The R-Effect in Methyl Transfers
for 15 min. A sample of 0.026 mmol of fresh 2 fluoroborate
was placed in the methanolic sample and allowed to stand for
3 h at 30 °C. The 2 fluoroborate salt dissolved immediately in
the solution. After 3 h a gcms sample was introduced into the
inlet of the GC equipped with an MSD that we have previously
used for our work.8,13-15 The column was a methylsilicon gum
cross-linked capillary (12 ft × 0.33 µm). Analysis of the MS
and results was obtained on software supplied with the GCMS.
This system was previously8 found to be linear in grams ((5%)
for samples determined on it. The krel was obtained by the ratio
of integrated areas of the methyl products of the reactions,
adjusted where appropriate by ratios of molecular weights. The
krel values for the other phenolate anion were determined
similarly. The results are summarized in Table 1, and plotted
in Figure 2.
AgCl electrode and 5 nM ferrocene as an internal standard.
The potentials are corrected to the SCE electrode.
Com p u ta tion a l ch em istr y was performed by Hyperchem
v. 5.5 or v. 6 on Gateway computers operating Windows 98 at
366 or 400 mHz. The methyl cation affinity (MCA) and the
methyl radical affinity (MRA) of dibenzothiophene were com-
puted as reported for aryl methyl sulfides.13 The values were
(PM3) -97.80 eV for MCA and -16.49 eV for MRA.
Resu lts
The kinetic data from reaction III are summarized in
Table 1, and plotted in Bro¨nsted-type plots in Figure 2.
The slopes of the two R-effect plots by GCMS and HNMR
are very similar, thus the data are at least internally
consistent. The sizes of these slopes are both smaller than
those reported for GNMBH anions reacting with aryldi-
methylsulfonium ions.14 This ânuc value was reported as
The lower line in Figure 2 shows the goodness of fit for the
plot of the phenolates (R ) 0.999). The last point on the line
at 15 pKa units was obtained by competition of 4-methoxy-
phenolate vs 4-methoxyNMBH anion that was already deter-
mined vs 3-nitrophenolate anion. Correction for the krel of the
4-methoxyNMBH anion gave the point for 4-methoxyphenolate
on the phenolate line. The topmost line of Figure 2 shows the
G-NMBH anion line. This inverse extrapolated point gave us
a measure of the validity of the pair of lines for the phenolates
and the G-NMBH anions because, apparently, one line would
predict points on the other line.
0.85, with a âlg value14 of 0.42.
Me
Comparison of these ânuc values indicates that GNBMH
R-nucleophiles in reaction III transfer less charge than
in the reaction of GNMBH anions with substituted
aryldimethylsulfonium ions. In reaction III the slope for
the substituted phenolates is about that for substituted
phenolates reacting with methyl-substituted benzene-
sulfonates (0.31),8 whereas for the substituted aryldi-
methylsulfonium case it was 0.45. Both values are in the
normal range for a normal SN2 reaction.
From these data we obtain the result that reaction III
with R-nucleophiles is more like a normal SN2 reaction
than methyl transfers from substituted aryldimethyl
sulfonium ions. However, an R-effect of about 1.137 (logkR
- log knormal) is still expressed. This R-effect is greater
than any expressed in the substituted aryldimethylsul-
fonium case (0.222 to 0.926),12 but considerably smaller
than R-effects from 1-naphthyldimethylsulfonium (1.512),
or 9-anthracenyl (1.835).13
Deter m in a tion s of kr el for G-NMBH Ion s. An a lysis by
GCMS. Competition reactions with both G-NMBH and 3-ni-
trophenolate were set up as for krel for the phenolates, as above.
After an initial line was established (topmost in Figure 2)
selected points were verified with competitions with nearly pKa
matched phenolates.
The identities of the G-NMBHOCH3 peaks on the gc trace
were verified as previously reported,3 by spiking samples of
the reaction mixtures with authentic material. In some cases
small amounts of impurities overlapped the areas of the
desired product peaks, and had to be eliminated by a manual
integration routine, supplied by the software. A line was
dropped connecting the point of intersection of each impurity
with the known peak with the baseline by means of the mouse
pointer. This line was extended along the baseline until the
known peak joined the baseline and the areas were recorded
digitally. This somewhat-less-than-optimum method of deter-
mining the areas gave the triangular points on the top line of
Figure 2. The scatter is apparent by looking at the figure. We
thought it necessary to validate this line because of the
difficulty in obtaining the exact areas of small peaks on the
gc. The next section gives the attempt at validation by HNMR.
The response of the R-effect to electron demand can
be determined by subtracting the regression equation for
Bro¨nsted-type plots for substituted N-methylbenzohy-
droxamates from the regression equation for the nor-
mal nucleophiles (phenolates) and differentiating the
result to get ∂ R-effect/∂ pKaH ) (â(nuc;R-nucleophile)
-
â(nuc;normal nucleophile)).15 It is worthwhile noting that the
response of (III) to increasing electron demand in the
R-nucleophiles (GNMBH) is less for (III), 0.586-0.309 )
0.277, than for the substituted aryldimethylsulfonium
case, 0.43.15
Deter m in a tion s of kr el for G-NMBH An ion s vs 3-n itr o-
p h en ola te a n d 4- Nitr op h en ola te An ion s via HNMR.
Several G-NMBH anions were allowed to compete with 3-nitro-
and 4-nitrophenolate ions in methanol-d4. The HNMR spectra
were obtained after concentrating the solutions to ca. 2-3 mL.
Integral traces were obtained digitally for the products. The
three clearest points (no overlapping peaks) were taken as
indicating the krel by this method. These values appear as
Plotting the size of the R-effect vs the reduction
potentials gives Figure 3. The regression coefficient for
the three aryldimethylsulfonium reaction with 3-nitro-
phenolate vs 4-ClNMBH, determining the R-effect, shows
a very good fit for the three aryldimethylsulfonium ions
implying that these three ions are of the same type of
dependence on the SET parameter. There is a much
different dependence for (III) on the SET-type parameter.
The size of the R-effect is more like the phenyldimeth-
ylsulfonium value than like a sulfonium cation between
the 1-naphthyl and 9-anthracenyl cases as would be
expected from the size of the reduction potential.
1
circles in the middle line of Figure 2, and as H NMR in Table
1. The NMR has been described previously.8
Deter m in a tion of th e Red u ction P oten tia ls of S-
Met h yld ib en zot h iop h en iu m F lu or ob or a t e, P h en yld i-
m eth ylsu lfon iu m , 1-Na p h th yld im eth ylsu lfon iu m , a n d
9-An th r a cen yld im eth ylsu lfon iu m F lu or obor a tes. These
parameters were obtained in acetonitrile, as previously re-
ported for the aryldimethylsulfonium fluoroborates.14 The
values were 1388 mV for S-methyldibenzothiophenium, 957
mV for phenyldimethylsulfonium, 1252 mV for 1-naphth-
yldimethylsulfounium, and 1528 mV for 9-anthracenyldi-
methylsulfonium. The standard errors in all were (5 mV.
Discu ssion
These values were obtained by cyclic voltammetry, using
acetonitrile solutions containing 0.05 M tetrabutylammonium
tetrafluoroborate, at a sweep rate of 100 mV/s with an Ag/
The expectation that reaction III would give less
R-effect than with aryldimethyl sulfonium ions is not
J . Org. Chem, Vol. 68, No. 5, 2003 1813