Activation of C-H Bonds by Bidentate Phosphorus Ligand Complexes

7 Activation of C-H Bonds by Bidentate Phosphorus Ligand Complexes of Iron

Downloaded by GEORGETOWN UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch007

S. D. ITTEL, C. A. TOLMAN, A. D. ENGLISH, and J. P. JESSON E. I. du Pont de Nemours and Co., Inc., Central Research and Development Dept., Experimental Station, Wilmington, DE 19898

Iron complexes containing bidentate alkyl and aryl phosphorus ligands cleave a variety of C-H bonds under mild conditions. Hydrido acetylide complexes were prepared by oxidative addition of primary acetylenes in the Fe(DPPE) and the Fe(DMPE) systems [DPPE = bis(diphenylphosphino)ethane, D M P E = bis(dimethylphosphino)ethane]. The Fe(DMPE) system also cleaves C-H bonds of activated methyl groups, aromatic compounds, and certain other sp hybridized molecules. The C-H cleavage reactions are reversible, resulting in equilibrium mixtures of isomeric products in many cases. Studies of substituted benzenes show that while product stability is favored by electron withdrawing substituents, steric effects play a predominant role in the determination of product distribution. 2

2

2

2

/ ^ n e o f the basic goals of i n d u s t r i a l catalysis is t h e a c t i v a t i o n o f C - H b o n d s i n a v a r i e t y of t r a n s f o r m a t i o n s . this r e a c t i o n is w e l l e s t a b l i s h e d (1,2,8) inate b o n d activation.

I n heterogeneous catalysis,

b u t o f t e n suffers f r o m i n d i s c r i m -

M o r e r e c e n t l y i t has b e c o m e

clear that C - H

a c t i v a t i o n c a n b e a c c o m p l i s h e d b y h o m o g e n e o u s c a t a l y t i c systems i n v o l v i n g t r a n s i t i o n m e t a l c o m p l e x e s (4,5,6).

T h e H - D exchange r e a c t i o n

has b e e n c a t a l y z e d b y a v a r i e t y of h i g h o x i d a t i o n state c y c l o p e n t a d i e n y l p o l y h y d r i d e c o m p l e x e s (7,8,9,10) P t * (2,11,12). 2

a n d e l e c t r o p h i l e s s u c h as P d

2 +

and

T h e s e s i m p l e e x c h a n g e reactions p o i n t t h e w a y t o w a r d

more fruitful synthetic applications. T h e exchange reactions b y p o l y h y d r i d e s a n d l o w - v a l e n t species a r e g e n e r a l l y a g r e e d ( J O ) to i n v o l v e o x i d a t i v e a d d i t i o n o f t h e C - H b o n d to 0-8412-0429-2/79/33-173-067$05.00/0 © 1979 American Chemical Society In Inorganic Compounds with Unusual Properties—II; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

68

INORGANIC COMPOUNDS W I T H UNUSUAL

PROPERTIES

II

f o r m C - M a n d M - H b o n d s , exchange of D f o r H , a n d finally, r e d u c t i v e elimination

yielding

the exchanged

product.

T h e intermediate

aryl

h y d r i d o species g e n e r a l l y w e r e n o t d e t e c t e d b u t w e r e p o s t u l a t e d o n t h e p r e c e d e n t of i r r e v e r s i b l e a d d i t i o n of a r y l C - H b o n d s to t h e h i g h l y r e a c t i v e transient i n t e r m e d i a t e s M ( C H ) , M = m o l y b d e n u m (13) 5

or R u ( D M P E )

(14),

5

(15,16)

2

a n d tungsten

2

[DMPE

— bis(dimethylphosphino)-

ethane]. A s p a r t of o u r c o n t i n u i n g s t u d y i n v o l v i n g t h e c h e m i s t r y o f l o w - v a l e n t t r a n s i t i o n m e t a l complexes b e a r i n g p h o s p h o r u s l i g a n d s (17),

w e were


i n t r i g u e d b y a l i t e r a t u r e r e p o r t of t h e zero-valent, f o u r - c o o r d i n a t e c o m plex F e ( D M P E )

It h a d already been shown that

(18).

2

zero-valent

complexes of i r o n are m o r e electron r i c h t h a n t h e i r s i m i l a r l y l i g a t e d n i c k e l analogs; [ F e ( P ( O M e ) ) 3

5

is p r o t o n a t e d b y t h e w e a k a c i d m e t h a n o l

b u t s t r o n g acids a r e r e q u i r e d t o p r o t o n a t e

Ni(P(OMe) ) ] 3

(19) It

(20).

4

a p p e a r e d that a c o o r d i n a t i v e l y u n s a t u r a t e d , e l e c t r o n - r i c h species s u c h as Fe(DMPE)

2

s h o u l d h a v e a w e a l t h of i n t e r e s t i n g c h e m i s t r y .

lights of that c h e m i s t r y

T h e high-

a n d t h e c h e m i s t r y of c l o s e l y

(21,22,23,24)

r e l a t e d species ( 2 5 ) are r e p o r t e d here. Reduction

of

Fe(DMPE) Cl 2

T h e r e d u c t i o n (22)

2

of F e ( D M P E ) C l 2

2

was carried out i n T H F

u s i n g t w o e q u i v a l e n t s of s o d i u m n a p h t h a l e n i d e .

T h e expected product,

F e ( D M P E ) , w a s n o t o b t a i n e d f o r reasons w h i c h w i l l soon b e c o m e a p 2

parent.

Instead,

t w o other

products

were

obtained.

T h e first w a s

F e ( D M P E ) H ( C i H ) , analogous to p r e v i o u s l y r e p o r t e d r u t h e n i u m a n d 2

o s m i u m (15,16)

0

7

c o m p o u n d s , except that i t exists i n s o l u t i o n as a n e q u i -

l i b r i u m m i x t u r e of cis a n d trans isomers.

T h e second product was a

z e r o - v a l e n t c o m p l e x , b u t i n s t e a d of b e i n g f o u r c o o r d i n a t e i t is five c o o r d i nate, h a v i n g the structure s h o w n i n 2 a n d d i s p l a y i n g a n A B

cis-1

4

trans-l

In Inorganic Compounds with Unusual Properties—II; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

3 1

P{ H} 1

7.

ITTEL ETA L .

Bidentate

Phosphorus

Ligand

69

Complexes

2


N M R s p e c t r u m . T h e o r i g i n of this s e c o n d c o m p l e x is r e a d i l y a s c e r t a i n e d b y r e a c t i n g one e q u i v a l e n t of D M P E w i t h t w o e q u i v a l e n t s o f 1. T h e react i o n , w h i c h takes p l a c e i n a b o u t o n e h o u r is c o n v e n i e n t l y f o l l o w e d b y 3 1

P { H ) N M R . Resonances a t t r i b u t a b l e to 1 d i s a p p e a r f r o m t h e s p e c t r u m 1

as resonances a t t r i b u t a b l e to 3 appear.

F i n a l l y , 1 a n d 3 are c o m p l e t e l y

c o n s u m e d , g i v i n g 2.

A B C spin system 4

3 T h e f o r m a t i o n of 3 a n d 2 b y r e a c t i o n of D M P E w i t h 1 explains t h e source of 2 i n t h e p r e l i m i n a r y r e d u c t i o n . M o r e s i g n i f i c a n t l y , i t d e m o n strates that 1 c a n r e d u c t i v e l y e l i m i n a t e a C - H b o n d to g i v e free n a p h t h a lene a n d t h e u n d e t e c t e d i n t e r m e d i a t e species " F e ( D M P E ) . " T h i s reac2

t i o n is the first step f o r almost a l l of the f o l l o w i n g c h e m i s t r y . T h e r u t h e n i u m a n d o s m i u m analogs of 1 h a v e b e e n i n the l i t e r a t u r e f o r m a n y years

(15,16)

b u t because t h e y

are m u c h less p r o n e

to

eliminate

n a p h t h a l e n e , t h e y h a v e s h o w n l i t t l e of t h e c h e m i s t r y d e s c r i b e d b e l o w f o r the i r o n complexes. Formation

of Five-Coordinate,

Zero-Valent

Species

T h e s e reactions are o u t s i d e t h e scope of C - H a c t i v a t i o n b u t a r e m e n t i o n e d here because t h e y i l l u s t r a t e some of t h e properties of t h e Fe(DMPE)

2

system.

T h e " F e ( D M P E ) " g e n e r a t e d f r o m 1 reacts w i t h 2

C O to f o r m F e ( D M P E ) C O (22). 2

T h e c a r b o n y l stretching frequency of

F e ( D M P E ) C O is o b s e r v e d at 1812 c m ' , e x c e p t i o n a l l y l o w f o r a n o n 2

1

b r i d g i n g carbonyl, indicating a very h i g h electron density o n the metal.



70

INORGANIC

COMPOUNDS WITH UNUSUAL

PROPERTIES

II

6 P h o s p h o r u s l i g a n d s also f o r m F e ( D M P E ) L complexes (22),

limited

2

o n l y b y steric interactions. P(OMe)

(6 =

3

L i g a n d s r a n g i n g i n cone angle (26)

1 0 7 ° ) to P ( 0 - o - t o l y l )

nate complex, but P P h

3

3

(0 — 1 4 1 ° ) f o r m the

from

five-coordi-

1 4 5 ° ) does not c o o r d i n a t e t h r o u g h p h o s -

(0 =

p h o r u s ; rather, it behaves as a s u b s t i t u t e d b e n z e n e ( v i d a i n f r a ) . P h o s p h o r u s l i g a n d s a n d C O also react w i t h a v a r i e t y of F e ( D P P E ) (25)

complexes

[ D P P E = b i s ( d i p h e n y l p h o s p h i n o ) ethane] s u c h as

(DPPE) (C H ) 2

2

4

(4)

and F e ( D P P E ) H 2

2

(5)

to f o r m F e ( D P P E ) L 2

complexes, b u t the fastest r e a c t i o n is o b s e r v e d w i t h t h e species, 6. systems;

2

Fe-

ortho-metalated

Steric l i m i t a t i o n s are m u c h m o r e severe i n the F e ( D P P E )

2

the largest p h o s p h o r u s l i g a n d c o o r d i n a t e d w a s P ( O M e ) P h , 2

h a v i n g a cone a n g l e of 1 1 6 ° . A v a r i e t y of u n s a t u r a t e d m o l e c u l e s react w i t h 1 to f o r m TT c o m p l e x e s E t h y l e n e f o r m s the simplest c o m p l e x of this t y p e :

(22).

The

1

H N M R resonances of the ethylene are s h i f t e d u p f i e l d to 0.59 p p m

below Me Si. 4

T h i s shift, one

of the

largest

ever r e p o r t e d

(27),

is

a n o t h e r i n d i c a t i o n of the h i g h e l e c t r o n d e n s i t y o n t h e i r o n . T h e e t h y l e n e c o m p l e x a n d c o m p l e x e s of other u n a c t i v a t e d olefins s u c h as p r o p y l e n e a n d b u t a d i e n e ( w h i c h is rj ) 2

u n d e r g o a f l u x i o n a l process o n t h e N M R

t i m e scale w h i c h e q u i l i b r a t e s the f o u r D M P E p h o s p h o r u s n u c l e i .

Com-

plexes of a c t i v a t e d olefins s u c h as a c r y l o n i t r i l e are s t e r e o c h e m i c a l l y r i g i d


7.

Bidentate

ITTEL E T A L .

Phosphorus

Ligand

Complexes

as a result of stronger b o n d i n g to t h e o l e f i n (28,29).

T h e TT c o m p l e x e s

of d i s u b s t i t u t e d acetylenes s h o w m a r k e d shifts i n v =c; d i p h e n y l a c e t y l e n e 0

shows a C = C s t r e t c h i n g f r e q u e n c y at 1720 c m " , 502 c m " l o w e r t h a n t h e 1

free acetylene.

1

A z o b e n z e n e also f o r m s a TT c o m p l e x , a d d i n g o n e m o r e

m e t a l to those o b s e r v e d to f o r m TT complexes w i t h diazenes

(SO,SI).

A n t h r a c e n e f o r m s a n u n u s u a l rj c o m p l e x i n w h i c h o n e o f t h e D M P E 4


m o l e c u l e s is m o n o d e n t a t e

1,3-cyclohexadiene

(22);

f o r m s a n analogous

complex formed w i t h butadiene.

c o m p l e x i n contrast t o t h e rj

2,

A p p a r e n t l y t h e i n a b i l i t y of the c y c l i c

species to a d o p t a t r a n s o i d g e o m e t r y favors t h e m o r e c o m m o n -q c o o r d i 4

n a t i o n e n o u g h to o v e r c o m e t h e chelate effect of t h e D M P E . Activation

of s p C—H Bonds 3

E a r l y i n o u r i n v e s t i g a t i o n of 1, a c e t o n i t r i l e w a s u s e d as a solvent f o r N M R studies. S u r p r i s i n g l y , this solvent r e a c t e d w i t h 1 to f o r m a transh y d r i d o c y a n o m e t h y l c o m p l e x of t h e t y p e :

Later, m a n y compounds h a v i n g activated m e t h y l groups w e r e f o u n d to u n d e r g o s i m i l a r reactions; e t h y l acetate, d i m e t h y l s u l f o x i d e , e t h y l c y a n o acetate, a n d m a l o n o n i t r i l e are just a f e w of t h e c o m p o u n d s s t u d i e d

(23).

A c e t o n e f o r m s a s i m i l a r c o m p l e x b u t i f a d e f i c i e n c y o f acetone is u s e d , t h e d i m e r i c species f o r m s . S i m i l a r observations w e r e m a d e i n the r e l a t e d systems [ I r ( P M e ) ] 3

4

+

and [ I r ( D M P E ) ] b y Herskovitz 2

+

(32,33).



72

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

T h e r e a c t i o n of 1 w i t h c y c l o p e n t a d i e n e

(23)

yields a complex i n

w h i c h the ^ - c y c l o p e n t a d i e n y l g r o u p is n o n f l u x i o n a l (34). the r e a c t i o n

of 4 w i t h c y c l o p e n t a d i e n e

yields

II

(25)

In

contrast,

the k n o w n

(35)

compound:

A f t e r o x i d a t i v e a d d i t i o n of the C - H b o n d , steric c r o w d i n g is r e l i e v e d b y d i s s o c i a t i o n of one D P P E a n d s u b s e q u e n t i ^ - c y c l o p e n t a d i e n y l coordination. T h e r e a c t i o n of c o m p o u n d s h a v i n g a c t i v a t e d m e t h y l groups w i t h 4 , 5, or 6 fails to y i e l d h y d r i d o a l k y l c o m p l e x e s . R e a c t i o n of 4 w i t h /?-diketones (36) p r e s u m a b l y proceeds t h r o u g h a n i n t e r m e d i a t e s i m i l a r to t h e c y c l o p e n t a d i e n y l c o m p l e x , b u t the final o v e r a l l r e a c t i o n i s :


7.

ITTEL E T AL.

Bidentate Phosphorus Ligand Complexes H

R 2Fe(DPPE) C H 2

2

4

+ 2


2

P

x

R •> 2 C H 2

II

II

0

4

P h

2

| ^ 0

73

4

+ DPPE

+

0

Ph P

+

2

Ph P 2

R

R R (1)

K i n e t i c measurements i n d i c a t e that loss o f n a p h t h a l e n e f r o m 1 to y i e l d " F e ( D M P E ) " is t h e r a t e - l i m i t i n g step f o r t h e f o r m a t i o n of these 2

DMPE

complexes.

T h e reactions

to g i v e

five-coordinate,

zero-valent

species a n d t h e reactions to g i v e n e w h y d r i d o a l k y l species a l l take p l a c e o n a p p r o x i m a t e l y t h e same t i m e scale.

I n addition, the trans-hydrido

a l k y l complexes represent o n l y t h e t h e r m o d y n a m i c p r o d u c t s of t h e C - H c l e a v a g e reactions. W h e n t h e r e a c t i o n is f o l l o w e d b y

3 1

P and * H N M R ,

a cis i n t e r m e d i a t e is o b s e r v e d . A s a n e x a m p l e , the r e a c t i o n of 1 w i t h a c e t o n i t r i l e f o l l o w e d as a f u n c t i o n of t i m e b y 100 M H z h y d r i d e N M R s p e c t r a is s h o w n i n F i g u r e 1. A n i n i t i a l p a t t e r n at — 12.5 p p m a t t r i b u t a b l e to t h e cis i s o m e r appears i n i t i a l l y a n d t h e n d i s a p p e a r s as t h e trans i s o m e r at —23.2 p p m appears.

H y d r i d e resonances

f r o m t h e c i s - a n d trans-

h y d r i d o a r y l species are absent b e c a u s e t h e spectra w e r e r u n i n C e D

6

( v i d e i n f r a ) ; b u t i t is also p r o b a b l e that t h e r e d u c t i v e e l i m i n a t i o n of arene takes p l a c e t h r o u g h t h e cis isomers. T h e s e observations a n d a n a l o gous results i n other systems s t r o n g l y suggest, t h o u g h d o n o t necessitate, a c o n c e r t e d three-center i n t e r m e d i a t e f o r t h e o x i d a t i v e a d d i t i o n .


74

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

100MHz 'HNMR SPECTRA IN REACTION OF DFe(C D^)(dmpe) fi

P


WITH CrhCN

t = 313 MIN

t =1350 MIN -15 -20 CHEMICAL SHIFT (PPM)

-10

-25

Figure 1. Hydride NMR spectra (100 MHz) of the reaction of acetonitrile with Fe(DMPE) D(C D ) (1 dissolved in C D ) taken as a function of time 2

6

5

6

6


II

7.

ITTEL E T AL.

Activation

Bidentate

Phosphorus

Ligand

75

Complexes

of sp C—H Bonds

E a r l i e r w e n o t e d that d i s u b s t i t u t e d acetylenes r e a c t w i t h 1 to g i v e 7r-bonded complexes.

A c e t y l e n e a n d m o n o s u b s t i t u t e d acetylenes

to g i v e t r a n s - h y d r i d o a c e t y l i d e complexes (23).

The D P P E

react

complexes,

3, 4 , a n d 5, also react w i t h acetylenes to g i v e t r a n s - h y d r i d o a c e t y l i d e complexes

Iso-electronic

(25,37).

h y d r i d o cyanide complexes

are o b -

tained u p o n reaction w i t h h y d r o g e n cyanide. T h e reaction of H C N w i t h Fe(DMPE) H(C 2

1 0

H )

is the first r e a c t i o n r e p o r t e d h e r e

7

(23)

that


p r o c e e d s b y a m e c h a n i s m different f r o m those a l r e a d y d i s c u s s e d . T h e r e a c t i o n , w h i c h is over essentially u p o n m i x i n g , seems to i n v o l v e electrop h i l i c attack p r i o r to loss of n a p h t h a l e n e . Activation

of Aromatic

C—H Bonds

(24)

D i s s o l u t i o n of 1 i n b e n z e n e - d f o r m s a n e q u i l i b r i u m m i x t u r e of f o u r 6

species g i v e n b y : K cis-1 + C D ^ ± m - F e ( D M P E ) D ( C D ) + \\ \\ trans-1 trans-Fe ( D M P E ) D ( C D ) 6

6

2

6

2

The

3 1

P { H } N M R singlet of trans-l

5

6

Ci H 0

8

(2)

5

decreases i n i n t e n s i t y , t h e A B C D

1

s p i n system a t t r i b u t a b l e to cis-1 appears to loose r e s o l u t i o n , a n d a 1 : 1 : 1 t r i p l e t a t t r i b u t a b l e to the trans d e u t e r i d e appears.

T h e a p p a r e n t loss of

r e s o l u t i o n of the cis isomers is a t t r i b u t a b l e to s u p e r p o s i t i o n of t h e n o r m a l A B C D s p i n system a n d to another one h a v i n g a n a d d i t i o n a l 1 : 1 : 1 s p l i t t i n g f r o m t h e d e u t e r i u m c o u p l i n g . T h e o b s e r v e d intensities i n d i c a t e a n e q u i l i b r i u m constant f o r R e a c t i o n 2 of 0.08; F e ( D M P E )

2

has a n affinity f o r

n a p h t h a l e n e m o r e t h a n one o r d e r of m a g n i t u d e greater t h a n t h a t f o r benzene. T h e r e a c t i o n of toluene w i t h 1 c o u l d c o n c e i v a b l y l e a d to f o u r d i f ferent trans-isomeric p r o d u c t s b y attack at the ortho, m e t a , p a r a , o r a l p h a positions.

T h e c a r b a n i o n c h e m i s t r y of t o l u e n e w o u l d p r e d i c t attack at

the m e t h y l g r o u p b u t , as i n other t r a n s i t i o n m e t a l arene a c t i v a t i o n studies (8,9,10),

this is n o t f o u n d .

The

3 1

P N M R s p e c t r u m of the t o l u e n e

a d d u c t s d i s p l a y s t w o singlets a t t r i b u t a b l e to t r a n s - t o l y l a d d u c t s i n a n i n t e n s i t y ratio of 1:1.6 i n a d d i t i o n to resonances

of c i s - t o l y l a d d u c t s .

Selective m o n o - d e u t e r a t i o n experiments s h o w that t h e w e a k e r d o w n f i e l d resonance is a t t r i b u t a b l e t o t h e p a r a i s o m e r w h i l e t h e stronger o n e is a t t r i b u t a b l e to t h e m e t a isomer; n o ortho i s o m e r is o b s e r v e d .

If o n e

corrects f o r statistical effects, t h e p r e f e r e n c e f o r p a r a , m e t a , a n d o r t h o isomers is 1:0.8:0. W e a t t r i b u t e this d i s t r i b u t i o n of isomers p r i m a r i l y t o


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steric effects, a v i e w w h i c h is s u p p o r t e d b y t h e o b s e r v a t i o n that 1 dissolves i n p - x y l e n e a n d m e s i t y l e n e w i t h n o r e a c t i o n ; a l l of t h e a r o m a t i c C - H b o n d s are ortho to a m e t h y l g r o u p . T h e s e results are also consistent w i t h the o b s e r v a t i o n of o n l y t h e 2 - n a p h t h y l i s o m e r of 1; a 1 - n a p h t h y l i s o m e r w o u l d b e ortho s u b s t i t u t e d a n d therefore sterically h i n d e r e d . S u b s t i t u t i o n o f t r i f l u o r o m e t h y l f o r t h e m e t h y l of t o l u e n e

enhances

the s e l e c t i v i t y f o r t h e p a r a p o s i t i o n over m e t a , a n d a g a i n n o o r t h o is o b s e r v e d . C o m p e t i t i v e studies s h o w that a t r i f l u o r o m e t h y l g r o u p activates b e n z e n e b y almost f o u r orders o f m a g n i t u d e , a n d m e t h y l deactivates i t b y Downloaded by GEORGETOWN UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch007

almost o n e o r d e r o f m a g n i t u d e . T h i s is a m a r k e d c h a n g e f r o m p r i o r C - H a c t i v a t i o n studies i n w h i c h there w a s less t h a n o n e o r d e r of m a g n i t u d e c h a n g e b e t w e e n t h e most a c t i v a t i n g a n d d e a c t i v a t i n g substituents ( 8 , 9 , J O ) , b u t i t s h o u l d b e n o t e d that t h e earlier studies w e r e k i n e t i c d i s t r i b u -

-15 CHEMICAL SHIFT (PPM) Figure 2.

Hydride

-20

NMR spectra (100 MHz) of the (a) benzonitrile para-tolunitrile adducts to Fe(DMPE) 2


and (b)

7.

ITTEL ET A L .

Bidentate

Phosphorus

Ligand

77

Complexes

tions w h i l e w e are m e a s u r i n g t h e r m o d y n a m i c d i s t r i b u t i o n s .

Fluorine

s u b s t i t u t i o n s u r p r i s i n g l y y i e l d s o n l y one p r o d u c t , the trans-ortho a d d u c t . T h i s o b s e r v a t i o n is a result of t h e l i m i t e d steric effect of a fluoro s u b s t i t u ent c o m b i n e d w i t h its p o w e r of a c t i v a t i o n . E l e c t r o n w i t h d r a w a l t y p i c a l l y activates t h e entire r i n g w i t h ortho > p a r a >

meta.

A s a f u r t h e r e x a m p l e of these effects, b e n z o n i t r i l e w i t h its s t r o n g l y a c t i v a t i n g c y a n o g r o u p , shows t h e greatest selectivity f o r p a r a over m e t a f o r trans isomers. T h e s i t u a t i o n is c o m p l i c a t e d b y t h e o b s e r v a t i o n of t w o n o r m a l a n d one s o m e w h a t a b n o r m a l cis isomers ( F i g u r e 2 ) . T h e n o r m a l Downloaded by GEORGETOWN UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch007

isomers are a t t r i b u t a b l e to m e t a a n d p a r a species; ortho.

t h e u n u s u a l o n e is

W h e n the r e a c t i o n is c a r r i e d o u t w i t h p - t o l u n i t r i l e , o n l y t h e

cis-ortho i s o m e r is o b s e r v e d . A s p a c e - f i l l i n g m o l e c u l a r m o d e l of cis-orthob e n z o n i t r i l e c a n b e c o n s t r u c t e d , b u t i t is n o t p o s s i b l e to b u i l d a m o d e l of the trans-ortho isomer. Activation

s p C — H Bonds

of Non-Aromatic

2

B e n z a l d e h y d e reacts w i t h 1 to g i v e :

T h i s c o m p l e x does n o t e x t r u d e C O n o r does F e ( D M P E ) H ( C H ) insert 2

C O to y i e l d t h e a b o v e .

6

5

P r o p i o n a l d e h y d e reacts w i t h 1 to g i v e a r e l a t e d

a c y l h y d r i d e , b u t F e ( D M P E ) C O is also f o r m e d . S u c h c o m p l e x e s 2

have

b e e n p r o p o s e d as i n t e r m e d i a t e s i n t h e d e c a r b o n y l a t i o n o f a l d e h y d e s b y RhCl(PPh ) 3

3

( 3 8 ) b u t never b e f o r e o b s e r v e d .

A s w e h a v e a l r e a d y p o i n t e d o u t , s i m p l e olefins f o r m 7r-bonded c o m plexes w i t h " F e ( D M P E ) . " S t i m u l a t e d b y r e c e n t w o r k w i t h r u t h e n i u m 2

( 3 9 ) , w e f o u n d that r e a c t i o n of e t h y l m e t h a c r y l a t e w i t h 1 f o r m s t h e v i n y l hydride complex:


78


II

a l t h o u g h i t w a s n o t d e m o n s t r a t e d c o n c l u s i v e l y that t h e m e t h y l g r o u p is cis to t h e m e t a l . A n a t t e m p t to p r e p a r e t h e analogous c o m p l e x w i t h m e t h a c r y l o n i t r i l e u n e x p e c t e d l y gave a 7r-olefin c o m p l e x . Other Reactions of Interest W e h a v e a l r e a d y p o i n t e d o u t that 1 reacts w i t h e t h y l acetate at a m e t h y l C - H b o n d a n d w i t h e t h y l benzoate at t h e m e t a a n d p a r a p o s i t i o n s . It w a s therefore s u r p r i s i n g that s i m i l a r reactions w i t h t h e c o r r e s p o n d i n g Downloaded by GEORGETOWN UNIV on August 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch007

m e t h y l esters gave p r o d u c t s r e s u l t i n g f r o m cleavage of t h e o x y g e n m e t h y l b o n d s . I n a d d i t i o n to M e C = O a n d P h C = O , t h e substituent R c a n b e P h a n d M e ( M e O ) P == O . T h e o n l y a p p a r e n t e x p l a n a t i o n f o r this differe n c e i n b e h a v i o r b e t w e e n m e t h y l a n d e t h y l is steric; t h e m e t h y l c o n e angle ( 9 0 ° ) is s m a l l e r t h a n that of a n e t h y l ( 1 0 2 ° )

(26).

Me

T r i m e t h y l s i l a n e reacts w i t h 1 to g i v e a m i x t u r e of the cis- a n d transh y d r i d o trimethylsilyl complexes. b e c a u s e t r i m e t h y l s i l a n e represents

T h i s r e a c t i o n is of p a r t i c u l a r interest t h e o n l y case i n w h i c h w e c a n b e

c e r t a i n that there is n o f o r m of p r e c o o r d i n a t i o n b e f o r e attack to g i v e t h e h y d r i d o species.

I n a l l other cases—arenes, acetylenes, a c t i v a t e d m e t h y l

g r o u p s , a n d others—there is some c o n c e i v a b l e m o d e o f c o o r d i n a t i o n p r i o r to t h e attack o n the C - H b o n d Kinetics and Thermodynamics The

of C—H

discussion i n the preceeding

Activation sections

d e a l t w i t h systems

at

e q u i l i b r i u m , b u t just as i n t h e earlier e x a m p l e i n v o l v i n g a c e t o n i t r i l e , i t is p o s s i b l e to m o n i t o r t h e progress of these reactions b y a c o m b i n a t i o n o f spectroscopic techniques. w i t h acetophenone.

A n i n t e r e s t i n g e x a m p l e is t h e r e a c t i o n o f 1

O n e o f a series of h y d r i d e N M R spectra t a k e n as a

f u n c t i o n of t i m e is s h o w n i n F i g u r e 3. T h e first p r o d u c t s o b s e r v e d w e r e the m e t a - a n d p a r a - a r y l h y d r i d e a d d u c t s .

Later the cis-methyl adduct

d e v e l o p e d , a n d finally, t h e t r a n s - m e t h y l a d d u c t ( b a r e l y v i s i b l e i n t h e figure)

d e v e l o p e d as t h e system a p p r o a c h e d e q u i l i b r i u m .


7.

ITTEL E T AL.

Bidentate

Phosphorus

Ligand

79

Complexes

100 MHz H NMR SPFCTRiJM IN REACTION OF DFe(C

Activation of C-H Bonds by Bidentate Phosphorus Ligand Complexes

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