Polar substituent effects in the solvolysis of 2-methyl-2-exo-norbornyl 2,4-dinitrophenyl ethers

POLAR

SUBSTITUENT

EFFECTS

2-METHYL-2-EXO-NORBORNYL

C.A. Institute

of Organic

A comparison

Summary.

and secondary shifts

of polar

a better

ETHERS

& A. Waldner

University

substituent derivatives

mechanistic

is a widespread

R = H, solvolyse tertiary

the former

with

belief

anchzmerlc

analoques

of Basel,

4056

effects

In the

lndlcates

that

model

(kA) process

that

secondary

assistance

1s thought

cation

2, the

classical

cation

4 l. This -

the

of the following

of tertiary

that

3, R = H, react

classical

light

2,4_DINITROPHENYL

OF

one

Easel,

Switzerland.

solvolysis

of tertiary

a gradation

of electron

involving

the classical-non-

ion dichotomy.

There

their

Chemistry,

2-exo-norbornyl

provides

classical

Grob

IN THE SOLVOLYSIS

latter

and secondary

without

to lead

such

however,

mechanistic

comparison

the C(1)+(6)

2-exo-norbornyl

assistance.

Furthermore, i.e.

to an unbridged,

i.e.

cannot

effects

be upheld

4

R

CH3

derivatives.

lb

Lb+

X

Br

5 First phenyl ethers

order

rate

constants

for

the reaction

of tertiary

non-

in

in the solvolysis

2

R

A,

whereas

bridged,

dichotomy

of substituent

derivatives d-bond,

to a synunetrlcally

(kc) process,

rigorous

-1

from

2-exo-norbornyl -_

exo-2,4-dlnltro-

ethers La - _ 3h (X = ~,~-(PJO~)~C~H~O) and for the corresponding 2 in 80 vol.8 ethanol at 100“ C are listed In the following -5

endo TABLE.

TABLE

exo-ethers R=

k(s-1) -2

CH3

4.74

b)

H

c)

(CB3)2CH

2.70 x LO-* -2 6.34 x 10

d)

CH2Br

e)

COOCH3

x 10

2.23 x 1O-3 -4 3.97 x 10 -5 5.46 x 10

0COCH3

9)

coo-

1.27

h)

CH20H

2.78 x 10

When stituent line well known

elevated -1.30

rates

a special

for the exo ethers

stancially smaller

larger

than

tosylates

than

5

I, X = OTs

controlled

in the W-like reflected (CH3j2CH

to 13

rate

4 is less

cations

-6.

tertiary

endo

adamantanes

ethers

7 4 -

-5

the

(plot 21, as

effect

from

the

ethers

2

439

ratios

in k

rates

would

which

'1

latter and

are

lead

to

9 of

1, is sub(plot 2), but

secondary

exo-

of 2 and 2 are mainly strongly

attracting that

than

sub-

constant

of plot

decrease

correlate

for

4

conformation

1) and for the

be expected

The

processes

are more

at C(6)

inductive

fit the regression

(3 = -0.75

for 3 and 1 show

2, plot

c

slope

i.e. as the electron

3 also

the

The reaction

sickle-like

(TABLE)

257

reported

ionization

An the

exo ethers

(Figure

13

are omitted.

4, respectively.

by substituents

log k for the tertiary

52

against

and CH2OH

of R, and these

the g-values

affected

105

1) the pornts

for the recently

3 than -

for R = OCOCH3,

cations

COO-

for the endo

effects

exo/endo

In addition

increases.

groups

of -2.0

conformation

In the

1, plot

(Figure

, Evidently,

by the polar

491

x lo -6

3h are plotted

3, as calculated

the 9 value

171

1.08 x lo -4

of 16 and

that

1.58 x lo_4 -4 1.29 x 10 2.13 x 10 -5

2.89 x lo -4

electron-donating

by factors

272

4.18 x 10 -6

-3a

6; for R 3

if the electrofugal

to exert

k(exo) k(endo)

1.74 x 1o-4

7.69

x lo-1 -2

log k for the ethers constants

-5

k(s-')

a)

f)

endo-ethers

-3

from

transmitted -5. This

491 for R =

power

ionization

ionization

well

with

l-substituted

like processes.

is also

of R to tertiary

to secondary

log k for the 3-bromoHowever,

the

423’

0 -1

-2 -3

-4

-5 -6 05 Figure

05

0 1

Plot

(1)

log

k

1.5

1 -3

YS

67

(2)

Plot

2 log

k

_5

2.5 vs

OF

-1

-2

-3

-4

log

-5

7

-6

Ftgurc

2

-5 Plots

of

-4 log

k

A

-2

-3 vs

(1)

Log

k

5,

(2)

-1 log

k

1

0

,

(3)

log

k

1

k

9 values

are

scmewhat

the unstrained

showing

important

exo ethers

3 and -

the

the two series

(slope 0.66)

by substituents

for R = H, which The correlation

tertiary placement C(2).

This

induced

_4 and 6,

should

stronger

bonding

tural

formulae

cipation),

rndicates

WC thank

The only further

electrons

be stronger between

this

in the

3 suggests

polarization,

that

implies

symbolized

stronger

of the transmission Professor

Paul

but

exception

than

for

the

are

only

effects

R. Schleyer,

degree of the 6

here

.

of the secondary

cationic

arrows

of drscenters

lead

of conventional basis

and

at

in the cations

-6 and therefore

On this

3),

in different

in the degree

symbolism

polarization,

2, plot

involved

incipient

cation

log k for the

is the deviation

by horizontal

In the

of polar

von

same way

differ

bridging.

longitudinal

(-1.20)

_1 5 (Figure

factors

towards

and C(2).

3 -

between

the reactions

in the secondary

C(6)

correlation

exo tosylates

that

ethers

4.

1 and 3, respectively,

as in 2, and

descrrptions

secondary

at C(6).

of the R-C(G)-C(1)

norbornyl

is the excellent

arc affected

in plot

derivatives,

-7 (-1.14)

result

that

point

for the strained

bromoadamantanes

The most tertrary

larger

bridgrng

to struc-

(d-parti-

as in _ 4 and 5, are alternative in electron

Erlangen,

Eor

deficient

stimulating

molecules. criticism.

REFERENCES For

a recent

Brown,

with

2) C.A. Grob 3)

C.A.

authoratative comments

& A. Waldner,

Grob,

B. Schaub

review

by P. von

Tetrahedron

C.A.

Grob,

ibid.

5) W. Fischer,

C.A.

Grob,

G. von

6)

e.g.

"The Nonclassical

Letters

& P1.G. Schlageter,

4) W. Frscher,

1979,

see

R. Schleyer;

61,

1588

Sprecher

Plenum

Ion Problem"

Press,

New York

by H.C. 1977.

1980,

Helv.

chim.

acta 63,

57

(1980).

(1978). & A. Waldner,

Tetrahedron

Letters

1905. the rates

3 follow the inductive order i-Pr) CH3 ) H, whereas the order is H > i-Pr > CH3 in the secondary series 1. This reversal LS probably caused by the easier and stronger salvation of C(6) in the secondary

in the tertiary

cation

5, R = H.

series