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Catalysis Science & Technology
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ARTICLE
Journal Name
Conversely, the cyclobutane carboxylic acid apparently Lewis and mainly Brønsted acid catalysis). WhVeienw Awrtiecle Ohnalivnee
DOI: 10.1039/C5CY01538C
underwent a ring opening reaction followed by carbon skeletal investigated the activity of others irons salts, we found that the
rearrangement, resulting in 2-methyl-2buten-carboxylic acid. solubility of salts show to be another important point on this
This intermediate product was esterified with β-citronellol, reaction. Insoluble iron salts in acetonitrile (FeSO4 and Fe2(SO4)3
and gave a product identified by GC-MS analysis as β- were much less active than iron nitrate. Conversely, the totally
citronellyl 2-methyl-2-butenolate (i.e. 2-butenoic acid, 2- soluble catalyst FeCl3 was more active than Fe(NO3)3. However, it
methyl-, 3,7-dimethyl-6-octenyl ester) (Figure 13).
was less selective because of higher formation of oligomers. The
following order of reactivity for acetate selectivity was found: β-
citronellol > geraniol > nerol. The tertiary terpenic alcohols (i.e. α-
terpineol and linalool) did not underwent esterification. We verified
that in absence of solvent, the formation of oligomers compromise
significantly the reaction selectivity. Although two processes
depending of H+ ions to occur, oligomerization is notably lower
when we have iron(III) nitrate catalyst in solution. The reaction
scope was extended to the other carboxylic acids. We have found
that Fe(NO3)3 catalyst was able to esterify alkyl acids with β-
citronellol. However, it failed on benzoic acid esterification, a
resulted attributed to lowering of electrophilicity of carbocation
intermediate, due to electron donating effect of aromatic ring.
Finally, we carried out reactions with alone HNO3 acid, and we
could verify that this Brønsted acid was less active and selective
than iron(III) catalyst.
O
COOH
OH
O
Fe(NO3)3
+
+
H2O
CH3CN, 333 K
Figure 13. Carbon skeletal rearrangement followed by
esterification with β-citronellol of cyclobutane carboxylic acid
in presence Fe(NO3)3catalyst.
Previously, we had found that iron(III) cations are able to
provoke rearrangement of carbon skeletal in cyclic compounds (i.e.
monoterpenes), where carbon-carbon bounds are initially broken to
originate more stable compounds [19]. Herein, iron (III) nitrate
promoted the ring opening of cyclobutane carboxylic acid providing
an unsaturated carboxylic acid, which was converted to ester in
presence of β-citronellol in CH3CN solutions (Figure 13).
The Fe(NO3)3-catalyzed esterification of pentanoic acid (i.e.
valeric acid) with β-citronellol provided mainly β-citronellyl valerate
Acknowledgements
(ca. 41
% selectivity). As secondary products were formed
The authors are grateful for the financial support from CAPES,
CNPq, FAPEMIG (Brazil).
dihydromyrcenyl valerate besides a mixture of minority products
(i.e. combined selectivity ca. 30 %).
Benzoic acid was the only carboxylic acid that did not converted
to ester in Fe(NO3)3-catalyst esterification reactions with β-
citronellol. We can be explain this observation if we analyze the
rate-determining step (i.e. rds) of esterification reactions. After the
protonation of carboxylic acid carbonyl group, it require that
alcohol hydroxyl attacks the carbonylic carbon atom, which should
have an electrophilic character [39,40]. Nonetheless, the group
attached to the carbon atom can increase or decrease these
electrophilicity, through the withdrawing or donating electron
effect, respectively. Because ethyl group has higher electron
donating capacity than methyl group, the esterification of acetic
acid was more effective than that propionic acid.
When we analyze the actuation of benzyl group, we realize that
besides steric hindering, which hamper the attack of alcohol
hydroxyl group on carbonylic carbon atom, the -system provide
electron density to the carbocation, make it less electrophilic,
hampering that alcohol attack [40].
Notes and references
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acetates was developed. Iron (III) nitrate hydrate, an inexpensive
and commercially available catalyst was used in absence or
presence of solvent. Among metal nitrate catalysts assessed,
Fe(NO3)3 was the most selective towards formation of acetate
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