Rates of reactions of Ferrocenylcarbocations with water

i

:-I-.:;: 3_::F~_:_.f;:_~.~_,._~~.~,,:.: . --:;~~~,~-~:,r_.-:_~~.’ :.: . . _. --I:_;l.i:.: - ‘. - ‘-.. T_ A’_>; I;;_‘;: _.y‘..Y T;\,:, _,: .:. .:

>_::.-_.+: _.,_:_:;,I_*‘~’

__._

.‘-.,_,:_~.r.~-~:,::---

r_.

~~

.-.~.. .-.-.:--.:

_.

_= --;-;-*-

.:

-..T-;-~ ‘~~+-:.:<,=~._,

.:. -.::_=_. .=;-

:y_:_

;

.T6.-,.-

:r.

_‘.z.,:

.__.;.:_~+

.~

.

_.

. . .

;.

. . . .._ I.-

_-

_...-v

.

y.:

:_-_.I_

_:I’.;.: ::_:;._-. -._: -.

;-;-.y::y

.:

~.

‘i_

:

.‘Y___

i: :

_ .,

_. __

_I

. .

.

.

ml.

1

..

~.. ._

_;

.-..

.~.

_

_

-.

.

-~ : 1 ...._j.._ ;.:.y. ~__~.~.’ -.-._ ._~.::_,I._ I --1-T .’ .’ : . ..--. .._ , .: y. f ,.‘.. ,:.. :-~.~~Iw;r’Es’.:o.~‘~~~I*Ns..oF~ ~EP,~CENYLCAEE~CA~CATIONS WITH- WATER .- ._ z--_ -- ._-. .;'..,. _.-::__c~ietiti::A;~ 3tmtcm* -am.-iv~~son earrasco . _: .cf-Chemistry,' Univeisity of dalifornia, Santa .-:Depa -. ..,..

_.

:

: .

.-._

:_.

-.

::_r~~,p~el~n~~-c~~ca-tidn:.

_

-.

;rtment .._:.. --

&lichia

B,arbara,

930-16 --(U.S.A.:): _

._ -....a& m- : :

:

... l

__ Wiiliau.~S.

Watts

__Schoo_l.of gbysical

Sciences,_ New University

of Ulster,

Coleraine

(Received March 9ti,.1977)

.-.

: Secondary

‘.towards.waCer -are

ferrocenylcarbenium

than

-formed more

,~-0Me

and;

when

ions

.tertiary &x~logues

rapidly

from alcohols.

(F&-U&)

(FcifAr, or Fc&Ar) The

g = 't-Bti'or 1-adam-+yl,

_. _: in&o .cati@n- is ~&za~dea.

are much

cations

the

acid

more

reactive

,and, in

are stabilized conversion

of

acid, by alcohol

.~_.

..

: . : _ . :I:-AsDart'of a.study of the .rates.of reactions of metal-stabilized : ..~.. :-_~arhoc.=t~ons..with nucleoph;les,.we have investigated the spontaneous .-.. ._ --.re~ctipns.._wit%~_ater of the. _ferrocenylcarbeni&n ions Ia-i .which were .:-.. ~. -_. .,pkep_ared.botb asisolable-t_etrafluoroborate.salts and, in solution, as .- .. : corresponding~ alcohols in : for addition of water to and

(If-i) substituted-

aryl/alkyl-

---cations_a_-rin-.~~;.~ablk .. --‘- ,_&. _-.r.: -I- -. Y_ :.--&getberl-Gith~the ::. __.::_.. _. _... .___ -:- : ~1 .-I_. __ -r.

second-orders

r+te

constants,

.. -.. ‘-

..

__

. -. .-. -_ . . :;.

.- --

-c_.

.

.~

1.

_

'b _. ::62.. 58.b

15 b .-.. . 172,b

I -I

-,.44-o

135;c

A

_I

41:"

960."

-0.49

1.37

.x5.90

0.60

0.72

0.46

2.19

1.58

o;20

15.9.

2.80

:,0,58

-0.0036

-1.31

0.86

0.014

-0.88

y342

--.0.24

O-CC52

-0.76

-375

0.32.

Y483

d.0092

7.4e

I

I

~0.64 12.f

8.ge

76.f

0.82 f

L

b-

" At 25.0°C

= R&f.

.e.0.25 Pi H2Sba -~.

Reacticy indicated

0_..125. M H2S04

M HCl

d 0.125

M HCl

c43

of Ia,b'with

(T&le)

c 0.0125

water

were

followed

in_ H20 and in H2d:MeCN

king

at the wavelengths a Durrum

stopped-flow

~spec~ro~hotom&zet.~~~Ty&.call~; [email protected]&_~ mixture of cation and . .alcoholi$ HCliR 2O.or H 2 SO 4rH20tMeCN was added to H20 or H20:MeCN so _ :; ~_ that-t~e'obserGeiiI‘fiiTSf;orae~ rat& ?zonstant was the sum.of the forward .. ~~.i _-. -.. :-'~d~:rev~sk~:rat~-:colistants. (kf +.k$H+]) , ‘.. Reactions

which oi.Ic-i

were with

separated- using water

werefollowod

and aryl moieties. values

The kf carbocations

water

(Ia,b) are much

showing

kf,

overcomes

nance.

that

groups

steric-hindrance

Ia,b.

account

If-i,

the modest

kf more

supzressfpn

steric

group

markedly

reduces

trends.

of resqIc-e

[S] have-suggested

ooncexitration- at-the

formal

ferrocenylcarbocations-

For the aryl/alkyl~systems

by the-E-mcthoxy

of%-conjugation

addition. of

.tertiaq:cations

secondary

enhancements

between

group-further

the I-centre

.-

indi.-

&the

-. effects. are-much inctieases.k&

k 3: for the.arylj&kyl

hindrance

the..$econdary

i@h&bition

studies

charge

aryl group. Substitu&t

in&e

Recent~1?&N?4R

for the observed rate

to nucleophilic

due-to-stekc

of tertiaryas.compared-with

may

As expected6

reatitive than the-teitiary &c-i). .in If-i which-destabilize the &&also

is significantly.gr&ater

which

cate.

pattern.

more

group.reduces

than in the secondary

1 -centre

~.

any rate enhancement

A E-methoxy

that there

._

show. a different

the bulky.alkyl

However, reduce

1.

larger

.the _

for

on,‘.k,thanon

mono?:hd

di-ary&

_kf. p E%nethoxy 1 .. sys,tems:Ip-e_ but .:

systems :If-i..presumably.-reflecting

to.alcohol.heterolysis...+lky

substituents

reduce

kr

steric

,by..

for. hetero.lysisj : : -: (nucleophiles add.tid depart-preferentially:.,from~ tb~~&~&&Ztion). .:_.._ .. -. and by inhibiting .iesonknce. stabili&&n bf~'~he.-transitior.stpte;. ‘.. These effects also.explain.the-~hi~h~_~~r~~ct~~it~es_.4f Itho.second-5 .i-:. . -.: ~:.overthe _t&tiary aIcohols _ -, :- .-;. ,_Y_~: .:.-.~..- : -.zy. --:-.:‘-_. .;.---.‘--..‘: _-~.: _ ,; .. .-.I L. .__-_ .; ..__:___ .: ~. The.obseFved-- dep&&& -of-.k '--a&-k ..& -s~rud~~~.~~h~ra~-~ijith b- _r forcing

the- alcohol

-.

into an unfavour~~e'c~nformation-

,-

-f,

;.-

r

-.::__;__:_--_. .-_-.~:~I_~~:~~1__~.--:--_',=.~ _,-'_:~.q r-.

carbocation-jike._-tt‘ar;sjtion state,- -,-si_t~uon;'~~rar..~_~:.~~~r.':_

-IL;:

1 .See -R. 11;.Boyd ifi."Solute-Solvent aad C:D.-Ritchie, 2

Marcel

Dekker,

J. J. Jaruzelski

N. C. Deno,

Interactions", New York,

eds. J. F. Coetzee

1969,

and A. Schriesheim,

Chapter J. Amer.

3. Chem.

SOC.,

-.77 (1955) 3044. 3 .$, A: Hill

and R. Wiesner,

J. Amer.

Chem.

Sot.,

(1969) 509; E. A.

Hill& -44Ci 5 ._

J.--0rgauometal. C&em., 24 (1970) 457. . Ai .&n&n and S. K. Huang,'J.~Amer. Chem.

91

S. Braun,

T. S. Abram

and W. E. Watts,

Sot.,

97

-J. Organometal.

(1975) 2140. Chem.,

97

-- (1975, 429.. 6:: E. A.-Hill

and W. J. Mueller,

-Tetrahedron

Letters,

(1968) 2564.