Selectivity as a mechanistic tool in solvolytic reactions.

~trahedxm

Letters lio. 9. pp 637 - 640, 1975.

Peryon

printed in Great Br1tal.n.

Re8a.

SELECTIVITY AS A WlXHANISTIC TOOLIN SOLVOLYTIC RBACTIONS. Addy Pross Department of Chemistry, Ben-Gurion University

of the Negev,

Beer-Sheva. Israel. (Received in DK 6 Jammry 1975; accepted for publioation Harris has recently molecules

exhibit

reported that in the solvolysis

greater nucleophiltcity

aqueous solut ion1. This inversion abilityofwater

of adamantyl arenesulphonates,

than those of ethanol

in the relative

nucleophilic

to form a double hydrogen bond in the solvent

oxygen atoms of the arenesulphonate form such a structure

leaving group (I).

16 Jauuary 1975)

This,

(kethanol kwater
‘H---O’

R-4

S-

‘Ar

/Et

which maintains the normal nucleophilic

in Table 1 indicate

this interpretation

) are listed

philic

> 1.

We therefore

chlorides.

kethanol’kwater

in the rel-

of alkyl chlorides

to be incorrect. all

of which exhibit

the s-e

( which should exhibit

with the alkyl chlorides

slrrlar

showing the normal nucleo-

conclude that it is not the leaving group but the substrate

637

No account was

Furthermore it can be seen that

as well as benzhydryl pnitrobenzoate

behaviour to the arenesulphonates order f.e.

of ethanol was observed.

to the solvolysrs

as the adamantyl arenesulphonates.

tosylates

where

order.

taken of 1-adamantyl, 2-adamantyl and era 2-norbornyl inverse nucleophilicities

chlorides

of arenesulphonates produces an inversion

of ethanol and water, in contrast

listed

,o

S‘Ar

it was stated that in the solvolysisofalkyl

Thus it was concluded that the solvolysrs nucleophilicities

0,

‘H---O’

(111

no double hydrogen bond can form, the usual* greater nucleophrlicity

and 2-octyl

to ethanol which cannot

in contrast

H---O \, 0

In support of this proposal

1-octyl

to the

separated ion pan to the two

(1)

The results

in an

(II).

R_._O-

ative

water

skeleton that

638

Ho. 9 TABLE 1

Relative

Nucleophlllclty

Substrates

showing

of

Benzhydryl

and Water in Solvolysis

I-Adamantyl

chlorldeb

0.576

7.15

2-Adamantyl

chlorldeb

0.169

4.91

exo-2-Norbornyl

2.06

2-Adamantyl

tosylateC

0.548

2.08

2-Adamantyl

brosylateC

0.460

2-Adamantyl

benzenesulphonateC

0.432

chlorldeb

chlorxdeb

chlorldeb

tosylated

1.92

Z-Octyl

tosylated

1.01

kE’kW

from Ref. (ROEt)

1.

’

the ratlo Result

of

taken

(ROH) was obtained

determines

the relative

solvolysis

scheme3.

the nucleophlllcity from Ref.

by glc.

1 -

form stable

that

carbonlum

kl and k2 Involve substrates

AnTable

the very

1

a

greater

SOlVent

cannot ions,

backslde

Separated

undergo have only

attack

1 whxh

of

chlorldeb

fall

0.843

-)r -

1

nucleophllic

k3 available

selectlvlty

in the kg product the water

category

separated

by considering

s

on the formatlon

determlnlng

that

This

effect

with

X-

k4

water

is

rise

that

ion.

since The

derlvatlves, a stronger

is presumably

the fact

ion pair 1s essentiallylrreverslble.This would give

route,

carbonrum

and norbornyl

Thus It appears

combined

- The latl’

and which do not readily

of a free

are the adamantyl

Ion pair

+

1 P

substitution

step

taken

the Winstein

R+

k3

as the maJor product

detennlnlng

Result

5.

P

and k4 depends

show Inverse

in Ref.

R+ll X-

k2

b

of water

(ROEt) . (EtOH) from kE kw = ( ROH) .(

This may be understood

backside

lnto this

to that

from data

P

than ethanol stability

ethanol

Data obtained

R+X-

kl

compounds which

nucleophile

of

calculated

order.

P Substrates

4

d

e Result

nucleophlllc

RX

1

kE%

3.45

E-nltrobenzoatee

measures

kE’kW 4 1

18.96

1-Octyl

a

showing

Ethanol:

chlorldeb

a-(p-Chlorophenyl)ethyl

Benzhydryl

Substrates

In 70% Aqueous

kE kw

chlorldeb

a-Phenylethyl

Reactions

kE kW > 1

E,p’-Dlmethylbenzhydryl pMethylbenzhydry1

Ethanol

due to

formatlon

to the greater

of

the

639

No. 9

formation

of

the greater

alcohol stability

water molecule, between the

product of

being

the two ions

leaving

ethanol

and water

product

formation

ethanol

and water,

expected steps. not

Since

this

of

this

is

intimate

or all

average that

to be more nucleophilic

technique

Acknowledgement

to hold. of

it of

of is

the

in aqueous

being

more acidic,

for a) a

itself

more readily

stabilises

bond.

for

kl to k4 , the relative

the selectivity selectivity

of nucleophilic

of attack

to favour

ethanol

solvolyses,

of nucleophilicitres

substrates

of

able

attack

each of four

ethanol

is,

to undergo

nucleophilicities

as the better

reaction

selectivity

studies

of

kg kW is rate

determining

nucleophile,

in fact,

may be used as a highly

mechanism and further

work to

are now in progress. We wish

causes

interposes

k2 and k4, the normal order

ethanol

The reason

it

is

generally

than water

can be seen that

solvolytic

, more readily

As a consequence,

expected

of water.

may stem from two possible

b) water,

hydrogen kl,

nucleophilicity

molecule

the routes

or more quantitatively.

weighed

ion pair

ion pair,

steps,

expected

average

greater

than an ethanol

determining

to observe

elucidation

separated

a more effective

by several

In conclusion in the

the

to be the weighed

surprising

found

bulky

group through

For the product of

the water

less of

and the apparent

to thank Dr

D

Kost for

helpful

discussions.

extend

the

subtle capabilities

tool

woo. y

References 1. J. M. Harris, A. Becker, D. C. Clark, J. F. Fagan, and S. L.

Kennan,

Tetrahedron

Lette.rs,

3813 (1973). 2. T. W. Bentley,F. L. Schadt, and P. v. R. Schleyer.J.

Amer.

3. S. Winstein. B. Appel, R. Baker, and A. Diaz. Cham. sot.,

Chsm.

spec.

SOO.,

PubI.,

4. J. M. Harris, J. F. Fagan, F. A. Walden , and D. C. Clark, Tetr&w&on S. D. J. McLennan and P. L. Hartm,

Tetrahedron

Lett.,

4215 (1973).

94, No.19,

993 (1972).

109 (1965).

titt., 3023 (1972).