Reactivity and effects of stabilization of diphenylphosphinate

Spojevi 4-fluorbenzhidril-difenil-fosfinat, 4-metilbenzhidril-difenil-fosfinat, 4,4'- dimetilbenzhidril-difenil-fosfinat i 4-metoksibenzhidril-difenil-fosfinat sintetizirani su reakcijom odgovarajućeg benzhidrilnog bromida sa srebrovom soli difenil-fosfinata u acetonitrilu uz vrlo visoko iskorištenje (iznad 90%). Konduktimetrijskom metodom su određene konstante brzine solvolize sintetiziranih spojeva u 60%-tnoj vodenoj otopini acetonitrila te 60%-tnoj i 80%-tnoj vodenoj otopini etanola. U navedenim otapalima u prvom stupnju razgradnje spojeva nastaju različito supstiturani benzhidrilni kation, odnosno elektrofug i difenil-fosfinatna izlazna skupina, odnosno nukleofug. Izmjerene vrijednosti logaritama konstanti brzine solvolize, log k korelirane su s Ef vrijednostima korištenih referentnih elektrofuga te su dobiveni pravci ovisnosti log k/Ef visokog stupnja korelacije (R2 > 0,994) koje opisuje LFER (linear free energy relationship) jednadžba: log k = sf (Ef + Nf). Iz korelacijskih dijagrama log k/Ef određeni su parametri nukleofugalnosti Nf i sf za difenil-fosfinatnu izlaznu skupinu: u 60%-tnom acetonitrilu Nf/sf = 0,08/0,86; u 60%-tnom etanolu Nf/sf = 0,49/0,89; u 80%-tnom etanolu Nf/sf = 0,19/0,93. Ako se uzmu u obzir postojeće prethodno određene Nf vrijednosti drugih nukleofuga u različitim otapalima, difenil-fosfinatna skupina spada u skupinu umjereno reaktivnih izlaznih skupina. U skladu s očekivanjima, heterolitička reakcija je brža u otapalu s većim udjelom vode (60%-tni etanol u odnosu na 80%-tni). Budući da je acetonitril aprotično otapalo, u etanolnim vodnim otopinama su određene veće Nf vrijednosti difenil-fosfinatne skupine. Uslijed različitih nagiba pravaca (sf) log k/Ef u 60%-tnom acetonitrilu i 80%-tnom etanolu, dolazi do njihove konvergencije, tj.inverzije relativne reaktivnosti. NBO (Natural bond orbital) analizom je ispitana delokalizacija elektronske gustoće stabilne konformacije supstrata i izlazne skupine te je ustanovljeno da na stabilnost izlazne skupine tijekom solvolitičkog prijelaznog stanja utječe negativna hiperkonjugacija u kojoj sudjeluju fosfor i dva kisikova atoma vezana za fosfor.The following compounds were synthesized: 4-fluorobenzhydryl diphenylphosphinate, 4-methylbenzhydryl diphenylphosphinate, 4,4'-dimethylbenzhydryl diphenylphosphinate, 4-methoxybenzhydryl diphenylphosphinate from the corresponding benzhydryl bromide and silver diphenylphosphinate in acetonitrile giving very high yields (above 90%). Synthesized compounds were subjected to solvolysis in 60% aq. acetonitrile, 60% aq. ethanol and 80% aq. ethanol. In mentioned solvents, in the first stage of the degradation of the compounds, X, Y-substituted benzhydryl cation, i.e. electrofuge and diphenylphosphinate leaving group, i.e. nucleofuge are formed. The measured values of the logarithms of the rate constants of solvolysis, log k are correlated with Ef parameters of the used reference electrofuge and plots log k vs Ef of the high degree of correlation (R 2 > 0,994) were constructed. Using correlation diagrams and LFER (linear free energy relationship) equation log k25°C = sf (Nf + Ef), nucleofuge-specific parameters Nf and sf for diphenylphosphinate leaving group were obtained: in 60% aq. acetonitrile Nf/sf = 0,08/0,86; in 60% aq. ethanol Nf/sf = 0,49/0,89; in 80% aq. ethanol Nf/sf = 0,19/0,93. If the nucleofuge – specific parameters of other leaving groups are taken into account, diphenylphosphinate belongs to the group of intermediate strong nucleofuges. As expected, heterolytic reaction is faster in solvent with higher amount of water (60% aq. ethanol vs. 80% aq. ethanol). Since acetonitrile is an aprotic solvent, in aqueous solutions of ethanol were determined higher Nf values of diphenylphosphinate. Due to different slopes of correlation log k/Ef in 60% aq. acetonitrile and 80% aq. ethanol, convergence appears, i.e. inversion of relative reactivity. Delocalisation of the electron density of stable substrate conformation and leaving group was examined by NBO (natural bond orbital) analysis. It has been found that the stability of the leaving group during the solvolytic transition state is influenced by the negative hyperconjugation in which phosphorus and two oxygen atoms bound to phosphorus are involved