Theoretical Study of the Chemical Properties and the Reaction Pathway of Decarbonylative Alkylative Esterification of Styrenes with Aliphatic Aldehydes

Using inexpensive and available aliphatic aldehydes as an alkyl source is a useful and cost-eﬀective way to extend the chain of benzyl esters; this decarbonylative alkylative esteriﬁcation of styrene derivatives has been used for organic synthesis and medical chemistry. A cocatalyzed decarbonylative alkylative esteriﬁcation of styrene derivatives with aliphatic aldehydes and iodo-benzenediacetate to provide chain elongated benzoates was investigated by the density functional theory, and quantum theory of atoms in molecules analysis has been used. The chemical properties and the reaction pathway between styrene and aldehyde derivatives in the presence of PhI(OAc) 2 and Co(OAc) 2 have been studied. Chemical properties of styrene and aldehyde derivatives for detecting the stability of products were studied using HOMO and LUMO, potential electronic chemical, global hardness, and global electrophilicity power. The molecular electron potential results show that the styrene and its derivatives are electron donors and aldehyde derivatives are electron acceptors. The localized orbital locator, electron location function analysis, and quantum theory of atoms in the molecule have been used to study the active sites for interactions between reactants. The decarbonylative alkylative esteriﬁcation was restricted exclusively to cobalt catalysts. The step of ligand exchange was the rate-determining step for this reaction.


Introduction
Aldehydes are inexpensive and readily available compounds that are used in the reactions of decarbonylative couplings by rhodium, cobalt, or ruthenium [1][2][3].Aldehydes are used in regioselectivity reactions of the CHO-functional group such as Diels-Alder [4] and domino oxa-Michael-aldol [5].Different ways of removing functional groups from organic molecules are very important in the industry [6,7].Direct esterification of alkenes is an important step in converting large petrochemical molecules to esters as target molecules or synthetic intermediates [8].In the past, alkenes were esterified by the addition of electrophilic acids.In the esterification reaction of alkenes, an alkyl group and an ester group are attached to the C � C bond simultaneously.is reaction increases the reactivity of the products.erefore, alkylative esterification of alkenes is an interesting and useful conversion that is challenging at the same time.In this regard, the carbonylation of aldehydes by metal has been considered for decades [9][10][11].In early 1965, Tsuji [12] decarbonylation of aldehydes by stoichiometric quantities of Wilkinson catalysts has been reported to produce a mixture of alkanes, alkenes, and stable inorganic products.PhI(OAc) 2 is used as an oxidizing agent in organic chemistry which is capable of oxidizing cobalt to its higher valence [13][14][15].
Several methods have been proposed regarding esterification of alkenes which can mainly be divided into two categories: for type I, by connecting alkyl and ester groups which are correlated to alkenes and to generate alkyl radicals via decarboxylation [16,17].For the type II, the ester and alkyl groups onto the alkenes which are uncorrelated and coming from different sources [18,19].
Generally, experimental methods establish indirect information about the reactions and products geometries; getting the details of mechanisms is difficult.One of the main aspects of recognizing the appropriateness of the theory for predicting interactions is based on the measurement criteria with laboratory properties and measured values.According to the frontier molecular orbitals theory, the high amount of highest occupied molecular orbital (HOMO) energy indicates the tendency of the molecule to lose electrons.Also, low amounts of lowest unoccupied molecular orbital (LUMO) energy indicate the ability of the molecule to accept electrons from other molecules.Quantum theory of atoms in molecule (QTAIM) analysis [20], the localized orbital locator (LOL) [21], and electron location function (ELF) [22] were calculated on the M05-2X to explain properties which can be used to connect the changes in charge of atoms in the molecules.M05-2X is a suitable choice, which can also reasonably represent dispersion interaction.In the present paper, quantum descriptors, including the energy of gap (ELUMO-EHOMO), hardness (η), dipole moments, chemical potential (μ), and electrophilicity index (ω) have been calculated and discussed.eoretical calculations have been done to elucidate the stability of productions.Initially, the use of a radical initiator resulted in the removal of the carbonyl group from the aldehyde.Alkyl radicals of aldehyde derivatives are highly active and attack the styrene conjugate that produces the radical compound.
eoretical calculation method calculates the chemical properties of molecules by considering the effects of electron correlation.
eoretical calculations require less cost and time compared to laboratory methods.
e energy and pathway of reaction between reactants are achieved by using the M05-2X level of theory [27].e main goal of this work is to characterize the electronic and structural nature and pathway of reaction of the corresponding reactants and products.e optimized structures are depicted in Figure 1.
e reaction pathway of decarbonylative alkylative esterification of styrene derivatives with aliphatic aldehydes has been calculated.Styrene, isobutyraldehyde, and iodobenzenediacetate were chosen as the model substrates.DTBP (di-tert-butyl peroxide) has been used as the radical initiator of the reaction.e use of DTBP leads to the removal of aldehyde hydrogen from isobutyraldehyde, and then the carbonyl group is separated from the aldehyde structure.As a result, the active alkyl radical group (II) remains.In the following intermolecular hydrogen atom abstraction of the isobutyraldehyde, spontaneous decarbonylation and insertion in the C � C bond of styrene provide benzyl radical III that coordinates to the Co(II) catalyst to create Co(III) complex IV.Addition of PhCO 2 − to the reaction medium results in the exchange of ligand in complex IV, and this ligand exchange crosses transition state 1 (TS1).e reductive elimination in structure V with TS2 results in the formation of 3-methyl-1-phenylbutyl acetate (3a) and Co I (OAc).PhI(OAc) 2 can oxidize the Co I (OAc) compound and recover the Co II (OAc) 2 catalyst (Scheme 1).
e energy profile of the reaction pathway is shown in Scheme 2. e energy of transition states TS1 and TS2 for all compounds (kJ/mol) (reaction A: styrene and its derivatives (1a-1l) with isobutyraldehyde; reaction B: aldehyde derivatives (2b-2k) with styrene) are reported in Table 1.
e reaction energy for all the products obtained is reported in Figure 2. Various styrene derivatives bearing electron donating or withdrawing substituents on the phenyl moiety smoothly underwent this three-component decarbonylative cascade reaction to afford the desired alkylated esters in good yields, such asmethoxy (1b), tert-butyl (1c), methyl (1d), chloromethyl (1e), tirfluoromethyl (1f ), and halo (1g-1l).Among them, the optimized reaction condition could be applied to the styrene derivatives with chlorosubstituted at para, meta, and ortho position (1g, 1h, and 1i), Journal of Chemistry and similar yields were obtained, which reveal no obvious substituent effect.e energy of the chlorine substitution reaction in styrene derivatives is the same for all positions.e reaction energy of styrene derivatives with bromosubstituted at para and meta position (1k and 1l) is similar, which is characterized by substitutions have no effect.Cobalt(II) acetate tetrahydrate (Co II (OAc) 2 ) has been employed as an efficient catalyst.e 1m molecule is a suitable substrate and can be transformed into the corresponding product 3m; this reaction is sensitive for the steric changes on the alkenes, as observed in the experiment; α-methyl styrenes are inactive in the alkylative esterification.
To clarify the nature of electronic transitions in the corresponding reaction, molecular properties, such as chemical potential (μ), are calculated as follows: And, global electrophilicity index where I is the ionization potential, and equal to E HOMO , and A is electron affinity and can be introduced as A � E LUMO .e energy level of the boundary molecular orbitals provides useful information on the chemical reaction indicator.
erefore, the reaction path can be examined in more detail.μ, η, S, and ω of all reactants are presented in Table 2.As can be seen in Table 2, the μ values of aldehyde radicals (II), styrene and its derivatives are negative,   Journal of Chemistry erefore, it can be concluded that the transfer of electrons from styrene and its derivatives to aldehyde radicals (II) is done.Various styrene derivatives bearing electron donating or withdrawing substituents on the phenyl moiety (1a-1l) smoothly underwent this three-component decarbonylative cascade reaction to afford the desired alkylated esters in good yields.Products formed with aldehyde radicals are thermodynamically more stable than products with styrene derivatives.
ese values indicate the transfer of electron charge from HOMO styrene and its derivatives to LUMO aldehyde radicals.
e values of LUMO-HOMO indicate which of these compounds present high chemical stability.μ less than zero represents a spontaneous reaction.e electrophilicity is a descriptor of the reactivity that allows a quantitative classification of the global electrophilic nature of a molecule within a relative scale.ω shows measure of energy lowering due to the maximum electron flow between the donor and the acceptor.According to the definition, this index measures the tendency of chemical species to accept electrons.A more reactive nucleophile is characterized by lower μ and ω values, and then a good electrophile is characterized by high μ and ω values.e greater μ, η, and ω differences between compounds led to faster reactions.According to the obtained HOMO and LUMO of Figure 4, it can be concluded that ethylene double bonds of styrene derivatives have as electron donors and the aldehyde radicals have the role of electron acceptors.
Electron density is a crucial factor in understanding the reactivity of electrophilic and nucleophilic sites and the interaction of hydrogen bonds.erefore, MEP of these compounds was simulated to predict this reaction from attacks on nucleophilic and electrophilic sites for all the compounds using the M05-2X level of the optimized geometry.
Computed MEPs for PhI(OAc) 2 , styrene and aldehyde derivatives are depicted in Figure 5.
e blue and red surfaces indicate positive and negative potential regions, respectively.In MEP images, as can be seen, the focus of the red color is on the phenyl ring of styrene; due to the concentration of HOMO orbitals on the phenyl ring, it can be concluded that styrene derivatives tend to give electrons from the phenyl ring.Aldehyde radicals play an electron accepting role, as can be seen on the aliphatic part of aldehyde, the focus of the LUMO orbitals is very low, and the blue color in the images MEP indicates a lack of electrons in this part.e MEP plots of PhI(OAc) 2 , styrene, and aldehyde derivatives suggest that the red surfaces, which are rich in electrons, interact with the blue surfaces.
One of the most useful methods for the examination of bond strength is QTAIM; this method provides accurate quantitative information about the electronic structure.is theory explains the concepts of chemical bonding and their associated properties.Topological parameters are a suitable tool for determining the bond strength.Surface analysis based on covalent bonds provides ELF and LOL maps, which indicates regions of molecular space where an electron pair is more likely to be found [22].ELF and LOL images have drawn using Multiwfn.e chemical concept of LOL and ELF is similar because they both depend on the kinetic energy density.ELF images were created by considering the electron pair density, and LOL show that gradients of      localized orbitals are maximized when localized orbitals overlap.
Color and contour maps of ELF and LOL for the molecule of styrene (1a), isobutyraldehyde (2a), and 3-methyl-1-phenylbutyl acetate (3a) are shown in Figure 6. e blue regions around hydrogen atoms show the delocalized electron cloud around it.e covalent regions are seen between bonds in 3a molecule, indicated by red color with high electron density, electron density Laplacian and ELF values, and the electron depletion regions between valence shell and inner shell are shown by the blue circles around the carbon.From Figure 6(d), the high ELF regions are seen around double bond and oxygen atoms of styrene and aldehyde derivatives indicating the presence of highly localized bonding and nonbonding electrons, respectively.

Conclusion
In this research, the calculations are done at the M05-2X level.
e reaction between styrene derivatives with isobutyraldehyde in the presence of PhI(OAc) 2 and Co(OAc) 2 was investigated by theoretical methods.Using cheap and readily available aliphatic aldehydes as an alkyl source is a good way to stretch the chain of benzyl esters and would render this decarbonylative alkylative esterification cascade reaction attractive for organic synthesis and medicine chemistry.e reaction energy of styrene derivatives with bromo and chloro substituted at para and meta positions is similar, which is characterized by substitutions having no effect.Co II (OAc) 2 has been used as an efficient catalyst.e greater the difference in values in the parameters μ, η, and ω between the reactants, and the faster the electron transfer and the faster the reaction.According to the shown HOMO and LUMO, we can say that styrene derivatives have as electron donors and the aldehyde radicals have the role of electron acceptor.Contour map of electron density, electron density Laplacian, LOL and ELF of styrene (1a), isobutyraldehyde (2a), and 3-methyl-1-phenylbutyl acetate (3a) molecules were depicted.e blue regions around hydrogen atoms show the delocalized electron.e covalent regions are seen between bonds, indicated by red color with high electron density.e reaction pathway of decarbonylative alkylative esterification of styrene derivatives with aliphatic aldehydes has been computationally studied.e energy of transition states TS1 and TS2 for all compounds were investigated.A mechanism including the Co II -Co III -Co I interconversion was proposed based on the mechanistic and experimental studies.

Data Availability
e authors express appreciation to the Research Council of the Islamic Azad University of Kerman for supporting this investigation.

Scheme 2 :Figure 2 :Figure 3 :
Scheme 2: e energy profile of the reaction pathway at the M05-2X level of theory (unit: kJ/mol).

Table 1 :
Energy of transition states TS1 and TS2 for all compounds (kJ/mol).