Enantioselective dehydrogenation of secondary carbinols by ruthenium(II) chiral phosphine complexes in the transfer hydrogenation of olefins

INORG. NUCL. CHEM. LETTERS Vol. 12, pp 837-842, 1976. PergamonPress. Printed in Great Britain.

ENANTIOSELECTIVE RUTHENIUM(II)

DEHYDROGENATION

CHIRAL PHOSPHINE HYDROGENATION

K. OHKUBO*,

T. AOJI,

OF SECONDARY CARBINOLS BY COMPLEXES

IN THE TRANSFER

OF OLEFINS

K. HIRATA,

and K. YOSHINAGA

Department of Synthetic Chemistry, Kumamoto University, Kumamoto 860, Japan

(Received 19 July 1976) Ru(II)

triphenylphosphine

transfer hydrogenation

complexes are known to catalyze the

of olefins by primary or secondary

carbinols (i) : RR'CHOH + ~C=C~ RuCl2(PPh3)3 When the above reaction phosphine complexes,

or RuH 2(PPh 3)4)RR,C= O + I C H C H ~i

is carried out with Ru(II)

an enantioselective

racemic RR'CHOH could be expected. of l-phenylethanol,

dehydrogenation

Indeed,

of

the dehydrogenation

I, by an in situ prepared RuCI2(NMDP) 3

(NMDP=(+)-neomenthyldiphenylphosphine) line complexes

chiral

(2) or isolated crystal-

(3) such as Ru2CI4(DIOP) 3 resulted in the appreci-

ably predominant

consumption

with an almost quantitative enantioselectivity

of one of the enantiomers formation of acetophenone.

was very low but reproducible,

(TABLE i) The

and the opti-

cal purity of I obtained by fractional distillation without any contaminants increasing reflected

possessing

conversion,

the optical rotation

increased with

obeying a pseudo-first-order

rate law

in the constant kR/k S values during the reaction:

kR R-(+)-I ~ S-(-)-I

chiral Ru(II)

complex,

olefin> acetophenone

kR/ks = ln{ [R-I]/[R-I]0}/In{ [S-I]/[S-I]0 } The chiral phosphine stituents

ligands possessing

seem to be more effective

ing order of the enantioselective

different

as reflected

abilities,

and bulky subin the follow-

RuCI2(NMDP) 3>RuCI 2-

(o-AMPP)3>Ru2CI4(DIOP)3>RuCI2(BMPP)3>RuCI2(PMPP)3>RuCI2(P-AMPP)2 (PPh3) , and NMDP was found to be the most effective ligand in the present reaction.

837

838

Enantioselective Dehydrogenation of Secondary Carbinols TABLE 1

Enantioselective Complexes

Abilities

of Some Ru(II)

in the Transfer Hydrogenation

Chiral Phosphine

of Benzalacetone

([Benzalacetone]0/[I]0=0.84) 23 b Complex a Conc. Time Cony. [a]D

by I

at 180°C

(mM) RuCI2(NMDP) 3

(hr)

(%)

O.P. 105kR 105kS

(de@.)

(%)

kR/ks

(sec -I) (sec -I)

8.0

5.0 38.2

-1.199

2.284 2.804

2.548

i.I00

RuCI2(o-AMPP) 3 i0.i

14.0 38.1

-0.684

1.303 0.9744

0.9228 1.056

Ru2CI4(DIOP) 3

4.0

4.0 57.9

-0.923

1.758 6.131

5.887

1.041

RuCI2(BMPP) 3

8.0

9.0 49.3

-0.469

0.897 2.125

2.070

1.027

RuCI2(PMPP) 3

4.0

3.0 40.5

-0.124

0.236 4.829

4.758

1.009

RuCI2(P-AMPP) 2- 8.0 (PPh 3)

5.0 63.5

+0.192 0.365 5.503

5.543

0.993

a. o-AMPP=(-)-o-anisylmethylphenylphosphine;

DIOP=(-)-2,2-di-

methyl-l,3-dioxolan-4,5-bis(methylene)bis(diphenylphosphine); BMPP=(+)-benzylmethylphenylphosphine;

PMPP=(-)-propylmethylphenylphosphine; p-AMPP=(-)-p-anisylmethylphenylphosphine, b. [~]D23 23 -52.5 ° , c 2.27 in CH2CI 2 (ref.7). The experimental error of [~]D' O.P., or rate constant was within +0.002 °, +0.003%, -I sec , respectively.

The selectivity

kR/kS)

(defined by

was found to vary from

olefin to olefin in the dehydrogenation (TABLE 2) with the quantitative parallel with the acetophenone

or +0.001

of I by RuCI2(NMDP) 3

saturation of the olefin in formation.

The change in the selec-

tivity by the olefin is probably due to a newly produced chiral field induced by the olefin in its coordination

to an active

RuCI2(NMDP) 2 (4) formed via the following reaction: RuCI2(NMDP) 3. {Ru}

+

RICH=CHR2.

~ RuCI2(NMDP) 2 {Ru}+ NMDP "

Since there is some phosphobetaine

~Ru} s ,. RI~H-"~HR 2 ketones

and the phosphobetaine

ed is then bound in a new catalytically unsaturated

ketones resulted

{II}

(2)

formation with the chiral

ligand in the cases of two unsaturated and PhCH=CHCOMe,

(i)

(5), PhCH=CHCOPh

(R~CH(Ph)CH=C(R)O) active species,

in the most remarkable

form-

the

effect on the

induction of the chirality. The complex

{II} seems to participate

in the enantioselective

Enantioselective Dehydrogenation of Secondary Carbinols TABLE Enantioselection Hydrogenation Olefin

2

of I by RuCI2(NMDP) 3 (8 mM)

of O l e f i n s a

Temp.

Time Conv.

_[e]~3

O.P.

(°C)

(hr)

(%)

(deg.)

(%)

165

5

18.7

0.616

1.174

benzalacetone

839

in the T r a n s f e r

,s f::!l[:::!] AAH #b

_AAS #b

1.121

1.03

2.12

170

5

27.9

0.892

1.699

I.Ii0

21.47

32.

180

5

38.2

1.199

2.283

i.i00

22.50

30.59

190

5

60.3

1.106

2.109

1.047

benzalacetophenone 180

8

17.3

1.055

2.010

1.273

2.19

4.41

190

8

46.4

2.264

4.313

1.148

195

8

81.2

2.514

4.788

1.059

31.48

13.79

180

8

26.2

0.053

0.i00

1.036

0.94 c

2.00 c

195

8

41.8

0.026

0.050

1.002

trans-

stilbene

1540cj 48 18 c] 2-ethylhexyl methacrylate

180

8

11.0

0.289

0.570

1.051

ethyl cinnamate

180

8

36.1

0.050

0.095

1.003

none

165

5

11.3

0.035

0.067

1.011

180

5

19.4

0.023

0.044

1.004

0.18 c

0.40 c

1451cj 4989cj a.

[NMDP]~/[RuCI,(PPh~)~]^=6

b. AAH = AHs-AH R in k c a l / m o l unit.

AH ~, and AS ~ are w i t h i n

e.u.,

{II}

+

Experimental

of RR'CHOH

RR'CHOH

The p a r t i c i p a t i o n

errors of rate and

-H +

in the following way: ~

H

) [ R R ' C H O - - { R u } ~ RI] ~HR 2

of the free proton

H+

l-phenylethanol

includes

in Reaction(4)

(3)

has been

of b e n z a l a c e t o p h e n o n e

by

w i t h RuCI2(PPh3) 3 (6). In R e a c t i o n

the rate d e t e r m i n i n g

tion of the e - c a r b o n - b o u n d

{III}

) RR'C=O+RICH2CH2R2+{Ru}(4)

in the t r a n s f e r h y d r o g e n a t i o n

the d e u t e r a t e d (3) w h i c h

in e.u.

2%, Z0.03 kcal/mol,

> [RR'C~'O--{Ru}-CHRICH2R2]

confirmed

-AAS += - A S ~ - A S

respectively.

dehydrogenation

{III}

[olefin]~/[I]^=0.84.

c. R o u qr h l y e s t i m! a t e d value.

constants, +0.007

and

unit and

step

(viz. , the abstrac-

h y d r o g e n by the Ru m e t a l

(6)), a

840

Enantioselective Dehydrogenation of Secondary Carbinols

couple of RR'CHOH,

(R,R')=(Ph,Me),

ed different selectivities

(Ph,Et), and

(PhCH2,Me) , show-

(TABLE 3), but the selectivity order

was not simply reflected in the bulkiness of the substituents and R'). At any rate,

(R

it is notable from the linear Arrhenius-

dependence of each rate constant

(kR or k S ) being kept constant

during the reaction that the difference ! in the entropies of activation between the enantiomers,

AAS ~, which became large in

the case of high selectivity,! was in parallel with that in their entropies of activation,

AAH T. This phenomenon was also seen in

the effect of the olefins on the ! enantioselectivity

(TABLE 2).

In view of the fact that the AAH T values measure the difference in the energies for the dehydrogenation of each enantiomer (especially,

for the rate determining Reaction(3)

abstraction of the carbon-bound hydrogen),

involving the

they should be zero or

negligibly small, because the energy required for the hydrogen abstraction is equivalent for each enantiomer under a constant O-Ru coordination distance.

Therefore,

there are two reasonable

explanations concerning the above phenomenon:

(a) the enantio-

selection and the hydrogen abstraction in Reaction(3)

occur

simultaneously during the coordination of the carbinols to the complex {II}, that is, the rate-determining

step is somewhat

complicated due to the coordination and the hydrogen abstraction in the coordination sphere and

(b) the hydrogen abstraction

occurs after the coordination of the carbinol to the complex, but it is expected at the different coordination distance between the enantiomers.

In the latter case, the much closer coordination of

an enantiomer than that of the other may result in an enhancement of the selectivity for one of the enantiomers, because the close coordination makes the effect of asymmetric fields more optically favorable

(AST becomes negatively large)

the energy required for the hydrogen abstraction small). In this sence, the idea

and decreases

(AH~ becomes

(b) seems more plausible,

because the enantiomer showing the smaller AH ~ value required more negative AS ~ value than the other in the same carbinol. In the absence of olefins,

the selectivity was substantially

decreased by the formation of a side reaction product, mesobis(l-phenylethyl) ether in the dehydrogenation of I: where (Ru) and (Ru-H) denote the Ru(II) chiral phosphine complex and the hydrido-complex

respectively.

Enantioselective Dehydrogenation The intermediate tion,

because

{IV}

seems to be required

the selectivity

(8 mM)

forma-

being kept almost constant

during

RR'CHOH R R'

Temp. (°C)

Me

Ph (I)

Ph

of Secondary

in the Presence

Et (I')

841

for the ether

TABLE Enantioselection

of Secondary Carbinols

3 Carbinols

by RuCI2(NMDP) 3

of Benzalacetone a

Time Conv. (hr) (%)

-[e]D b (de~.)

26.17

0.840

O.P " (%) 1.600

105k R 105k S (sec-l) (sec -I)

kR/k S

1.268

i.iii

160

7

1.141

170

6

34.28 0.829

1.579

2.017

1.871

1.078

180

5

47.32

0.912

1.737

3.658

3.465

1.056

190

2

41.46

0.719

1.370

7.628

7.248

1.052

170

7

26.26

0.549

1.373

1.264

1.155

1.094

180

5

38.72

0.328

0.820

2.766

2.675

1.034

190

3

36.41 0.401

1.003

4.285

4.099

1.045

PhCH 2 Me

170

12

31.12

0.018

0.089

0.861

0.865

0.995

(I'')

180

6

27.39

0.015

0.074

1.478

1.485

0.995

190

4

39.93

0.018

0.089

3.533

3.546

0.996

Enantiomer

AH + (kcal/mol)

R-(+)-I

23.03

S-(-)-I

23.76

R-(+)-I'

23.94

S-(-)-I'

24.86

S-(+)-I''

27.97

R-(-)-I''

27.99

AAH + (kcal/mol)

AS + (e.u.)

AAS + (e.u.)

-28.70

0.73

1.49

-27.21 -27.62

0.92

1.91

-25.71 -19.58

0.02

0.i0

-19.48

a. [NMDP]N/[RuCI~(PPh~) ] =3 and [benzalacetone] /[carbinol] =i. b I': [aT 17-20 +40.01 ~c05, C6H6) (ref.8); I'':0[~] 25 -20.27 (c 5, C2H5OC2H5) AS ~ are w i t h i n RR'CHOH

+

(Ru)

RR'CHOH

+

H+

(ref.9). +0.09 _H +

The experimental

kcal/mol

and +0.08

) [RR'CHO--(Ru)] RR 'CHOH 2 +

(Ru-H)

{IV}0 _H2

> (Ru) +

errors

e.u.,

o~ AH + and

respectively.

{IV} ---~ RR'C=O + > (RR'CH)2 ° 1 ~H 2

(Ru-H)

842

Enantioselective Dehydrogenation of Secondary Carbinols

the dehydrogenation

of RR'CHOH without olefin

expected without the enantioselection intermediate

of RR'CHOH via the

{IV} in the ether formation process.

ether was almost equal to that of the ketone tion of RR'CHOH without olefins. tive mechanism

The mol% of the

in the dehydrogena-

The more detailed enantioselec-

is now under investigation.

We thank the Ministry of Education Scientific

(i0) cannot be

Research

for a grant-in-aid

for

(No. 175450).

R e f e r e n c e s and Notes i. Y.Sasson and J.Blum, T.Nishiguchi, 48, 1585

M.Kobayashi,

(1975),

R.F.Voigt,

2167

and K.Fukuzumi,

and references

2. Strictly speaking, 3. The complexes

Tetrahedron Lett.,

(1971); H.Imai,

Bull.Chem.Soc.Jpn.,

therein.

the structure of the complex is uncertain.

except Ru2CI4(DIOP) 3 (B.R.James,

Chem.Comm.,

574

D.K.W.Wang,

(1975)) were prepared by the reac-

tion of the chiral phosphines with RuCI2(PPh3) 4 in hexane. results of NMR spectra and elementary to be published, 4. K.G.Caulton,

and

analyses,

The

which are now

were reasonable.

J.Amer.Chem.Soc.,

96, 3005

(1974), and references

therein. 5. L.Marko and B.Heil,

Catalysis

Rev., 8, 269

6. Y.Sasson

and J.Blum,

J.Org.Chem.,

7. U.Nagai,

T.Shishido,

R.Chiba,

2!i, 1701 8. J.Kenyon,

40, 1887

(1973). (1975).

and H.Mitsuhashi,

Tetrahedron,

(1965). S.M.Partridge,

and H.Phillips,

J.Chem.Soc.,

207

(1937). 9. J.Kenyon,

H.Phillips,

and V.P.Pittman,

J.Chem.Soc.,

1072

(1935). 10. K.Ohkubo, 183

K.Hirata,

(1976).

K.Yoshinaga,

and M.Okada,

Chem. Lett.,