Nucleofugality of phenyl and methyl carbonates

Article abstract of DOI:10.1021/jo701379n

(Chemical Equation Presented) 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°C. The LFER equation, log k = s_f(E_f + N_f), was used to derive the nucleofuge-specific parameters (N _f and s_f) for phenyl carbonate (1LG) and methyl carbonate (2LG) leaving groups in a given solvent in S_N1 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 (N_f = -0.84 ± 0.07 in 80% v/v aq EtOH) than methyl carbonate 2LG (N_f = -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 (ΔΔE_aff = 11.7 kcal/mol at the B3LYP/6-311++G(d,p) level and ΔΔE_aff = 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.

Full text of DOI:10.1021/jo701379n

Nucleofugality of Phenyl and Methyl Carbonates
Bernard Denegri and Olga Kronja*
Faculty of Pharmacy and Biochemistry, UniVersity of Zagreb, Ante KoVacˇic´a 1, 10000 Zagreb, Croatia
kronja@pharma.hr
ReceiVed June 26, 2007
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 °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 (∆∆Eaff ) 11.7 kcal/mol at the B3LYP/6-311++G(d,p) level and ∆∆Eaff ) 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.
Introduction
We assumed that in solvolytic conditions some aryl or alkyl
carbonates can also undergo heterolytic R-O bond cleavage.
A similar reaction was observed with allyl carbonates in which
the carbonate acts as a leaving group in palladium-catalyzed
substitution with a nucleophile.3
Alkyl and aryl carbonates react with strong nucleophiles,
yielding substitution products. The reaction is second-order and
proceeds via a tetrahedral intermediate formed in an initial
addition step to the carbonyl carbon atom, followed by displace-
ment of the alcohol (phenol) in a stepwise or concerted manner.1
This addition/elimination mechanism can be easily related to
the nucleophilic substitution that carboxylic esters typically
undergo. It has long been accepted that, if stabilized carbocation
intermediates are produced by cleavage of the R-O bond in
carboxylic esters, the hydrolysis mechanism changes, and in
weakly basic to acidic medium, the esters hydrolyze in a SN1
manner, producing the carbocation intermediate and the car-
boxylate ion (leaving group) in the rate-determining step.2
Therefore, we set out to examine solvolytic reactions that
proceed via the SN1 route in which a carbocation intermediate
arises from departure of an alkyl or aryl carbonate anion. We
have chosen to investigate the benzhydryl derivative of methyl
and phenyl carbonates for several reasons. First, benzhydryl
cations are such stable carbocations that their salts exist as stable
compounds.4 Even more important is that using the benzhydryl
(2) Bunnett. J. F. J. Am. Chem. Soc. 1961, 83, 4978-4983.
(3) (a) Verhelst, S. H. L.; Wennekes, T.; van der Marel, G. A.; Overkleeft,
H. S.; van Boeckel, C. A. A.; van Boom, J. H. Tetrahedron 2004, 60, 2813-
2822. (b) Tsuji, J.; Shimizu, I.; Minami, I.; Ohashi, Y.; Sugiura, T.;
Takahashi, K. J. Org. Chem. 1985, 50, 1523-1529.
* To whom correspondence should be addressed. Phone: +385 1 481 8301.
Fax: +385 1 485 6201.
(1) (a) Gresser, M. J.; Jencks, W. P. J. Am. Chem. Soc. 1977, 99, 6963-
6970. (b) Castro, E. A.; Andujar, M.; Toro, A.; Santos, J. G. J. Org. Chem.
2003, 68, 3608-3613. (c) Castro, E. A.; Compodonico, P; Toro, A.; Santos,
J. G. J. Org. Chem. 2003, 68, 5930-5935 and references cited therein.
(4) (a) Mayr, H.; Schneider, R.; Schade, C.; Bartl, J.; Bederke, R. J.
Am. Chem. Soc. 1990, 112, 4446-4454. (b) Hagen, G.; Mayr, H. J. Am.
Chem. Soc. 1991, 113, 4954-4961. (c) Bartl, J.; Steenken, S.; Mayr, H. J.
Am. Chem. Soc. 1991, 113, 7710-7716.
10.1021/jo701379n CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/04/2007
J. Org. Chem. 2007, 72, 8427-8433
8427

Denegri and Kronja
carbonates enabled us to determine the ability of alkyl and aryl
carbonate anions as the leaving groups quantitatively and relate
their solvolytic behavior to commonly used leaving groups.
Analogously to Mayr’s procedure in construction of the
nucleophilicity/electrophilicity scales,5 the nucleofugality/elec-
trofugality scales based on studying the solvolysis rates of a
large variety of substituted benzhydryl substrates with different
leaving groups in various solvents were developed recently. The
absolute rate of the heterolysis reaction (SN1) can be estimated
with reasonable accuracy according to the following three-
parameter LFER equation:6
leaving groups (e.g., chloride, bromide, tosylate, p-nitrobenzoate,
etc.).6a In order to calculate the nucleofugality parameters (Nf)
-
and the slope parameters (sf) for phenyl carbonate (C6H5OCO2
;
1LG) and methyl carbonate (CH3OCO2-; 2LG), logarithms of
the first-order rate constants were plotted against Ef. Excellent
linear correlations were obtained (Table 2). The nucleofuge-
specific parameters calculated from the equation given above
are presented in Table 2. Selected plots of log k against Ef are
given in Figure 1 (for all correlation lines, see Supporting
Information).
In order to obtain nucleofuge-specific parameters (Nf and sf)
for 1LG and 2LG in commonly used solvents in which kinetic
data could not be collected at conventional temperatures by
log k ) sf (Ef + Nf)
(1)
conventional methods, log k values were plotted against YOTs
.
in which k is the first-order rate constant (s-1), sf is the
nucleofuge-specific slope parameter, Nf is the nucleofugality
parameter, and Ef is electrofugality parameter.
From log k/YOTs plots obtained in 60-90% aq ethanol (v/v) for
both 1 and 2, the reaction rates in pure ethanol and in 50% aq
ethanol were obtained by extrapolation. Similarly, the extrapo-
lated rates for 1 and 2 were extracted for 70% aq methanol
(v/v) and for 1 in 80% aq acetone (v/v). By applying log k/Ef
correlations, the nucleofuge-specific parameters were calculated
in pure and 50% ethanol (v/v), in 70% methanol (v/v), and in
80% acetone (v/v) from extrapolated kinetic data (see Tables 1
and 2). YOTs has been chosen because of the similarity in
structure of the tosylate as a leaving group and the carbonates,
although correlations of log k with YCl and log k with YBr gave
the same results.
The nucleofugality (Nf) is obtained for the leaving group in
a given solvent, while the electrofugality parameters (Ef) are
independent variables that refer to the carbocation generated in
the heterolysis reaction (SN1). Such an approach separates the
contributions of electrofuge and nucleofuge to overall solvolytic
reactivity. Predefined parameters are Ef ) 0.00 for dianysyl-
carbenium ion (X ) Y ) 4-OCH3) and sf ) 1.00 for chloride
nucleofuge in pure ethanol.6a,c In this special type of the linear
free energy relationship, nucleofugality (Nf) of the given leaving
group is defined as the negative intercept on the abscissa of
log k (25 °C)/Ef plot.
According to the kinetic measurements, it is evident that
solvolysis of all substrates 1 and 2 follow SN1 type displacement
reaction. The most important parameters that indicate formation
of the positive charge in transition state are the slopes of the
reactions (Table 2), which are in the same range as sf obtained
in solvolysis of benzhydryl derivatives with other nucleofuges
studied earlier.6a For example, the corresponding slopes obtained
in solvolysis of benzhydryl tosylates are in the range between
0.75 and 0.89, while those of chlorides are between 1.02 and
0.98. Since Ef values correlate very well with Hammett σ+
values (r2 ) 0.992), the slope parameter can be easily related
to F+, which was already suggested by Bentley, stating that sfEf
is compatible with F+σ+.7 Therefore, sf values obtained show
that the extent of the positive charge on the reaction center
generated in the transition state in solvolysis of benzhydryl
carbonates is comparable to the extent of positive charge
generated in solvolysis of benzhydryl derivatives with common
leaving groups that follow the SN1 route. This conclusion rules
out the addition/elimination mechanism established for numer-
ous carbonates.
Results and Discussion
Kinetic Studies. A series of benzhydryl phenyl carbonates
(1) and benzhydryl methyl carbonates (2) were prepared from
the corresponding benzhydrols and phenyl chloroformate and
methyl chloroformate, respectively, according to the methods
presented in the Experimental Section. The substrates were
designed to enable all measurements at 25 °C, by adjusting the
electrofugality of the benzhydrylium system (selecting appropri-
ate X and Y on the benzhydryl rings). The solvolysis rates were
measured in various solvents at 25 °C conductometrically or/
and by potentiometric titration of the liberated acid. Details are
given in Kinetic Methods (Experimental Section). The first-
order rate constants are presented in Table 1.
Product Analysis. In order to get additional evidence for
the solvolysis mechanism, we analyzed the reaction products.
Series of benzhydryl phenyl carbonates 1 (X ) 4-OCH3, Y )
4-OPh; X ) 4-OCH3, Y ) 4-CH3; and X ) 4-OCH3, Y ) H)
and benzhydryl methyl carbonates 2 (X ) 4-OCH3, Y )
4-OCH3; X ) 4-OCH3, Y ) 4-OPh; X ) 4-OCH3, Y ) 4-CH3;
and X ) 4-OCH3, Y ) H) were solvolyzed for approximately
10 reaction half-lives in 100% methanol and 100% ethanol, and
the products were isolated and analyzed by means of NMR.
The key product that can rule out the second-order addition/
elimination reaction and prove SN1 displacement reaction is the
benzhydryl methyl ether (for methanolysis) and benzhydryl ethyl
ether (for ethanolysis) since these products can arise only if
substitution on the secondary benzhydryl carbon occurs. In pure
Electrofugality parameters (Ef) for all benzhydryl cations that
are generated in solvolysis of the carbonates investigated here
have previously been determined and also used as a basis set
of Ef values for determining Nf and sf parameters of numerous
(5) (a) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker,
B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.; Schimmel, H. J.
Am. Chem. Soc. 2001, 123, 9500-9512. (b) Mayr, H.; Kempf, B.; Ofial,
A. R. Acc. Chem. Res. 2003, 36, 66-77.
(6) (a) Denegri, B.; Streiter, A.; Juric´, S.; Ofial, A. R.; Kronja, O.; Mayr,
H. Chem.sEur. J. 2006, 12, 1648-1656. (b) Denegri, B.; Ofial, A. R.;
Juric´, S.; Streiter, A.; Kronja, O.; Mayr, H. Chem.sEur. J. 2006, 12, 1657-
1666. (c) Denegri, B.; Minegishi, S.; Kronja, O.; Mayr, H. Angew. Chem.,
Int. Ed. 2004, 43, 2302-2305.
(7) Bentley, T. W. Chem.sEur. J. 2006, 12, 6514-6520.
8428 J. Org. Chem., Vol. 72, No. 22, 2007

Nucleofugality of Phenyl and Methyl Carbonates
TABLE 1. Solvolysis Rate Constants of X,Y-Substituted Benzhydryl Phenyl Carbonates (1) and of X,Y-Substituted Benzhydryl Methyl
Carbonates (2) in Various Solvents Measured at 25 °C
b
b
substrate
solventa
100M
X, Y
Ef
kc (s-1
)
substrate
solventa
X, Y
Ef
kc (s-1
)
1
4-Me,4′-Me
4-MeO,H
-3.47 (1.19 ( 0.04) × 10-4
-2.06 (1.53 ( 0.05) × 10-3
(1.48 ( 0.01) × 10-3 d
1
50A50W 4-Me,4′-Me
-3.47 (3.64 ( 0.06) × 10-4
(3.60 ( 0.23) × 10-4 d
4-MeO,H
4-MeO,4′-Me
4-MeO,H
4-MeO,4′-Me
4-MeO,4′-PhO
4-MeO,4′-MeO
-2.06 (3.61 ( 0.07) × 10-3
-1.29 (1.56 ( 0.05) × 10-2
-2.06 (7.28 ( 0.15) × 10-5
-1.29 (4.56 ( 0.14) × 10-4
-0.81 (1.35 ( 0.05) × 10-3
0.00 (9.31 ( 0.15) × 10-3
-2.06 (2.16 ( 0.04) × 10-4
-1.29 (1.34 ( 0.02) × 10-3
-0.81 (3.56 ( 0.09) × 10-3
0.00 (2.36 ( 0.06) × 10-2
-2.06 (4.49 ( 0.05) × 10-4
-1.29 (2.82 ( 0.04) × 10-3
-0.81 (6.60 ( 0.06) × 10-3
0.00 (4.36 ( 0.06) × 10-2
4-MeO,4′-Me
-1.29 (8.87 ( 0.23) × 10-3
4-MeO,4′-PhO -0.81 (2.58 ( 0.08) × 10-2
2
100M
90M10W 4-Me,H
-4.68 (3.30 ( 0.05) × 10-5
-3.47 (4.06 ( 0.04) × 10-4
-2.06 (4.48 ( 0.12) × 10-3
(4.08 ( 0.10) × 10-3 d
4-Me,4′-Me
4-MeO,H
90M10W 4-MeO,H
4-MeO,4′-Me
4-MeO,4′-Me
-1.29 (2.40 ( 0.06) × 10-2
-4.68 (9.31 ( 0.16) × 10-5
-3.47 (1.03 ( 0.03) × 10-3
(1.05 ( 0.03) × 10-3 d
80M20W 4-Me,H
4-MeO,4′-PhO
4-MeO,4′-MeO
4-Me,4′-Me
80M20W 4-MeO,H
4-MeO,H
4-MeO,4′-Me
-2.06 (9.36 ( 0.08) × 10-3
-1.29 (5.03 ( 0.08) × 10-2
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
70M30W 4-Me,4′-Me
4-MeO,H
-3.47
-2.06
-1.29
-3.47
-2.06
-1.29
2.30 × 10-3e
1.86 × 10-2e
9.55 × 10-2e
1.17 × 10-5e
3.09 × 10-4e
1.58 × 10-3e
70M30W 4-MeO, H
-2.06
-1.29
-0.81
0.00
-2.06
-1.29
-0.81
0.00
9.12 × 10-4e
5.62 × 10-3e
1.23 × 10-2e
7.94 × 10-2e
1.23 × 10-5e
7.59 × 10-5e
3.47 × 10-4e
2.75 × 10-3e
4-MeO,4′-Me
4-Me,4′-Me
4-MeO,H
4-MeO,4′-Me
4-Me,4′-Me
4-MeO,H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO, H
100E
100E
90E10W
-3.47 (7.22 ( 0.17) × 10-5
-2.06 (1.27 ( 0.03) × 10-3
(1.31 ( 0.02) × 10-3 d
4-MeO,4′-Me
-1.29 (7.17 ( 0.13) × 10-3
90E10W
80E20W
70E30W
60E40W
50E50W
-2.06 (6.13 ( 0.17) × 10-5
-1.29 (3.44 ( 0.08) × 10-4
-0.81 (1.14 ( 0.01) × 10-3
0.00 (8.46 ( 0.06) × 10-3
-2.06 (1.39 ( 0.04) × 10-4
-1.29 (8.20 ( 0.08) × 10-4
-0.81 (2.16 ( 0.03) × 10-3
0.00 (1.57 ( 0.02) × 10-2
-2.06 (2.88 ( 0.11) × 10-4
-1.29 (1.53 ( 0.05) × 10-3
-0.81 (3.58 ( 0.07) × 10-3
0.00 (2.55 ( 0.05) × 10-2
-2.06 (5.22 ( 0.16) × 10-4
-1.29 (2.76 ( 0.08) × 10-3
-0.81 (5.76 ( 0.12) × 10-3
0.00 (3.97 ( 0.15) × 10-2
4-MeO,4′-PhO -0.81 (2.18 ( 0.06) × 10-2
80E20W
70E30W
60E40W
50E50W
4-Me,4′-Me
-3.47 (1.89 ( 0.02) × 10-4
(1.97 ( 0.04) × 10-4 d
4-MeO,H
4-MeO,4′-Me
-2.06 (2.71 ( 0.06) × 10-3
-1.29 (1.42 ( 0.03) × 10-2
4-MeO,4′-PhO -0.81 (3.93 ( 0.13) × 10-2
4-Me,H
4-Me,4′-Me
-4.68 (3.47 ( 0.05) × 10-5
-3.47 (4.17 ( 0.10) × 10-4
(4.48 ( 0.16) × 10-4 d
4-MeO,H
4-MeO,4′-Me
4-Me,H
-2.06 (5.00 ( 0.15) × 10-3
-1.29 (2.47 ( 0.05) × 10-2
-4.68 (7.85 ( 0.12) × 10-5
-3.47 (8.55 ( 0.07) × 10-4
(8.65 ( 0.16) × 10-4 d
4-Me,4′-Me
4-MeO,H
-2.06 (8.76 ( 0.17) × 10-3
-1.29 (4.09 ( 0.04) × 10-2
4-MeO,4′-Me
4-Me,4′-Me
4-MeO,H
-2.06
-1.29
-0.81
0.00
8.13 × 10-4e
4.37 × 10-3e
7.94 × 10-3e
5.37 × 10-2e
-3.47
-2.06
-1.29
-3.47
-2.06
-1.29
1.41 × 10-3e
1.29 × 10-2e
6.61 × 10-2e
3.55 × 10-6e
8.13 × 10-5e
4.47 × 10-4e
4-MeO, 4′-Me
4-MeO, 4′-PhO
4-MeO, 4′-MeO
4-MeO,4′-Me
80A20W 4-Me,4′-Me
4-MeO,H
70A30W 4-MeO, 4′-Me
4-MeO, 4′-PhO
-1.29 (1.33 ( 0.08) × 10-4
-0.81 (3.04 ( 0.03) × 10-4
0.00 (2.29 ( 0.06) × 10-3
-2.06 (7.78 ( 0.06) × 10-5
-1.29 (3.87 ( 0.13) × 10-4
0.00 (6.02 ( 0.13) × 10-3
-2.06 (2.39 ( 0.01) × 10-4
-1.29 (1.12 ( 0.02) × 10-3
0.00 (1.50 ( 0.01) × 10-2
4-MeO,4′-Me
4-MeO, 4′-MeO
70A30W 4-Me,4′-Me
4-MeO,H
-3.47 (2.94 ( 0.09) × 10-5
-2.06 (4.69 ( 0.08) × 10-4
(4.00 ( 0.00) × 10-4 d
60A40W 4-MeO, H
4-MeO, 4′-Me
4-MeO, 4′-MeO
4-MeO,4′-Me
-1.29 (2.32 ( 0.04) × 10-3
50A50W 4-MeO, H
4-MeO, 4′-Me
4-MeO,4′-PhO -0.81 (5.09 ( 0.02) × 10-3
60A40W 4-Me,4′-Me
-3.47 (1.04 ( 0.01) × 10-4
-2.06 (1.33 ( 0.02) × 10-3
(1.39 ( 0.06) × 10-3 d
4-MeO, 4′-MeO
4-MeO,H
4-MeO,4′-Me
-1.29 (5.85 ( 0.03) × 10-3
a Binary solvents are v/v at 25 °C. A ) acetone, E ) ethanol, M ) methanol, W ) water. b Electrofugality parameters are taken from ref 6a. c Average
k values (determined conductometrically unless otherwise noted) from three to six runs. Errors shown are standard deviations. d Determined titrimetrically
by means of a pH-Stat. e Extrapolated from log k/YOTs plot.
alcohols, the substrates were quantitatively converted to p-
methoxybenzhydryl methyl ether and p,p-dimethoxybenzhydryl
ether, respectively, unambiguously excluding addition/elimina-
tion mechanism.
Quantum Chemical Calculations. Kinetic measurements
revealed that phenyl carbonate (1LG) is a better nucleofuge
than methyl carbonate (2LG). Carbonates 1 react about 20 times
faster in all solvents used than the corresponding methyl
carbonates 2 (Tables 1 and 2). Also, the slopes (sf) obtained in
solvolysis of 1 are lower by about 0.1 than that obtained in
solvolysis of 2, indicating that the substituents on the benzhydryl
ring contribute slightly more to the solvolysis rates of methyl
carbonates than that of phenyl carbonates.
In order to rationalize the greater reactivity of the phenyl
carbonates than methyl carbonates, we carried out quantum
chemical calculations to establish the affinity of the leaving
groups toward benzhydrylium ion and also to get structural
features of the phenyl carbonate anion (1LG) and methyl
carbonate anion (2LG) that are responsible for greater reactivity
of 1. Calculations were performed using the Gaussian 03
J. Org. Chem, Vol. 72, No. 22, 2007 8429

Denegri and Kronja
TABLE 2. Nucleofugality Parameters Nf and sf for Phenyl
Carbonate (1LG) and Methyl Carbonate (2LG) in Various Solvents
leaving
group
b
solventa
Nfb
sf
rc (n)d
1LG
100M
-1.05 ( 0.13
-0.70 ( 0.12
-0.41 ( 0.14
-0.18 ( 0.25
-1.55 ( 0.09
-1.01 ( 0.07
-0.84 ( 0.07
-0.66 ( 0.09
-0.49 ( 0.10
-0.34 ( 0.23
-2.19 ( 0.00
-1.85 ( 0.08
-1.51 ( 0.07
-1.17 ( 0.15
-1.99 ( 0.02
-1.66 ( 0.05
-1.43 ( 0.08
-1.19 ( 0.09
-2.22 ( 0.11
-2.01 ( 0.06
-1.84 ( 0.07
-1.73 ( 0.11
-1.59 ( 0.13
-1.51 ( 0.17
-2.74 ( 0.32
-2.42 ( 0.02
-2.09 ( 0.00
0.88 ( 0.04
0.83 ( 0.03
0.79 ( 0.03
0.73 ( 0.07
0.98 ( 0.02
0.93 ( 0.02
0.87 ( 0.02
0.83 ( 0.02
0.79 ( 0.02
0.76 ( 0.06
0.96 ( 0.00
0.85 ( 0.02
0.80 ( 0.02
0.74 ( 0.03
1.02 ( 0.01
0.98 ( 0.02
0.95 ( 0.03
0.93 ( 0.04
1.15 ( 0.04
1.04 ( 0.02
0.99 ( 0.02
0.94 ( 0.03
0.90 ( 0.04
0.87 ( 0.06
0.97 ( 0.09
0.92 ( 0.01
0.87 ( 0.00
0.9982 (4)
0.9990 (4)
0.9984 (4)
0.9953 (3)
0.9998 (3)
0.9995 (4)
0.9994 (4)
0.9995 (4)
0.9993 (4)
0.9966 (3)
1 (3)
0.9996 (4)
0.9999 (3)
0.9992 (3)
0.9999 (4)
0.9997 (4)
0.9989 (4)
0.9984 (4)
0.9988 (4)
0.9996 (4)
0.9995 (4)
0.9986 (4)
0.9977 (4)
0.9959 (4)
0.9959 (3)
0.9999 (3)
1.0000 (3)
90M10W
80M20W
70M30We
100Ee
90E10W
80E20W
70E30W
60E40W
50E50We
80A20We
70A30W
60A40W
50A50W
100M
90M10W
80M20W
70M30We
100Ee
90E10W
80E20W
70E30W
60E40W
50E50We
70A30W
60A40W
50A50W
2LG
FIGURE 1. Selected plots of log k (25 °C) versus Ef for the solvolysis
of X,Y-substituted benzhydryl phenyl carbonates (1) and benzhydryl
methyl carbonates (2) in binary aqueous solvents of methanol, ethanol,
and acetone (v/v).
1LG and 2LG are presented in Figure 2, along with some
important parameters (bong lengths, NBO charges, and angles).
The second value for each datum in Figure 2 represents the
same parameter obtained by employing the solvent model
(PCM-B3LYP/6-311++G(d,p) level).
Both phenyl and methyl carbonates converged to energy
minimum structures (seven for phenyl carbonate and six for
methyl carbonate; see Supporting Information) which can be,
in analogy to rotamers of esters, thioesters, and chloroformates,
characterized as cis,cis-planar, cis,trans-planar, and trans,cis-
planar.9 The cis and trans refer to the position of the alkyl or
aryl substituents toward the carbonate carbonyl group. Because
of the large benzhydryl group, trans,trans-planar rotamers for
both 1 and 2 were not located as energy minimum structures.
For both carbonates 1 and 2, the most stable rotamer is the
cis,cis-planar (Figure 2).
a Binary solvents are v/v at 25 °C. A ) acetone, E ) ethanol, M )
methanol, W ) water. b Errors shown are standard errors. c Correlation
coefficient. d Number of data points. e Calculated from rates extracted from
log k/YOTs by applying log k/Ef correlations.
program suite.8 Geometries of carbonates 1 and 2 (X ) Y )
H) were fully optimized at B3LYP/6-311G(d,p) and B3LYP/
6-311++G(d,p) levels, and those of the anions 1LG and 2LG
at B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), and MP2/6-
311++G(d,p). Stationary points were characterized as minima
(no imaginary frequencies) by calculation of the harmonic
vibration frequencies. The unscaled B3LYP and MP2 frequen-
cies were used to calculate the zero-point energies. Total
energies and unscaled zero-point vibrational energies along with
Mulliken and NBO charges are given in the Supporting
Information. In order to check the influence of the solvent on
the structure of leaving groups 1LG and 2LG and their affinities
toward benzhydrylium ion, we applied the polarizable continuum
solvent model (PCM) with dielectric constant for methanol and
optimized of the most stable conformers of methyl and phenyl
carbonate ions at the B3LYP/6-311++G(d,p) level.
The affinity of the benzhydryl cation toward a given
carbonate, presented with the process indicated below (R is CH3
and C6H5, respectively)
Ph2CH+ + ROCOO- f Ph2CHOCO2R
is calculated according to the following equation, taking the
calculated energies for the most stable conformer:10
∆Eaf ) Ecalc(cis,cis-planar Ph2CHOCOOR) -
[Ecalc(Ph2CH+) + Ecalc(ROCOO-)] (2)
Optimized B3LYP/6-311++G(d,p) structures of the most
stable conformers of 1 and 2 and those of the leaving groups
Affinities along with some relevant data are presented in
Table 3. The results show that the affinity of the benzhydryl
cation toward 2LG (CH3OCO2-) is larger than that toward 1LG
(C6H5OCO2-) by more than 10 kcal/mol (∆∆Eaff ) 15.4 kcal/
mol at the B3LYP/6-311G(d,p) level and ∆∆Eaff ) 11.7 kcal/
mol at the B3LYP/6-311++G(d,p) level, respectively); that is,
the phenyl carbonate anion is more stabilized than the methyl
carbonate anion, which is in accordance with experimental
(8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2004.
(9) (a) Matzner, M.; Kurkjy, R. P.; Cotter, R. J. Chem. ReV. 1964, 64,
645-687 and references cited therein. (b) Nagy, P. I.; Tejada, F. R.; Sarver,
J. G.; Messer, W. S. J. Phys. Chem. A 2004, 108, 10173-10185 and
references cited therein.
(10) Schindele, C.; Houk, K. N.; Mayr, H. J. Am. Chem. Soc. 2002, 124,
11208-11214.
8430 J. Org. Chem., Vol. 72, No. 22, 2007

Nucleofugality of Phenyl and Methyl Carbonates
FIGURE 2. Optimized B3LYP/6-311++G(d,p) structures of benzhydryl phenyl carbonate 1 (the most stable conformer), benzhydryl methyl
carbonate 2 (the most stable conformer), phenyl carbonate ion 1LG, and methyl carbonate ion 2LG in the gas phase. Selected bond lengths (angstroms),
angles (degrees), and B3LYP/6-311++G(d,p) NBO charges (in parentheses) are given for the gas phase (first values) and in methanol (second
value; PCM-B3LYP/6-311++G(d,p) level).
TABLE 3. Affinities of Benzhydryl Cation toward R-carbonate Anions (1LG and 2LG), Group NBO Charges, and C-OR Bond Lengths in
1LG and 2LG
group NBO charges
a
leaving
group
level of
theory
∆Eaff
R-oxy
moiety
carboxylate
moiety
C-OR bond
length (Å)
(kcal mol-1
)
1LG (R ) Ph)
B3LYP/6-311G(d,p)
-120.18
-113.53
-22.51
-0.624
-0.562
-0.440
-0.571
-0.437
-0.411
-0.374
-0.431
-0.376
-0.438
-0.560
-0.429
-0.563
-0.589
-0.626
-0.569
1.680
1.605
1.444
1.598
1.479
1.465
1.405
1.461
B3LYP/6-311++G(d,p)
PCM- B3LYP/6-311++G(d,p)b
MP2(full)/6-311++G(d,p)
B3LYP/6-311G(d,p)
B3LYP/6-311++G(d,p)
PCM- B3LYP/6-311++G(d,p)b
MP2(full)/6-311++G(d,p)
2LG (R ) CH3)
-135.60
-125.23
-25.21
a ∆Eaff ) Ecalc(cis,cis-planar Ph2CHOCOOR) - [Ecalc(Ph2CH+) + Ecalc(ROCOO-); Ecalc values were corrected for (unscaled) ZPE. b Polarizable continuum
solvent model with dielectric constant for methanol.
findings that 1 solvolyzes faster than 2. Calculations employing
the solvent model (PCM) show the same but less pronounced
effect (∆∆Eaff ) 2.70 kcal/mol).
Structural Features and Reactivity of Carbonates. The
outstanding feature of both carbonates is that the negative charge
is delocalized to three oxygen atoms almost equally rather than
only two oxygens that are involved in resonance (see NBO
charges in Figure 2). This observation suggests that, besides
resonance, negative hyperconjugation11 occurs in both ions 1LG
and 2LG, presented in Scheme 1.
Because of the negative hyperconjugation, the C-OR bonds
are longer in both anions 1LG and 2LG than in corresponding
(11) (a) Forsyth, D. A.; Yang, J.-R. J. Am. Chem. Soc. 1986, 108, 2157-
2161. (b) Wiberg, K. B.; Rablen, P. R. J. Am. Chem. Soc. 1993, 115, 614-
625. (c) Nilsson Lill, S. O.; Rauhut, G.; Anders, E. Chem.sEur. J. 2003,
9, 3143-3153.
J. Org. Chem, Vol. 72, No. 22, 2007 8431

Denegri and Kronja
Interestingly, the effect of stabilization of two nitro groups
in 3,5-dinitrobenzoate (DNB) is smaller than the effect of the
phenoxy group in 1LG but larger than the effect of the methoxy
group in 2LG. However, the stabilization effect of both the
phenoxy and the methoxy group is larger than that of only one
nitro group, so the corresponding p-nitrobenzoates solvolyze
by 1 and 2 orders of magnitude slower, respectively. Figure 3
also shows that the solvent effect is the largest on chlorides,
somewhat less on bromide and tosylate, but much less pro-
nounced on weaker leaving groups in which charge delocal-
ization occurs.
Finally, according to eq 1, from earlier published Ef param-
eters for numerous electrofuges and from Nf parameters
presented here (Table 2), one can determine how to handle
carbonates during preparation. For example, the half-life of allyl
methyl carbonates (Ef ≈ -6 for any allyl cation)6a in 50% (v/
v) aqueous acetone at room temperature is about 2 years (k ≈
10-8), while the half-life of cumyl phenyl carbonate (Ef ) -5.0
for cumyl cation) in 100 MeOH at 25 °C is about 1 day (k ≈
10-6).
Experimental Section
FIGURE 3. Relative nucleofugalities Nf for typical leaving groups in
various solvents. Mixtures of solvents are given as v/v: solvents: E )
pure ethanol, 80E ) 80 aq ethanol, M ) methanol, 80M ) 80% aq
methanol.
Substrate Preparation.
NMR spectra are presented in the Supporting Information.
4-Methoxy-4′-phenoxybenzhydrol. The title compound was
prepared by a standard Grignard reaction of 4-phenoxyphenylmag-
nesium bromide (5.0 g, 20.1 mmol of 4-bromodiphenyl ether, and
0.8 g, 32.9 mmol of magnesium) with 4-methoxybenzaldehyde (2.5
g, 18.4 mmol) in ether. Recrystallization from light petroleum/ether
(4:1) afforded white crystals of 4-methoxy-4′-phenoxybenzhydrol
(3.12 g, yield ) 55%).
SCHEME 1
4-Methoxy-4′-methylbenzhydrol. The title compound was
prepared by a standard Grignard reaction of 4-methylphenylmag-
nesium bromide (8.0 g, 46.8 mmol of 4-bromotoluene, and 1.2 g,
49.4 mmol of magnesium) with 4-methoxybenzaldehyde (4.5 g,
33.1 mmol). Recrystallization from light petroleum/ether (4:1)
afforded white crystals of 4-methoxy-4′-methylbenzhydrol (4.9 g,
yield ) 65%).
4,4′-Dimethoxybenzhydrol, 4-Methoxybenzhydrol, 4,4′-Dim-
ethylbenzhydrol, and 4-Methylbenzhydrol were prepared by the
reduction of the commercially available substituted benzophenones
with sodium borohydride in methanol.
carbonates 1 and 2. Also, RO-C-CO angles are increased. This
trend is much more pronounced in phenyl carbonate anion 1LG
than in anion 2LG (see details in Figure 2). By detailed
comparison of the anion structures obtained computationally,
it could be seen that the amount of negative charge generated
is distributed differently in 1LG and 2LG since in 1LG the
phenyl group efficiently accommodates the negative charge to
its o- and p-positions by resonance. It is illustrative to compare
the distribution of the negative charge between the carboxylate
moiety and the RO group of the anion (Table 3). Even though,
in both cases, the oxygen in the RO group has a considerable
amount of negative charge, it is evident that in 1LG the phenoxy
group carries more partial negative charge than the methoxy
group in 2LG. In accord with this observation is an unusually
long C-O bond (1.6 Å) in 1LG. Therefore, because of more
efficient delocalization of the negative charge due to negative
hyperconjugation in 1LG, the solvolytic reactivity of 1 is larger
than that of 2. It should be noted that the same but less
pronounced structural features for anions were obtained by
applying the solvent model calculation.
It is useful to compare the leaving group ability (nucleofu-
gality) of carbonates and commonly used leaving groups. The
relative nucleofugalities are presented in Figure 3. The nucle-
ofugality of carbonates is comparable to those of some car-
boxylates in which the carboxylate anion is stabilized by
negative inductive effect of the substituent on the carboxylate
group. Thus, according to Nf values, the ability of leaving groups
decrease as follows:
4-Methoxy-4′-methylbenzhydryl phenyl carbonate (1, X )
4-OCH3, Y ) 4′-CH3). A solution of phenyl chloroformate (4.4
g, 28.0 mmol) in benzene (20 mL) was added dropwise to the
previously prepared vigorously stirring solution of 4-methoxy-4′-
methylbenzhydrol (4.3 g, 18.9 mmol) and pyridine (4.5 g, 57.0
mmol) in benzene (50 mL). The reaction mixture was stirred for
12 h under the atmosphere of argon at ambient temperature. The
solid pyridinium chloride was then removed by filtration, while
the excess of pyridine was removed by dilute hydrochloric acid
(vigorous stirring). The benzene layer was separated and washed
with aqueous sodium bicarbonate (1×) and water (2×) in the
separatory funnel. After drying over anhydrous sodium sulfate,
benzene was evaporated in vacuo to give pale yellow oil (4.6 g;
yield ≈ 70%).
Additional purification (column chromatography, preparative
chromatography, TLC, vacuum distillation, methods for crystal-
lization) resulted with decomposition of the carbonate. According
to the NMR spectra (see Supporting Information), the crude product
did not contain the starting materials and other noticeable impurities.
4-Methoxy-4′-phenoxybenzhydryl phenyl carbonate (1, X )
4-OCH3, Y ) 4′-OPh) was prepared from the 4-methoxy-4′-
phenoxybenzhydrol (1.5 g) according to the procedure described
for 4-methoxy-4′-methylbenzhydryl phenyl carbonate (1, 4-X )
4-OCH3, 4′-Y ) CH3), yielding the title compound as a pale yellow
OTs- > Br- > Cl- > CF3CO2- > C6H5OCO2- (1LG) >
DNB > CH3OCO2- (2LG) > PNB
8432 J. Org. Chem., Vol. 72, No. 22, 2007

Nucleofugality of Phenyl and Methyl Carbonates
TABLE 4. Experimental Concentrations (c0) of Proton Sponge
Base and the Substrates 1 and 2 in Kinetic Measurements of the
Solvolysis Rates of the Substituted Benzhydryl Carbonates
oil; yield ≈ 75%. 1, X ) 4-OCH3, Y ) 4′-OPh, was very unstable,
and its decomposition started with complete removal of benzene.
4-Methoxybenzhydryl phenyl carbonate (1, X ) 4-OCH3, Y
) H). 4-Methoxybenzhydrol (2.0 g) was treated according to the
procedure for preparation of 1, X ) 4-OCH3, Y ) 4′-CH3. After
drying over anhydrous sodium sulfate and evaporation of benzene
in vacuo, a residue was recrystallized from light petroleum affording
white crystals (yield ≈ 77%).
conductivity method
titrimetric method (pH-Stat)
c0(PSB)b
(mM)
end-point
(pH)
c0(S)
(mM)
solventa
c0(PSB)/c0(S) b
100M
90M10W
80M20W
90E10W 45.0-110.0
80E20W 45.0-60.0
70E30W 15.0-30.0
60E40W
70A30W 15.5
8.0-16.0
4.5-16.0
3.0-8.0
4.0-6.0
1.5-2.5
10.0
10.0
10.0
10.5
10.5
10.0
10.0
10.5
10.0
10.0
7.0
5.0
7.0
4.0-7.0
5.0
4.0-6.0
4.0-5.0
2.0-5.0
5.0
4,4′-Dimethylbenzhydryl phenyl carbonate (1, X ) Y )
4-CH3). The procedure is the same as described above for 1, X )
4-OCH3, Y ) H. Starting with 4,4′-dimethylbenzhydrol (2.0 g),
the title compound was obtained as white crystals (yield ≈ 70%).
4-Methylbenzhydryl phenyl carbonate (1, X ) 4-CH3, Y )
H). The procedure is the same as described above for 1, X )
4-OCH3, Y ) H. From 4-methylbenzhydrol (1.5 g), the title
compound was obtained as white crystals (yield ≈ 68%).
4,4′-Dimethoxybenzhydryl methyl carbonate (2, X ) Y )
4-OCH3). 4,4′-Dimethoxybenzhydrol (1.0 g, 4.1 mmol) and pyri-
dine (2.0 g, 25.3 mmol) were dissolved in dry benzene (20 mL). A
solution of methyl chloroformate (1.5 g, 10.5 mmol; commercially
available) in benzene (30 mL) was then added dropwise to the
previously prepared vigorously stirring solution. Stirring was
continued for the next 12 h in the argon atmosphere at ambient
temperature. The solid pyridinium chloride was removed by
filtration, while the excess of pyridine was removed by dilute
hydrochloric acid (vigorous stirring). The benzene solution was
washed with water in a separatory funnel (2×) and dried over
anhydrous sodium sulfate. Solvent and other possible volatile
materials were then evaporated in vacuo, yielding pale yellow oil
(1.0 g, yield ≈ 80%). Additional purification (column chromatog-
raphy, preparative chromatography, TLC, vacuum distillation,
methods for crystallization) resulted with decomposition of the
carbonate. According to the NMR spectra (see Supporting Informa-
tion), the crude product did not contain the starting materials and
other noticeable impurities.
1.5-2.5
25.0-55.0
15.0-30.0
4.0-15.0
1.5-3.0
5.0-15.0
2.5-4.5
1.5-3.0
4.5-8.0
60A40W
50A50W
8.0
3.0-4.5
4.0-5.0
a Binary solvents are v/v at 25 °C. A ) acetone, E ) ethanol, M )
methanol, W ) water. b PSB ) Proton sponge base: [1,8-bis(dimethy-
lamino)naphthalene]; S ) substrate (1 and 2).
tions for each given aqueous binary mixture presented in Table 4.
The optimal concentration range of the proton sponge base in each
aqueous binary mixture was determined to fulfill the following
criteria: (A) Correlation coefficients of the fitted conductivities were
r > 0.995; (B) deviations among rate constants calculated at 2τ1/2
,
3τ1/2, and 4τ1/2 for an individual run were within 5%; (C) deviations
of the rate constants for a given carbonate in the given concentration
range of the base were within experimental error (Table 1); (D)
rate constant for the solvolysis of the substituted benzhydryl
chlorides determined under the same conditions agreed within an
experimental error or were slightly decreased with respect to the
rate constants determined without a presence of the proton sponge
base (see Table S1 in the Supporting Information); and (E) the
values of the rate constants obtained conductometrically and
titrimetrically were the same in the limits of the experimental error
(Table 4).
4-Methoxy-4′-phenoxybenzhydryl methyl carbonate (2, X )
4-OCH3, Y ) 4′-OC6H5). The procedure is the same as described
above for 2, X ) Y ) 4-OCH3. From 4-methoxy-4′-phenoxyben-
zhydrol (1.0 g), the title compound was obtained as pale yellow
oil; yield ≈ 80%.
Titrimetric Method. Titrimetric rate constants for solvolysis of
the substituted benzhydryl phenyl carbonates were obtained by
means of pH-Stat with end-point set at pH ) 10.0-10.5, which
was the lowest pH that provided a complete ionization of the
liberated phenyl carbonic acid (Table 4). Titrimetric experiments
were carried out by Radiometer Analytical workstation TIM856.
Typically, 30 mL of (4.0-7.0) × 10-3 M solution of carbonate
was thermostated at 25 °C, and the liberated phenyl carbonic acid
was continuously titrated by using a 0.06-0.10 M solution of NaOH
in the same solvent mixture. First-order rate constants were obtained
by least-squares fitting of data to the first-order kinetic equation
for 3-4 half-lives.
4-Methoxy-4′-methylbenzhydryl methyl carbonate (2, X )
4-OCH3, Y ) 4′-CH3). The procedure is the same as described
above for 2, X ) Y ) 4-OCH3. From 4-methoxy-4′-methylben-
zhydrol (1.5 g), the title compound was obtained as a pale yellow
oil; yield ≈ 80%.
4-Methoxybenzhydryl methyl carbonate (2, X ) 4-OCH3, Y
) H). 4-Methoxybenzhydrol (1.5 g) was treated according to the
procedure described for 2, X ) Y ) 4-OCH3. After removal of
benzene and other volatile materials in vacuo, the residue was
recrystallized from the light petroleum, yielding white crystals (yield
≈ 80%).
Kinetic Methods.
Solvents were purified and dried according to the standard
procedures.
Acknowledgment. We gratefully acknowledge the financial
support of this research by the Ministry of Science, Education
and Sport of the Republic of Croatia (Grant No. 006-0982933-
2963). We thank Prof. Herbert Mayr and Prof. Zlatko Mihalic´
for their helpful comments.
Conductivity Method. Freshly prepared solvents (30 mL) were
thermostated at 25((0.01) °C for several minutes prior to addition
of the substrate. Typically, 10-40 mg of substrate was dissolved
in 0.10-0.15 mL of dichloromethane and injected into the solvent.
Concentration of the carbonates in the reaction mixtures was 10-3
to 5 × 10-3 M.
Increase of conductance during solvolysis was recorded auto-
matically by means of WTW LF 530 conductimeter using a Pt
electrode LTA 1/NS. Rate constants were obtained by least-squares
fitting of the conductivity to the first-order kinetic equation for 3-4
half-lives.
Supporting Information Available: Calculated energies, group
charges, and Cartesian coordinates for optimized geometries of
carbonates, 1H NMR and 13C NMR spectra of 1 and 2, and
corresponding alcohols, correlations of log k versus Ef for solvolyses
of the substituted benzhydryl carbonates. This material is available
free of charge via the Internet at http://pubs.acs.org.
In order to achieve a complete ionization of a liberated weak
acid, in solvolysis of substituted benzhydryl methyl carbonates and
substituted benzhydryl phenyl carbonates, a proton sponge base [1,8-
bis(dimethylamino)naphthalene] was added in a range of concentra-
JO701379N
J. Org. Chem, Vol. 72, No. 22, 2007 8433