Abstract
The substitution of unsymmetrical tetra-alkyltins, reaction (1; R = Me, Et, Pr n, Bu i, and Pe neo), in solvent methanol at 30 C has been studied, R-SnPr[i/3] + Hg(OAc)[2] → Pr[i/3] SnOAc + RHgOAc (1) The reactions have been shown to follow second order kinetics, first order in each reactant. The order of reactivity was found to be: R - Me > Et > Pr[n] >> Bu[i] > Pe[neo] > Pr[i]. This order is quantitatively and qualitatively very similar to the sequences found in previous work for substitution of symmetrical tetra-alkyltins by mercury(II) chloride and by mercury(II) acetate in solvent methanol. By analogy with these former reactions, reaction (1) is deduced to proceed with retention of configuration at the carbon atom undergoing substitution. The substitution of tetraethyltin by mercury(II) carboxylates, reaction (2; R' = Bu[t], Et, Me, ClCH[2]CH[2], MeOCH[2], and CICH[2]) has been studied kinetically both in the protic Et[4]Sn + Hg(OCOR')[2] → Et[3]SnOCOR' + EtHgOCOR' (2) solvents methanol and tertiary butanol and in the aprotic solvents acetonitrile and acetone at 30 °C, The reactions have been shown to follow second order kinetics, first order in each reactant. The order of reaction rates observed for reaction (2) both in methanol and tertiary butanol was: R' = Bu[t] < Et < Me < ClCH[2]CH[2] < MeOCH[2] < CICH[2]. Stability constants for the Hg[2+] / RCO[2-] systems in methanol have been calculated and together with the observed rate constants for reaction (2) have been used to deduce the actual rate constants for attack of the species RCO[2]Hg[+], (RCO[2])[2]Hg, and (RCO[2])[3]Hg[-] on tetraethyltin; it is suggested that reaction (2) in methanol and tertiary butanol takes place by the S[E]2(open) mechanism, The reactivity sequence found for reaction (2) in solvent acetone and acetonitrile was: R' = Bu[t] < Et < Me < ClCH[2]CH[2] < MeOCH[2] < CICH[2]. The effect of changing the electrophile on the rate of reaction (3; R = Me, and Et; X = Cl, I, and OAc) in solvent acetone, the dissection of activation parameters into initial R[4]Sn + HgX[2] < R[3]SnX + RHgX (3) state and transition, state effects, and kinetic salt effect studies, all suggest that reaction (3) in solvent acetone and acetonitrile proceeds with a mechanism most probably intermediate between S[E]2(open) and S[E]2 (cyclic). lodinolysis of symmetrical tetra-alkyltins, reaction (4; R = Me, Et, Pr[n], Bu[i], and Pe[neo]), in solvent methanol R[4]Sn + I[2] → R[3]SnI + RI (4) at 20°C revealed the following reactivity sequences R = Me > Et > Pr[n] > Bu[i] ? Pe[neo] > Pr[i].