Differential bridging in the solvolysis of 6- and 7-substituted 2-exo and 2-endo-norbornyl p-toluenesulfonates

Tetrahedron Letters,Vol.24,No.31,pp Printed in Great Britain

3195-3198,1983

0040-4039/83 $3.00 + .OO 01983 Pergamon Press Ltd.

DIFFERENTIAL BRIDGING IN THE SOLVOLYSIS OF 6- AND 7-SUBSTITUTED 2-EXO- AND 2-ENDO-NORBORNYL p-TOLUENESULFONATES

Peter Flury & Cyril A. Grab* Institute of Organic Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland.

SUMMARY. The large difference between the reaction constants p, for the solvolysis of the 6-exo- and 7-anti-substituted norbornyl tosylates 3 and 1, respectively, is further proof for the hypothesis that 2-norbornyl cations are anisotropic with regard to the transmission of polar effects and that through space induction involves graded bridging.

Evidence was recently presented that the ionization rates of bicyclic sulfonates in protic solvents are strongly affected by the bridging of the cationic center to a pentacoordinate R-carbon atom, as in 1 i. It was also suggested that the differences between the rates of epimeric sulfonates are due mainly to differential bridging strain in the respective transition states

2

and that effective 1,3-bridging requires that the back lobe of the

R-C@ orbital overlap strongly with the incipient p-orbital of the cationic center, as illustrated in -2

3

.

)c:

R~,c-------~:s’

)p

:H&Ts ’ R&f

R,&OTS R&

OTs

l(p*=-2.0)

Z(PI =-0.78)

4

3195

H

6

3196

This

condition

is satisfied

2-exo-norbornyl

tosylate

lines)

and intersect.

converge

the cation

overlap

Hence,

bridging

tial bridging

should

Among

of 425 for 2 and 2

other

substituted

data,

pI

tosylates

a through

bond

bridging,

these pI

and a through values

or negligible

R

4 Further the rates

support

which

component

of the 6-exo-

can be roughly

values

constants partitioned involves

6: into

graded

in the exo ion pair -4

ion pair -6.

b) CH3

fl

C) CH20Ac

g) Br

dl CH2Br

h) CN

Cl Qo&

is supplied

and the products

tosylates

accompany

of the reac-

4 PI

substituent

.

e) COOCH3

Ii

bonds

1

at 70.0'

before

is strong

Differen-

of the high exe/

and that the latter

that bridging

for this hypothesis

of transmitting

the strains

confirm

(in 80 % ethanol)

and 2-exo-norbornyl

space

As shown

induction

anion.

state.

for solvolysis

to the inductive

that

in the endo

R=a)

01s

number

of the rates

. Assuming

cause

of

2, R = H,

upon the magnitude

respectively,

3 and 5 in 80 % ethanol.

5

of the R-substituents

was based

orbitals

epimer

in the transition

from by dotted

by the departing

(R = H) in 80 % ethanol

of -2.0 and -0.78,

(indicated

from the 2-endo

to be the major

this conclusion

the sensitivity

and weak

considered

axes

4, derived

hand the corresponding

is blocked

be weak or negligible

was,therefore,

constants

side

cation

the orbital

On the other

as long as the endo

endo rate ratio

measure

3, R = H, where

in the ion pair 5, R = H, derived

cannot

tion

in the 2-norbornyl

7 -

(b-h) and 8

in these

study

of the 7-anti-substituted (b-h), respectively.

compounds

the bridging

by the present

While

2-endo the

is the same as in 2 and 5

of C2 with

of

C6 and C7 are clearly

3197

different. at first

ing ,

Thus,

but do not intersect,

as illustrated

unusually effect

7 converge axes at C2 and C7 in the ion pair -9 from -

the orbital

indicating

in -10, generates

low pI value

of -0.72

of the substituents

that overlap

a large

strain.

for the series

at C7 must

7 -

is weak

This

(a-h)

6

and that bridg-

is supported

by the

. The residual

then be due mainly

to through

inductive

bond

induc-

tion. The

value

pI

much

smaller

ably

larger

than

the presence

creases

for the 6-exo-substituted for the epimeric

of the anion

value.

This

bond

were

with

(b-h) afforded of the 7-anti-

reasons.

part

However,

due to

C2 in 11

of its positive

charge

from C 1. Deshielding

by the practically

identical This

to

of C 1 in-

at C7 and hence

-12 and 13, respectively

would

raises

rates not be

important.

different

-21

-20

7:3 containing

1.1 equiv.

yields

of triethylamine

of the same products,

and the 3-endo-substituted

The alcohols

2, but consider-

at C2 and C7 is hindered

the substituents

-19

In dioxane/water

respectively.

densitiy

is supported

and 7-syn-chlorides

-18

mainly

electron

transmit

8 (a-h) is -

7. In the ion pair 11 derived -

for geometrical

interaction

conclusion

if bridging

tosylates

and thereby

2-exo-tosylates

stereoisomers

of the orbitals

and also

like C2, withdraws

of the 7-anti-

1. and 8

lobes

bridged

its through

the case

for the 7-anti-substituted

of the rear

be strongly

C6 which,

the pI

that

than that

from -8 overlap

should

of -0.97

-14 from 7 are formed

the tosylates

which

2-exo-norbornanols with

inversion,

consisted 14 and 15, -

those

from 8 -

3198

with

retention

bridging cursor

in the cation

of -15. When

ide -18 were

nortricyclenes with

the transmission

is strongly

substituted

or bromine

leads

small

The cation

C2, C6

to 11, the pre-

amounts

of the epox-

16 also yielded -

2-exo-alcohols

consider-

19 by a C6, C2 hydride -

-20 by 1,3-elimination.

our earlier

of inductive

dependent

preferential

of the latter

by O-3 participation.

In conjunction that

-16. Rearrangement

of the 7-syn

beside

at C2, thus confirming

R in 15 was chlorine

formed

able amounts shift,

of configuration

work

1,2,3

substituent

on hitherto

unrecognized

these

results

effects

show clearly

in norbornyl

stereoelectronic

cations

factors.

REFERENCES R. Hanreich

1) C.A. Grob,

& A. Waldner,

C.A. Grob

& A. Waldner,

Hanreich,

G. von

C.A. Grob,

Sprecher

Tetrahedron

3235

of this work

Int. Ed. Engl.

21, 87

Helv.

see C.A. Grob,

2312

Angew.

(1981); Grob,

2298

R.

(1981);

(1981).

Chemie

94,

87

(1982);

(1982).

& U. Zutter,

Tetrahedron

Lett.

4) C.A. Grob

& B. Schaub,

Helv.

Acta 65,

Chim.

23,

2849 1720

(1982). (1982) and earlier

papers

therein.

5) derived

from the pK of 4-substituted

Schaub

& M.G.

Schlageter,

6) It is noteworthy what

3231 C.A.

Chim. Acta 64,

ibid. 64,

Grob

cited

Lett. 22,

(1981); W. Fischer,

& A. Waldner,

B. Giinther & R. Hanreich,

2) for a review

3) C.A.

ibid. 22,

larger

applies

in trifluoroethanol

nucleophilic

and 2-endo-norbornyl 7) In 80 vol.% respectively. 2-norbornyl

57

ethanol

tosylates

react

Hornback,

that participation

J. Amer.

in Germany

study

a rate

& J.M.

these

in 80 8 ethanol.

therefore,

are 1.57 x 10

However,

which

this also

cannot

be concluded

essentially

undergo

that

2-exo-

by a kc process. -4

of the acetolysis

and 1.63 x 10 of all four

-4

,

7-chloro-

12/13 of 1.54 was found by P.G. Gassman -Sot. 94, 7010 (1972). These authors con-

of the C 1, C6 bond

18 April

2,

2, 2 and 1. are some-

ratio

Chem.

B.

(1980).

tosylates

It can,

In a careful tosylates

than

1-adamantyl

at 70.0'

see C.A. Grob,

for the tosylates

attack.

cluded

(Received

ibid. 3,

that the p, values

to 3-substituted

rearside

quinuclidines;

1983)

is not important.