2740 J . Org. Chem., Vol. 62, No. 9, 1997
Ta ble 2. Su m m a r y of r-Effect Da ta for G-NMBH An ion s
Fountain et al.
ref
Me
electrophile
ânuc
âlg
R-effect
45
13
charge type
solvent
p-nitrophenylacetate
benzyl bromide
methyl arenesulfonates
0.23
0.31
0.86
0.80
0.85
δ-,δ-
δ-,δ-
δ-,δ-
δ-,δ-
δ-,δ+
12% EtOH/H2O
12% EtOH/H2O
methanol-d4
H2O-20% dioxane
methanol-d4
14
15
4
4
0.44
0.42
2.5-3.5
3.5-4.8
phenyldimethylsulfonium
0.6-8.45
this work
on the O- atom in methoxide is spread into the H atoms,
giving a shorter C-O bond than in methanol. Similarly,
Shi and Boyd, using the Bader population analysis at
high levels of theory (MP2//6-31++G**),18 point to nega-
tive charge deposited on H atoms in SN2 transition states.
A similar occurrence is possibly the case with the
present systems. Accepting the putative sum of bond
orders to C in Table 3 for normal nucleophiles as ca. 0.88
(averaged), then for the G-NMBH-Me2S+Ar system an
excess of 0.14 electron/H atom would have to be accom-
modated over the amount in a normal nucleophilic attack.
The necessity for H atoms to bear all this excess charge
is lessened by the ability of the leaving group to withdraw
charge in the transition state. The phenyl sulfates,
arenesulfonates, and methyl sulfides are apparently quite
good at this charge withdrawal; thus they show the
R-effect. With a less good leaving group no R-effect, or a
much diminished one, would occur. Reports of no R-effect
with halide leaving groups19 are consistent with less
ability to take away negative charge from the C atom in
the TS. These ideas are consistent with Bordwell’s
discussion of factors previously recognized as favoring
electron transfer from a nucleophile to an electrophile,
RX.20 A strong electron receptor in R is one such
prominent factor.21
Ta ble 3. Su m m a r y of Bon d Or d er In for m a tion fr om
Br o1n sted Typ e a n d Br o1n sted Typ e-Lew is P lots for
Nu cleop h ilic Atta ck on Meth yl Ar en esu lfon a tes a n d
P h en yld im eth ylsu lfon iu m Ion s
BO
nuc
(ânuc
leaving grp
Me
system
)
(1.0-âlg
)
ΣBO
G-NMBH-MeO3SAR
G-NMBH-Me2S+Ar
G-PhO- MeO3SAR
G-PhO-Me2S+Ar
0.86
0.85
0.56
0.58
0.54
0.46
1.42
1.43
0.85
0.91
0.31a
0.45
a
Computed analytically from data obtained by competition
experiments
group) are 0.56 and 0.58 between the C atom and the
methyl aryl sulfide leaving groups. Some comment on
Me
the meaning of this difference in the size of âlg is in
order. Small differences in this parameter are accepted
in the literature as meaningful.27 With methyl arene-
sulfonates a difference of 0.45-0.47 indicated a change
from neutral ethanol to ethoxide.27a The SN2 displace-
Me
ment of allyl from arenesulfonates had a âlg ) 0.51,
indicating π assistance, exo-norbornyl arenesulfonates
Me
had âlg ) 0.57, and cyclobutyl had 0.55. In each case
Me
the larger âlg was associated with a greater electronic
assistance in promotion of bond cleavage between C and
the O3SAr group. The regression coefficients for these
literature determinations ranged from 0.992 to 0.998.
Figure 1 shows very comparable R values for the present
work, and the difference (0.44-0.42 ) 0.02) is consistent
with the interpreted literature differences. These facts
Other lower LUMO (LL) substrates,22 such as >CdO
groups, exhibit greater R-effects because they disperse
the negative charge from the C atom to the O atom,
where it is more accessible to H-bonding in donor
solvents.4 This idea of an intrinsic electronic R-effect
which is modified by extrinsic factors, such as transfer
of excess charge to atoms where H-bonding can stabilize
it, can be substantiated within the current models of
nucleophilic behavior. The next sections discuss this.
Shaik’s SCD model for the SN2 transition state26
combined with Hoz’s model for the R-effect22 afford
reasonable models to accommodate our data. These
models are particularly compelling because of the known
ability of aryldimethylsulfonium ions to undergo SET
Me
indicate that interpretation of our differences in âlg is
Me
warranted. Apparently the smaller âlg is associated
with the better nucleophile toward aryldimethylsulfo-
nium ions in the present case. Apparently the methyl
aryl sulfide responds more to the transfer of large
amounts of charge than the arenesulfonates. The exact
nature of the response of methyl arenesulfonates to
nucleophiles is yet nearly unexplored.8 The difference
between the ânuc value for the phenolates (0.45)8 and the
G-NMBH anions (0.85) and the âlgMe values for these two
anionic systems indicates something fundamentally dif-
ferent occurs in their respective transition states.
Table 3 shows the sums of the bond orders to C at the
transition state for the two nucleophilic-electrophilic
systems. A trend is clear that the G-NMBH anions
contribute more charge to the SN2 transition state (TS)
than the phenolate “normal nucleophiles”.
•
concomitantly with expulsion of a group, such as CH3 .
Saeva et al.23 have specifically examined this point and
report that, in contrast to preliminary capture of an
electron to give a sulfuranyl or aryl radical anion,24
electrochemical data are consistent with a concerted σ
S-C bond breaking concomitant with electron accep-
tance. This conclusion was subsequently modified by
Save`ant, Saeva, et al.25 who show in electrochemical
experiments that capture of an electron in SET transac-
The major question implicit in Table 3 is “Where does
the extra charge in the TS go?” At least a partial answer
comes from the study of AIM17-based bond lengths and
charge distributions in methanol and methoxide by
Wiberg.16 In this study the excess charge distribution
(18) Shi, Z.; Boyd, R. J . J . Am. Chem. Soc. 1989, 111, 1575.
(19) Gregory, M. J .; Bruice, T. C. J . Am. Chem. Soc. 1967, 89, 4400.
(20) Bordwell, F. G.; Clemmens, A. H. J . Org. Chem. 1981, 46, 1036.
(21) Kornblum, N. Angew. Chem., Int. Engl. Ed. 1975, 14, 734.
(22) Hoz, S. J . Org. Chem. 1982, 47, 3545.
(23) Saeva, F. D.; Morgan, B. P. J . Am. Chem. Soc. 1984, 106, 4121.
(24) Beak, P.; Sullivan, T. A. J . Am. Chem. Soc. 1982, 104, 4450.
(25) (a) Andrieux, C. P.; Robert, M.; Saeva; F. D.; Save`ant, J .-M. J .
Am. Chem. Soc. 1994, 116, 7864. (b) Saeva, F. D.; Breslin, D. T.; Martic,
P. A. J . Am. Chem. Soc. 1989, 111, 1328.
(26) Shaik, S. S.; Schlegel; H. B.; Wolfe, S. Theoretical Aspects of
Physical Organic Chemistry; J ohn Wiley & Sons, Inc.: New York, 1992;
Chapter 4.
(14) (a) Dessolin, M.; Laloi-Diard, M.; Vilkes, M. Tetrahedron Lett.
1975, 2405. (b) Dessolin, M.; Laloi-Diard, M. Bull. Soc. Chim. Fr. 1971,
2946.
(15) (a) Aubort, J . D.; Hudson, R. F. Chem. Commun. 1970, 937; (b)
938; (c) 1378.
(16) Wiberg, K. B. J . Am. Chem. Soc. 1990, 112, 3379.
(17) Bader, R. F. W. Acc. Chem. Res. 1985, 9, 18 references therein.