Hydrolysis of aryl orthocarbonates by general acid catalyzed and spontaneous processes. Characterization of the water reaction of (ArO)3COAr′ and (ArO)3CN3

Article abstract of DOI:10.1021/ja00092a009

Twenty-four aryl orthocarbonates of formula (Aro)₄C, (ArO)₂C(OAr′)₂, or (ArO)₃COAr′ have been made by coupling of the copper phenoxides with carbon disulfide, and their hydrolyses have been examined in 60% water-40% acetonitrile (I = 1.00 M, KCl) at 70.0 °C. Their hydrolysis by general acid catalyzed and, in the case of (ArO)₃COAr′ when Ar′ = p-nitrophenyl and p-cyanophenyl, spontaneous processes yields aryl carbonates. The initial, rate-determining event for both processes is the cleavage of the bond between the central carbon and the least basic phenoxy group: with mixed orthocarbonates of phenols differing in pK by <1 unit, parallel pathways are observed. The spontaneous reactions of (X-C₆H₄O)₃C-O-P-C₆H ₄NO₂ and (X-C₆H₄O)₃C-O-p-C₆H₄CN where the σ value of X varies from -0.23 to 0.22 are 4-10 times slower than reactions of p-nitrophenyl tetrahydropyrans reported in the literature and give rise to nonlinear Hammett plots. However, plots against the pK_a of X-C₆H₄OH are linear, and the derived β(reaction center) values indicate substantial buildup of positive charge on the central carbon atom. A similar plot is obtained for the spontaneous reactions of five azides (XC₆H₄O)₃CN₃ for which common ion inhibition experiments have been performed. The derived selectivities (M^-1) of the tris(aryloxy)carbenium ion intermediates between azide ion and water vary from 8.5 × 10³ to 6.6 × 10³ as the σ value of X changes from -0.28 to 0.06. The lifetime of the tris(aryloxy)carbenium ion intermediates in water is therefore likely to be >10^-6 s. The slow formation and slow hydrolysis of tris(aryloxy)carbenium ions suggests that (in the formation direction) development of conjugative stabilization lags behind carbon-oxygen bond cleavage and (in the hydrolysis direction) precedes carbon-oxygen bond formation. The stereochemistry of tetraaryl orthocarbonates immediately suggests reasons why this should be so.