Ispitivanje Kalbe-Šmitove reakcije naftoksida alkalnih metala

Abstract

Mehanizam karboksilacije fenoksida različitih metala bio je tema brojnih eksperimentalnih i teorijskih istraživanja. Mada postoje brojni eksperimentalni rezulati o Kolbe-Šmitovoj reakciji naftoksida alkalnih i zemnoalkalnih metala, mehanizam obrazovanja hidroksinaftoevih kiselina do sada nije ispitivan teorijskim metodama. U okviru ove doktorske disertacije ispitan je mehanizam karboksilacije naftoksida natrijuma, kalijuma, rubidijuma i cezijuma u položajima 1 i 3. U slučaju natrijum 2-naftoksida (NaphO-Na) ispitan je i mehanizam reakcije u položaju 6. Ispitivanje mehanizma karboksilacije izvedeno je u korelaciji sa ispitivanjem strukturnih i elektronskih osobina učesnika u reakciji. Rezultati ove doktorske disertacije dobijeni su pomoću metoda funkcionala gustine koje su implementirane u programski paket Gaussian: B3LYP, B3LYP-D2 i M06- 2X u kombinaciji sa bazisnim skupovima LANL2DZ, 6-311+G(d,p) za ugljenik, kiseonik i vodonik i Def2-TZVPD za metal. Metode B3LYP-D2 i M06-2X dizajnirane su za modeliranje nekovalentnih interakcija, te su korišćene da bi se što bolje opisali disperzioni efekti na kratkim i srednjim međuatomskim rastojanjima (≤ 500 pm). Proračuni su izvedeni za gasovitu fazu. Urađena je NBO (Natural Bond Orbital) analiza većine optimizovanih geometrija radi utvrđivanja efekata koji proističu iz elektronske strukture. Istraživanje mehanizma Kolbe-Šmitove reakcije naftoksida alkalnih metala (NaphO-M) pomoću funkcionala B3LYP pokazalo je da, kao i u slučaju fenoksida alkalnih metala, reakcija započinje građenjem stabilnog intermedijernog kompleksa V- M. Što se tiče reakcionih puteva u položajima 1 i 3, ugljenik iz SO2 vrši elektrofilni napad na S1 prstena, što vodi do nastanka metalnog 2-hidroksi-1- naftoata (E1-M). Iznenađujuće je da prelazno stanje za elektrofilni napad na S3 naftalenskog prstena nije pronađeno. Građenje 3-hidroksi-2-naftoata (E3-M) objašnjeno je 1,3-premeštanjem SO2M grupe. Treba istaći da prelazna stanja za 1,3-premeštanje kod reakcija NaphO-Rb i NaphO-Cs nisu pronađena. Reakcija u položaju 6 ispitana je samo za NaphO-Na. Ni ovde nije nađeno prelazno stanje za elektrofilni napad na S6 naftalenskog prstena. Nađeno je da reakcioni put 6 8 započinje elektrofilnim napadom na S8, što posle uzastopnih premeštanja CO2Na grupe vodi do građenja intermedijera D6-Na. Dalje se događa homolitičko raskidanje veze C6-H pri čemu nastaje radikal koji vezivanjem za atom vodonika gradi sporedni proizvod reakcije natrijum 6-hidroksi-2-naftoat (E6-Na). Rezultati dobijeni pomoću funkcionala M06-2X i B3LYP-D2 su međusobno u dobroj meri konzistentni, a dosta se razlikuju od rezultata dobijenim pomoću metode B3LYP. NaphO-Na i SO2 mogu da nagrade dva kompleksa: V-Na (već opisan) i S-Na. Dok V-Na ne može da se dalje transformiše do reakcionih proizvoda, SO2 grupa u S-Na zauzima idealan položaj za elektrofilni napad u sva tri položaja: 1, 3 i 6. Svaki reakcioni put se odigrava preko odgovarajućeg prelaznog stanja u kom se gradi nova S-S veza, i intermedijera D-Na. U sledećem, bimolekulskom reakcionom koraku, dva intermedijera D-Na razmenjuju protone susedne SO2 grupama. Ovi intermolekulski reakcioni koraci zahtevaju značajno manje energije aktivacije u poređenju sa intramolkulskim prelazom protona sa ugljenika na kiseonik. Bez obzira na primenjenu metodu mogu se izvesti sledeći zaključci: U poređenju sa reakcijama karboksilovanja naftoksida u položajima 1 i 3, reakcioni put u položaju 6 je i kinetički i termodinamički daleko nepovoljniji. Putevi 1 i 3 su kompetitivni, pri čemu put 1 zahteva niže energije aktivacije ali put 3 vodi do stabilnijeg proizvoda. Ovaj zaključak je u saglasnosti sa eksperimentalnim rezultatima koji su pokazali da na dosta niskoj temperaturi od 20oS nastaje samo E1-M, dok sa povećanjem temperature raste prinos proizvoda E3-M i E6-M. Pošto se Kolbe-Šmitova reakcija izvodi na visokim temperaturama glavni poizvod reakcije je termodinamički najstabilniji E3-M.

The mechanism of the carboxylation reaction of different metal phenoxides has been a subject of numerous experimental and theoretical investigations. In spite of the fact that there are many experimental results on the Kolbe-Schmitt reaction of alkali and alkaline earth metal naphthoxides, the mechanism of formation of hydroxy naphthoic acids has not been investigated by means of theoretical methods. Within this doctoral dissertation the mechanism of the carboxylation reaction in the positions 1 and 3 was examined for the naphthoxides of sodium, potassium, rubidium, and cesium. In the case of sodium 2-naphthoxide (NaphO-Na) the mechanism in the position 6 was also examined. The examination of the carboxylation reaction mechanism was carried out in correlation with the investigation of structural and electronic properties of the participants in the reaction. The results of this doctoral dissertation were obtained by means of the density functional methods which are implemented in the Gaussian program package: B3LYP, B3LYP-D2, and М06-2X. These methods were applied in combination with the following basis sets: LANL2DZ, 6-311+G(d,p) for carbon, oxygen, and hydrogen, and Def2-TZVPD for metals. The B3LYP-D2 and М06-2X methods have been designed for modeling noncovalent interactions, and thus, they were used to describe dispersion effects at short and medium interatomic distances (≤ 500 pm). The calculations were performed for the gas-phase. The NBO (Natural Bond Orbital) analysis was performed for majority of the optimized geometries for explanation of the effects which result from the electronic structure. The investigation of the mechanism of the Kolbe-Schmitt reaction of the alkali metal naphthoxides (NaphO-М) was carried out by using the B3LYP functional. Similarly to the case of the alkali metal phenoxides, it was shown that the reaction begins with the formation of a stable intermediate complex В-М. As for the reaction pathways 1 and 3, the carbon from СО2 performs an electrophilic attack on C1 of the ring, which leads to the formation of the metal 2- hydroxy-1-naphthoate (Е1-М). It is surprising that a transition state for an electrophilic attack on C3 of the naphthalene ring was not revealed. The formation of 3-hydroxy-2-naphthoate (Е3-М) was explained with a 1,3-shift of the СО2М group. It should be pointed out that transition states for the 1,3-shift in the reactions of NaphO-Rb и NaphO-Cs were not revealed. 10 The reaction in the position 6 was examined only in the case of NaphO-Na. Again, a transition state for an electrophilic attack on C6 of the naphthalene ring was not revealed. It was found that reaction pathway 6 begins with an electrophilic attack on C8 followed with few consecutive rearrangements of the CO2Na group, and yields the D6-Na intermediate. Further, a homolytic cleavage of the C6-H bond takes place, and a radical is built. This radical reacts with a hydrogen atom, and in this way the minor reaction product sodium 6-hydroxy-2-naphthoate (E6- Na) is built. The results obtained by using the М06-2X and B3LYP-D2 functionals are mutually consistent to a great extent. At the same time, they are significantly different from those obtained with the B3LYP method. NаphO-Na and СО2 can build two complexes: В-Na (already described) and С-Na. While В-Na cannot transform to yield the reaction products, the СО2 group in С-Na takes an ideal position for аn electrophilic attack in all three positions: 1, 3 and 6. Each reaction pathway occurs via a corresponding transition state where a new С-С bond is being formed, and the intermediates D-Na. In the next, bimolecular reaction step, two D-Na intermediates exchange the protons adjacent to the СО2 groups. These intermolecular reaction steps require significantly lower activation energies in comparison to intramolecular proton transfer from the carbon to the oxygen. Independently of the applied method, the following conclusions can be made: In comparison to the carboxylation reactions of naphthoxides in the positions 1 and 3, reaction pathway in the position 6 is both kinetically and thermodynamically unfavorable. Pathways 1 and 3 are competitive, where pathway 1 requires lower activation energies, but pathway 3 leads to the more stable product. This conclusion is in agreement with the experimental results which showed that at rather low temperature of 20оС only Е1-М is formed, whereas the yields of the products Е3-М and Е6-М increase with the increasing temperature. Since the Kolbe-Schmitt reaction is performed at high temperatures the major reaction product is thermodynamically most stable E3-М.