. The catalyst was sequestered from the
column, while the eluted product was isolated after removal of the
solvent under vacuum with a yield of 85 %.
2.2. Proposed catalytic cycle for transesterification
Transesterification of ML was extended to other selected alcohols
aiming to produce esters that have already demonstrated industrial
applicability (Table 4). Except for the synthesis of 2c, these reactions
are described for the first time in this paper, and both catalyst loadings
of 0.05 % and 0.5 % mol/mol were therefore screened in order to find
customized conditions. Good to excellent yields for most of the primary
alcohols were achieved within 18 h even with the lowest loading, while
at 0.5 % these substrates were converted nearly quantitatively (entries
1–7). The data demonstrate the robustness of the system also in com-
parison with the other few examples of iron(III) homogeneous catalyzed
transesterification of benchmark substrates [20,22]. The higher catalyst
The proposed mechanism for the transesterification is sketched in
Scheme 2 and is a revisiting of the mechanism described by Ohshima
[20]. In excess of alcohol, the dinuclear catalyst c1 undergoes a ligand
substitution with elimination of water and formation of the bis-μ-alk-
oxide species I. One of the two bridges is then cleaved by ML which
coordinates the iron center as in II. This activates the carbonyl group
against the nucleophilic attack of the alkoxide ion in the proximity with
the obtainment of a tetrahedral intermediate (III), while the position
left free by the alkoxide ion is occupied by another molecule of alcohol.
At this point, a sequence of electronic shifts returns the ester product
and methanol, reestablishing the dinuclear species I.
Fig. 2. Example of 1H NMR spectrum for a reaction mixture (entry 3 of Table 3).
3

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Fig. 4. 1H NMR spectra of the reaction mixture over time in the conditions of entry 4.
Table 4
Transesterification of methyl levulinate with alcohols.
Entry
1
Product
Use
Green plasticizer [23]; fuel additive [24]. 95
Yield,a % (cat. loading 0.05% mol/mol) Yield,a % (cat. loading 0.5% mol/mol)
1
97
1
2
3
4
Fuel additive [25].
95
1
> 99
> 99
> 99
Cosmetics [26]
> 99
> 99
Green plasticizer [23]; cosmetics [26].
5
6
Coating [27].
67
85
1
1
95
95
1
1
Fuel additive [3].
7
Fuel additive [25].
81
1
97
1
8
9
–
56
22
2
2
85
98
1
1
Cosmetics [28].
10
a
–
38
2
91
2
1H NMR yield; ratio ROH : MLA = 1.5 : 1; T=100 °C.
4

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Scheme 2. Proposed mechanism for the transesterification of methyl levulinate MLA.
To validate the presence of intermediates of type I, dimer c1 was
heated in n-butanol for 2 h. Removal of the solvent returned an orange
solid, and its IR spectrum in nujol showed a clear shift of the band
attributed to the asymmetric Fe-O-Fe stretching compared to that of c1
(824 cm−1 vs 815 cm−1). This evidence suggests the effective forma-
tion of the proposed species.
the reaction temperature was set at 120 °C and the vessel was equipped
with a short column of molecular sieves [19g] in order to trap the water
and shift the equilibrium towards the products. The catalytic runs are
reported in Table 5.
According to the premise, yields were already considerable without
catalyst, and its presence served to sustain the reaction when its pro-
gression reduced the Brønsted acidity of the system. For example, the
yield improved from 74 to 95 % with n-octanol (entry 2). It should be
emphasized that this information is usually not mentioned in the cat-
alytic reports of Table 2, which therefore do not clarify the contribution
of the added catalyst to the yield.
Finally, to disclose the identity of the catalyst at the end of the re-
action, diethyl ether was added to the reaction mixture described by
entry 1 of Table 3 (0.5 % mol/mol) with consequent precipitation of a
brown solid. Its IR spectrum confirmed the presence of the dimeric
species of type I, and therefore the same powder was reused to prove
the persistence of activity: in the second run, performed at loading of
0.05 % mol/mol, the yield of n-butyl levulinate was 93 %, which in-
dicates that the complex is still active and may be re-used.
A comparison between Tables 2 and 5 discloses that this homo-
geneous solvent-free system is competitive with literature results: the
loading of catalyst is generally smaller, and a convenient ROH: LA can
be adopted. Thus, n-hexyl, n-octyl and benzyl levulinate 2b, 2c and 2g
were obtained in yields comparable to those reported so far, while es-
terification with 2-ethylhexanol and geraniol to produce 2d and 2e are
described for the first time in this paper.
2.3. Esterification of levulinic acid (LA)
Aiming to verify the flexibility of catalyst c1, we also attempted the
esterification of levulinic acid with primary alcohols. Since in ester-
ification [29] the intrinsic acidity of the substrate already provides a
contribution of Brønsted acid catalysis, it is expected that conversions
are already substantial without any added catalyst, and the introduc-
tion of a Lewis acid is intended to give further impulse to the reaction
[29].
These results disclose that complex c1 displays significant Lewis
activity also in esterification. This is also confirmed by the comparison
with rare examples of literature [30] which describe the use of homo-
geneous iron(III) catalysts for esterification in less favorable reaction
conditions (refluxing xylene, up to 30 h).
The reactions were performed under convenient conditions and
with a dedicated setup to allow the progressive subtraction of water:
5

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Table 5
acid. It is demonstrated the ability of a simple dimeric iron(III) complex
to catalyze the homogeneous transesterification of methyl levulinate
and the esterification of levulinic acid without solvents and at low
catalyst loadings. As far as we know, this is the first systematic study
based on a homogeneous catalytic system for these transformations,
which have been so far a prerogative of heterogeneous catalysis.
Therefore, the ensemble of results represents a significant extension in
this emerging sector of biomass transformation, and demonstrates the
emerging role of iron complexes in homogeneous catalysis under
friendly conditions [34].
Esterification of levulinic acid with alcohols.
Entry
Product
Yield,a % (no
catalyst)
Yield,a % (cat.
loading 0.5%
mol/mol)
1
2
3
70
74
78
1
1
1
80
95
95
1
1
1
4. Experimental section
4.1. General
Reagents and solvents were purchased from Sigma-Aldrich. Known
compounds were characterized comparing to the NMR spectra with
those present in literature. These and all the new compounds have been
characterized with a Bruker Avance Ultrashield 400 operating at proton
frequency of 400 MHz of with a Varian 500 Oxford at proton frequency
of 500 MHz. The following abbreviations were used for describing NMR
multiplicities: s, singlet; d, doublet; t, triplet; q, quartet; hept, heptet; m,
multiplet; dd, doublet of doublets; dt, doublet of triplets; heptd, heptet
of doublets; td, triplet of doublets; tt, triplet of triplets. Complexes c1
and c2 were prepared according to literature methods. [35]
4
5
63
58
1
1
87
82
1
1
a
1H NMR yield; ratio ROH : LA = 1.5 : 1; T=120 °C; reaction time=18 h.
4.2. Transesterification catalytic runs
Methyl levulinate (1.30 g, 10.0 mmol) and the appropriate amount
of iron(III) catalyst, according to Tables 3, 4, were mixed in a round
bottom flask. The alcohol was added in one portion in the desired
amount, according to Tables 3, 4. The flask, connected to a cold trap,
was heated in an oil bath at 100 °C under magnetic stirring. After 18 h,
the cooled reaction mixture was analyzed by 1H NMR spectroscopy.
One run was performed on larger scale aiming at isolating the product.
A mixture containing methyl levulinate (6.50 g, 50.0 mmol), n-butyl
alcohol (5.55 g, 75.0 mmol) and c1 (0.016 g, 0.025 mmol) was stirred
18 h at 100 °C. The mixture was filtered on a thin layer of Florisil, which
was extracted with ethyl acetate (2 mL). The product was isolated after
removal of the solvent under vacuum (7.3 g, 85 %).
4.3. Preparation of intermediate I
The iron(III) catalyst c1 (0.033 g, 0.050 mmol) and n-butanol
(11.1 g, 150 mmol) were mixed in a round bottom flask. The flask,
connected to a cold trap, was heated in an oil bath at 100 °C under
magnetic stirring for 2 h. Then the alcohol was removed under vacuum
and an orange powder was obtained. Selected IR signals (nujol,
Fig. 5. Dimeric association found in the solid state for [Fe(Salen)(X)] com-
plexes (X = Cl– or RCO2–).
2.4. Proposed catalytic cycle for esterification
ν) = 1654, 1629, 1598, 1541, 906, 861, 824 cm−1
.
It is known that carboxylic acids RCO2H transform dimer c1 into the
corresponding carboxylate [Fe(Salen)(RCO2)] [31]. These species were
studied in the solid state where a dimeric association was postulated,
similar to what found for the chloro-derivative c2 (Fig. 5).
4.4. Catalyst recover and recycle
Methyl levulinate (1.30 g, 10.0 mmol) and the iron(III) catalyst c1
(0.033 g, 0.050 mmol) were mixed in a round bottom flask. The alcohol
n-butanol (0.556 g, 7.50 mmol) was added in one portion. The flask,
connected to a cold trap, was heated in an oil bath at 100 °C under
magnetic stirring. After 18 h the reaction was cooled, and diethyl ether
was added. A brown solid precipitated, which was recovered by cen-
trifugation. The solid was identified as the intermediate I, and was re-
used for a second catalytic run at a loading of 0.050 % mol/mol. The
yield of product was 93 %.
However, it is reasonable that the corresponding monomeric species
c3 (Scheme 3) are instead present in solution, exactly as observed for
the same c2 [32]. Therefore, a likely [33] mechanism of esterification
involves initial protonation of c3 (Scheme 3), and upon coordination to
the iron center (I’), the carbonyl group of levulinic acid is activated
towards the nucleophilic attack of the alcohol. The intermediate ox-
onium species II’ eliminates the product and water, leaving the metal
ion ready to activate another molecule of substrate.
3. Conclusions
4.5. Esterification catalytic runs
This work is part of the flourishing research aimed at the prepara-
tion of innovative bio-based products through valorization of levulinic
Levulinic acid (0.580 g, 5.00 mmol) and the iron(III) catalyst c1
(0.016 g, 0.025 mmol) were mixed in a round bottom flask. The alcohol
6

M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
Scheme 3. Proposed mechanism for the esterification of levulinic acid LA.
was added in one portion (7.50 mmol). The flask, equipped with a short
column of molecular sieves, was heated in an oil bath at 120 °C under
magnetic stirring. After the catalytic runs, the cooled reaction mixture
was analyzed by 1H NMR spectroscopy.
(t, J =6.8 Hz, 2 H), 2.19 (s, 3 H), 2.02 – 1.92 (m, 1 H), 1.86 (dhept,
J = 7.0, 2.8 Hz, 1 H), 1.73 – 1.64 (m, 2 H), 1.52 – 1.41 (m, 1 H), 1.41 –
1.32 (m, 1 H), 1.08 – 0.94 (m, 2 H), 0.89 (dd, J = 6.8, 1.3 Hz, 6 H), 0.87
– 0.83 (m, 1 H), 0.75 (d, J =7.0 Hz, 3 H); 2j: δ 4.80 – 4.68 (m, 1 H),
2.74 (t, J =6.6 Hz, 2 H), 2.55 (t, J =6.6 Hz, 2 H), 2.19 (s, 3 H), 1.86 –
1.79 (m, 2 H), 1.74 – 1.65 (m, 2 H), 1.58 – 1.47 (m, 1 H), 1.45 – 1.22
(m, 5 H).
1H NMR data (500 MHz, CDCl3, δ)= 2a: 4.07 (t, J =6.7 Hz, 2 H),
2.74 (t, J =6.6 Hz, 2 H), 2.57 (t, J =6.6 Hz, 2 H), 2.19 (s, 3 H), 1.66 –
1.54 (m, 2 H), 1.43 – 1.31 (m, 2 H), 0.92 (t, J =7.4 Hz, 3 H); 2b: 4.06 (t,
J =6.8 Hz, 2 H), 2.74 (t, J =6.4 Hz, 2 H), 2.57 (t, J = 6.5, 2 H), 2.19 (s,
3 H), 1.67 – 1.57 (m, 2 H), 1.39 – 1.22 (m, 6 H), 0.88 (t, J =6.9 Hz,
3 H); 2c: 4.06 (t, J =6.8 Hz, 2 H), 2.75 (t, J =6.6 Hz, 2 H), 2.57 (t, J
=6.6 Hz, 2 H), 2.20 (d, J =5.0 Hz, 3 H), 1.74 – 1.50 (m, 2 H), 1.40 –
1.08 (m, 10 H), 0.88 (t, J =6.8 Hz, 3 H); 2d: 4.11 – 3.92 (m, 2 H), 2.78
(t, J =6.6 Hz, 2 H), 2.61 (t, J =6.6 Hz, 2 H), 2.22 (s, 3 H), 1.59 (m, 1 H),
1.48 – 1.14 (m, 8 H), 1.00 – 0.84 (m, 6 H); 2e: 5.33 (t, J =6.5 Hz, 1 H),
5.08 (t, J =6.7 Hz, 1 H), 4.59 (d, J =7.1 Hz, 2 H), 2.75 (t, J =5.7 Hz,
2 H), 2.58 (t, J =6.6 Hz, 2 H), 2.19 (s, 3 H), 2.14 – 2.07 (m, 2 H), 2.07 –
2.00 (m, 2 H), 1.69 (s, 3 H), 1.68 (s, 3 H), 1.60 (s, 3 H); 2f: 4.22 (t, J
=4.7 Hz, −OCH2CH2OCO-, 2 H), 3.62 (t, J =4.7 Hz, -OCH2CH2OCO-,
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
CRediT authorship contribution statement
Massimo Melchiorre: Investigation, Validation. Raffaele
Amendola:
Investigation.
Vincenzo
Benessere:
Validation,
2 H), 3.53 (q,
J =7.2 Hz, CH3CH2O-, 2 H), 2.75 (t, J =6.5 Hz,
Visualization. Maria E. Cucciolito: Conceptualization, Writing - ori-
ginal draft. Francesco Ruffo: Supervision. Roberto Esposito:
Conceptualization, Writing - original draft.
CH3COCH2CH2OCO-, 2 H), 2.61 (t, J =6.6 Hz, CH3COCH2CH2OCO-,
2 H), 2.19 (s, −COCH3, 3 H), 1.21 (t, J =7.0 Hz, CH3CH2O-, 3 H); 2 g:
7.39 – 7.27 (m, Ph-, 5 H), 5.12 (s, PhCH2O- 2 H), 2.77 (t, J =6.4 Hz,
CH3COCH2CH2OCO-, 2 H), 2.63 (t, J =6.5 Hz, CH3COCH2CH2OCO-,
2 H), 2.19 (s, −COCH3, 3 H); 2h: 4.93 – 4.83 (m, 1 H), 2.74 (t, J
=6.5 Hz, 2 H), 2.55 (t, J =6.3 Hz, 2 H), 2.19 (s, 3 H), 1.58 – 1.42 (m,
2 H), 1.35 – 1.22 (m, 8 H), 1.19 (d, J =4.9 Hz, 3 H), 0.86 (t, J =6.9 Hz,
3 H); 2i: 4.67 (td, J = 10.9, 4.3 Hz, 1 H), 2.74 (t, J =6.6 Hz, 2 H), 2.55
Acknowledgements
MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca),
Italy, PON Ricerca e Innovazione 2014-2020, grant code: AIM1829571
- 1.
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M. Melchiorre, et al.
MolecularCatalysis483(2020)110777
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