Transition Metal Hydrides - American Chemical Society

9

Downloaded by UNIV OF ILLINOIS URBANA on October 19, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch009

Ruthenium and Rhodium Hydrides Containing Chiral Phosphine or Chiral Sulfoxide Ligands, and Catalytic Asymmetric Hydrogenation BRIAN R. JAMES, RODERICK S. MC MILLAN, ROBERT H. MORRIS, and DANIEL K. W. WANG Department of Chemistry, University of British Columbia, British Columbia, Canada V6T 1W5 The Ru(II) complexes Ru Cl (diop) , HRuCl(diop) , and RuCl (diop) , were synthesized and found to be effective in solution for catalytic asymmetric hydrogenation of some prochiral olefinic substrates. The systems, including some kinetic data, are compared with the related HRh(diop) -catalyzed systems; 2

2

4

3

2

2

2

(-)diop = 2R,3R-(-)-2,3-0-isopropylidene-2,3-dihydroxy-1,4bis(diphenylphosphino)butane. Two trimeric Ru(II) complexes, [RuCl L ] were made, where L is either the chiral sulfur-bonded R-(+)-methyl para-tolyl sulfoxide or (S,R;S,S)(+)-2-methylbutyl methyl sulfoxide; the latter functions as an asymmetric hydrogenation catalyst, but optical yields of only 15% were attained. There is evidence for hydride intermediates. Ru(II) catalysts containing (—)dios, the 2R,3R-bis-(methyl sulfinyl)butane analog of (—)diop, give optical yields up to 25% with excess enantiomer opposite to that obtained for the corresponding (-)diop system. The cationic rhodium complex [(norbornadiene)Rh(PPh )(dios)] is a precursor to an active hydrogenation catalyst, but no asymmetry has been observed in products. 2

2

3

+

3

A

s y m m e t r i c synthesis ( I ) has gained new m o m e n t u m w i t h the potential . use of homogeneous catalysts. T h e use of a transition metal complex w i t h c h i r a l ligands to catalyze a synthesis a s y m m e t r i c a l l y f r o m a p r o c h i r a l substrate is beneficial i n that resolution of a n o r m a l l y obtained racemate product m a y be avoided. In certain catalytic hydrogénations of olefinic bonds, optical purities a p p r o a c h i n g 100% have been attained (2,3, 4,5); hydrogénations of ketones (6, 0-8412-0390-3/78/33-167-122/ $05.00/0 © American Chemical Society In Transition Metal Hydrides; Bau, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

9.

Ruthenium and Rhodium

JAMES E T AL.

123

Hydrides

7) a n d imines (6) a n d h y d r o f o r m y l a t i o n experiments (8) have thus far achieved m u c h lower enantioselectivities.

C a t a l y t i c a s y m m e t r i c syntheses using m e t a l

complexes have also been concerned w i t h h y d r o s i l y l a t i o n (7, 9, 10),

carbon-

carbon bond formation as i n oligomerization (11), a n d , more recently, oxidations i n v o l v i n g peroxides (12, 13).

T h e metal complexes have been used homoge-

neously i n solution or on various supports

(7,14).

T h e use of c h i r a l tertiary phosphine ligands has been studied most w i d e l y , but other chiral ligands such as carboxylic acids (15), imines (8,16),

amides (17),

amines (18), alkoxides (19), a n d hydroxammates (13) have been investigated,


a n d we reported recently on some sulfoxide systems (20, 21 ). T h e detailed pathways of asymmetric induction d u r i n g catalysis are not well understood even for the more w i d e l y studied hydrogénations (8, 22),

and

matching substrates with the most suitable transition-metal ohiral catalyst remains very m u c h an e m p i r i c a l art. W e are not aware of kinetic studies except our o w n on catalyzed a s y m m e t r i c hydrogénations, although they usually are assumed to follow well-studied n o n c h i r a l analogs; for example, R h C l P 3 * systems, where

R = P P h gives 2R, 3R-(—)diop R = S O C H gives 2R, 3i?-(-)dios

X3

Me

2

3

O^^CTLR H

CH—CH.—CH—CH,— X

CH

CH

X

(

CH

; i

;

MBMSO

MPTSO

P * is an optically active tertiary phosphine, likely w i l l resemble the R h C l ( P P h 3 ) 3 system (23).

H o w e v e r , even i n this exhaustively studied system, both h y d r i d e

a n d / o r unsaturate routes are feasible (23, 24); by v a r y i n g conditions, the choice of route c o u l d affect stereoselectivity.

Most a s y m m e t r i c hydrogénations have

used prochiral olefinic acid substrates, and these systems have not been thoroughly studied even w i t h n o n c h i r a l catalysts. This chapter reports principally on studies w i t h r u t h e n i u m chiral phosphine a n d c h i r a l sulfoxide complexes a n d their use for catalytic hydrogénation.

We

have used the f a m i l i a r d i o p l i g a n d , [ 2 R , 3 R - ( — ) - 2 , 3 - O i s o p r o p y l i d e n e - 2 , 3 - d i h y droxy-l,4-bis(diphenylphosphino) butane] (7); a related chiral chelating sulfoxide l i g a n d dios, the bis(methyl sulfinyl)butane analog (21);

(S,R;S,S)-(+)-2-meth-

y l b u t y l m e t h y l s u l f o x i d e ( M B M S O ) , chiral i n the a l k y l group; a n d R - ( + ) - m e t h y l p a r a - t o l y l s u l f o x i d e ( M P T S O ) , c h i r a l at sulfur.

P r e l i m i n a r y data on some cor-

responding Rh(I) complexes are presented also. W e have a i m e d to characterize hydride species with the intent to study them as hydrogénation catalysts; mechanistic studies should a i d eventually i n u n d e r -

In Transition Metal Hydrides; Bau, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

124

TRANSITION M E T A L HYDRIDES

standing the enantioselective processes better.

W e have published brief reports

(20, 25, 26) on some of the studies described i n this chapter. Results and Discussion Complexes Containing D i o p .

A phosphine exchange reaction using

R u C l ( P P h ) 3 a n d d i o p (~1:2) r e a d i l y yields the n e u t r a l , green 2

3

complex

Ru2Cl4(diop)3 (Complex 1) containing a bridging diop ligand. A single i>(Ru-Cl) at 310 c m

- 1

a n d the proton-decoupled

3 1

P N M R data ( F i g u r e 1) can be i n t e r


preted i n terms of five-coordinate, square p y r a m i d a l geometry at each ruthenium. Such a structure generally gives an A B X pattern that is s i m p l i f i e d if / A X w h i c h seems reasonable here.

=

7BX>

T h e spectra are interpreted r e a d i l y w i t h J\\ =

7BX = 30 H z and strong trans coupling, /

B

A

310 H z . T h e electronic spectrum

=

w i t h m a x i m a at 455 a n d 700 n m also resembles that of the five-coordinate R u C l ( P P h ) 3 with m a x i m a at 480 and 750 n m (27,28). 2

3

O f interest, the expected

octahedral R u C l ( d i p h o s p h i n e ) complexes are obtained w i t h P h P ( C H ) „ P P h 2

2

2

2

2

ligands w i t h η = 1, 2, 3 while the diphenylphosphinobutane(dpb) derivative w i t h η = 4 (comparable to diop) again gives the b r i d g e d d i m e r product (25, 26, 29), presumably because of steric problems.

1 Consistent with its unsaturated character, C o m p l e x 1 i n the solid state readily absorbs 2.0 m o l C O at 1 a t m to give R u C l ( d i o p ) ( C O ) , i/(CO) at 2010 c m 2

4

the d p b analog behaves s i m i l a r l y (29).

3

2

- 1

;

Interestingly, i n toluene solution the

R u C i 4 ( d i o p ) 3 complex absorbs 3.0 m o l C O and solution i r gives bands at 2100 2

a n d 1990 c m

- 1

.

W h a t must be a m i x t u r e of carbonyls has not been separated,

but assuming that the R U - P A b o n d is cleaved i n solution, l i k e l y products are six-coordinate R u C l ( d i o p ) ( C O ) 2

a n d R u C l ( d i o p ) ( d i o p * ) ( C O ) , where d i o p *

2

2

refers to a monodentate, d a n g l i n g diop. Reactions of R u C l 4 ( d i o p ) 3 w i t h H 2

2

at ambient conditions i n d i m e t h y l -

acetamide solution (dma) also i n d i c a t e d the existence of d i o p * C o m p l e x 1 absorbs 1.0 m o l H occurs i n toluene.

2

complexes.

per d i m e r a c c o r d i n g to R e a c t i o n 1; no reaction

T h e yellow hydride (Complex 2) was prepared independently

by a phosphine exchange m e t h o d f r o m H R u C l ( P P h ) 3 a n d diop( 1:2.5) a n d is 3

characterized by an absorption m a x i m u m at 375 n m (c = 2000); subtracting this spectrum from that of the products of Reaction 1 leaves an absorption curve close to that measured for R u C l ( P P h 3 ) , w i t h a broad m a x i m u m around 500 n m (28, 2

30).

2

This spectrum is attributed to R u C l ( d i o p ) . 2

W e have been unable to isolate

this i n a pure form, but it is almost certainly like R u C l ( P P h ) , a chloride-bridged 2

3

2


9.


JAMES E T AL.

125

Hydrides

30°C


-60°C

67-2 Z/

31.8 Z/

x

173 Z/

B

-24.7 -26.3

A

Figure I. Observed P - | H ) spectrum of Ru Cl (dioph in C D C / or toluene, relative to 85 % H P 0 , downfield shifts are reported as positive 3 1

2

3

4

3

4

d i m e r (27). Solutions of R u C l ( P P h ) also do not react w i t h H (SO) but do react readily i n the presence of 1 m o l P P h to y i e l d H R u C l ( P P h ) . S i m i l a r l y , in the presence of added diop, C o m p l e x 1 absorbs 2.0 m o l H a c c o r d i n g to R e action 2, w h i c h gives a second method of synthesizing the hydride. P r e l i m i n a r y kinetic data on Reaction 1 showed a complex dependence on r u t h e n i u m , w h i c h was consistent w i t h predissociation (such as Reaction 3) i n d i c a t e d by the C O uptake experiments. Such H a n d C O gas-uptake studies encouraged us to at tempt to synthesize the five-coordinate Complex 3, even though the six-coordinate R u C l ( d i o p ) isomer d i d not exist. C o m p l e x 3 was prepared simply by treating C o m p l e x 1 w i t h excess diop or by c a r r y i n g out a R u C l ( P P h ) / d i o p (1:10) ex change reaction. T h e N M R spectrum of C o m p l e x 3 showed the same A B X pattern of C o m p l e x 1 plus a sharp line of correct intensity at a m u c h higher field caused by the d a n g l i n g phosphorus, close to that of uncoordinated diop. T h e extra lines at higher field i n F i g u r e 1 (—26.3 and —24.7 δ) are attributed to small amounts of C o m p l e x 3 a n d free d i o p , respectively. C o m p l e x 3 also reacts w i t h H (1:1) to give H R u C l ( d i o p ) (cf. Reaction 2). 2

3

2

2

3

3

3

2

2

2

2

2

2

3

3

2

R u C l ( d i o p ) + H -> H R u C l ( d i o p ) + R u C l ( d i o p ) + H C l 1 2 2

4

3

2

2

2

diop Ru Cl (diop) + 2 H — > 2HRuCl(diop) + 2HC1 2

4

3

2

2


(1)

(2)

126


R u C l ( d i o p ) — RuCl (diop)(diop*) + RuCl (diop) 2

4

3

2

(3)

2

3 T h e h i g h f i e l d H N M R of H R u C l ( d i o p ) at 2 5 ° C i n C D C l J

2

3

or toluene-cf

8

consists of seven e q u a l l y spaced bands w i t h intensities roughly i n the ratios 1 : 2:3:4:3:2:1 (Figure 2), and, based on conductivity measurements i n nitromethane (Λ = 19 c m o h m 2

- 1

M

_

1

; usually

60 for a 1:1 electrolyte), this spectrum was

interpreted as a trigonal-bipyramidal cation complex w i t h an associated chloride anion (25, 26).


ohm

- 1

T h e h y d r i d e , however, is n o n c o n d u c t i n g i n d m a (Λ

A f ; we found a value of - 1

0.9 c m

2

60 for a 1:1 electrolyte) and is almost certainly

so i n the N M R solvents used, chloroform and toluene.

Thus the structure i n these

solvents a n d i n the solid state is now considered to be the neutral six-coordinate cis C o m p l e x 2.

T h e septet results if

H

p = 7ΗΡ ' = 14.2 H z a n d / p a

3

H

e

=

7ΗΡ ' 6

=

28.8 H z . Such an assignment i m p l i e s that Pe a n d Pe are exchanging r a p i d l y at the temperature studied (25°C), a n d there is precedent i n the literature for such behavior (SI ).

T h e proton-decoupled

3 1

P N M R spectra ( F i g u r e 2) show a c o n

sistent symmetrical A X type pattern (/pp = 40 H z ) , and the uncoupled spectrum 2

2

shows the expected splitting pattern. Further support for Complex 2 comes from reaction of the hydride i n toluene with 1 m o l C O to yield HRuCl(CO)(diop)(diop*); the room temperature H N M R in toluene consists of two triplets centered at 16.4 τ ( / ( t r a n s ) = 115 H z , /PH(CIS) PH

= 23 H z ) , consistent w i t h Structure 4.

T h e d a n g l i n g phosphorus produces an

intense sharp h i g h f i e l d line even at room temperature (cf. F i g u r e 1).

More

detailed variable-temperature N M R studies on these interesting hydrides are i n progress, but our present approximate analyses are consistent w i t h C o m p l e x 2 and Structure 4.

Presumably, steric factors again prevent a trans arrangement

of two chelating d i o p moieties.

Figure 2. Observed high field H spectrum and P-\H] spectrum of HRuCl(diop) , rela tive to 85 % H3PO4; in C D C / 3 or toluene, -30°C 1

2


9.

JAMES E T AL.

127

Ruthenium and Rhodium Hydrides

Catalytic Hydrogénation Using the Diop Complexes. catalyst system is effective for a s y m m e t r i c hydrogénation.

T h e HRuCl(diop)2 D m a solutions of

Complexes 1,2, or 3 readily catalyze the homogeneous hydrogénation of activated


olefinic substrates at 3 0 ° - 6 0 ° C a n d 1 a t m H

2

(Table I).

C o m p l e x 2 a n d its

precursor, C o m p l e x 3, give similar m a x i m u m hydrogénation rates for the same conditions and concentrations w h i l e C o m p l e x 1 only has one-half the r u t h e n i u m available as the potential h y d r i d e catalyst (see

Reaction 1) a n d shows about

one-half the a c t i v i t y i n terms of total r u t h e n i u m . O p t i c a l yields of u p to 60% were obtained for some prochiral α,β-unsaturated carboxylic acid substrates.

We

have not attempted to o p t i m i z e such yields; as usual, use of (+)-diop gives one conformer i n excess w h i l e use of (—)-diop gives the opposite conformer

(25).

T h e d i o p system is the most effective of the Ru(II) c h i r a l phosphine c o m plexes that we have f o u n d for a s y m m e t r i c hydrogénation (25, 26). drogénation rates are about V

50

The hy-

as large as those using H R u C l ( P P h ) 3 under 3

corresponding conditions (32) but are reasonably efficient nevertheless.

For

example, 1 M solutions of atropic a c i d are converted q u a n t i t a t i v e l y to 2-phenylpropionic acid (40% enantiomeric excess (ee)) i n one day w i t h 1 0

2

M catalyst

at 1 a t m H . 2

Preliminary kinetic data on the catalyzed hydrogénation of acrylamide using H R u C l ( d i o p ) generally show a first-order dependence on h y d r o g e n , between 2

a first- a n d a zero-order on both r u t h e n i u m a n d substrate, a n d an inverse dependence on added diop at lower substrate concentrations.

These dependences

are consistent w i t h the m e c h a n i s m o u t l i n e d below (Reaction 4) a n d the corresponding rate l a w ( E q u a t i o n 5).

T h e less than first-order dependence on r u -

t h e n i u m is reflected i n the denominator t e r m ([diop]) w h i c h contributes at lower substrate concentrations; at zero-order substrate, the a l k y l is f u l l y f o r m e d

(K-

[olefin] >> [diop]), and the rate is strictly first-order i n r u t h e n i u m w i t h no inverse d i o p dependence.

T h e mechanism closely resembles that of H R u C l ( P P h s ) 3 (32,


128

TRANSITION M E T A L

HYDRIDES

T a b l e L Hydrogénation of U n s a t u r a t e d O r g a n i c Substrates U s i n g H o m o g e n e o u s C h i r a l R u t h e n i u m a n d R h o d i u m Catalysts

Substrate

\ a,b

Itaconic acid

2ΟΓ3°· HRh[(+)diop] [RuCl (MBMSO) ] ' RuCl [(-)dios] [(-)ddios]* RuCl [(-)dios] [(-)ddios]' 6

2

2


Product % ee

% Hydrogénation

Catalyst

2

a

3

Maximum Rate X 10* (M sec'

1

100 70 100 50

R(+),23 R(+),38 R(+), 20 R(+), 15^

5.80 10.0 58.3 —

50

R(+),25

—

c

C

d

e

2

2

1

\a,b

A t r o p i c acid

2a,b

2[(-)diop] HRh[(+)diop] RuCl [(-)dios] [(-)ddios]* a

6

2

100

R(+),8

—

100 100 100 100

R(-),-40 R(-),27 S(+),27 R(-),37

8.73 — — 0.28

c

d

2

20

\a,b

2-Acetamidoacrylic acid

HRh[(+)diop] [RuCl (MBMSO) ] < [RuCl -

2

2

2

S(+),4

—

100 100 100

S(-),59 S(-),56 R(+), 1.5

1.34 0.33 —

90

S(-),0.5

—

c

d

3

2

(MBMSO) ] ' RuCl [(-)dios] [(-)ddios]* 2

3

y

2

60

\a,b

Citraconic acid

HRh[(+)diop] [RuCl (MBMSO) ] 2

2

2

3

a

γ a,b

Acrylamide

2a, b

HRh[(+)diop] [RuCl (MBMSO) ] 2

2

2

S(-),7

—

100 0.0 0.0

0.0 0.0 0.0

3.25 0.0 0.0

100 100 100 100

— — — —

c

e

42.7 80.0 2.8 2.5* C

C

d

3

0.7-1.3 M substrate, except for rate data {see c,d,k). Complex 1, Ru Cl [(+)diop]3; Complex 2, H R u C l ( ( + ) d i o p ] ; Complex 3, RuCl [(+)diop]K + ) d i o p ] ; 6 0 ° C in dma, 1 0 M R u (monomer), 1 atm H . Measured using 4 X 1 0 M R u (monomer). 0.2 M substrate at 6 0 ° C in dma, 1 atm H . 1.5 Χ 1 0 M R h , 0.04 M substrate, in 1-butanol-toluene (2:1); 3 0 ° C except for atropic acid at 5 0 ° C ( J 5 , 36). 2 X 1 0 M R u (monomer), 4 0 ° C in dma, 4 atm H , 10 days. ^ T h e ee is determined by optical rotation measurements (21) and by using a chiral shift reagent on the dimethyl ester of the α-methylsuccinic acid product. * 1.50 X 1 0 M R u , 0.5 M substrate, 5 5 ° C in dma, 3 atm H 7 days (27). As for g but at 7 0 ° C . « 2 Χ 1 0 M R u (monomer), 0.4 M substrate, 6 0 ° C in dma, —100 atm, 10 days. > 1.5 Χ 1 0 M R u (monomer), 0.4 M substrate, 4 0 ° C in dma, 4 atm H , 7 days. * Measured using 1 0 M R u (monomer), 0.4 M substrate at 7 0 ° C in dma, 1 atm H . a

b

2

4

2

- 2

c

d

e

2

2

3

2

- 3

2

2

2

2 )

h

- 2

- 2

2

3


2

9.

JAMES E T AL.

33,34).

129


T h e corresponding diphos complex, *rans-HRuCl[Ph2P(CH2)2PPh2]2

(25, 26), is completely ineffective for hydrogénation under similar conditions presumably because of less labile phosphine ligands; the more h i n d e r e d d i o p l i gands of H R u C l ( d i o p ) are replaced r e a d i l y by olefinic substrates. 2

A nonre-

duction of a- and β-methylcinnamic acids and mesoconic (methylfumaric) acids suggests that steric factors governing olefin coordination are important, a n d the selectivity pattern again resembles that of the H R u C l ( P P h ) catalyst (32, 33, 3

3

34). Also i n c l u d e d i n T a b l e I are data for hydrogénation of the same substrates


catalyzed by the analogous r h o d i u m complex, H R h [ ( + ) - d i o p ]

2

(35, 36).

Con-

sidering the different conditions used ( 3 0 ° C i n 2:1 l - b u t a n o l - t o l u e n e solution), the optical yields are remarkably similar and give the same conformer i n excess. Internal olefins again were not reduced.

O u r suggested m e c h a n i s m (35, 36)

corresponds closely to that outlined i n Reaction 4, except that i n the r h o d i u m system, kinetic data indicate that the diop ligand does not dissociate completely but becomes monodentate and dangles i n solution to provide a coordination site for the substrate.

T h e isolation of the RuCl2(diop)(diop*) complex reported i n

this chapter, a n d of Rh(I) carbonyls containing.monocoordinated diphosphines (37) adds credence to our postulated mechanism.

Chiral Sulfoxide Ligands.

T h e M B M S O l i g a n d , a clear, colorless o i l at

2 0 ° C , was synthesized starting f r o m 95% S(—)-2-methylbutanol, w i t h successive conversion of the - O H group to - B r , - S H , and - S C H 3 and finally to - S O ( C H s ) using hydrogen peroxide i n acetone [v(SO) = 1025 c m ] . - 1

T h e S-chirality at the carbon

center remains unchanged throughout the synthesis, a n d because the f i n a l o x i dation step is nonstereospecific, the l i g a n d is prepared as a m i x t u r e of two d i a stereomers, (S,R) a n d (S,S).

* H N M R measurements on solutions of the l i g a n d

in the presence of a chiral lanthanide shift reagent (Kiralshift E 7 , A l f a Chemicals) showed equal amounts of each isomer.

F o r example, the S - C H 3 singlet is split

into two equal-area singlets separated by 0.30 p p m

(38).

T h e synthetic route to R ( + ) - M P T S O is o u t l i n e d i n Reaction 6.

T h e ex-

perimental detail followed that of Boucher and Bosnien (39) and is a modification of literature methods (40, 41).

C h l o r i n a t i o n of the para-toluene sulfinate gives

the s u l f i n y l chloride that was converted to the (—)menthyl sulfinates.

This d i -

astereomer mixture was resolved by fractional crystallization, and the subsequent G r i g n a r d reaction, that is k n o w n to proceed by direct inversion at the sulfur (42),


130


gave the solid, white chiral sulfoxide i n an overall yield of 25%, w i t h optical purity of 96% R ( + )

;

*(SO) at 1055 c m " . 1

W e reported recently on the (—)dios l i g a n d synthesis f r o m L-(+)-tartaric a c i d a n d a related ddios l i g a n d , the d i h y d r o x y d e r i v a t i v e that resulted f r o m a c i d cleavage of the isopropylidene acetal group (21 ). Complexes C o n t a i n i n g C h i r a l Sulfoxides.

T h e b l u e solutions obtained

by refluxing R u C l * 3 H 2 0 i n polar solvents under H have provided a useful route 2

3

to Ru(II) complexes (43, 44).

As described i n the experimental section, treatment

of such m e t h a n o l i c solutions w i t h monodentate sulfoxides (Rurligand =

1:2)

y i e l d e d the t r i m e r i c [ R u C l 2 L ] species (where L = c h i r a l ligands M B M S O a n d


2

3

M P T S O ) a n d a p o l y m e r i c complex [ R u C ^ L ^ J n (where L is r a c e m i c m e t h y l p h e n y l sulfoxide). T h e [ R u C ^ L ^ t a complexes are y e l l o w - g o l d , neutral species, a n d the m o lecular weights i n benzene correspond to a trimer.

G o u y magnetic measurements

show moments of about 0.6 B M per t r i m e r at 2 0 ° C w h i c h , i n v i e w of the air sensitivity of the compounds, is consistent w i t h d i a m a g n e t i c Ru(II) complexes. Strong ir bands at 1105 and 1110 c m " , respectively, for the M B M S O and M P T S O 1

complexes are attributed to u(SO) of S-bonded sulfoxide (45, 46); m e d i u m broad bands at 330 a n d 325 c m

- 1

, respectively, are probably t e r m i n a l R u - C l stretches.

T h e presence of ir bands caused by O-bonded sulfoxide expected i n the 9 0 0 - 9 8 0 cm

region (45, 46) is equivocal since this region is complicated by other bands

- 1

present i n the free ligand. H N M R studies are useful for detecting the presence of S- or O - b o n d e d

l

sulfoxide.

In dmso complexes, for example, the m e t h y l protons shift d o w n f i e l d

f r o m the free l i g a n d value by ~ 1 p p m i n S-bonded dmso a n d by ~ 0 . 1 p p m i n O - b o n d e d complexes (45).

For [RuCl (MBMSO) ] 2

2

3

i n C D C 1 , b r o a d peaks 3

caused by the protons of the 7- a n d δ-carbons are found i n the 0.7-1.75 δ region; the jS-carbon protons are at 1.85-2.50 δ, a n d the α-carbon protons are at 2.8-4.2 δ.

T h e peak areas show that m a i n l y S-bonded sulfoxides are present, but the

possibility of some O-bonded species can not be ruled out entirely for the system because of the close p r o x i m i t y of the peaks.

T h e N M R spectrum of [ R u -

C l 2 ( M P T S O ) 2 ] i n C D C 1 , however, consists of three b r o a d resonance regions 3

3

at 2.04-2.58 δ (protons of the p a r a - t o l y l m e t h y l group), 3.34-3.96 δ (S-bonded sulfoxide m e t h y l protons), a n d 6.44-7.90 δ (aromatic protons).

T h e r e are no


9.


JAMES E T A L .

131

Hydrides

resonances attributable to O-bonded sulfoxide; thus, both trimers are considered to contain only S-bonded sulfoxide.

A linear structure such as Structure 5 seems

plausible, but triangular clusters of R(II) are k n o w n (47,48). of [ R u ( C 0 M e ) ( H 0 ) ] 3 w i t h b r i d g i n g acetates (μ 2

2

T h e existence (48)

0.4 B M per R u ) suggests

2

3

that Structure 6 w o u l d also be a reasonable f o r m u l a t i o n . T h e gold solid [ R u C l ( C H S O C 6 H ) ] , isolated f r o m the m e t h y l p h e n y l 2

3

5

2

n

sulfoxide reaction, is diamagnetic or feebly paramagnetic a n d shows a S-bonded *>(SO) at 1130 c m " a n d i>(Ru-Cl) at 330 c m " . 1

T h e l i m i t e d solubility suggests

1

a p o l y m e r i c structure.


W e reported recently (21) the synthesis a n d characterization of r u t h e n i u m complexes c o n t a i n i n g chelating c h i r a l sulfoxides; RuCl (dios)(ddios), R u C l ( d 2

2

dios) , a n d R u C l ( d d i o s ) ( d m s o ) ( C H O H ) were made either via the blue solutions 2

2

3

or v i a sulfoxide exchange w i t h c i s - R u C l ( d m s o ) (49,50). 4

C a t i o n i c Rh(I) c o m

plexes of the type [Rh(diene)(PPh )(sulfoxide)] ,

w i t h a range of sulfoxides i n

2

+

3

c l u d i n g c h i r a l ones, have also been synthesized (46). C a t a l y t i c H y d r o g é n a t i o n U s i n g the C h i r a l S u l f o x i d e C o m p l e x e s .

The

[ R u C l ( M B M S O ) ] trimer d i d effect asymmetric hydrogénation of the t e r m i n a l 2

2

3

olefins, 2-acetamidoacrylic a n d itaconic acids, u n d e r homogeneous conditions in d m a (see Table I). A 15% enantiomeric excess was obtained for hydrogénation of itaconic to R - m e t h y l s u c c i n i c a c i d using

4 a t m H at 4 0 ° C w h i l e the a m i 2

d o a c r y l i c a c i d gave N - a c e t y l a l a n i n e i n only

1.0% ee.

Trisubstituted olefins

such as a- a n d β-methylcinnamic, citraconic, a n d mesaconic acids were not r e d u c e d at a l l . T h e [ R u C l ( M P T S O ) ] system was more interesting because it 2

2

contained chiral sulfur centers.

3

Activated olefins, i n c l u d i n g prochiral ones, were

hydrogenated readily i n d m a at 60° C even at 1 a t m H . 2

U n f o r t u n a t e l y , the

reductions occur concomitant w i t h r u t h e n i u m m e t a l f o r m a t i o n a n d probably are catalyzed heterogeneously; no stereoselectivity was observed.

The M P T S O

complex itself is stable under H a n d produces no metal; thus, reduction to metal 2

likely occurs v i a a n unstable h y d r i d o - o l e f i n complex. T h e M B M S O complex r e a d i l y hydrogenates a c r y l a m i d e i n d m a at 70° C a n d 1 a t m H , at rates convenient for k i n e t i c studies. 2

Such a n investigation re

vealed a kinetic dependence on R u of one t h i r d (51); this is consistent w i t h the t r i m e r complex dissociating to a s m a l l extent to an active m o n o m e r i c catalyst. D m a yellow solutions of this t r i m e r ( 1 0

2

M) d i d absorb small amounts of H

in the absence of substrate ( 5 % based on a 1 H per R u reaction). 2

sorption increased to

2

This ab

2 0 % to produce an orange solution i n the presence of

proton sponge [the strong base l,8-bisdimethylamino(naphthalene)], presumably by p r o m o t i n g f o r m a t i o n of intermediate hydrides v i a a reaction such as 7 (30). W e have been unable to detect any h i g h field * H N M R signals i n these solutions, and we have experienced similar difficulties even for concentrated solutions of H R u C l ( P P h ) i n d m a . S i m i l a r studies on base-promoted h y d r i d e f o r m a t i o n 3

3

f r o m c i s - R u C l ( d m s o ) have g i v e n m e t a l h y d r i d e species detectable at 28 r i n 2

4

dmso-d6; yellow to orange color changes were observed again (51).

U n d e r similar

conditions, the RuCl (dios)(ddios) complex leads to higher optical yields than 2


132


the [ R u C l 2 ( M B M S O ) ] 3 trimer (see Table I), presumably because of the rigidity 2

of the chelate ligands. T h e data indicate decreasing asymmetric i n d u c t i o n w i t h increasing temperature that c o u l d result f r o m greater l i g a n d dissociation, less restriction on m o t i o n of coordinated substrate, a n d hence less stereoselective catalytic species.

Interestingly, the (—)dios system, that has the same R chirality

as (—)diop at carbons atoms 2 a n d 3, gives the enantiomer i n excess opposite to the H R u C l ( d i o p ) system for hydrogénation of itaconic a n d 2-acetamidoacrylic 2

acids.

Differences i n substrate b i n d i n g could be an important factor; i n addition

to π-bonding a n d carboxylate coordination, Η-bonding is a possibility i n the


sulfoxide systems ( - C O 2 H or — N H w i t h sulfoxide oxygen or - O H of ddios). T h e cationic r h o d i u m complexes [Rh(diene)(PPh3)(sulfoxide)]" ", i n w h i c h 1

all the monodentate sulfoxides are O-bonded (46), tend to give metal on treatment w i t h H 2 i n solution, even i n the presence of c o o r d i n a t i n g olef i n i c substrates; the O - b o n d e d sulfoxides presumably are not strong enough 7r-acce£tors. estingly, a [ ( N B D ) R h ( P P h ) ( d i o s ) ] 3

Rh(PPh )(acetone)] 3

+

+

Inter

species f o r m e d i n situ f r o m the [ ( N B D ) -

cation (46) i n d m a absorbs 3 m o l H at ambient conditions, 2

p r o d u c i n g norbornane.

Reaction 8 seems l i k e l y , although the metal h y d r i d e

has not been detected by N M R . T h e H N M R indicates that the i n situ reactant l

only has O - b o n d e d dios (21 ) w h i l e the product shows resonances t y p i c a l of S-

bonded dios (21); such bonding reasonably appears to stabilize the hydride.

The

hydrogenated solutions are active homogeneous hydrogénation catalysts under m i l d conditions, but the use of prochiral substrates has yielded products with zero enantiomeric excess.

T h e substrates m a y displace the c h i r a l l i g a n d .

Experimental G e n e r a l . D r i e d , degassed reagent grade solvents were used throughout, and d m a was purified as described previously (52). Complexes were made under inert atmospheres using Schlenk-tube techniques. Micro-analyses were done by P. Borda of this department. T h e hydrogénation procedures and the work-up of products for d e t e r m i n i n g optical yields are reported elsewhere (21 ). Itaconic acid (Eastman) and 2-acetamidoacrylic acid (Fluka) were C P grade; atropic a c i d was prepared a c c o r d i n g to the literature (53). R u t h e n i u m and rhodium trichlorides were obtained as trihydrates from Johnson Matthey L i m i t e d . ( + ) D i o p was obtained f r o m Strem C h e m i c a l s ; a literature synthesis p r o v i d e d (—)diop (7). R a c e m i c m e t h y l p h e n y l sulfoxide was a Κ & Κ product. Sulfoxide L i g a n d s . (S,R;S,S ) - ( + ) - 2 - M E T H Y L B U T Y L M E T H Y L S U L F O X I D E ( M B M S O ) . 80 g 95% ( S ) - ( - ) - 2 - m e t h y l b u t a n - l - o l (Κ & K ) was titrated w i t h B r in d r y D M F i n the presence of t r i p h e n y l p h o s p h i n e a c c o r d i n g to the m e t h o d of W i l e y et al. (54). T h e c h i r a l b r o m i d e was converted to the m e r c a p t a n v i a an 2


9.


JAMES E T A L .

133

Hydrides

isothiourea hydrobromide salt that was then decomposed b y aqueous N a O H (55). T r e a t m e n t of the mercaptan i n aqueous N a O H w i t h iodomethane (56) gave ( S ) - 2 - m e t h y l b u t y l m e t h y l sulfide that was separated as a clear l i q u i d b y steam distillation; ôÇjfép 0 . 9 0 - 1 . 7 0 (multiplet, 9 H , C H - C H - C H - C H ) , 2.1 (singlet, 3 H , S - C H ) , 2 . 6 0 - 2 . 7 8 (multiplet, 2 H , - C H - S ) . T h i s sulfide reacted overnight at room temperature w i t h H 0 i n acetone; the v a c u u m distillate fraction ( 6 5 - 7 0 ° C , 0.2 m m ) was dried over B a O and was vacuum distilled to give the clear oily sulfoxide i n 3 6 % overall y i e l d ; b p 6 3 ° C , 0.1 m m ; [ « f o = + 2 0 . 3 ; neat. 3

3

3

2

3

2

2

2

2 5

Found:

C , 5 3 . 9 0 ; H , 10.64.

C H i S O requires C , 5 3 . 6 8 ; H , 10.51. 6

4

ôÇjgf

13

0 . 9 0 - 1 . 2 0 (multiplet, 8 H , C H - C H - C - C H ) , 1.25-1.60 (multiplet, 1 H , - C H - ) , 3

2

3

2 . 4 0 - 2 . 8 0 (triplet, 2 H , - C H - S ) , 2.55 (singlet, 3 H , S - C H ) . 2

3


(R)-(-f ) - M E T H Y L P A R A - T O L Y L S U L F O X I D E ( M P T S O ) .

Sodium para-tol-

uene sulfinate (Eastman) (96 g) was added slowly to 2 0 0 m L t h i o n y l c h l o r i d e under N , a n d the m i x t u r e was stirred overnight. Excess S O C l was flash evaporated, a n d the para-toluenesulfinyl chloride, a n orange, viscous, a i r - a n d water-sensitive l i q u i d , was distilled f r o m the reaction m i x t u r e [bp 8 7 ° C , 0.01 m m , 7 8 g (99%)]. T h i s chloride, 71 g i n 1 0 0 m L anhydrous ether, was a d d e d to 64 g (—)menthol i n 100 m L anhydrous e t h e r / 3 5 m L d r y p y r i d i n e under N at dry ice-acetone temperature; the mixture was stirred a n d w a r m e d to 2 0 ° C over 1 hr. T h e n , 100 m L of 1 M aqueous H C l was added; ether extractions ( M g S 0 dried) y i e l d e d an oily residue that was dissolved i n 3 0 m L petroleum ether a n d stored at 0 ° C . M e n t h y l para-toluenesulfinate (88 g) was collected a n d recrystallized from acetone as colorless rods; [ a I D —197 ( C 1, acetone), w h i c h agrees with the literature (57). M e t h y l magnesium iodide solution (5.3 g M g , 3 5 g C H I , 100 m L ether) was added under N to 3 0 g m e n t h y l sulfinate i n 2 5 0 m L ether at 0 ° C ; after stirring at 2 0 ° C for 0.5 hr, 150 m L of saturated N H C 1 solution were added, a n d the aqueous phase was made just basic w i t h aqueous a m m o n i a . T h e dried ether layer was evaporated, and the residue was diluted with 5 0 m L hexane and cooled to 0 ° C to give 6.4 g of sulfoxide; further ether extractions y i e l d e d another 5 g. Recrystallization f r o m cyclohexane gave w h i t e flakes; m p 7 4 ° C , [ a j o = + 1 4 3 ° ( C 1, acetone), w h i c h agrees w i t h the literature (58). F o u n d : 2

2

2

4

25 =

3

2

4

2 5

C , 6 2 . 5 2 ; H , 6.64; O , 10.53; S, 2 0 . 8 2 .

C H i o S O requires C , 6 2 . 3 4 ; H , 6.49; O , 8

10.39; S, 20.78. %fâ 2.45 (singlet, 3 H , C H - ) , 2.74 (singlet, 3 H , C H - S ) , 7.46 (quartet, 4 H , A r ) . Ruthenium Complexes. M-DIOPBIS[DICHLOROMONO(DIOP)RUTHENIh

3

UM(II)], R U C L ( D I O P ) . 2

O n e g r a m of R u C l ( P P h )

3

4

3

2

3

3

(59) a n d 0.9 g d i o p (1:1.7

mole ratio) were refluxed i n 100 m L hexane under N for 16 hr; the green precipitate was washed w i t h hexane a n d v a c u u m d r i e d (80%). F o u n d : C , 60.6; H , 5.3; C I , 7.8. C 9 H 0 C l P R u requires C , 6 0 . 7 ; H , 5.2; C l , 7.7. 2

3

9 6

6

4

6

2

CHLOROHYDRIDOBIS(DIOP)RUTHENIUM(II), H R U C L ( D I O P ) . 2

One

gram

of H R u C l ( P P h ) d m a (60) a n d 1 g diop were refluxed i n 100 m L hexane under A r for 2 4 hr; the yellow precipitate was washed w i t h hexane a n d v a c u u m d r i e d 3

(60%).

3

Found:

C , 6 5 . 2 ; H , 6.0.

C

6 2

H 6 5 0 C l P R u r e q u i r e s C , 6 5 . 6 ; H , 5.8. 4

4

DICHLOROBIS(DIOP)RUTHENIUM(II), RUCL (DIOP)(DIOP*). 2

One

gram

of R u C l ( d i o p ) a n d 1 g d i o p were stirred i n 2 5 m L benzene under A r at 2 0 ° C for 1 0 hr; the residue that was obtained b y evaporation was washed thoroughly w i t h hexane to y i e l d a light green powder (70%). F o u n d : C , 6 3 . 3 ; H , 5.7. C 2 H 6 4 0 C l P R u requires C , 6 3 . 7 ; H , 5.5. 2

6

4

3

4

2

4

TRIMERIC DICHLOROBIS[(S,R;S,S)-(+)-2-METHYLBUTYL M E T H Y L SULFOXIDE]RUTHENIUM(II), [ R U C L ( M B M S O ) ] . 2

2

3

A total of

1.1 m L of the sulf-

oxide was a d d e d to the blue solution f o r m e d b y r e f l u x i n g 1 g of R u C l * 3 H 0 i n 4 0 m L C H O H under H , a n d the r e f l u x i n g continued under H for t w o days; f i l t e r i n g to remove metal a n d the r e m o v a l of the C H O H left a b r o w n o i l . 3

3

2

2

3


2

134

TRANSITION METAL HYDRIDES

Benzene, 40 m L , was added and the solution was freeze dried; this gave 1.75 g (97%) of the complex. Found: C , 33.1; H , 6.6; C l , 16.2; S, 14.7. C36H8406S C1RRU3 requires C , 32.7; H , 6.4; C l , 16.1; S, 14.6. Mol wt 1275 g/mol (benzene), asdas 0.7-1.75 (multiplet, 16H, C H - C H - C - C H ) , 1.85-2.5 (multiplet, 2H, - C H - ) , 2.8-4.2 (multiplet, 10H, C H - S - C H - ) . 6

3

3

TRIMERIC

2

DICHLOROBIS[(R)-(+)-METHYL

RUTHENIUM(II), [ R U C L ( M P T S O ) ] . 2

2

3

3

2

PARA-TOLYL SULFOXIDE]-

A total of 1.3 g of the sulfoxide was added

to a methanolic blue solution formed from 1.0 g R u C l * 3 H 0 , and the refluxing continued under H for four days. The brown solution was pumped to dryness, and 15 m L C H C 1 was added; the solution was filtered, and when ether was added, the complex precipitated (1.2 g, 60%). Found: C , 40.4; H , 4.4; C l , 14.5. C 8 H 6 o 0 S C U R u requires C , 40.0; H , 4.2; C l , 14.7. Mol wt 1360 g/mol (benzene). offiS 2.04-2.58 (multiplet, 6H, C H - A r ) , 3.34-3.96 (multiplet, 6H, S-CH ), 6.44-7.90 (multiplet, 8 H , Ar). 3

2

2

3


4

6

6

3

3

3

P O L Y M E R I C DICHLOROBIS(METHYL P H E N Y L SULFOXIDE)RUTHENIUM(II),

[Ru-CL (CH SOC H ) ] . A total of 1.3 g of the sulfoxide was added to the methanolic blue solution formed from 0.75 g R u C l 3 H 0 , and the refluxing under H continued overnight. The gold solid that separated was filtered, washed with ethanol and acetone, and vacuum dried (0.9 g, 65%). Found: C , 36.9; H , 3.7; C l , 15.9. C i H 0 S C l R u requires C , 37.2; H , 3.6; C l , 15.7. 2

3

6

5

2

n

3

2

2

4

1 6

2

2

2

Acknowledgment We thank the National Research Council of Canada for financial support, including a scholarship (R.H.M.), and Johnson Matthey Ltd. for loan of the ru thenium and rhodium salts. Literature Cited 1. Morrison, J. D., Mosher, H. S., "Asymmetric Organic Reactions," American Chemical Society, Washington, D. C., 1976. 2. Knowles, W. S., Sabacky, M. J., Vineyard, B. D., Weinkauff, D. J., J. Am. Chem. Soc. (1975) 97, 2567. 3. Tanaka, M., Ogata, I., J. Am. Chem. Soc., Chem. Commun. (1975) 735. 4. Achiwa, K., J. Am. Chem. Soc. (1976) 98, 8265. 5. Fryzuk, M. D., Bosnich, Β., J. Am. Chem. Soc. (1977) 99, 6262. 6. Levi, Α., Modena, G., Scorrano, G., J. Am. Chem. Soc., Chem. Commun. (1975) 6. 7. Dumont, W., Poulin, J-C., Dang, T-P., Kagan, H. B., J. Am. Chem. Soc. (1973) 95, 8295. 8. Pino, P., Consiglio, G., Botteghi, C., Salomon, C., ADV. CHEM. SER. (1974) 132,

295. 9. Yamamoto, K., Hayashi, T., Zembayashi, M., Kumada, M., J. Organomet. Chem. (1976) 118, 161. 10. Corriu, R. J. P., Moreau, J. J. E., J. Organomet. Chem. (1974) 64, C51. 11. Bogdanovic, B., Angew. Chem. Int. Ed. (1973), 12, 954. 12. Yamada, S., Mashiko, T., Terashima, S., J. Am. Chem. Soc. (1977) 99, 1988. 13. Michaelson, R. C., Palermo, R. E., Sharpless, K. B., J. Am. Chem. Soc. (1977) 99, 1990. 14. Takaishi, N., Imai, H., Bertelo, C. Α., Stille, J. K., J. Am. Chem. Soc. (1976) 98, 5400. 15. Sbrana, G., Braca, G., Giannetti, E., Intern. Conf. Organomet.Chem.,7th, Venice, 1975, p. 188. 16. Hirai, H., Furuta, T., J. Polym. Sci., Part Β (1971) 9, 729.



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17. McQuillin, F. J., "Homogeneous Hydrogenation in Organic Chemistry," p. 98, Reidel, Dordrecht, 1976. 18. Ohgo, Y., Kobayashi, K., Takeuchi, S., Yoshimura, J., Bull. Chem. Soc. Jpn. (1972) 45, 933. 19. Carlini, C., Politi, D., Ciardelli, F., J. Am. Chem. Soc., Chem. Commun. (1970) 1260. 20. James, B. R., McMillan, R. S., Reimer, K. J., J. Mol. Catal. (1976) 1, 439. 21. James, B. R., McMillan, R. S., Can. J. Chem. (1977) 55, 3927. 22. Knowles, W. S., Sabacky, M. J., Vineyard, B. D., A D V . C H E M . SER. (1974) 132, 274. 23. Tolman, C. Α., Meakin, P. Z., Lindner, D. L., Jesson, J. P., J. Am. Chem. Soc. (1974) 96, 2762. 24. James, B. R., "Homogeneous Hydrogenation," p. 400, Wiley, New York, 1973. 25. James, B. R., Wang, D. K. W., Voigt, R. F., J. Am. Chem. Soc., Chem. Commun. (1975) 574. 26. James, B. R., Wang, D. K. W., Inorg. Chim. Acta (1976) 19, L17. 27. Hoffman, P. R., Caulton, K. G., J. Am. Chem. Soc. (1975) 97, 4221. 28. James, B. R., Markham, L. D., Inorg. Chem. (1974) 13, 97. 29. Bressan, M., Rigo, P., Inorg. Chem. (1975) 14, 2286. 30. James, B. R., Rattray, A. D., Wang, D. K. W., Chem. Commun. (1976) 792. 31. Miller, J. S., Caulton, K. G., J. Am. Chem. Soc. (1975) 97, 1067. 32. Markham, L. D., Ph.D. dissertation, University of British Columbia (1973). 33. James, B. R., "Homogenous Hydrogenation," p. 83, Wiley, New York, 1973. 34. Hallman, P. S., McGarvey, B. R., Wilkinson, G., J. Chem. Soc. A (1968) 3143. 35. Cullen, W. R., Fenster, Α., James, B. R., Inorg. Nucl. Chem. Lett. (1974) 10, 167. 36. James, B. R., Mahajan, D., Isr. J. Chem. (1977), in press. 37. Sanger, Α., J. Chem. Soc., Dalton Trans. (1977) 120. 38. Whitesides, G. M., Lewis, D. W., J. Am. Chem. Soc. (1971) 93, 5914. 39. Boucher, H., Bosnich, B., J. Am. Chem. Soc., (1977) 99, 6253. 40. Holloway, J., Kenyon, J., Phillips, H., J. Chem. Soc. (1928) 3000. 41. Axelrod, M., Bickart, P., Jacobus, J., Green, M. M., Mislow, K., J. Amer. Chem. Soc. (1968) 90, 4835. 42. Jacobus, J., Mislow, K., J. Am. Chem. Soc. (1967) 89, 5228. 43. Gilbert, J. D., Rose, D., Wilkinson, G., J. Chem. Soc. A (1970) 2765. 44. James, B. R., McMillan, R. S., Inorg. Nucl. Chem. Lett. (1975) 11, 837. 45. McMillan, R. S., Mercer, Α., James, B. R., Trotter, J., J. Chem. Soc., Dalton Trans. (1975) 1006. 46. James, B. R., Morris, R. H., Reimer, K. J., Can. J. Chem. (1977) 55, 2353. 47. Rose, D., Wilkinson, G., J. Chem. Soc. A (1970) 1791. 48. Spencer, Α., Wilkinson, G., J. Chem. Soc., Dalton Trans. (1972) 1570. 49. Mercer, Α., Trotter, J., J. Chem. Soc., Dalton Trans. (1975) 2480. 50. Evans, I. P., Spencer, Α., Wilkinson, G., J. Chem. Soc., Dalton Trans. (1973) 204. 51. McMillan, R. S., Ph.D. dissertation, University of British Columbia (1976). 52. James, B. R., Rempel, G. L., Discuss. Faraday Soc. (1968) 46, 48. 53. Ames, G. R., Davey, W., J. Chem. Soc. (1958) 1794. 54. Wiley, G. Α., Hershkowitz, R. L., Rein, Β. M., Chung, B. C., J. Am. Chem. Soc. (1964) 86, 964. 55. Rabjohn, N., Ed., Org. Synth. (1963) 4, 937. 56. McAllen, D. T., Cullum, T. V., Dean, R. Α., Fidler, F. Α.,J.Am. Chem. Soc. (1951) 73, 3627. 57. Herbrandson, H. F., Dickerson, R. T., Jr., Weinstein, J., J. Am. Chem. Soc. (1956) 78, 2576. 58. Mislow, K., Green, M. M., Laur, P., Melillo, J. T., Simmons, T., Ternay, A. L., Jr., J. Am. Chem. Soc. (1965) 87, 1958. 59. Stephenson, Τ. Α., Wilkinson, G., J. Inorg. Nucl. Chem. (1966) 28, 945. 60. James, B. R., Markham, L. D., J. Catal. (1972) 27, 442. R E C E I V E D July 19, 1977.


Transition Metal Hydrides - American Chemical Society

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