Nucleofugality of Phenyl and Methyl Carbonates

Abstract

A series of X,Y-substituted benzhydryl phenyl carbonates 1 and X,Y-substituted benzhydryl methyl carbonates 2 were subjected to solvolysis in different methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The LFER equation, log k = sf(Ef + Nf), was used to derive the nucleofuge-specific parameters (Nf and sf) for phenyl carbonate (1LG) and methyl carbonate (2LG) leaving groups in a given solvent in SN1 type reaction. Kinetic measurements showed that phenyl carbonates solvolyze one order of magnitude faster than methyl carbonates. Optimized geometries of 1LG and 2LG at B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), and MP2(full)/6-311++G(d,p) levels revealed that negative charge delocalization in carbonate anions to all three oxygen atoms occurs due to negative hyperconjugation. Phenyl carbonate (1LG) is a better leaving group (Nf = -0.84 +/- 0.07 in 80% v/v aq EtOH) than methyl carbonate 2LG (Nf = -1.84 +/- 0.07 in 80% v/v aq EtOH) because of more pronounced negative hyperconjugation, which is characterized with a more elongated RO-C bond and more increased RO-C-CO angle in 1LG than in 2LG. Calculated affinities of benzhydryl cation toward methyl and phenyl carbonate anions (DeltaDeltaEaff = 11.7 kcal/mol at the B3LYP/6-311++G(d,p) level and DeltaDeltaEaff = 2.7 kcal/mol at the PCM-B3LYP/6-311++G(d,p) level in methanol, respectively) showed that 1LG is more stabilized than 2LG, which is in accordance with greater solvolytic reactivity of 1 than 2.

Solvolytic Reactivity of 2,4‐Dinitrophenolates

“…On the other hand, the s f values for DNPs are higher than those for phenyl carbonates in all solvents examined (Figure 2). [3] This discrepancy indicates that, for phenyl carbonates, both the earlier TS and the delocalized partial negative charge diminish the m OTs parameter in comparison to chlorides, whereas reduction of the m OTs values for DNP mainly arise from the intense charge delocalization in the LG moiety of the late TS. Consequently, the solvation effect is less important for DNP than for phenyl carbonate, which is not surprising considering that two nitro groups on the phenyl ring take part in accommodating the developing negative charge in the TS by resonance.…”

“…[3] 4-Fluorobenzhydryl 2,4-Dinitrophenyl Ether: Freshly cut potassium (1.0 g, 24.7 mmol) was added to a previously prepared stirring solution of 4-fluorobenzhydrol (5.0 g, 24.7 mmol) in anhydrous benzene (30 mL), and the solution was stirred for 2 h under an atmosphere of argon in an ice-cold bath. A solution of 1-fluoro-2,4-dinitrobenzene (9.2 g, 49.4 mmol) in benzene (10 mL) was then added dropwise with vigorously stirring and reaction mixture was stirred further for 1 h. The brown precipitate was filtered off and benzene was evaporated in vacuo to give a pale-yellow oil.…”

“…Such an approach separates the contributions of electrofuge and nucleofuge in overall solvolytic reactivity. Predefined parameters are: E f = 0.00 for dianisylcarbenium electrofuge (X = Y = 4-OCH 3 ) and s f = 1.00 for chloride nucleofuge in pure ethanol. [1a] According to equation (1), the nucleofugality (N f ) of the given leaving group is defined as the negative intercept on the abscissa of log k vs. E f plot.…”

“…ates [3] have already been determined. The latter leaving groups are produced by cleavage of the C-OC(O) bond in the heterolysis step in solvolysis.…”

Nucleophilicities and Nucleofugalities of Organic Carbonates

“…[2,3] Recently, Denegri and Kronja have studied the nucleofugality (leaving group ability) of phenyl and methyl carbonate (k 1 in Scheme 1) in different solvents. [4] tert-Butyl carbonates (tBocO-R), the most prominent carbonates, were not included in this study. As these data are of particular interest for synthetically working chemists, we now report on the nucleofugality of the tBocO group.…”