Short Communication Mechanism 1 Octene Isomerization and Hydrosilylation Reaction Catalysed Ru3co12

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An-Najah J. Res., Vol. II, No. 8, 1994)

ilo:nal S. Hilal et al.

COMMUNICATION

ON THE MECHA NISM O F 1- OCTEN E

ISOMERIZATION AND

HYDRO SILYLATIO N REACTION CATALYSED BY

Ru3(C

0

) 1 2

Hikmat S. Hilal*', Waheed Jondi

2

Shukri Khalar

j

. 6 1 1

rn L ,X.,L.JI

. , J 1

z , )L I

L J I

, I

el z . . . . 1 1

r

'y •

r p

js-1.11

. , 1 1 1 . 1 1

, I1

zALJI

i st

Isom erizat ion and hydrosi ly lation react ions of term inal olefins

have been reported under therm al and p hotochem ical cond itions using

R u 3

C0)

, 1 , a n d H R u

3

(C O )

, 2 ,

0

In a very recent work, we reported E

L I

t ha t cata lyses b oth

Isom er iza t ion and hyd ros i ly la tion reac t ions o f 1 - oc tene (eq . 1) . I t

h a s b e e n f o u n d t h a t th e i s o m e r iz a t io n r e a c t io n o c c u r s v i a lo w e r

n u c l e a r it y c a t a ly t ic s p e c i e s th a t re s u lt fr o m f ra g m e n t a t io n o f t h e

m other c lus te r 1. Ev idence in favou r of concu rrent c lus te r ca ta lys is

t

1 C hem istry Departmen t , An-Najah N. , University, Nablus, West -Bank

2 C hem istiy Department , A n-Najah N. , University, Nablus, Weat -Bank

3 Chem istry Deparunen t , An-Najab N. , Universi ty, Nablus, West -Bank

175



On The M echanism Of 1 -Octene Isomerization and fiyclrosilylation Reactions...

(Short Communication)

was also reported. On the other hand, the hydrosilylation reaction

occurred via cluster catalysis at first and after some-time fragment

catalysis occu rred

1 1

J .

C5C„CH,-CH,-CH,Si(0E01

C

5 H„CH

2

-CH-CH

2 t Et0)3 SiH

ioxane

C 5

H„C

H

In this comm unication, we report our discussions regarding plausible

mechanistic pathways for the hydrosilylation and isomerization

reactions of 1 - octene using cluster 1.

t

XP RIM NT L

A ll chemicals used w ere purchased from A ldrich Chemical Co.

in pure form. IR spectra were measured on a Pye Unicam SP200

spectrophotometer.

Catalytic experiments were conducted as follows : a stirred

thermostated 100 m l round - bottomed flask reactor was charged with

the cluster, the solvent dioxane (enough to make total volume 10.0

ml), the silane and the internal standard reference (benzene , 5.0 m l).

The olefin was then syringed via a suba-seal stopper. A cooled

condenser column was connected to the reactor to prevent evaporation.

176



Au-Najah .1. Res., VoL II, No. 8, 1994)

ilcrnat S. Hilal et al.

Throughou t the course of the reaction, small aliquot were taken

after specified times. Each aliquot was immediately chilled in a

stoppered sample tube, and then IR analyzed.

T he hydrosilylation reaction was m onitored by follow ing the

rate of disappearance of Si- H IR band at 2210cm

relative to the

standard reference band at 196 0 cm . The isomerization reaction was

monitored by two methods. In one method, the appearance of the

product C

5

H

(C H

2

),Si OE0

3

, 2, was monitored. This was achieved by

measuring the 1250cm IR band relative to reference band at 196 0cm7

In the other method, the isomerization reaction was monitored by

following the rate of consum ption of 1-octene ( IR band at 16 50cni )

less the rate of consump tion of (E t0)

3 Sill ( IR band at 2210cm-1 ).

RESUL TS AND DISCUSSION

The Isomerization Reactions:

At 60°C or lower, 1- octene underwent the isomerization

reaction. Only ... trans- 2-octene was detected. No isomerization

reaction occurred in the absence of the silane. Turnover number

(T.N = m oles of trans- 2-octene/ moles of 1) values of up to 700 were

obtained within 11 min. The product was identified by its boiling

range (120-122°C). Fingerprint IR spectral comparison of reaction

product with literature

]

values and with authentic sample spectra were

conclusive.

The reactions showed a first order dependence on the silane

and on the olefin, when relatively low concentrations w ere used (2 .0

and 1.0M or lower for the olefin and the silane respectively). These

177



On The Mechanism Of I-Octene Isomer ization and Hydrosilylation Reactions...


resu lts wer e ob tained by the method of initial rates. Plots of ln(initial

rate) vs. In[reactant} were constructed.

At 70°C or higher, similar r esu lts have been ob tained w ith

higher T.N. valu es (higher than 700 w ithin 1 1 min.).

How ever , at 70°C or higher, the olefin isomer ization was not

the only reaction that took place, vide infra . The reaction has been

conducted at different temperatu res r anging from 50 to 70°C . The

activation energy has been calcu lated and fou nd to be 25.0 Kcal mot'.

Below 45°C, no detectable reaction was observed. In the temperatu re

range 60-70°C, mixtu res of isomer ization and hydrosilylation reaction

products were observed .

As repor ted earlier the L ake's kinetic criterionm has been

u sed to see if catalysis occu rs b y the intact clu ster 1 or b y other

fragment species that may r esu lt du ring the r eaction. The resu lts

indicated a fragment catalysis w ith the possibility for concu r rent

cluster catalysis'''.

To accou nt for the above resu lts. a two - rou te mechanism,

which pr esents both cluster and fragment catalytic pathways, has been

proposed(scheme I).

Steps 1) and 6) show the role of the tertiary silane in

activating the clu ster. 1 is know n to be u nreactive to molecu les such

as oleflns 1 8 1

. This exp lains the lack of isomer ization in the ab sence of

the tertiary silane. It is well established that 1 becomes more reactive

when activating ligands are added such as CH

3

CN' 9 1

, Ph2

m11 11]

and

others

1 1 2 1

. Ox idative- addition of the Si-H addendu m at metal atom

178



An-Najah J. Res., Vol. II, No. 8, 1994)

ikmat S . Hilal et aL

clusters has been proposed for other catalytic systems

113 )

. It is assum ed

here that the equilibrium in step (1) should be shifted more toward

the reactant side, videlicet k, < Ic

y

This was evidenced by the lack of

significant shift in the catalytic m ixture visible spectra. This assum p-

tion is also consistent w ith theory. In step (1 ), the forw ard reaction

Scheme I

L

(00)

4

u — s i R 3

0

R

o i ,

— H

C O

RuC0

4

S

R 3 )

R u

C O /

3 \

i1

/

C

-

R

(COlfu— RuCALSiR3)

R u

C 0 1 3

1 ?

1

C O L R u - - R u C 0 ) 4 p iR 3 1

R u

-

H

1 C 0 1 3 \ C

i

t .

4

H

\;\

R

r

H ) S

-p

- C - C =

C

R

3

S

1 3

• :

P

3

s

00 ,

4

-

P ;

S i • - k u

CO), C --,c

9

`

Ru (CO)

3

2

H

R

H

179



On The Mechanism Of 1-Octene isomerization and Hydrosilylation Reactions...

Short Communication)

involves an increase in the Ru formal ox idation state. Su ch process

is not favored in metal atoms that are su rr ou nded by n-acidic CO

ligands.

Each catalytic cycle shown in Scheme I explains the kinetics.

Assuming that step (1 ) is a qu ickly established equ ilibriu m, and that

step (2 ) in cycle A, (or step 7 in cycle B ), is a slow step, and u sing

steady state approximation,u

4 1

, for 6 in cycle A (or for 1 0 in cycle B)

then:

k

r a t e

[Ru

3

CG)

] REt0)3

SiHnolef in ]

k

k

k

[olefild

At low olefin concentrations, and knowing that step (2 ) is slow,

viz k 1 » k

, then:

K

r a t e

k _

[Ru3 C0 )121 REt0 )

3SiHliolefin]

which is consistent with the exp er imentally determined kinetics

` )

Step (3 ) is an insertion reaction that is well know n for many other

catalytic systems

5 1

. Step (4 ) is a deinser tion r eaction that is well

know n'''. Step (5) is a Lew is base dissociation reaction, followed by

a lewis base association reaction to form 7.

180



An-Najah J .

Res. , Vol

II. No. 8

* (1994)

ikm at S. Hal et aL

The H ydrosilylation Reaction:

In a recent study o f the 1- octene hydrosilylation reactionst

i

l ,

i t has b een s tated that c luster catalys is occurred at the b eginning of the

reaction. After 15

m in, ind ica t ion o f con cur rent

fragment catalysis

were apparent. Based on the discussion of Schem e I, vide sup ra,

Scheme

I I h a s b e e n p r o p o s e d . Sc h e m e I I sh o w s tw o r e a s o n a b le

pathw ays fo r the 1 -oc tene hyd r os ily l a tion .

Schem e I I

F A

C O ) 4

C O ) 4 R U

°

u

►

u-C-C-C-R

C0

R

-

C

-

C

-

C S i=

[

C : 4 - ,R u

C-C-S

C-P

4 H-SiE

_ S i

R u

i

S i

C 0

- 1

181



On The Mechanism Of 1-Octene lsommization and Hydrosilylation Reactions....


Path A involves catalysis by intact cluster. Step (1) is an

insertion of the olefm molecule between Ru and H in complex 11.

t

This is an anti-Markovnikov's process. Step (2) is a binuclear

reductive-elimination of the product molecule.

Path B involves catalysis by fragment species. Step (3 ) is an

insertion reaction of the olefm molecule between R u and Si atoms.

Step (4) is an oxidative-addition of a new silane molecule to

completethe sixteen valence shell electrons at Ru. Step (5) is a

reductive-elimination, of the produ ct molecules, followed by a Lew is

base association. Similar reaction pathways are well know nm.

ACKNOWLEDGMENTS

This work has been conducted in full in the laboratories of An-

N ajah N ational U niversity. The authors wish to thank S.Al-D ado,

E.M . Sawalha, H. D iab, S. Abu-Baker, 0. Doud, N . Taha, and A . Al-

Sayed, all of this department.

18 2



An -Najah J. Res . , Vol . II , No. 8, (1994)

ikrnat S. F ilial et al.

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Short Communication Mechanism 1 Octene Isomerization and Hydrosilylation Reaction Catalysed Ru3co12

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