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Determination of regioselectivity of cycloaddition reactions from electronegativity of the active centres
txl4&4020/89 53 oo+ 00 0 1989 Pergamon Press plc
TerrahedronVol. 45, No 21, pp 6129 to 6740, 1989 Pnnted m Great Bntarn
DETERHINATION OF REGIOSELECTIVITY OF CYCLOADDITIQN REACTIONS FROn ELECTRONEGATIVITY OF THE ACTIVE CENTRES S.K. Pal
Department of Chemistry, Presidency College, Calcutta - 700 073, India. (Recemed in UK 22 March 1989)
Abstract
: An equation has been derived in terms of inherent electronegativityof the active centres, which can be utilised to determine the regioselectivityof the cycloaddition reactions.
haroductioo by
the
: Regioselectivity of methods2
which
were
cycloaddition reactions1 was
applicable to M.Os
and
determined
previously
inapplicable to GVB
orbitals3.
But pericyclic reactions can be explained both in terms of M.0s.l and GVB orbitalsl. So, the
purpose
of this communication is to report a novel technique to determine
the regioselectivityof the cycloaddition reactions, which
is independent of the type
of the orbitals used to explain the reactions. Dvivatfon of an Eqn. : The relationship between electronegativity and energy is 5 known . Paar has shown the equivalence of Sanderson's electronegativity and 8 have drawn attention to the chemical potential (dE/dN)6. Both Sanderson and Parr
well
electronegativity equalisation as
a
possible guiding principle in chemical reactions.
With these ideas in mind, an equation has been derived here in terms of electronegativity of the active centres in order to determine the regioselectivityof the nonpolar cycloaddition reactions. Let the active centres of the cycloaddition reactions shown are all carbon atoms and
xA,
xB,
of the
,
,
D
active centres A
respectively. Since the
are
all
uncharged
B
species,
energy
neglected for
simplicity and
energy
has
for the the
the
energy
change
covalent ’ bond two
active is
may so
been
the be
covalent
formation
between
centres. Since
energy
term
difference in size and
proportional
/A 4al2r
+
LB
D, 2s
,A%D +?
/* 4112~ \B
+
c\ 2n D/
2z10z \B
C'
considered
associated with
to
the
in Fig.1
are the inherent electronegativities
XD
C and
all
terms
only
and
active centres
electrostatic
term
xc
-C
o?l / ....(i)
/A-C +
2z/oz \B
Jz /
- D
....(ii)
covalent inversely
in energy 6729
Fig. 1. Cycloaddition reactions
6730
of
S. K. PAL
the
participating
which
is
the
related
covalent
difference tion
reaction
being
the E2 AE
the
When
-
the
then
Kl
=K.
bond
of
energy
A
XC)
( XB
E2 = Kl active
I
change
centres
= K2 = K (say)
(XA-
-
XB)
C -
are
XD)
the
of
the
Similarly,
( XB
-
carbon
the
XC)
with
XD)
( XA
the
(XB-
-
Xc)
bonds
reaction
Xc)
A
- D
XB -
X C},
associated
1,
So,
( XB
order
with
(ii) in Fig.
constant.
-
reactants
X D)(
change
same
A E = El
of
I’ ( X A -
cycloaddition
K2 I
becomes,
(XA-
formation
energy
that
cycloaddi-
cycloaddition
proportionality
-
atoms
(i)
the
parameter said
electronegativity
ends
= Kl
the
inherent
a be
the
1, El
D in
can
in a concerted
with
-
such
it
since
both
in Fig.
K2 being
/
to
is so
Now,
associated
B
all
orbital,
(i)
and
and then eqn.
(Xc-
at
X D), XD)
the
inversely
occurs
reaction
electronegativity of
centres.
constant.
( XA
size
active
formation
bonds
inherent
and
proportional
proportionality
formation
= El
is
cycloaddition
= K2 I ( XA -
since
energy
participating
so the
C in the
and
the
term
the
covalent
simultaneously,
Kl
both
energy
between
and B -
orbitals
to
-
of
x,)....(i)
hybridisation,
E2
(XA-
Xc)
(XB-
‘D) . . . . . . . . (ii)
Section
I
Salient
features
(ii)
:
when
of
AE
and
XC
=
arises
only
=
-
XD, when
over
high
denominator of
case,
over
the of i.e. be
ve.
-
I
the
order
explain
-
a
why
reaction,
the
active
(ii) b e
expected
reaction9.
electronegativity the
complex
a of
will
be
more
then
the
identical
with
maximum
the
value
of
the
(ii).
Other
to
acid
active with
to
then
two
Here, electronegative
be
more
to
E2)
favoured
provided
the
X C or
El)
the
the
reverse
order
vice-versa
: In
the
now
If
at
( AE favoured
is
XD)I(
be maximum
combination
hand
are
Eqn.
will joined
(ii)
can than
in
a
to
will
XA
then factor
regioselective
joined
acid,
+ ve one
will
(e)
more
a catalyst
directly
right
least )
favoured
corresponds
be
electronegativity change.
as
-
(which
more
denominator
the
Lewis XC
be
change
energy
used
will
(d)
inherent
the
of
I
Regioselectivity
low
then
X D <
case
centre
(
X B or
So, here high - low electronegative
Here,
is
reaction
=
-
will
ve
reaction
fraction fraction
El )
-
energy
cycloaddition
over
XA
arise.
I low
favoured.
maximum.
having
formation
high
attachment in
If
XA#XB#XC#XD,
corresponds
But
Lewis
not
is
+ ve.
ve.
be
does
X B and
-
catalysed When
>
favoured
(b)
corresponds
denominator
now
zero
-
(which
electronegative
to
centres
high
always
more ve.
corresponds
XA
(which
associated
acid
So,
is
is
be +
When
ve.
(ii)
low
(c)
>XCorXA
the
(ii)
combination
Then the increases). 10 will increase in eqn. XD
-
low
process
in
case
eqn.
in eqn.
Lewis
uncatalysed
participating
in
-
X D.
XD
(which
centres
is
#
attachment
denominator
of
denominator
which
XC
always
will AE =
regioselectivity
i.e.
favoured.
active
high
in
0
centres
But
more
the
provided
=
(i)
situation
XBand is
active
+
Reaction
XB and
XA> (ii)
regioselectivity one
#
numerator of
of
reverse the
AE
(a)
opposite
electronegative
is
the
high
E2)
together
the
low is
attachment
to
the
-
combination
this
then
in eqn.
of
:
For
(i)
numerator
attachment
(ii)
ve.
XA
casesmayarise: the
Eqn.
cycloaddition
the
substituent
increase
-
XD)
side
of
(
XB
(say v -
‘D)
eqn. (ii) Will
6731
Regioselectivityof cycloadditions remain
unchanged.
negative will
for
A E if
Obviously,
the
Lewis
acid
ca$alyed
for
reaction
uncatalysed
reaction
regioseZectivitg
i.e.
will of
be
the
more
reaction
increase.
section
II :
Application
of
Eqn.
cycloaddition
reactant
substituents
of
of the
the
Cl
values and
-
of
is
other
to
active
CO2Me.
centre
In
be
calculated to the
centre
be
in
case
the
electronegativity
of
substituent active
joined
to
the
active
centre
and
b
values
of
as
stated
electronegativities each
reactant
tion
reaction
can
be
(ii)
(see
when
the
inherent of Table
the
sign
predicted. be
has
of
assumed
of
centres
to
be high
positive
value
centres.
prediction
high the
A E means and
active
case, with
of
use
Although
a
for
of
be
9%
>=< H 1
b = 8.86) of
the
CHO and the
inherent
residue
directly
determined
from
CHO 9.96 CO2Me
2 1
the
from -
low
associated
with
the
high/low
-
of
AE
few
examples
Fig.
(ii)
El
by
can
b
reactants in eqn.
high/low
observation
ring
example,
change
regioselectivity
with the experimental
turn
of
energy -
in
of
calculating
of the
b
a and
electronegativity
for
and
in
determined
-
a and b values
which
centres
cycloaddition be
the
then
the
= 10.80,
inherent
a cycloaddiby
1 are inserted A E can
In this
associated
been
value
the
OH (a
a
a
electronegativity
from
the
residue
the
in
inherent
electronegativities
mentioned
from
calculated
and
centre
After
the
As
active
the inherent
a and b values
considered,
of
I) .
be
= 12.85)
ring the
predicted
Section
active centres in
a
active
the
1
from
substituents.
the regioselectivity
eqn.
then
be
are
viz.
can
an 11
method
centre
Similarly,
calculated
above,
of
b
of
Huheey ’ s
compound
centre,
its
by active
2).
can be
the
electronegativity
= 7.17,
(Fig.
the a
determining,
the
H (a
9.16 (C,)
inherent
joined
substituents
found
The
can
active
the
:
(ii)
values
to
negative
which
regioselectivity
mode of
combination
and
high
energy
low
means
low
change
use
of
for
many other
has
E2.
has
So,
found
to
easily
combination
here of
negative
the
attachment
furnished
been
be
been assumed of
attachment
been
thermal
can
mode
electronegative
only
(ii)
eqn.
-
electronegative
high-low have
2
active of
the
in
Table
be
in agreement
and photochemical
1 the
cycloaddition
reactions18. Polar is
Effect
similarly
active mode
centres of
identical order
:
If will
least
one
then the
of
will
be of
active
eentres
the
that
involve
In such active
in
each
electronegativity
; then according one
electronegativity .
electronegativity
the
inherent
be indentical
combination inherent
of
at
substituted
such
to Section I (d) joining
cases, centres
of of
mere in
the
cycloaddition above,
substituted
the
active
determination
each
reactant
similarly
preferred
centres of
cycloaddition
the
having relative
reactant
is
6732
S. K. PAL Table
Zycloaddition reaction type
1
Reactants with inherent electronegativities of their active centres A
_ ve
QXJ
9.34~
+ 2)
Sign ~Predicted of regioselectivity AR
Observed regioselectivity
Rd
B
/-%CO,M. 7.68 L?-
-
h-h/l-l (high-high/ low-low)
h-h/l-l
12
h_l
13
7~*77
I i
‘~:a
Ph Jr::,”
+ ve
~hi~;~~ow~
7.95 8.17 Me -OS+OEt 8.38
Ph
(2 + 2) Thermal 0
)1 -0,-o Ph 13.21
8.05 0
8.36
+ ve
h-l
h-l
+ ve
h-l
h-l
14
9.37
ClF2C
CF2Cl
9.37
Y d 9.98 7.68
(2 + 2) Photo
9’044
h -
h/l
-
h -
h/l-l
-1
h-hl l-l
15
ve
h-h/l-l
16
C02Me
8.05
Me0
-ve
d
9047 OAC /i-
9.89 8.05
.b
0
9.89 U
0
+ ve
h-l
h-l
17
9.89 '
0
sufficient -
low
to
of
active
the
quaIitatively
of
for the
from
this active
commonly the
the
attachment)
tivity
and
determine
mode of
basis
in of
the
purpose centres
mode
centres polar we by
use
in
the
nature
of
of the in
of
of
not
Huheey’s
organic
centres.
each
effect
need
combination
active
the
method.
to Now,
can
be
exerted
the
them
with
into
four below).
or
high
electronega-
be
the
(other
low
of
then
inherent
(see
-
order
can
substituents
categorised by
high/low
present
calculate the
-
relative
reactant
substituents
require
effect
high
And the
cycloaddition
chemistry polar
(i.e.
determined
active
centres
electronegativity than types For
hydrogen) (I-IV) all
on these
6733
Regioselectivityof cycloadditions four types lative
can
-
follows For
of
and
I
effects this
case
of
both
the
centres
directly
will
joined
substituent (ii) For type
to
decreases
etc.
etc.
etc.
(a)
(b)
In this
the
(D)
is
much (Fig. the
will
the
of the distance
: - I and + R effects of the ethylenic
stronger
than
R
much higher carbon
(Fig.
as
are present
joined
will
decrease.
This
and
the
effect
carbon of
the
!
directly
effect
at
-1
3a).
carbon
carbon +
3
be
remote
(d)
decreases
to the subetituent is
obvious
with
the
because increase
3b).
III substituent case,
increase than
but that of the remote
of the distance For type
but
substituent
with the increase
II substituent
effect
9
(cl Fig.
increase the,
the electronegativity increase
I
CH=CH2,
Br,
carbon
ethylenic
active
-
Cl,
CHO,
are
In
will
COMe,
substituents
electronegativity
Here,
J=Ph,
subetituent(W): R
-
present:
(iii)
the
M=CH3,
D=OMe ,
W=CO2Me,
: type
-1
such
from
the
‘d
0
ethylene
determined
effect
g”
CI”
in
reactant
substituted
be
polar
(i)
centres
active
1
the re-
electronegativfty
cycloaddition
a5
a5
of
the
Of
a
of eubstituents,
kder
(J)
: Conjugative
conjugative
effect
of
Table
effect the
is present substituent
: will
try
- 2 1
Compound*
Type of substituent
Relative order of electronegativity of the active centres
* S = Substituent
to
decrease
the
6134
K.
s.
electronegativity carbon
of
ethylenic joined
of
the
carbons
to the
gativity
of
both
the
substituent substituent
the
ethylenic
will
and the
try
latter
If
IV substituent
the
substituent
Although
the
than
that
the
latter
(hi)
be
CH2
(CH3
or
carbon
jugative
will
effect.
that
the
the
adjacent
will
be
and
less
on
more
because
on
the
decrease carbon
remote
carbon
adjacent
experience
carbon
additional
an
both
adjacent
the
the
group,
bearing
the
ted ethylenic Similarly,
the
negativity
of
substituted
and
is present the
CH3 I a
will
in Table
Il. 10.
the
active A
compounds mined Since
as
CH2 group
higher
electron
becomes
unique.
slightly
greater
is
donating
Reactant A (4v)
Reactant B (2n)
centres
in
dienes
1
$”
in
5 (Fig.4)
depicted to
in
the
CHO
2
d
2 S=Ph,
20
OMe
/ \
c S=C02H,
gCOZH
21
Me
of
CN gs
4 <
22 19b
S=CH=CH2, C02Me
of
cycloaddition
3)
and
have the
Section
19
G
can
effect
electronegativity (Table
Ref.
S=Ph ,G02H, Me
electro-
the Table 2,the
of
than
- 3
.
1
3
2. Following
& B
according
of
polar
power
S
have been summa-
centres 2 -
order
the
more
S
will
All these
order
reactants
3d).
2-substituted
be
:
situation
Table
carbon
(Fig.
active
from
the substituent. relative
relative the
determined
rised
carbon
the
directly
Hence, the electrone-
substituent
then
will CH3/CH2 group than that of the unsubstitu-
be higher
be
ethylenic
the
Sp2
electron
toward the induction) (by carbon of methyl hybridised SP3 or methylene group. So the electroneof
the ,both
carbon
pull
gativity
of of
at the
effect
carbon
the
the
is
Of
remote
but
effect
hypercon-
electronegativity and
to
to
result
The
decrease
will
attached
exert
appreciable
joined
has
charge ethylenic
T I
electronegativity
at the romote carbon.
of
former
because of its greater llb capacity . Such a substituent when
be
: Hyperconjugative
the
whereas the
3~).
electronegativity
hydrogen,
CH2)
will
directly
(Fig.
CH3 or
inherent
of
increase
effect
carbon
than that of the remote carbon
(iv) For type
carbons
to
and negligible
ethylenic
PAL
I
in
been deter23 diagrams . (d)
Ph
the
above,
5
riCOzMe
0
zob
6735
Regioselectwity of cycloadditions RO
--.X0
[ 2
Ph
@I R = Me
b) R=Et
(R =Cl,Me,CHO,CO2H)
c) R = PhCH2
4
5
3
Fig.4
the favoured combination will involve attachment of the active centres having identical 25 24 electronegativity,so dimerization of the compounds 2 and 3 and cycloaddition reaction involving 4 and
5 ,will go through high-high/low-low electronegativeattachment of the
active aentres. Similarly, the
cycloaddition reactions involving the substrates A
and
B of Table 3 can be predicted to occur by high-high/low-low electronegativeattachment of the active centres because the inherent electronegativityof the methylene carbon active centre in each substrate (A or B) can be expected to be identical. All these predictions are in accord with the experimental observations. Mw(lrsswnt
: The
disagreement between the predicted regioseiectivityand
observed
regioselectivitymay
arise mainly due to (i) steric interaction and (ii)lonepair-lone26 pair or dipole-dipole interaction . As for example, the thermal (2+2) cycloadditionreac-
Table - 4
Zycloaddition Reactantswithinherent electroSign reactiontype negativities oftheir activecentres of AE
Predicted regioselectivity
Observed regioselectivity
Ref.
B
A 0 (2+2)
Ph 8.36
Thermal
Ph’=;;2;
.
mve
h-h/l-l
h-l
27
-ve
h-h/l-l
h-l
13
8.22
(4+2) Thermal
0 9.89
9.91 N='C-e=o 7.68
6736
S. K. PAL
Me L
,COPh c-c
6
(4
(b)
Me0
(cl
(d)
tion between the substrates A and B (TabIe 4) favours high - iow electronegativeattach27 ment of the active centres rather than high-high/Pow-Iow eIeetronegative attachment although eqn.(ii) predicts the Iatter. The
atetic fnfaract&m in ihe high-highrlow-low
approach is probabIy responsible for the disagreement (PQ.5a). Another case of dfsagree13 ment was observed in the thermal (4+2) cycloadditionreaction between 1 - cyanoethylene and cycloheptenone (Table 4). Although eqn.(ii) predicts high-high/low-low electronegative attachment of the active centres, yet the reaction goes through high-low eIectronegative joining of the active centres because in the former approach serious lone pair - lone pair interaction 28a is present (shown by
double' headed arrow in Fig.5b). In the case 28b.c 29 of intramolecular cycloaddition reaction ring strain ortransannularinteraction may be present which
may
sometimes become
too important to jeopardise the prediction by
eqn.(ii).
DiSCUSSiOll
:
Although cycloaddition reactions with heterodienes and
have not been considered here yet there is no reason why
heterodienophiles
eqn.(ii) will not be applicable
for such thermal reactions if electrostaticenergy term does not play any important role. 30 As a typicaI example of such thermal reactions, the compound 2a is found to react with compound 6 in benzene at r.t. to produce compound 7 (96%) (Fig.5) by high-highllow-low electronegativeattachment of the active centres which is in accord with the prediction by Section I(d). But photocycloadditionreactions involving heterodienes and heterodienophiles are complicated
because
of the possibilityof excitation by
nonbonding elec-
trons. In our electronegativitymethod,
the inherent electronegativitiesof the active centres
in the excited state of a molecule have been assumed to be identical with those in the ground state and regioselectivityhas been determined accordingly. Although this is merely an approximation and may
not necessarily hold good for all photoexcited molecules
yet our method has been found to be suitable in predicting the regioselectivityof many
6137
Regiosckctivity ofcycloadditions
photocycloaddition reactions on of
the
the basis of this approximation. The
clectronegativity method
independent of the type
is
its wider
applicability because
most advantage the
method
is
of the orbital8 used to explain the cycloaddition reactions.
So a practical organic chemist will find this method very useful as it does net involve advanced chemical calculation.Of course, sometimes solvent effect31 or homoconjugation3' plays
an
important role on
the
regioselectivity of
the
cycloaddition reaction for
which due considerationshould then be made.
Referencesandnetee:
1 (a).
Hoffmann, R.; Woodward, R.B. The Cotwavation
04 Ohbitat Symm~thy,
Verlag-
Chemie. Academic Press. 1970.
(b). 2 (a). b).
Gleiter, R.G.; Bohm. M.C. Pwre t; Appt. Houk. K.N.
J.Am.
Chem.
Epiotis. N.D. J.Am.
Sot.
Chem.
1973,
Sot.
1973.
3.
Goddard. W.A.; Ladner. R.C.J.Chem.
4.
Goddard. W.A. J.Am.
Chsm.
95, 95,
1983,
55,237.
4092. 5624.
Phyb.1969, 51, 1073.
Chem. Sot. 1972, 94, 793; Pal. S.K. hd.
'J.Chem.l987,26B(4)
312-317. 5.
Klopman, G. Chemic&
Reactivity
and
Re~~tiion
Path& Klopman. G. Ed. Wiley New
York. 1974, 66. 6.
Thomson. C.(Sr.Reporter). JhcoxcticaP Chemidthy
The Royal Society of Chemistry.
Iondon. 1981, Vol 4. 7.
Sanderson. R.T. Chemicd Bond4 and Bond Eneagy 2nd Edn. Academic Press. New
York. 1976.
a.
Paar. R.G.; Donnelly. R.A.; Levy. M.; Palke. W.E. J.Chem. Phy6.1972, 68.3801.
9.
Fleming.I. FaontieaO&it&
and O/Lganic Reuctions John Wiley & 8ons.l976;Brid-
ges. A.J.; Fisher. J.W. Jetaahe&on 10.
Let
xD*
L&k&5
1983,
24(5),
447450.
represents the inherent electronegativity of the active centre D in theca-
talysed reaction, so
the said fraction will then be
converted into
(XB -xD*) (XC -xD&(xA-xD*) In the uncatalysed reaction, let BE is - ve and xA> xB & XD>XC
.
- xD i.e. the numerator of the said As XD increases to xD*, then xc - xD*>k fraction will increase. Now let us consider two cases :
s. K.
6738
Ci)
v&en
Here,
XA
ds
iaereaeeeto
ZD
so* 0,
c-x, -
- xD*t)
Since
the
numerator
,the said fraction (ii)
when
Suppose, Here,
and x* >
> XD
then
xDg
x* -
XA -
but
XD = 1akand to
XB -xD=
( XA - xD*)
hand side
-
is less
( ‘A - xD*) (XB -xD*)<(al the numerator Here also, of the said fraction
(b).
J.E.
Huheey.
J.
J.E.
will
13.
reaction reaction,
- x)
( lb/
-x
Phy6.
Ibl<(xA
+ x)
( lb1 -
Ial 1 - x2 than zero then in both the
-xD) (XB -xD) but denominator
increases
Chem. 1965,
69,
3284 ; ibid.
- phincip~ed
; Tunick.
Tetnahednon
15.
Margaretha.
16.
Ogino.
17.
Mitra.
decreases,
so
the
value
1966, 70.2086;
Huheey.
J.E.
A.A.
ibid, 1969,
Lettm
Hurd. C.D.
R.D.;
3rd
and reactivity
04 &u&tie
1983.
of electronegativity
was considered centre
in calculating
the electrone-
by Huheey I s method. 30, 3676;
Barchtold.
C.A.;
3679.
Kitahara. Y . ; Uyehara. T. Chem and lnd. Kimbrough.
then
XD
lb1
of unknown substituentlactive
14.
:
lb1
Cunnigham . R . : Damishefsky . W. J. O&g. Chem. 1965,
1to.S.
of
than 1a I 1b I i.e.
lno&ganic Chemidtiy
Ciabattoni . J .
value
the
1966, 31, 2365.
100% eaualisation gativity
12.
so
increase.
Edn. Harper International.
(cl .
XD
decreases,
la I is equal to zero or greater
1 bl
O&g. Chem.
-X,&X,-
xD* = 1b 1 + x
= ( /aI
( XB - xD*)
cases
right
xg
sad
= XD + x where x is a + ve quantity.
xA - xD* = la 1 - x and XB -
Huheey.
denominator
and XB - XD*>XB -
Now, when
J.
xp ‘XA “XD xg - x,).
xD* in the catalysed
= Ial
11 (a).
(
5
increase.
as XD increases
then,
reecsm
u%s?d.alyeed
Xl)*) <( XA -x,1
increases
till
tke
xA>xD and XB
XA - xD*
in
XD
PAL
1971, 354;
Shoji.
Y.;
Takeshita.
H.;
1815.
J.Am.
Chem.
Sot.
1960.
82,
1373;
Arena.
J.F.AngeW.
Chem. 1958, 70 631.
K.;
P. Hetv. Kubota.
T.:
Isogai.
Le Mahen. R.;
Chem. Sot. Sano. T.; dge.
Nelson.
1968, 90, Toda.
J.J.;
Baghi.
E.L.;
J.F.;
J.;
Chadha. Bogri.
Acta.
1974,
57,
1866.
Manaka. K. Chem. Letter
K. Tetiahehon
R.B.;
5570; Brown. 18.
Chem.
T.;
P.J.;
323; Ogino.
T.; Yamada.
1977, 2445.
Let&d
Bass.
1976,
T.D.;
Corey.
Kclin K.;
E.J.
Koch.
J.Am.
T.H.;
Chem. Sot. 1964. 86,
Chapman.
O.L.
J. Am.
1657. Tsuika.
Y. Chem.
N.K. ; Uskokovic. T.
Iethahedmn
Phanm.
M.R. Lettea
8t.dk
1983,
J. Am.
Chem.
1969,
1639.;
31(l), Sot.
Kelly.
Partri-
356-9.; 1973.
R.B.;
95,
532.;
2ameckik.
6739
Regioselectivity ofcycloadditions
Can.
J; ,Beekett. B.A. Jirkovsky H.M.; 19(a).
T.; Fishman.
Serve. P.Can.
Titov.
Y.A.;
Kharasch.
1. Ckem. 1972, 50. 3455.; Wiesher. M.Can.
Kuznetsova.
M.S.;
J.
C&m.
J. Chem.1969,
(b). Titov. Y.A.; Kuznetsova. Nauk. 1960, 1810, 1815.;
E.J.Am.
Akad.
Chem
A.I.Dokeady. Snyder.
Po0n.L.;
433.;
47,
Rosenberg.
47. 4295,
A.I.Dokeady.
Sternfeld.
1969,
K.:
Akad.
H-R.;
Nauk. SSSf?,1959,126,586.;
S0c.1939.
Poos.
Nauk.
61, 2318. SSSR.Otdel.Khim.
G.I. J.Am.Chem.
Soc.1949,
71, 1057. (c). Kuznetsova.
N.V.;
Kuznetsova.
A.I.;
Nazarov.
I.N. Zhua. Obbchei. Khim.
1955. 25, 88. 20(a).
Pasche.
W.; Lehmann.
E. Bea. 1935,
1941, 74, 524.; Alder. Ropp.
G.A.;
Lorenzi
Coyner.
F.J.;
K.; Vagt.
E.C.
Cristo.
J.Am.
S.J.
68, 1146.;
Chem.
Ibid.
Bluemenfeld.
H.; Vogt. W.Annaeen. Soc.1949,
G.ibid.
1949,
165,135.:
71, 1832.; Meek.
J.S.;
1830.
Gilchrist. (b). Wichterle. 0. CoeLn. Czech. Chem. Co~Un.Eng.Edn.l938,10,49?.; T.L.; Starr. R.G. Oqanic Reactions and Oh&f& Symmetiy 2nd Edn. Cambridge University Press. 1979. 21.
Alder. K.; Schumacher. M.; Wolff. D. Ann&n
22.
Snyder. H.R.; Poos. G.I. J.Am. Chem. Sot. 1950, 72, 4104.
23.
Higher and lower electronegativeactive centres have been depicted by a black
K. Annden
1949, 79, 564.; Vogt. W.; Alder.
1949. 120, 564.
solid circle and a vacant circle respectively in all the diagrams of this paper. 24 (a). Mizuno. K.; Hashizume. T.; Otsuji. Y. J. Chem. Sot. Chem. Commun. 1983. 14,
772-3. (b).
Greene. F.D.; Misrock. S.L.; Wolfe. J.R. J.Am. Chem. SOC. 1955, 77, 3852.; Applequist. D.E.; Little. R .L.; Friedrich. E.G.; Wall. R.E. ibdd.
1959,
81,
452.
1950, 201, 570.; Merrow. R.T.; Meek. J.S.;Ramney.
25.
Alder K.; Haydn. J. Ann&n
26.
Similar observation was noted in ref. 9.
27.
Fish R .H. J. 0q.j.Ckem. 1969, 34, 1127,; Koppitz.P.; Huisgen.R.; Feiler.L.A.
28 (a).
Another possibility is the presence of dipole-dipole interaction.
D.E.; Gristol. S.J. J.Am. Chem. %c.
1951, 73, 5563.
J.Am. Chem. Sot. 1974, 96, 2270. (b).
Seto. H.; Tsunoda. S.; Ikeda. H.; Fujimoto. Y.; Tatsuno. T.; Toshioka. H.Chem. Phtim. ?3uU . 1985, 33, 2594.
(cl.
Crimmins. M.T. Chem. Rev. 1988, 88, 1453-1473.
29.
Winkler. J.D.; Henegar. K.E. J.Am. Chem. Sot. 1987, 109, 2850. See also ref. 28 (c).
30.
Yamanchi. M.;
Katayama. S.; Baba. 0.; Watanaba. T.J. Chem. SOC. Chem.
Commun. 1983, 6, 281-2.
S. K.
6740
31.
Pardo. L.; Branchadell. V.; Oliva. A.; Bertran. J. Theo. Chem. 1983, 10, 255-60.
32.
PAL
,
Mahain. G.; Schwager. L.; Carrupt. P.A.; Vogel. P. Tetihedm 24(34), 3603-6.
l&&b
1983,
TerrahedronVol. 45, No 21, pp 6129 to 6740, 1989 Pnnted m Great Bntarn
DETERHINATION OF REGIOSELECTIVITY OF CYCLOADDITIQN REACTIONS FROn ELECTRONEGATIVITY OF THE ACTIVE CENTRES S.K. Pal
Department of Chemistry, Presidency College, Calcutta - 700 073, India. (Recemed in UK 22 March 1989)
Abstract
: An equation has been derived in terms of inherent electronegativityof the active centres, which can be utilised to determine the regioselectivityof the cycloaddition reactions.
haroductioo by
the
: Regioselectivity of methods2
which
were
cycloaddition reactions1 was
applicable to M.Os
and
determined
previously
inapplicable to GVB
orbitals3.
But pericyclic reactions can be explained both in terms of M.0s.l and GVB orbitalsl. So, the
purpose
of this communication is to report a novel technique to determine
the regioselectivityof the cycloaddition reactions, which
is independent of the type
of the orbitals used to explain the reactions. Dvivatfon of an Eqn. : The relationship between electronegativity and energy is 5 known . Paar has shown the equivalence of Sanderson's electronegativity and 8 have drawn attention to the chemical potential (dE/dN)6. Both Sanderson and Parr
well
electronegativity equalisation as
a
possible guiding principle in chemical reactions.
With these ideas in mind, an equation has been derived here in terms of electronegativity of the active centres in order to determine the regioselectivityof the nonpolar cycloaddition reactions. Let the active centres of the cycloaddition reactions shown are all carbon atoms and
xA,
xB,
of the
,
,
D
active centres A
respectively. Since the
are
all
uncharged
B
species,
energy
neglected for
simplicity and
energy
has
for the the
the
energy
change
covalent ’ bond two
active is
may so
been
the be
covalent
formation
between
centres. Since
energy
term
difference in size and
proportional
/A 4al2r
+
LB
D, 2s
,A%D +?
/* 4112~ \B
+
c\ 2n D/
2z10z \B
C'
considered
associated with
to
the
in Fig.1
are the inherent electronegativities
XD
C and
all
terms
only
and
active centres
electrostatic
term
xc
-C
o?l / ....(i)
/A-C +
2z/oz \B
Jz /
- D
....(ii)
covalent inversely
in energy 6729
Fig. 1. Cycloaddition reactions
6730
of
S. K. PAL
the
participating
which
is
the
related
covalent
difference tion
reaction
being
the E2 AE
the
When
-
the
then
Kl
=K.
bond
of
energy
A
XC)
( XB
E2 = Kl active
I
change
centres
= K2 = K (say)
(XA-
-
XB)
C -
are
XD)
the
of
the
Similarly,
( XB
-
carbon
the
XC)
with
XD)
( XA
the
(XB-
-
Xc)
bonds
reaction
Xc)
A
- D
XB -
X C},
associated
1,
So,
( XB
order
with
(ii) in Fig.
constant.
-
reactants
X D)(
change
same
A E = El
of
I’ ( X A -
cycloaddition
K2 I
becomes,
(XA-
formation
energy
that
cycloaddi-
cycloaddition
proportionality
-
atoms
(i)
the
parameter said
electronegativity
ends
= Kl
the
inherent
a be
the
1, El
D in
can
in a concerted
with
-
such
it
since
both
in Fig.
K2 being
/
to
is so
Now,
associated
B
all
orbital,
(i)
and
and then eqn.
(Xc-
at
X D), XD)
the
inversely
occurs
reaction
electronegativity of
centres.
constant.
( XA
size
active
formation
bonds
inherent
and
proportional
proportionality
formation
= El
is
cycloaddition
= K2 I ( XA -
since
energy
participating
so the
C in the
and
the
term
the
covalent
simultaneously,
Kl
both
energy
between
and B -
orbitals
to
-
of
x,)....(i)
hybridisation,
E2
(XA-
Xc)
(XB-
‘D) . . . . . . . . (ii)
Section
I
Salient
features
(ii)
:
when
of
AE
and
XC
=
arises
only
=
-
XD, when
over
high
denominator of
case,
over
the of i.e. be
ve.
-
I
the
order
explain
-
a
why
reaction,
the
active
(ii) b e
expected
reaction9.
electronegativity the
complex
a of
will
be
more
then
the
identical
with
maximum
the
value
of
the
(ii).
Other
to
acid
active with
to
then
two
Here, electronegative
be
more
to
E2)
favoured
provided
the
X C or
El)
the
the
reverse
order
vice-versa
: In
the
now
If
at
( AE favoured
is
XD)I(
be maximum
combination
hand
are
Eqn.
will joined
(ii)
can than
in
a
to
will
XA
then factor
regioselective
joined
acid,
+ ve one
will
(e)
more
a catalyst
directly
right
least )
favoured
corresponds
be
electronegativity change.
as
-
(which
more
denominator
the
Lewis XC
be
change
energy
used
will
(d)
inherent
the
of
I
Regioselectivity
low
then
X D <
case
centre
(
X B or
So, here high - low electronegative
Here,
is
reaction
=
-
will
ve
reaction
fraction fraction
El )
-
energy
cycloaddition
over
XA
arise.
I low
favoured.
maximum.
having
formation
high
attachment in
If
XA#XB#XC#XD,
corresponds
But
Lewis
not
is
+ ve.
ve.
be
does
X B and
-
catalysed When
>
favoured
(b)
corresponds
denominator
now
zero
-
(which
electronegative
to
centres
high
always
more ve.
corresponds
XA
(which
associated
acid
So,
is
is
be +
When
ve.
(ii)
low
(c)
>XCorXA
the
(ii)
combination
Then the increases). 10 will increase in eqn. XD
-
low
process
in
case
eqn.
in eqn.
Lewis
uncatalysed
participating
in
-
X D.
XD
(which
centres
is
#
attachment
denominator
of
denominator
which
XC
always
will AE =
regioselectivity
i.e.
favoured.
active
high
in
0
centres
But
more
the
provided
=
(i)
situation
XBand is
active
+
Reaction
XB and
XA> (ii)
regioselectivity one
#
numerator of
of
reverse the
AE
(a)
opposite
electronegative
is
the
high
E2)
together
the
low is
attachment
to
the
-
combination
this
then
in eqn.
of
:
For
(i)
numerator
attachment
(ii)
ve.
XA
casesmayarise: the
Eqn.
cycloaddition
the
substituent
increase
-
XD)
side
of
(
XB
(say v -
‘D)
eqn. (ii) Will
6731
Regioselectivityof cycloadditions remain
unchanged.
negative will
for
A E if
Obviously,
the
Lewis
acid
ca$alyed
for
reaction
uncatalysed
reaction
regioseZectivitg
i.e.
will of
be
the
more
reaction
increase.
section
II :
Application
of
Eqn.
cycloaddition
reactant
substituents
of
of the
the
Cl
values and
-
of
is
other
to
active
CO2Me.
centre
In
be
calculated to the
centre
be
in
case
the
electronegativity
of
substituent active
joined
to
the
active
centre
and
b
values
of
as
stated
electronegativities each
reactant
tion
reaction
can
be
(ii)
(see
when
the
inherent of Table
the
sign
predicted. be
has
of
assumed
of
centres
to
be high
positive
value
centres.
prediction
high the
A E means and
active
case, with
of
use
Although
a
for
of
be
9%
>=< H 1
b = 8.86) of
the
CHO and the
inherent
residue
directly
determined
from
CHO 9.96 CO2Me
2 1
the
from -
low
associated
with
the
high/low
-
of
AE
few
examples
Fig.
(ii)
El
by
can
b
reactants in eqn.
high/low
observation
ring
example,
change
regioselectivity
with the experimental
turn
of
energy -
in
of
calculating
of the
b
a and
electronegativity
for
and
in
determined
-
a and b values
which
centres
cycloaddition be
the
then
the
= 10.80,
inherent
a cycloaddiby
1 are inserted A E can
In this
associated
been
value
the
OH (a
a
a
electronegativity
from
the
residue
the
in
inherent
electronegativities
mentioned
from
calculated
and
centre
After
the
As
active
the inherent
a and b values
considered,
of
I) .
be
= 12.85)
ring the
predicted
Section
active centres in
a
active
the
1
from
substituents.
the regioselectivity
eqn.
then
be
are
viz.
can
an 11
method
centre
Similarly,
calculated
above,
of
b
of
Huheey ’ s
compound
centre,
its
by active
2).
can be
the
electronegativity
= 7.17,
(Fig.
the a
determining,
the
H (a
9.16 (C,)
inherent
joined
substituents
found
The
can
active
the
:
(ii)
values
to
negative
which
regioselectivity
mode of
combination
and
high
energy
low
means
low
change
use
of
for
many other
has
E2.
has
So,
found
to
easily
combination
here of
negative
the
attachment
furnished
been
be
been assumed of
attachment
been
thermal
can
mode
electronegative
only
(ii)
eqn.
-
electronegative
high-low have
2
active of
the
in
Table
be
in agreement
and photochemical
1 the
cycloaddition
reactions18. Polar is
Effect
similarly
active mode
centres of
identical order
:
If will
least
one
then the
of
will
be of
active
eentres
the
that
involve
In such active
in
each
electronegativity
; then according one
electronegativity .
electronegativity
the
inherent
be indentical
combination inherent
of
at
substituted
such
to Section I (d) joining
cases, centres
of of
mere in
the
cycloaddition above,
substituted
the
active
determination
each
reactant
similarly
preferred
centres of
cycloaddition
the
having relative
reactant
is
6732
S. K. PAL Table
Zycloaddition reaction type
1
Reactants with inherent electronegativities of their active centres A
_ ve
QXJ
9.34~
+ 2)
Sign ~Predicted of regioselectivity AR
Observed regioselectivity
Rd
B
/-%CO,M. 7.68 L?-
-
h-h/l-l (high-high/ low-low)
h-h/l-l
12
h_l
13
7~*77
I i
‘~:a
Ph Jr::,”
+ ve
~hi~;~~ow~
7.95 8.17 Me -OS+OEt 8.38
Ph
(2 + 2) Thermal 0
)1 -0,-o Ph 13.21
8.05 0
8.36
+ ve
h-l
h-l
+ ve
h-l
h-l
14
9.37
ClF2C
CF2Cl
9.37
Y d 9.98 7.68
(2 + 2) Photo
9’044
h -
h/l
-
h -
h/l-l
-1
h-hl l-l
15
ve
h-h/l-l
16
C02Me
8.05
Me0
-ve
d
9047 OAC /i-
9.89 8.05
.b
0
9.89 U
0
+ ve
h-l
h-l
17
9.89 '
0
sufficient -
low
to
of
active
the
quaIitatively
of
for the
from
this active
commonly the
the
attachment)
tivity
and
determine
mode of
basis
in of
the
purpose centres
mode
centres polar we by
use
in
the
nature
of
of the in
of
of
not
Huheey’s
organic
centres.
each
effect
need
combination
active
the
method.
to Now,
can
be
exerted
the
them
with
into
four below).
or
high
electronega-
be
the
(other
low
of
then
inherent
(see
-
order
can
substituents
categorised by
high/low
present
calculate the
-
relative
reactant
substituents
require
effect
high
And the
cycloaddition
chemistry polar
(i.e.
determined
active
centres
electronegativity than types For
hydrogen) (I-IV) all
on these
6733
Regioselectivityof cycloadditions four types lative
can
-
follows For
of
and
I
effects this
case
of
both
the
centres
directly
will
joined
substituent (ii) For type
to
decreases
etc.
etc.
etc.
(a)
(b)
In this
the
(D)
is
much (Fig. the
will
the
of the distance
: - I and + R effects of the ethylenic
stronger
than
R
much higher carbon
(Fig.
as
are present
joined
will
decrease.
This
and
the
effect
carbon of
the
!
directly
effect
at
-1
3a).
carbon
carbon +
3
be
remote
(d)
decreases
to the subetituent is
obvious
with
the
because increase
3b).
III substituent case,
increase than
but that of the remote
of the distance For type
but
substituent
with the increase
II substituent
effect
9
(cl Fig.
increase the,
the electronegativity increase
I
CH=CH2,
Br,
carbon
ethylenic
active
-
Cl,
CHO,
are
In
will
COMe,
substituents
electronegativity
Here,
J=Ph,
subetituent(W): R
-
present:
(iii)
the
M=CH3,
D=OMe ,
W=CO2Me,
: type
-1
such
from
the
‘d
0
ethylene
determined
effect
g”
CI”
in
reactant
substituted
be
polar
(i)
centres
active
1
the re-
electronegativfty
cycloaddition
a5
a5
of
the
Of
a
of eubstituents,
kder
(J)
: Conjugative
conjugative
effect
of
Table
effect the
is present substituent
: will
try
- 2 1
Compound*
Type of substituent
Relative order of electronegativity of the active centres
* S = Substituent
to
decrease
the
6134
K.
s.
electronegativity carbon
of
ethylenic joined
of
the
carbons
to the
gativity
of
both
the
substituent substituent
the
ethylenic
will
and the
try
latter
If
IV substituent
the
substituent
Although
the
than
that
the
latter
(hi)
be
CH2
(CH3
or
carbon
jugative
will
effect.
that
the
the
adjacent
will
be
and
less
on
more
because
on
the
decrease carbon
remote
carbon
adjacent
experience
carbon
additional
an
both
adjacent
the
the
group,
bearing
the
ted ethylenic Similarly,
the
negativity
of
substituted
and
is present the
CH3 I a
will
in Table
Il. 10.
the
active A
compounds mined Since
as
CH2 group
higher
electron
becomes
unique.
slightly
greater
is
donating
Reactant A (4v)
Reactant B (2n)
centres
in
dienes
1
$”
in
5 (Fig.4)
depicted to
in
the
CHO
2
d
2 S=Ph,
20
OMe
/ \
c S=C02H,
gCOZH
21
Me
of
CN gs
4 <
22 19b
S=CH=CH2, C02Me
of
cycloaddition
3)
and
have the
Section
19
G
can
effect
electronegativity (Table
Ref.
S=Ph ,G02H, Me
electro-
the Table 2,the
of
than
- 3
.
1
3
2. Following
& B
according
of
polar
power
S
have been summa-
centres 2 -
order
the
more
S
will
All these
order
reactants
3d).
2-substituted
be
:
situation
Table
carbon
(Fig.
active
from
the substituent. relative
relative the
determined
rised
carbon
the
directly
Hence, the electrone-
substituent
then
will CH3/CH2 group than that of the unsubstitu-
be higher
be
ethylenic
the
Sp2
electron
toward the induction) (by carbon of methyl hybridised SP3 or methylene group. So the electroneof
the ,both
carbon
pull
gativity
of of
at the
effect
carbon
the
the
is
Of
remote
but
effect
hypercon-
electronegativity and
to
to
result
The
decrease
will
attached
exert
appreciable
joined
has
charge ethylenic
T I
electronegativity
at the romote carbon.
of
former
because of its greater llb capacity . Such a substituent when
be
: Hyperconjugative
the
whereas the
3~).
electronegativity
hydrogen,
CH2)
will
directly
(Fig.
CH3 or
inherent
of
increase
effect
carbon
than that of the remote carbon
(iv) For type
carbons
to
and negligible
ethylenic
PAL
I
in
been deter23 diagrams . (d)
Ph
the
above,
5
riCOzMe
0
zob
6735
Regioselectwity of cycloadditions RO
--.X0
[ 2
Ph
@I R = Me
b) R=Et
(R =Cl,Me,CHO,CO2H)
c) R = PhCH2
4
5
3
Fig.4
the favoured combination will involve attachment of the active centres having identical 25 24 electronegativity,so dimerization of the compounds 2 and 3 and cycloaddition reaction involving 4 and
5 ,will go through high-high/low-low electronegativeattachment of the
active aentres. Similarly, the
cycloaddition reactions involving the substrates A
and
B of Table 3 can be predicted to occur by high-high/low-low electronegativeattachment of the active centres because the inherent electronegativityof the methylene carbon active centre in each substrate (A or B) can be expected to be identical. All these predictions are in accord with the experimental observations. Mw(lrsswnt
: The
disagreement between the predicted regioseiectivityand
observed
regioselectivitymay
arise mainly due to (i) steric interaction and (ii)lonepair-lone26 pair or dipole-dipole interaction . As for example, the thermal (2+2) cycloadditionreac-
Table - 4
Zycloaddition Reactantswithinherent electroSign reactiontype negativities oftheir activecentres of AE
Predicted regioselectivity
Observed regioselectivity
Ref.
B
A 0 (2+2)
Ph 8.36
Thermal
Ph’=;;2;
.
mve
h-h/l-l
h-l
27
-ve
h-h/l-l
h-l
13
8.22
(4+2) Thermal
0 9.89
9.91 N='C-e=o 7.68
6736
S. K. PAL
Me L
,COPh c-c
6
(4
(b)
Me0
(cl
(d)
tion between the substrates A and B (TabIe 4) favours high - iow electronegativeattach27 ment of the active centres rather than high-high/Pow-Iow eIeetronegative attachment although eqn.(ii) predicts the Iatter. The
atetic fnfaract&m in ihe high-highrlow-low
approach is probabIy responsible for the disagreement (PQ.5a). Another case of dfsagree13 ment was observed in the thermal (4+2) cycloadditionreaction between 1 - cyanoethylene and cycloheptenone (Table 4). Although eqn.(ii) predicts high-high/low-low electronegative attachment of the active centres, yet the reaction goes through high-low eIectronegative joining of the active centres because in the former approach serious lone pair - lone pair interaction 28a is present (shown by
double' headed arrow in Fig.5b). In the case 28b.c 29 of intramolecular cycloaddition reaction ring strain ortransannularinteraction may be present which
may
sometimes become
too important to jeopardise the prediction by
eqn.(ii).
DiSCUSSiOll
:
Although cycloaddition reactions with heterodienes and
have not been considered here yet there is no reason why
heterodienophiles
eqn.(ii) will not be applicable
for such thermal reactions if electrostaticenergy term does not play any important role. 30 As a typicaI example of such thermal reactions, the compound 2a is found to react with compound 6 in benzene at r.t. to produce compound 7 (96%) (Fig.5) by high-highllow-low electronegativeattachment of the active centres which is in accord with the prediction by Section I(d). But photocycloadditionreactions involving heterodienes and heterodienophiles are complicated
because
of the possibilityof excitation by
nonbonding elec-
trons. In our electronegativitymethod,
the inherent electronegativitiesof the active centres
in the excited state of a molecule have been assumed to be identical with those in the ground state and regioselectivityhas been determined accordingly. Although this is merely an approximation and may
not necessarily hold good for all photoexcited molecules
yet our method has been found to be suitable in predicting the regioselectivityof many
6137
Regiosckctivity ofcycloadditions
photocycloaddition reactions on of
the
the basis of this approximation. The
clectronegativity method
independent of the type
is
its wider
applicability because
most advantage the
method
is
of the orbital8 used to explain the cycloaddition reactions.
So a practical organic chemist will find this method very useful as it does net involve advanced chemical calculation.Of course, sometimes solvent effect31 or homoconjugation3' plays
an
important role on
the
regioselectivity of
the
cycloaddition reaction for
which due considerationshould then be made.
Referencesandnetee:
1 (a).
Hoffmann, R.; Woodward, R.B. The Cotwavation
04 Ohbitat Symm~thy,
Verlag-
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(b). 2 (a). b).
Gleiter, R.G.; Bohm. M.C. Pwre t; Appt. Houk. K.N.
J.Am.
Chem.
Epiotis. N.D. J.Am.
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Goddard. W.A.; Ladner. R.C.J.Chem.
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Let
xD*
L&k&5
1983,
24(5),
447450.
represents the inherent electronegativity of the active centre D in theca-
talysed reaction, so
the said fraction will then be
converted into
(XB -xD*) (XC -xD&(xA-xD*) In the uncatalysed reaction, let BE is - ve and xA> xB & XD>XC
.
- xD i.e. the numerator of the said As XD increases to xD*, then xc - xD*>k fraction will increase. Now let us consider two cases :
s. K.
6738
Ci)
v&en
Here,
XA
ds
iaereaeeeto
ZD
so* 0,
c-x, -
- xD*t)
Since
the
numerator
,the said fraction (ii)
when
Suppose, Here,
and x* >
> XD
then
xDg
x* -
XA -
but
XD = 1akand to
XB -xD=
( XA - xD*)
hand side
-
is less
( ‘A - xD*) (XB -xD*)<(al the numerator Here also, of the said fraction
(b).
J.E.
Huheey.
J.
J.E.
will
13.
reaction reaction,
- x)
( lb/
-x
Phy6.
Ibl<(xA
+ x)
( lb1 -
Ial 1 - x2 than zero then in both the
-xD) (XB -xD) but denominator
increases
Chem. 1965,
69,
3284 ; ibid.
- phincip~ed
; Tunick.
Tetnahednon
15.
Margaretha.
16.
Ogino.
17.
Mitra.
decreases,
so
the
value
1966, 70.2086;
Huheey.
J.E.
A.A.
ibid, 1969,
Lettm
Hurd. C.D.
R.D.;
3rd
and reactivity
04 &u&tie
1983.
of electronegativity
was considered centre
in calculating
the electrone-
by Huheey I s method. 30, 3676;
Barchtold.
C.A.;
3679.
Kitahara. Y . ; Uyehara. T. Chem and lnd. Kimbrough.
then
XD
lb1
of unknown substituentlactive
14.
:
lb1
Cunnigham . R . : Damishefsky . W. J. O&g. Chem. 1965,
1to.S.
of
than 1a I 1b I i.e.
lno&ganic Chemidtiy
Ciabattoni . J .
value
the
1966, 31, 2365.
100% eaualisation gativity
12.
so
increase.
Edn. Harper International.
(cl .
XD
decreases,
la I is equal to zero or greater
1 bl
O&g. Chem.
-X,&X,-
xD* = 1b 1 + x
= ( /aI
( XB - xD*)
cases
right
xg
sad
= XD + x where x is a + ve quantity.
xA - xD* = la 1 - x and XB -
Huheey.
denominator
and XB - XD*>XB -
Now, when
J.
xp ‘XA “XD xg - x,).
xD* in the catalysed
= Ial
11 (a).
(
5
increase.
as XD increases
then,
reecsm
u%s?d.alyeed
Xl)*) <( XA -x,1
increases
till
tke
xA>xD and XB
XA - xD*
in
XD
PAL
1971, 354;
Shoji.
Y.;
Takeshita.
H.;
1815.
J.Am.
Chem.
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1960.
82,
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J.F.AngeW.
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6739
Regioselectivity ofcycloadditions
Can.
J; ,Beekett. B.A. Jirkovsky H.M.; 19(a).
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Y.A.;
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22.
Snyder. H.R.; Poos. G.I. J.Am. Chem. Sot. 1950, 72, 4104.
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Higher and lower electronegativeactive centres have been depicted by a black
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1949, 79, 564.; Vogt. W.; Alder.
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26.
Similar observation was noted in ref. 9.
27.
Fish R .H. J. 0q.j.Ckem. 1969, 34, 1127,; Koppitz.P.; Huisgen.R.; Feiler.L.A.
28 (a).
Another possibility is the presence of dipole-dipole interaction.
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1951, 73, 5563.
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29.
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Yamanchi. M.;
Katayama. S.; Baba. 0.; Watanaba. T.J. Chem. SOC. Chem.
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S. K.
6740
31.
Pardo. L.; Branchadell. V.; Oliva. A.; Bertran. J. Theo. Chem. 1983, 10, 255-60.
32.
PAL
,
Mahain. G.; Schwager. L.; Carrupt. P.A.; Vogel. P. Tetihedm 24(34), 3603-6.
l&&b
1983,