(A) Cyclopropanes are a unique synthon within organic chemistry due to their properties
such as high ring strain and geometry, which gives cyclopropanes an unusual structure
with relatively shorter C–C and C–H bonds than in alkanes, and enhanced π-character
of C–C bonds than normal.[5 ] Langletz and collaborators have recently reported the stereoconvergent synthesis
of cyclopropanes using EDA and different styrenes in the presence of different photocatalysts
under irradiation with blue LEDs. Using, E -, Z - or E -, and Z -β-styrenes mixture, they observed the formation of the same product, a single diastereomer
with high yields.[6 ]
(B) Olefination, i. e., obtention of C=C double bonds, is an important step for organic
synthesis and it is chiefly achieved by using ylide compounds such as phosphorus reagents
in the Wittig reaction. Zou and co-workers have performed different reactions to first
investigate an olefination of aldehydes using EDA in the presence of triphenylphosphine
(PPh3 ) with iron(IV)–corrole complexes as catalysts. These complexes are analogues of porphyrin
tetrapyrrolic macrocycle used mainly as catalysts for oxidation, epoxidation, hydroxylation,
and insertion reactions. Their results showed an efficient method to afford trans -olefination products.[7 ]
(C) Pyrazoline derivatives are important synthons used in medicinal chemistry due
to their biological properties and also in the synthesis of fluorescent molecules
due to their chromophoric properties. Abeykoon and co-workers studied the synthesis
of pyrazolines from EDA using a metal porphyrin based metal-organic framework (Fe-Al-PMOF)
and iron tetrakis(4-methoxycarbonylphenyl)porphyrin (FeP). Their results indicate
the formation of diethyl maleate (DEM) and diethyl fumarate (DEF) with subsequent
formation of the pyrazoline ring by the reaction of DEM and DEF with more EDA. The
more the catalyst was washed, the more product was formed the next run, which is unusual
for this type of reaction, and it can be explained by the activation of more inner
catalytic sites in each wash.[8 ]
(D) 1,2-Rearrangement is a usual reaction in organic synthesis, when a substituent
moves from one atom to the next in an intramolecular reaction, forming different isomers.
Rahaman and co-workers have synthesized a broad range of 3-oxo-esters by reacting
EDA with different ketones and aliphatic aldehydes through a 1,2-shift using a Brønsted
acid catalyst. Their results revealed that aryl methyl ketones produced only aryl-migrated
products, while other ketones gave a mixture of products.[9 ]
(E) The pyrazole core is an important aza-heterocycle present in natural and synthetic
compounds, revealing different applications in synthetic and medicinal chemistry.
The synthesis of this core using diazo compounds and alkynes/alkenes with transition
metals as catalysts is well known and well established in the literature. Recently
Sandeep and co-workers studied a new strategy of synthesis of this core through a
metal-free reaction using β-aryl vinyl sulfonyl fluoride derivatives in a triethylamine-catalyzed
[3+2] cycloaddition, obtaining good to high yields.[10 ]
(F) Amino acids and amino alcohols can be obtained from β-hydroxy-α-diazo carbonyl
derivatives, and, therefore, these molecules are important, mainly, when it comes
to the synthesis of peptides and other macromolecules. Balou and co-workers presented
the synthesis of different α-diazo carbonyl derivatives using a solvent-free and green
protocol using potassium fluoride supported on nano clinoptilolite (KF/CP NPs) as
catalyst. A variety of aliphatic and aromatic aldehydes were reacted with EDA to give
compounds with moderate to high yields.[11 ]
(G) Another strategy for the synthesis of highly enantio- and diastereoselective cyclopropanes
was presented by Carreras and co-workers, where they synthesized different 1-boryl-2,3-disubstituted
cyclopropanes from alkenylboronates with EDA in the presence of catalytic amounts
of a chiral copper(I) complex. Besides the synthesis from alkenylboronates, the products
can be obtained from alkynes through a sequence of hydroboration–cyclopropanation
reactions. The yields varied from 27–96%.[12 ]
(H) Cressy and co-workers synthesized a family of new Rh(II)-based catalysts containing
tethered thioether ligands using microwave methods. These catalysts optimized cyclopropanation
using EDA and different olefins, enhancing the yields. The catalyst with better results
was the complex Rh2 (OAc)3 PhTOX, achieving 99% yield when styrene was used at 5 equivalents to EDA. Other substituted
styrenes were reacted and the cyclopropanes were obtained with a 54–99% yield.[13 ]
(I) Pyrazoles cores can be prepared by a 1,3-dipolar cycloaddition between an aliphatic
diazo compound and nitroalkenes followed by aromatization of the cycloadducts. Bykova
and co-workers used EDA to synthesize fused pyrazole from substituted 3-nitro-2H -chromenes in a new one-pot regioselective method, obtaining ethyl 3,4-dihydrochromeno[3,4-c ]pyrazole-1-carboxylates with yields varying between 68–87%. This strategy uses silver
acetate and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as bases and catalysts, increasing
the yield from 36–87%.[14 ]
(J) Imidazopyridines are an important nitrogen-based core, which is found in a variety
of natural products as alkaloids. Particularly, ethoxycarbonylmethylated imidazopyridines
are present in several drugs, such as zolpidem, alpidem, saripidem, etc.[15 ] Bhattacharjee and co-workers demonstrated the synthesis of alkyl-substituted imidazopyridines
by the reaction between imidazopyridines and EDA using Ru(byp)3 Cl2 as photoredox catalyst under the irradiation of 34 W LED lamp. The best yield was
obtained when a mixture of MeOH and H2 O (2:1) was used and the success of the methodology helped the authors to repeat the
reaction with other imidazoheterocycles, such as substituted imidazopyridines, imidazo[2,1-b ]thiazole, and benzo[d ]imidazo[2,1-b ]thiazoles with yield varying between 58–92%.[16 ]
(K) Yasukawa and co-workers developed a methodology for the olefination of deactivated
aromatic aldehydes with EDA. In the presence of a nitrogen-doped carbon-supported
Co/Cu bimetallic catalyst (NCI) and triaryl phosphine, various aldehydes were converted
into unsaturated esters in good yields, varying between 16–86%. The heterogeneous
catalyst allows the recovery and the reuse of the catalyst and, even after several
runs, there was no significant loss of reactivity.[17 ]
