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Tetrahedron Letters,Vol.23,No.6,pp 615-618,1982
Printed in Great Britain
THEORETICAL
Department
of
Department
Laboratoire
STUDIES
of
OF SN2
TRANSITION
STATES,
THE
Saul Wolfe* and D.J. Mitchell
Queen’s University,
Kingston,
Chemistry,
Ontario,
H. Bernhard Schlegel+
Wayne State University,
Detroit,
Chemistry,
ALPHA
Canada
Michigan,
C. Minot and 0. Eisenstein
ThBorique,
Bi?timent 490, Centre de Paris-Sud,
de Chimie
EFFECT
K7L 3N6
U.S.A.,
91405
48202
Orsay,
France
Swmnary: The quazitative HOMO-LVMO orbital interaction interpretation of the aZpha effect is
found to be invaZid for anionic nucleophiles. In any event, gas phase SN2 reactions of HOO- and
FO'Oshow no evidence of the effect. The role of a hydroxylic solvent requires greater consideration as the source of positive deviations from rate-equiZibrim plots.
The frontier
focuses
upon the
argument,
the
Therefore,
molecular
charge
efficiency
the
of
process
I-
> Br-
> Cl-
since
iodide
is
seems
thus
observed
experimentally
solvent,
the
mides
are
Cl-
> Br-
that
the
relevance
experimental
result
applicable
to
molecule
of
the
clusters
the
rates
the
manner
under
then
energy
change.
lose
ent
to
anions
to
a gas
of
the
water
are
phase
usual
is
such
ions
to
In the
the
the
the
to
incorporate
usual
qualitative
HOMO-LUMOenergy
HOMOor
the
It
changes
theory,g
and the
is
also
effects
the
trends,’
leaving
been
is
group
ability,
group,
as is
known5 that,
nature
inacetone
and alkyl
bro-
the
based
the
the
reaction
upon
is
strictly
stable
ion-
heats
of
Moreover,
reaction’
and leaving
intrinsic
treatment
of
barriers”
of
so
any particular
coordinate.
with
nucleophilicity
Marcus
to
theory
formation
can be correlated
of
straightforward,6
trends
that
the
of
not
ability
uncertain
concepts
from
group
because
ia in a treatment
follows
long
best
p-toluenesulfonates
solvent
the
SN2 reactions
has
the
gap.
lower-lying
order.
SN2 reactions,
phase
distinction
it
several
and leaving
uncertain.
conditions’
and also
However,
towards
of
gas phase
gas
by Marcus
also
of
data
solvent
indicate
are
large.
characteristics
fore,
quantitative
that
large,13
As discussed
SN2 reaction
bell-shaped
solvation
the
X- + CH3Y + XCH3 + Y-
CH3Y.
in
group
and overall
SN2 reactions
conducted
solvents.”
A variety
gas phase
reaction
higher-lying
nucleophile,
solvents.4
The same conclusion
in hydroxylic
the
LUMOof
proportional
the
nucleophilicity
of
of
their
of
the
are
best
halide
reverse
solution
(efficiencies)
ability
be the
FM0 method
treatment
suggested
the
predicted
in
inversely
HOMOlevels
to
the
the
is
X- to
expected
to follow
electronegativity
*
*
*
*
and the LUMO levels
are u
CI < ‘CBr
< ‘CC1 ’ ‘CF.’
to conform to the concepts
of nucleophilicity
and leaving
of
of
HOMOof
more rapidly
in hydroxylic
> I-,
Extension
the
predicted
reactivities
(FMO) treatment’
the
reaction
proceed
> F-,
The theory
from
the
will
LUMO. For X or Y = halogen,
i.e.,
orbital
transfer
causes
reaction
of
the
enthalpies
of
of
Clearly,
SN2 reactions
615
transfer
have
in
the
of
in heats
of
ion-molecule
solvation
profound
solution.‘a
effects
of
inorganic
of
addition
by Bohme,“*
disappearance
anions,
of
differences
recently
coordinate.
different
treatment
the
and that
anions
solvation
three
between
water
complexes
anions,
upon the
the
differ-
molecules
and restoration
and the
rates,
from
different
and,
there-
616
is
common to
correlate
the
in
solution
It
with
Briinsted
basicities
of
of
the
constant
rate
linear,
RO- ,
or
that
all
or
RS-,
correlations
for
of
effect
of
the
of
of
(iv)
action
plot.
of
of
a normal
cause
of
the
of
heteroatom
found
to
lie
nucleophiles
of
intrinsic
barriers
are
the
enhanced
centre,
this
rate
reactions
the
logarithm
nucleophile,
depends
or
i.e.,
all
Br6nsted
tertiary
carbon.”
be a special
upon both
With
is
centre,
now known to
relatively
reactivity
and is
effect
(i)
the
of
normally
has been
factors:
of
ground
electron
case
thermodynamic
closely
related
invariant,”
nucleophilic
manifested
discussed
mem-
and the
reagents
of
state;
(iii)
containing
as a positive
extensively,”
destabilization
transition
state
pairs
combination
A lower
the
deviation
and usually
ground
state
of
of
the
stabilization
in
the
products;
of
of
destabilization
is
the
an alpha-nucleophile.21
these
electron
HOMO-LUMOenergy
of
This
pairs,
difference
result
which
results,
orbital
leads
lies
to
higher
inter-
a HOMO
than
the
and is
considered
to
combination
observed
for
HOMO
be the
reactivity.
is
incorrect
is
if
also
lower
gas phase.
anions.
the
When the
taken
FM0 theory
second
first
into
account,
than
the
is
valid,
The out-of-phase
order
order
the
HOMOof
perturbational
Calculated
HOMOEnergy
effect
(i.e.,
electronegativity’z)
out-of-phase
HOMOs of
HO-,
a decrease
as seen
of
HOMOEnergies
(au)
from
in reactivity
The FM0 rationalization
TABLE.
Anion
for
represents
centre.
substantially
in the
of
alpha-nucleophiles.
adjacent
reacting
Therefore,
carbon,
is
factors.g”8
following
an alpha-nucleophile
the
addition
RNH2, different
secondary
correlation
the
to
FM0 theory,
argument
adjacent
Table.
to
increased
to
carbon,
a BrSnsted
in which
the
or
a plot
e.g.,
barrier)
stabilization
nucleophile.
attached
family,
activation
reaction
an out-of-phase
This
HOMOof
the
(ii)
the
the
The origin
solvation
between
that
relationship
refers
to
According
comprised
a given
R = primary
is
substitution
Typically,
K refers
to the conjugate
acid of the
a
in question
share the same nucleophilic
describe
(intrinsic
one or more of
reduced
nucleophilic
term diminishes.
adjacent
alpha-nucleophile;
of
nucleophiles.16
Even within
nucleophiles,
kinetic
on a BrEnsted
terms
etc.
rate-equilibrium
The alpha-effect”
a heteroatom
where
restrictions
and kinetic
a family
the
nucleophiles
all
these
a more general
(equilibrium)
bers
pK,,
the
RNH
2’
may be needed to
One reason
of
versus
provided
rates
(i)
thus
the
is
of
the
data
for
of
the
and FO- are
presented
predicted
becomes
heteroatom
effect
HOO-, ClO-
the
in the
alpha-
ambiguous.
Oxyanions a,b
of
HOMOEnergy
Anion
HO-
-0.0340
HOO-
-0.0868
CH30-
-0.0635
FO-
-0.1372
(au)
aAt the 4-31G level,
with full
geometry optimization
(1 au = 627.5 kcal/mol);
bin the computations
of M.M. Heaton, J. Am. Chem. Sot.,
100,
2004 (1978),
the HOMOof ClO- was found to lie 19.9
kcal/mol
below the HOMOof HO-, but the HOMOof HOO- was 6.3 kcal/mol
above that of HO-.
The
latter
is a result
of Heaton’s
use of the geometry of the hydroperoxyl
radical
rather
than the
anion.
Recomputation
of HOO-, using Heaton’s
basis
set and the fully
optimized
geometry,
lowers
the energy of the HOMOby 35.0 kcal/mol.
In terms
independent
of
the
interpretations,
Marcus
treatment
because
these
of
reactivity,
factors
are
(i),
(ii)
interrelated
and
(iii)
when the
do not
constitute
more general
rate-
6 17
equilibrium
correlation
a Marcus-type
involving
are
fifteen
any of
is
employed
to
(eq
exists
relationship
independent
hydrogen,
carbon,
[l])
probe
combinations
nitrogen,
for
the
alpha-effect.
in
the
gas phase
of
X and Y,
fluorine
oxygen,
We have
for
methyl
in which
or
the
sulfur,
found
transfer
entering
and in
recently*
that
reactions
and leaving
groups
which HOO- and FO- have
been included.
[II
AEXY
AEXyis
In eq
[ll,
rated
reactants,
intrinsic
of
the
the
AE’
barriers
of
in
completely
not
dynamic
that
this
conform
the
type
the
alpha-effect.
are
clustered
to
eq
the
[l],
for
not
accepted
valid.
around
for
the
the
transition
state
and the sepat
and AE:X and AEyy are the
reaction,
As shown by Brauman,”
gas phase
since
of
This
value
of
the
shows
and now does
(r
the
AEXy is
a good
of
to
exists,
it
which
and kinetic
a plot
alpha-nucleophiles
relationship
solvents,
alpha-effect,
conform
= 0.99)
of
Marcus-type
in hydroxylic
Figure
(-20
SN2 behavior
no experimental
thermodynamic
relationship
a single
AEXY
the
definition
RO- + CH3F + ROCH3 + F-,
A linear
between
change
reactions.
reactions
a combination
is
+ AE;y)}
measure
SN2 reaction.
However,
exists
difference
energy
degenerate
of
normal.
than
caveat
the
energy
potential
a gas phase
yet
rather
the
terms
alpha-nucleophiles
does
ab initioz3
is
for
efficiency
Thus,
FO- is
= Q(AExX + AE ) + $AE” + iAEo2/8(AE;,
YY
effects.
AEXy versus
the
accepted
because
the
could
is
incorporating
be argued
based
Figure
AE” for
test
for
solely
-4
AE”
Figure
1.
A plot of AEXy versus
AE” for ab initio
reactions
of HO-, CH30-, HOO- and FO- with CH F.
Data are in kcal/mol
from 4-31G level
computa 2.Ions
with full
geometry optimization.
that
eq
[l]
on thermo-
1 demonstrates
four
the
RO- intrinsic
kcal/mol).
HOO- and
reactions
existence
barriers”
of
of
618
Unfortunately, all experimental data relating to the alpha-effect refer to hydroxylic
solvents, in which the role of (iv) is expected to be maximal. According to recent work by
Benson,24 the difference in the heats of aquation of HO- and HOO- is 21.5 kcal/mol. Although the
existence of such a large solvation effect is not surprising,13 the possibility that this factor
alone may be responsible for the alpha-effect seems generally to have been discounted, despite the
recognition otherwise given to the important role of solvation effects.
It is known that the magnitude of the alpha-effect depends upon the reaction type, and
is smallest in an SN2 reaction.25 Clearly, it is necessary to extend the theoretical treatment of
rate-equilibriumrelationships from SN2 reactions to the more complex problems of additions to sp
and sp2 centres. It is also necessary to extend experimental investigations of the rate-equilibrium behavior of alpha-nucleophilesto a range of dipolar non-hydroxylic solvents. Finally, it
appears that the validity of the FM0 treatment of certain bimolecular reactions requires
reevaluation.
ACKNOWLEDGMENTS: This research has been supported by grants from the Natural Sciences and Engineering Research Council of Canada, and the Petroleum Research Fund, administered by the American
Chemical Society.
REFERENCES
AND
NOTES
t Fellow of the Alfred P. Sloan Foundation, 1981-1983.
1) I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley, New York, 1976.
2) F.A. Carey and R.J. Sundberg, Advanced Organic Chemistry, Part A, Plenum Press, page 13.
3) R.C. Bingham, J. Am. Chem. Sot., 97, 6743 (1975).
4) T.H. Lowry and K.S. Richardson, Mechanism and Theory in Organic Chemistry, Second Edition,
Harper and Row Publishers, page 333.
5) S. Winstein, L.G. Savedoff, S. Smith, I.D.R. Stevens, and J.S. Gall, Tetrahedron Lett., No. 9,
24 (1960); A.J. Parker, J. Chem. Sot. A, 220 (1966); A.J. Parker, Chem. Rev., 69, 1 (1969);
F.G. Bordwell and D.L. Hughes, J. Org. Chem., 46, 3570 (1981).
6) C. Minot and Nguyen Trong Anh, Tetrahedron Lett., 3905 (1975); C. Minot, PhD Thesis,
UniversitG de Paris Sud, Centre d'0rsay.
7) J.M. Riveros, A.C. Breda, and L.K. Blair, J. Am. Chem. Sot., 95, 4066 (1973).
8) S. Wolfe, D.J. Mitchell, and H.B. Schlegel, J. Am. Chem. Soc.,in press.
9) R.A. Marcus, J. Phys. Chem., 72, 891 (1968); A.O. Cohen and R.A. Marcus, ibid., 72, 4249 (1968).
10) W.J. Albery and M.M. Kreevoy, Adv. Phys. Org. Chem., 16, 87 (1978); M.J. Pelleriz and J.I.
Brauman, J. Am. Chem. Sot., 102, 5993 (1980).
11) For a gas phase SN2 reaction, the intrinsic barrier associated with X is defined as the energy
difference between the ion-molecule cluster [X.*.CH3X]- and the transition state [X*.CH3**X]-.
In the present work, these energies have been obtained at the 4-31G level, with full
optimization of all structures.8 The calculated intrinsic barriers of CH30-, HO-, HOO- and
FO- are (kcal/mol): 23.5, 21.2, 18.5 and 18.2, respectively.
12) W.J. Albery, Ann. Rev. Phys. Chem., 31, 227 (1980).
Conway, Ion Hydration in Chemistry and Biophysics, Elsevier, New York, 1981.
13) B.E.
14) D.K. Bohme and G.I. Mackay, J. Am. Chem. Soe., 103, 978 (1981).
15) J.F. Coetzee and C.D. Ritchie, Editors, Solute-Solvent Interactions,Volume 2, Marcel Dekker,
Inc., 1976,Chapter 12.
16) J.E. Leffler and E. Grunwald, Rates and Equilibria of Organic Reactions, John Wiley and Sons,
Inc., New York, 1963.
17) G. Klopman and R.C. Evans, Tetrahedron, 34, 269 (1978).
18) N. Agmon, Int. J. Chem. Kinetics, 13, 333 (1981).
19) N.J. Fina and J.O. Edwards, Int. J. Chem. Kinetics, 2, 1 (1973).
20) A.P. Grekov and V.Y. Veselov, Usp. Khim., 47, 1200 (1978).
21) G. Klopman, Editor, Chemical Reactivity and Reaction Paths, John Wiley, New York, 1974.
22) This point will be elaborated upon in a forthcoming full paper.
23) Computed at the 4-31G level with full optimization of all geometries.
24) S.W. Benson and P.S. Nangia, J. Am. Chem. Soe., 102, 2843 (1980).
25) E. Buncel, C. Chuaqui, and H. Wilson, J. Org. &em., 45, 3621 (1980).
(Received
in USA 8 October
1981)