Solvent effect on ground-state versus transition-state stabilization by metal ions. Contrasting metal ion behaviour in tetraglyme and ethanol for nucleophilic displacement at a phosphorus centre

Tck&xkonL.cocrs. Vol.31. No.45. pp 6513-6516.1990 Rmtcd in Grert Britain

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EFFECT ON GFtGUND-STATE VERSUS TBANSITIGN-STATE

BY METAL IGNS.

ETHANOL FUR NKLEGPHILIC

E. Buncel ag,

E.J.

DISPUCR4ENT

Dunna, Ng. van Truonga

aDepartment of Chemistry,

Queen’s

Establishment

Ottawa.

'Defence Research

Establishment

Suffield.

follows

The rate

the order

of reaction

state

stabilization

by interaction

(Li+

ethoxide

that

of p-nitrophenyl

crown ethers

anions

The kinetic

Purdon=.

K7L 3N6;

KlA 024;

Alberta.

Canada

diphenylphosphinate

is much faster.

TlA SK6

with

but in tetraglyme

alkali

metal phenoxide

the reactivity

which is accounted

through hydrogen-bonding

of p-nitrophenyl

is subject

for

order

on the basis

and of

is

of

the transition-

in the solvents

results

for

kinetic

the reactions + PhO

-->

metal

or inhibition)

of

lithium

Ph2P(0)OC6H4N02-p

by alkali effect

metal cations

on the rate

in the reaction

and tetraethylene

through hydrogen-bonding

Ph2P(0)OC6H4N02-p

behaviour

on differential

(catalysis

and cations

catalysis

have a marked retarding

ethanol

insight

diphenylphosphinate.

to electrophilic

on the contrasting

and transition-state

stabilizing

Hat,

as solvent

and cryptands

study affords

The

ground-state

rate

ion in ethanol

phenoxides

(tetraglyme).

Bay, Canada

Medicine

the reaction

We now report

alkali-metal

Shirley

Canada

with M+ in the two solvents.

> K+) and that

reaction.la’b

Kingston.

of the ground-state

We have reported (11, with

Bannardb and J.G.

, R.A.B.

LiOPh > KOPh in ethanol

KOPh >> LiOPh and the reaction differing

AT A PHGSPHGRUS CENTRE

University,

bDefence gesearch

Summary:

STABILIZATIGN

CONTRASTING MEML ION BEHAVICUB IN TETBAGLYME AND

glycol

dimethyl

ion interactions

and the role

of

of

of 1 with ether

with

solvent

the in

and complexation. and potassium

phenoxide

with 1,

Ph2P(O)OPh + p-N02C6H4*O-

1 in the solvents

ethanol

measured by standard stopped-flow

and tetraglyme

spectrophotometric

techniques

the appearance

reactions

proceeded in large

crown ethers and phenol,

(2)

of the spectral

quantitatively excess).

and cryptands; as described

added Li+

added crown ethers

Further

notable

the effect

the slower

the faster

absorption

and in all

in Figures

1 and 2. The rates

reactions

reactions

in ethanol,

in tetraglyme,

due to p-nitrophenoxide

cases

clean

results

of added salts

first

pertain (LiX,

order

to the effect KX),

anion.

kinetics

were

while by The

were followed

of addition

and the effect

of

of added water

below.

The key observations k(KOPh);

methods for

were used in the case of

following

(phenoxide

at 25’C are displayed

made are

and K+ salts

and cryptands

the following. (e.g.

For the reaction

as MC104) accelerate

have a rate-retarding

6513

effect.

in ethanol:

the reaction

(Li+

(1)

k(LiOPh)

> K+);

>

(3)

and k(KOPh + 2.2,2-cryptand)

=

[ MOPh], x

103(M)

Figure 1. Kinetic data for reaction of p-nltrophenyl diphenylphosphinate (1) with alkali metal phenoxides and benzyltrlmethylammoniua phenoxide (BTMAOPh)in ethanol at 25T.

Figure 2. Kinetic data for reaction of r with lithium and potasslun phenoxide in absence and liresence of cryptand oomplextng agents in tetraglyme at 25V.

6515

k(LiOPb effect

+ 2,1,1-cryptand); lc on the rate.

For the reaction small

retarding

reaction,

(41 added water and phenol

in tetraglyme:

effect

with

(Li+

concentr8tions

CC kMOPh1;

(21 added Li+

(31 added crotm ethers

> K+);

2.1.1-cryptand

2,2.2-cryptsnd

(11 k(LiOPhI

in small

having

a much greater

on the KOF% reaction;

(4)

effect

added water

and cryptamls

have little

or no

snd K+ h8Ve a

accelerate

on the LiOPh reaction

the than

and phenol have marked rate-inhibiting

effects. Importantly, the assumptions dissociated @TMXhl

the overall that

phenoxide

z 105.

represents

The following

EtOH, k(Li+PhO-1

> k(K+PhO-1

where k(Li+PhO-1

and k(K+PhO-1

k(PhO-I

is

the rate

The very

of 2,1,1-cryptand this

solvent.

cryptated

Li+

suggests

than free

phenoxide

We conclude

constants

that LiOPh exists

suggests

and PhO- in tetraglyme

k(PhO-I

free

On

or

phenoxide

one obtains the two solvents,

m k(K+PhO-1

due to ion paired

phenoxide coupled with

hand the smaller

compared to KOPh + 2.2,2-cryptand

of

For

>> k(Li+PM-1,

species

while

phenoxide.

of lithium

in tetraglyme

in ethanol,

thus hold for

tetraglyme,

the rate

compared to ethanol.

the reactivity

the benzyltrimethylammonium

relationships

and for

represent free

and that

phenoxide reactivity

reactivity

for

reactivity

On the other

free

is much greater

system reflects

(TGl,

> k(PhO-1;

constant

small

in tetraglyme

ion in tetraglyme

correspondingly

Co_4E

reactivity

the KOF% + 2,2,2-cryptand

reactivity that there

and that

of

the large

largely

accelerating

as unreactive

effect

aggregates

the LiOPh + 2,1,1-cryptand

is appreciable

the Li(2lll’PhO-

ion pairing

species

in

system

between the

has a lower

reactivity

ion.

that

the much smaller

is mainly due to stabilization

reactivity

of PhO- in ethanol

of the nucleophile

compared to tetraglyme

by hydrogen-bonding

in the hydroxylic

n

solvent.

In tetraglyme,

L

similarly

result

inhibition i.e.

from hydrogen-bonding

infers

that

the ground-state It

is evident

behaviour. both,

this

with

ground-state

would lead

the ground-state neutral

to a rate

interaction

substrate

(there

that alkali-metal

retardation

would involve

these H-bond donors.

ions have a marked effect

of H+ with

and, conversely,

is no direct

of PhO- with

or phenol would The

is much weaker in the transition-state,

acceleration’(catalysis1

the, transition-state

of added water

is predominant.

from interactions

A rate

effect

interactions

effect

from the results

which can arise

retarding

type of interaction

stabilization

differentially.

strongly

the powerful

would result a stronger

(inhibitlon).3

primarily

evidence

the ground-state,

for

if

is

with

reasonable

M* with phenoxide significant

M+ interacts

interaction It

on the kinetic

transition-state more

the to assume that

ion rather

association

or

than with the

of alkali-metal

ions

with the substrate). Thus the interaction because

of

the competing

the more effective

of M+ with PhO- should be much weaker in ethanol H-bonding

solvation

interaction

in the former

case

than in tetraglyme,

(PhO- ----

of K+ by TG would moderate the ground-state

HOEtI,

although

interaction

in that

6516

4

case.

The rateacceleration

interaction

with

M+ will

interact

because

PM-

more strongly

is no longer

by added W+.

The absolute

higher

density

charge

than for

nucleophilic

can hence be accounted

by the solvent,

effects

are expected

in the former,5

for

to the ground-state.

with PhCl- (ground-state

stabilized

bring

into

focus

displacement

hydrogen-bonding DMS&

relative

effect)

than with

and this

to be larger

by a stronger

Conversely,

will for

and hence the differences

lead Li+

the transition-state, to a rate

than for

will

in tegraglyme.

also

retardation

K+ due to the

bs larger

for

Li’,

K+.

Our results for

by M+ in ethanol

the transition-state

processes,

interactions.

in the former

which stabilize

solvent,

stabilization

the absolute

reaction

catalysis

have bearing 7

of

is greatly

processes

are much larger

is

not only

the solvent

where different

of

alters

to

and water

observed

and quantitatively

solvent

(comparable

between transition-state

as a solvent

of

role

by added phenol

ion catalysis

both qualitatively

Our demonstration

as a potential

and the powerful

retarded

hand, metal

Thus tetraglyme

on other

tetraglyme

of reaction

of the change in balance

but alters

on these processes.

will

On the other

by M+.

rates,

metal cations

been observed.

rates

though reactivity

as a result

ground-state

advantages

compared to ethanol,

The absolute

the nucleophile.

but not in tetraglyme

the relative

in ethanol and

dramatically the influence

of

dependence of metal ion

types of metal

ion effects

have

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