New Carbonylations Catalyzed by Transition Metal Complexes

19 New Carbonylations Catalyzed by Transition Metal Complexes Iwao Ojima, Zhaoda Zhang, Anna K o r d a , Patrizia Ingallina, and N u r i a Clos Department of Chemistry, State University of New York at Stony Brook, Stony Brook, N Y 11794

The Rh-catalyzed hydroformylation of N-allylamides gives isoaldehyde with good regioselectivity through chelation control. The Rh– and Co Rh2(CO) -catalyzed reactions of an N-methallylamide give a novel double carbonylation product and a pyrrolidine, respectively. These reactions are successfully applied to homoallylic systems to generate the corresponding azabicyclo[4.n.0]alkenes and alkanes. Intramolecular amidocarbonylation of 3-butenamide catalyzed by a Rh complex with excess phosphine ligands gives 3,4-dihydro-2-pyridone. The same reaction with excess P(OPh) affords a unique heterodimer selectively. The reaction of 4-pentenamide gives 5-methyl3,4-dihydro-2-pyridone exclusively, regardless of rhodium catalysts used. Reactions of hydrosilanes with 1-hexyne catalyzed by Co Rh (CO) and Rh (CO) at 25 °C under CO give (Z)-1-silyl-2formyl-1-hexenes, which are the products of "silylformylation". Novel Rh-Co complexes, (R Si) Rh(CO) -Co(CO) and RhCo(n-Bu-C≡ CH)-(CO) , are found to be important catalyst species for the reaction catalyzed by Co Rh (CO) . The catalytic cycle of the reaction is proposed. 2

12

3

2

2

12

4

12

3

2

n

4

S

2

2

12

H E L A T I O N - C O N T R O L L E D R E G i o s E L E C T I V E and stereoselective

reactions

have been studied extensively in the field of organometallic chemistry for organic synthesis. In the catalysis field, the asymmetric hydrogénation of dehydroamino acids and dehydropeptides (1-4), asymmetric epoxidation of allylic alcohols (5), and asymmetric isomerization of allylamines (6, 7) are

0065-2393/92/0230-0277$06.00/0 © 1992 American Chemical Society

278

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

excellent examples of the chelation-controlled methodologies to attain high stereoselectivity. However, to the best of our knowledge, no systematic studies have been performed on the application of chelation control to selective carbonylations. We describe here our recent results on the successful chelation control in hydrocarbonylations of N-allylamides and alkenamides catalyzed by rhodium and C o - R h mixed-metal complexes and other novel carbonylation reactions such as sequential double carbonylation (8).

New Syntheses of Nitrogen Heterocycles Through Amide-Directed Hydrocarbonyfotions Intramolecular Amidocarbonylation of IV-Alkenylamides. The hydroformylation of N-ally lace tarn ide was carried out by using a variety of rhodium catalysts [i.e., RhCl(PPh ) , RhCl(CO)(PPh ) , HRh(CO)(PPh ) , [Rh(dppb)(NBD)]Cl0 , R h ( C O ) , and a C o - R h mixed-metal complex, C o R h ( C O ) ]. Typical results are summarized in Table I. 3

4

2

2

4

3

3

2

3

3

l2

1 2

Table I. Hydrocarbonylation of N-Allylacetamide Yield

Catalyst Entry 1 2 3 4 5 6 7

(mol %) [Rh(dppb)(NBD)]ClCV (1.0) RhCl(PPh ) (1.0) RhCl(CO)(PPh ) (1.0) HRh(CO)(PPh )3 (1.0) Rh (CO) (0.25) Co Rh (CO)i (0.5) Co Rh (CO)i (1.0) 3

3

3

2

3

4

l2

2

2

2

2

2

2

c

Products Ratio

0

(%)

1

78 80 79 76 78 80 80

71 65 66 63 79 79 82

a

2 — — — 11 6 — —

3

4

5 7 7 13 6 21 18

24 28 27 13 9 — —

N O T E : All reactions were run by using a Pyrex reaction vessel (50 mL) in a stainless steel autoclave (300 mL) with 1.50 mmol of N-allylacetamide in T H F (3.6 mL) at 80 °C and 1200 psi of carbon monoxide and hydrogen ( C O / H = 1) for 18 h unless otherwise noted. The products were isolated by column chromatography on silica gel and identified by ' H and C NMR, IR, and mass spectroscopy. "Determined by *H NMR and G L C analyses. dppb is l,4-bis(diphenylphosphino)butane. N B D is norbornadiene. 60°C. 2

I 3

b

C

As Table I shows, the major product of the reaction is isoaldehyde (2methyl-3-acetylaminopropanal) (1) and the minor products are n-aldehyde (4-acetylaminobutanal) (2), 1-acetylpyrrolidine (3) and/or l-acetyl-2-formylpyrrolidine (4), which is the product of novel sequential double carbonylation (eq 1).

19.

OJIMA E T AL.

Carbonylations Catalyzed by Transition Metal Complexes 279

Ο CO, H Cat.

Η

2

Ο

Η

I

CHO

Η

CHO

1 (major)

(1)

2

I CHO

Hydroformylation of 1-alkenes catalyzed by rhodium complexes gives η-aldehyde as the predominant product. η-Aldehyde selectivity is increased when phosphine ligands are introduced [i.e., the n-to-iso ratio is in the range of 5-10 for phosphine-rhodium complexes and 1.1-2 for rhodium carbonyls (9) ]. Accordingly, good isoselectivities observed in the present system are opposite to those for the usual 1-alkenes. The C o - R h mixed-metal catalyst, C o R h ( C O ) , brings about substantially better product selectivity than other rhodium complexes (entries 6 and 7, Table I). This result implies the synergistic effects of the mixed-metal system: The Co (CO) -catalyzed hydrocarbonylation was reported (10, II) to give a mixture of three amino acids instead of amino aldehydes [i.e., 2-(benzoylamino)butanoic acid (45%), 2methyl-3-(benzoylamino)propanoic acid (8%), and N-benzoylproline (21%) 2

2

12

2

8

(10) l The results of the rhodium complex-catalyzed reactions shown in Table I are not as selective as those of the Co Rh (CO) -catalyzed reaction. Thus, there is a substantial synergistic effect when the two metals are combined. The observed unique isoselectivity is best interpreted by taking into account the amide-directed chelation control of regioselectivity. l-Acetyl-2-formylpyrrolidine (4), obtained as a minor product in rho dium-catalyzed reactions, is formed through a new type of amidocarbonylation via a hemiamidal (5) arising from n-aldehyde (2) followed by the sequential formation of an alkyl-Rh complex (6) and an acyl-Rh complex (7), as shown in Scheme I. In the final reductive elimination step, the acyl-Rh bond is 2

2

12

280

H O M O G E N E O U S TRANSITION

METAL CATALYZED

REACTIONS

Scheme 1. Proposed mechanism for sequential double carbonylation.

selectively cleaved by hydrogen to yield aldehyde (4). This reaction forms a sharp contrast to the cobalt-catalyzed amidocarbonylation, which gives the corresponding carboxylic acid exclusively (10-13). This new type of amidocarbonylation reaction provides the first example of rhodium-catalyzed sequential double carbonylation. Because this novel reaction has high potential as a synthetic method, we studied the reaction further to make it more selective. We employed N-(2-methyl-2-propenyl)benzamide as a substrate (eq 2). The results are summarized in Table II. Because of the 2-methyl group, initial hydroformylation became highly regioselective. Thus the reaction gave an expected 2-formylpyrrolidine (8) (1:1 diastereomer mixture) as the predominant product (almost exclusive in entry 3) together with a pyrrolidine (9) and a hemiamidal (10) in rhodium-catalyzed reactions (entries 1-8, Table II). In contrast to the rhodium-catalyzed reactions, the C o R h ( C O ) ^-catalyzed reaction gave 9 with >98% selectivity (entries 9 and 10, Table II). This reaction clearly demonstrates the synergistic effects of the mixed-metal system. 2

2

19.


OJIMA E T AL.

(2)

Table II. Hydrocarbonylation of iV-(2-Methyl-2-propenyl)benzamide Catalyst (mol %)

Entry 1 2 3 4 5 6 7 8 9 10

[Rh(dppb)(NBD)]C10 (1.0) [Rh(dppb)(NBD)]Cl(V (1.0) [Rh(dppb)(NBD)]Cl0 (1.0) RhCl(PPh ) (1.0) RhCl(PPh ) (1.0) HRh(CO)(PPh ) (1.0) Rh (CO) (0.25) Rh (CO) (0.25) Co Rh (CO) (0.5) Co Rh (CO) (0.5) 4

C

fc

4

3

3

3

3

3

4

12

4

12

2

2

l2

2

2

12

b

3

CO (psi)

H (psi)

Yield" (%)

600 1500 1700 600 1500 600 600 600 600 300

600 300 150 600 300 600 600 200 600 900

94 90 87 85 91 93 95 87 83 85

2

Products Ratio 10

8 27

62 87 >99.5 54 82 48 46

— —

46 7 13 20

—

—

98 100

2 0

11 13

— —

11 39 34 100^

d e

— —

N O T E : All reactions were run by using a Pyrex reaction vessel (50 mL) in a stainless steel autoclave (300 mL) with 1.50 mmol of N-(2-methyl-2-propenyl)benzamide in T H F (3.6 mL) at 100 °C for 18 h unless otherwise noted. The products were isolated by column chromatography on silica gel and identified by Ή and C NMR and mass spectroscopy. "Determined by ' H NMR and G L C analyses. *dppb is l,4-bis(diphenylphosphino)butane. N B D is norbornadiene. T h e reaction was run with 2.0 mol % of catalyst for 71 h. ^Containing 15% of n-aldehyde. 'Containing 18% of n-aldehyde. 'Containing 13% of n-aldehyde. 1 3

We looked into the mechanisms of these reactions and found that the hemiamidal (10) is the common intermediate to 8 and 9. Controlled exper iments using 10 revealed that 10 was actually converted to 8 (95% yield) in the presence of C O (1500 psi), H (300 psi), and R h C l ( P P h ) (1 mol %) at 100 °C for 18 h and that 10 was transformed to 9 (95% yield) in the presence of C o R h ( C O ) (1 mol %) at 100 °C and 1200 psi ( C O : H = 1) for 18 h. We also found that the hydrocarbonylation of 10 with C o ( C O ) (10 mol %) at 125 °C and 2000 psi ( C O : H = 1) for 20 h gives N-benzoyl-4-methylproline (11) cleanly in 72% yield (Scheme II). 2

2

2

3

1 2

3

2

2

2

8

Scheme U. Hydrocarbonylation of N-benzoyl-2~hydroxy-4-methylpyrrolidine.

9

19.

OJIMA E T AL.


The formations of 8, 9, and 11 are rationalized by taking into account the formylation, the hydrogenolysis, and the carboxylation, respectively, of an alkyl-metal complex 12 that is generated from 10 (Scheme II). The three reactions are competing processes. Carbon monoxide insertion is the predominant process for rhodium and cobalt catalysts; hydrogenolysis is almost exclusive for C o R h ( C O ) i . The rhodium catalyst gives aldehyde 8 through 2

2

2

exclusive hydrogenolysis of an acyl-metal complex 13, whereas the cobalt catalyst gives carboxylic acid 11 through exclusive hydrolysis of 13. Because the intramolecular amidocarbonylation has high potential as a new annulation method in organic synthesis, we applied these amide-directed hydrocarbonylations to 5-(2-methyl-2-propenyl)-2-pyrrolidinone (14) and 6-(2-methyl-2-propenyl)piperidinone (15) (Scheme III). The hydrocarbonylation of 14 and 15 with rhodium catalysts (1 mol % Rh)

such

Rh (CO) 4

as

HRh(CO)(PPh ) , 3

RhCl(PPh ) ,

3

3

at 100 ° C and 1800 psi ( C O : H

12

azabicyclo[4.3.0]-2-nonen-9-one

(16) and

2

RhCl(CO)(PPh ) , and

3

3

2

= 1) for 18 h gave 4-methyl-l4-methyl-l-azabicyclo[4.4.0]-2-de-

cen-10-one (17), respectively, in 60-65% yield. The product included small amounts of the corresponding saturated bicyclic lactams (18 and 19; 10-20%). The reactions catalyzed by C o R h ( C O ) 2

psi ( C O : H

2

= 0.5) for 18 h, gave

2

1 2

(0.5 mol %) at 125 °C and 1800

4-methyl-l-azabicyclo[4.3.0]-9-nonanone

(18) and 4-methyl-l-azabicyclo[4.4.0]-10-decanone

(19), respectively, in

75-80% yield. The Co (CO) -catalyzed reactions at 125 ° C and 2000 psi 2

(CO:H

2

8

= 1) with 10 mol % of the catalyst for 24 h gave 2-carbohydroxy-

4-methyl-l-azabicyclo[4.3.0]-9-nonanone

(20) and 2-carbohydroxy-4-methyl-

1- azabicyclo[4.4.0]-10-decanone (21), respectively, in only moderate yield (25-45%). The Rh-catalyzed hydroformylation of 14 or 15, carried out in the presence of ethyl orthoformate at 100 ° C and 1800 psi ( C O : H

2

= 1), gave

the corresponding homologous O-ethyl hemiamidals (22 and 23) in 65-80% yield. This product results through a sequential formation of the diethylacetal followed by cyclization. T h e subsequent Co (CO) -catalyzed carbethoxyla2

8

tion gave the ethyl ester of 24 or 25 in much better yield (60-75%). Treatment of 22 and 23 with trifluoroacetic acid in chloroform gave 16 and 17, respectively, in nearly quantitative yields. Construction of l-azabicyclo[4.n.O] systems through hydrocarbonylation can serve as a new annulation method in organic synthesis, especially for alkaloid syntheses.

Amide-Directed Hydrocarbonylation of Alkenamides.

The di-

hydro-2-pyridone skeleton is one of the important nitrogen heterocycles for pharmaceutical and agrochemical agents (14, 15). Simple dihydro-2-pyridones have been synthesized by the direct reaction of 2,4-pentadienoic acid or sorbic acid with ammonia (16) and by the sodium borohydride reduction of glutarimide (17). The 2,4-pentadienoic acid reaction gives a mixture of 3,6-dihydro- and 5,6-dihydro-2-pyridones; the reaction yields 3,4-dihydro2- pyridones selectively. We describe here new and convenient routes to

Ο

H

14 : η = 1 15:η = 2

Y^NH

18 : η = 1 19 : η = 2

16 : η = 1 17 : η = 2

2

1 2

3

3

OEt

y™

Ο 14:η = 1 15 : η = 2

η

=

1

· 23În = 2

CorfCO),

2

%

ο

Ο

( Ν

COOH

24 : η = 1 25 : η = 2

*C O O E^t

20 : η = 1 21 : η = 2

C02(CQ)g _ , CO,H ("^

Scheme 111. Amide-directed hydrocarbonylations.

2

2

3 3

RhCI(PPh ) CO, Η

CO, H

2

2

Co Rh (CO)

CO, H

HRh(CO)(PPh )

ο ζ

r ·< Ν m Ό 53 m >

>

r Ο

>

Η

m

δ

m •ζ m Ο α

Χ ο ο ο

19.

OJIMA E T AL.


3,4-dihydro-2-pyridones through intramolecular amidocarbonylation (18-20) of alkenamides, as well as a novel coupling reaction giving 6-(4-methyl-3pyrrolidin-2-on-l-yl)-2-piperidone (21, 22). First, the intramolecular amidocarbonylation of 3-butenamide was car ried out by using typical rhodium catalysts for hydroformylation (i.e., RhCl(PPh ) , RhCl(CO)(PPh ) , and R h ( C O ) at 80-100 °C and 1200 psi ( C O : H = 3/1 or 1/1) (eq 3). Results are shown in Table III. 3

3

3

2

4

12

2

Ο

26

27

28

As Table III shows, the reaction under those conditions gave a mixture of 3,4-dihydro-2-pyridone (26), 4-methyl-3-pyrrolin-2-one (27), and a heterodimer, 6-(4-methyl-3-pyrrolin-2-on-l-yl)-2-piperidone (28). As Scheme IV shows, 26 and 27 are formed via 4-formylbutanamide (30) and 3-formylbutanamide (31), respectively, whereas 28 is yielded via the crossed coupling of 26 (via 34) and 27 under the reaction conditions. Only the crossed coupling product, heterodimer 28, was obtained; no homocoupling of 26 or 27 was observed. To obtain 26-28 selectively, we examined the effects of phosphine ligands on the regioselectivity, as well as monomer-dimer selectivity of the reaction. As Table III shows (entry 3), the addition of 20 equiv of triphenylphosphine to R h C l ( P P h ) remarkably improved the selectivity for the formation of 26 (92%); no heterodimer 28 was formed. O n the other hand, when 10 equiv of triphenylphosphite was employed instead of triphenylphosphine for RhCl(PPh ) , heterodimer 28 was produced with excellent selectivity (94%) in 90% yield. Heterodimer 28 may serve as a useful intermediate for the synthesis of tricyclic or tetracyclic nitrogen heterocycles. Reaction conditions that afford 27 selectively have not been found so far. 3

3

3

3

Next we employed N-benzyl-3-butenamide as the substrate (eq 4). It was found that the N-benzylation of 3-butenamide favors the formation of a 2-pyrrolinone (27a) to some extent. The reaction catalyzed by R h C l ( P P h ) under 1200 psi of carbon monoxide and hydrogen ( C O : H = 1) gave 27a as the major product [e.g., 27a:26a = 2 (70% yield) at 100 ° C ; 27a:26a = 3 (64% yield) at 120 ° C ] The results contrast with those for 3-butenamide, in 3

2

3

286

H O M O G E N E O U S TRANSITION

oo to ι

co 3»

OJ

r-H

j t^- ^

1^ S 3

ι

Ρ-.

1

ο

I çu eu Ph

pu,

Ο Ο

OJ

ο

«

η J3

C

Pk -C

;Ϊ

η

«

o c v 9· Pm o

c c g

JZ

Ί3 OS

5?

Ρη

ο >Γ>>Ξ!> Ρμ

ε_ ε _ ,ε ρ ρ ο

^ ε

p ^ p ^ U U U Ο Ρ-. ϋ ϋ ϋ ϋ ϋ ϋ ^ ϋ

"g

pû

IJ'S'S'S
α)

REACTIONS

Scheme IV. Proposed mechanism for the formation of dihydropyridone (26), 4-methylpyrrolinone (27), and heterodimer (28).

3

ta oo -4

TO ce

H

?T

3 ts

TO S" O o

O*

ce

s

i

OS 's:

α.

TO

O

C* c

H

K

>

c

5

288


Ο

which 2-pyrrolinone (27) is the minor product. However, the addition of 20 equiv of triphenylphosphine to RhCl(CO)(PPh ) gave 26a with 91% selec 3

2

tivity in 98% yield, which was similar to the case of 3-butenamide. Because of the N-protection, no dimer formation was observed with this substrate. Amidocarbonylation of N-benzyl-3-butenamide catalyzed by C o ( C O ) 2

at 100 °C and 1470 psi ( C O : H

2

8

= 1) is reported to give 3-(N-benzylcarba-

moyl)-2-methylpropanoic acid in 69% yield; no formation of nitrogen heterocycles was observed (Ii).

Finally, the reaction of 4-pentenamide

was

examined in a similar manner (eq 5). Results are shown in Table IV.

29

Table IV. Synthesis of 5-Methyl-3,4-dihydro-2-pyridone (29) Through Intramolecular Amidocarbonylation of 4-Pentenamide Entry 1 2 3 4

Catalyst RhCl(PPh ) RhCl(COKPPh ) HRh(CO)(PPh ) Rh (CO)i 3

3

3

3

4

2

2

3

CO (psi)

H (psi)

Temperature (°C)

Time (h)

Yield" (%)

600 600 600 600

600 600 600 600

100 100 100 100

18 18 18 18

91 89 88 92

2

NOTE: All reactions were run with 1.50 mmol of 4-pentenamide and 0.015 mmol of a rhodium catalyst in T H F (3.6 mL) in an autoclave (300 mL) using a Pyrex reaction vessel (50 mL). The product, 29, was isolated by column chromatography on silica gel. "Isolated yield.

19.

Carbonylations Catalyzed by Transition Metal Complexes

OJIMA ET AL.

289

As Table IV shows, the reaction catalyzed by several rhodium complexes gave 5-methyl-3,4-dihydro-2-pyridone (29) as the sole product in excellent yield ( 8 8 - 9 2 % ) . Although the formation of seven-membered ring lactam is conceptually possible, such a product was not detected at all, even when 2 0 equiv of triphenylphosphine to R h C l ( P P h ) was employed as an additive. 3

3

The result clearly indicates that a "chelation control" is operative in this reaction. The result also strongly suggests that a similar chelation control is operating in the reactions of 3-butenamide and N-benzyl-3-butenamide as well. Thus, the effects of a large excess of triphenylphosphine to the rhodium catalysts cannot be accommodated by the blocking (or disruption) of the amide-directed chelation control, but can be interpreted as the regioselective hydroformylation of 3-butenamide (or iV-benzyl-3-butenamide) with the amide chelation intact. It is clearly indicated that the coordination of alkenamide to the rhodium catalysts is much stronger than that of triphenylphosphine.

Silylformytoion of 1-Alkyne Catalyzed by Rh-Co Mixed-Metal System The silicon version of hydroformylation of olefins, known as "silylcarbonylation", was discovered by Murai and co-workers (23). It is promoted by Co (CO) 2

to give silyl enol ethers of homologous aldehydes. In M u r a i s

8

silylcarbonylation, the silicon moiety always attaches to oxygen: No silicon migration is observed to the olefinic bond to form a silicon-carbon bond. As a part of our study of the catalysis of C o - R h mixed-metal complexes such as C o R h ( C O )

1 2

of C o R h ( C O )

1 2

2

2

2

2

and C o R h ( C O ) (20, 2 4 - 2 8 ) , we investigated the reactions 7

and C o R h ( C O )

7

with hydrosilanes in the presence and

absence of carbon monoxide, substrates, or both. O u r goal was to determine the active sites of these C o - R h mixed-metal catalyst systems by identifying active catalyst species as well as intermediates for catalytic cycles. To examine possible synergistic effects in the C o - R h mixed systems, R h ( C O ) 4

l 2

and

C o ( C O ) also were used as references. 2

8

We carried out the hydrosilylation of 1-hexyne with triethylsilane catalyzed by C o R h ( C O ) 2

2

1 2

at 2 5 °C in the presence of carbon monoxide (am-

bient pressure) in toluene. (Z)-l-Triethylsilyl-2-formyl-l-hexene (36c)

was

formed, in addition to usual hydrosilylation product, 1-triethylsilyl-l-hexene (37c)

(36c:37c

=

58/42) and some higher molecular weight

materials

(heavies). Dimethylphenylsilane gave 36a as the exclusive product under the same reaction conditions, although the formation of heavies was detected as well. This reaction yielding compound 36 from an alkyne is a new type of silylcarbonylation (i.e., silylformylation, which is different from Murais reaction.

290


While our study was in progress (29, 30), Matsuda et al. (31) reported the discovery of silylformylation catalyzed by R h ( C O ) by using a variety of alkynes and dimethylphenylsilane as the specific hydrosilane at 100 °C and 150-450 psi of carbon monoxide. We describe here our C o - R h mixedmetal version of silylformylation with mechanistic study. This procedure 4

12

unveiled the presence of ( R S i ) R h ( C O ) - C o ( C O ) (38: η = 2 or 3) as one of the key active catalyst species for the reaction. 3

2

n

4

HSiR = a : HSiPhMe ; b : HSiMe Et; c : HSiEt ; d : HSi(OMe ) 3

2

2

3

3

3

We carried out the reactions of 1-hexyne with a variety of hydrosilanes in toluene at 25 °C and ambient pressure or 150 psi of carbon monoxide in the presence of C o R h ( C O ) (substrate:catalyst = 1000) for 24 h; Rh (CO) was also employed for comparison purposes. The results are summarized in Table V. 2

2

1 2

4

1 2

As Table V shows, the structure of hydrosilane exerts a marked influence on the selectivity of the reaction (i.e., silylformylation vs. hydrosilylation). Trimethoxysilane clearly favors hydrosilylation, whereas dimethylphenylsi lane gives silylformylation product exclusively and trialkylsilanes give ca. 40:60 mixture of the hydrosilylation and silylformylation products. At 25 °C and ambient pressure of carbon monoxide, the reaction catalyzed by R h ( C O ) i is substantially faster than that catalyzed by C o R h ( C O ) , al though the ratio of the hydrosilylation to silylformylation is very similar. In both cases, considerable amounts of heavies are formed. At 25 °C and 150 psi of carbon monoxide, C o R h ( C O ) acts as an excellent catalyst, giving the silylformylation product (36) with 93-100% selectivity (entries 2, 4, and 6) except for H S i ( O M e ) (entry 8). Formation of the heavies is also substan tially decreased. Under the same conditions, the Rh (CO) -catalyzed re actions give a larger amount of heavies compared with the C o R h ( C O ) catalyzed ones, and H S i ( O M e ) does not give any silylformylation product (36) (entry 16). As C o ( C O ) is found to be virtually inactive even under forced conditions, it is apparent that there is a synergistic effect in the C o - R h mixed-metal system. 4

2

2

2

2

2

1 2

1 2

3

4

12

2

2

1 2

3

2

8

The regio- and stereochemistry of both silylformylation and hydrosil ylation deserve mention. All silylformylation products (36) have (Z)-l-silyl-2-

19.

Carbonyfotions Catalyzed by Transition Metal Complexes 291

OJIMA E T AL.

Table V. Reactions of Hydrosilanes with 1-Hexyne Catalyzed by C o R h ( C O ) i 2 2

2

and R h ( C O ) i 2 in the Presence of C a r b o n Monoxide 4

Product Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Catalyst

Hydrosilane

Co Rh (CO)i2 2

2

Co Rh (CO), Co Rh (CO)i Co Rh (CO)i Co Rh (CO)i Co Rh (CO)i Co Rh (CO)i Co Rh (CO) Rh (CO) Rh (CO) Rh (CO)i Rh (CO) Rh4(CO) Rh (CO) Rh (CO) Rh (CO) 2

2

HSiMe Ph 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

HSiMe Et 2

HSiEt

3

HSi(OMe)

3

l2

4

12

4

l2

4

2

4

l2

4

l2

4

12

4

l2

12

HSiMe Ph 2

HSiMe Et 2

HSiEt

3

HSi(OMe)

3

Condition"

(%)

36

A Β A Β A Β A Β A Β A Β A Β A Β

72 92 48 84 63 80 86 95 72 76 61 80 65 77 76 80

100 100 60 100 58 93 20 38 100 100

37

— — 40

— 42 7 80 62

— —

65 91 65 86

— —

35 9 35 14 100 100

N O T E : All reactions were run with 5.0 mmol of 1-hexyne, 5.5 mmol of a hydrosilane, and 5 X 10" mmol of a catalyst in toluene (7.5 mL) at 25 °C for 24 h. "Condition A: Reaction was carried out at ambient pressure of carbon monoxide in a Schlenck reactor; Condition B: Reaction was carried out at 150 psi of carbon monoxide in a stainless steel autoclave using a Pyrex reaction vessel (50 mL). Yield was determined by G L C analysis based on 1-hexyne consumed. determined by G L C analysis. 3

fc

formyl structure (i.e., the reaction is extremely regioselective as well as stereoselective). All hydrosilylation products (37) turn out to be (E)-isomers exclusively. This result makes a sharp contrast to the reported rhodium complex-catalyzed hydrosilylation of 1-alkenes, which gives a mixture of (Z)isomer (major) and (E)-isomer (minor) (32-35). As for the mechanism of silylformylation, the following facts should be taken into account. Silylformylation must include • a silicon shift from catalyst metal to an alkyne to form a 2-silyl1-alkylethenyl-RhCo complex, • subsequent formation of 3-silyl-2-alkylacryloyl-RhCo complex, • formation of 3-silyl-2-alkylacryloyl-RhCo hydride, and • reductive elimination giving the silylformylation product (36). To obtain structural information about active catalyst species in the silylformylation, we looked at the reaction of hydrosilanes with C o R h ( C O ) (30). The tetrametallic cluster (deep reddish brown) was converted to a C o - R h mixed-bimetallic complex (pale yellow) during the reaction. The 2

2

1 2

292


reaction of C o R h ( C O ) with 1-alkynes in the absence of hydrosilane gave the corresponding purple butterfly cluster complexes, C o R h ( C O ) ( R - C = C H ) . The structure of a triphenylphosphine complex, C o R h ( C O ) ( n - B u C = C H ) ( P P h ) , was determined by X-ray crystallography (36). 2

2

1 2

2

2

2

1 0

2

9

3

When a hydrosilane (4 equiv) was allowed to react with C o R h ( C O ) under carbon monoxide atmosphere in organic solvent (e.g., chloroform-d and n-hexane) at ambient temperature for an hour, novel ( R S i ) R h ( C O ) C o ( C O ) complexes (n = 2, 38A; η = 3, 38B) were quantitatively formed. The structure of 38 is suggested by the following facts: 2

3

2

1 2

2

n

4

• the number of silyl groups on the rhodium metal was unam biguously determined to be 2 by titration, with ferrocene as the internal standard, • the Ή N M R spectra of 38 [R Si = (a) P h M e S i , or (c) Et Si] do not show any R h - H signals (δ 0 to -20 ppm) and the Fourier transform IR spectrum of 38c prepared from E t S i - D and C o R h ( C O ) does not show any R h - D stretching band at all in the expected region (1600-1450 cm" ), and 3

2

3

3

2

2

1 2

1

• the silyl groups were replaced by R - N C (R = f-Bu, cyclohexyl, and 2,6-dimethylphenyl) to form [ ( R - N C ) R h ] [ C o ( C O ) +

4

4

These results clearly indicate that 38 is generated through oxidative addition of two molecules of hydrosilanes to a rhodium, followed by evolution of molecular hydrogen. These disilyl-RhCo complexes (38) are stable in nhexane under carbon monoxide for more than a week at ambient tempera ture, but decompose slowly under nitrogen and/or in chloroform. We carried out the reaction of ( P h M e S i ) R h ( C O ) - C o ( C O ) (38a-A and 38a-B) with 1 equiv of 1-hexyne at 25 °C and ambient pressure of carbon monoxide for 30 min (eq 7) (30). 2

2

n

4

The * H N M R monitoring of the reaction clearly demonstrated the for mation of 36a (with 5% Ε-isomer) accompanied by a small amount of 38a-B (38a-A disappeared) and a mixed-metal butterfly complex, R h C o ( n Bu-C==C-H)(CO) (39) T h j reaction strongly suggests that 38a is an active catalyst species or its direct precursor. 2

10

2

s

Next, we looked at the reaction of C o R h ( C O ) with 1-hexyne under carbon monoxide. The reaction of C o R h ( C O ) with 2 equiv of 1-hexyne at 25 °C and 75 psi of carbon monoxide in hexane for 20 h gave a novel R h - C o mixed-bimetallic alkyne complex bearing semibridging carbonyl ( v , 1935 c m ) , R h C o ( n - B u - C = C - H ) ( C O ) (40), as the major product. The com plex was isolated through column chromatography on silica gel under carbon monoxide as a reddish-orange solid (30). Complex 40 was then allowed to 2

2

2

2

1 2

1 2

c o

1

5

19.

OJIMA E T AL.


HS1R3 + 38a-A

^ΆίΖΓ^ W W

0

38a-B

0

CO

R Si=PhMe Si 3

2

react with 6 equiv of dimethylphenylsilane and 4 equiv of 1-hexyne at 25 °C and ambient pressure of carbon monoxide in chloroform for 1 h to give cleanly the silylformylation product 36a. The Η N M R spectrum of the reaction mixture only shows 36a, 40, and unreacted hydrosilane (eq 8) (30). This result clearly indicates that 40 is an active catalyst species or its direct precursor. !

Accordingly, there are two possible catalytic cycles for silylformylation at present. Either one or both catalytic cycles may be operative, depending on the reaction conditions. Mechanisms for the silylformylation of 1-hexyne using C o R h ( C O ) as the catalyst precursor, which can accommodate all the observations described, are proposed in Scheme V (30). 2

2

1 2

The unique disilyl-RhCo complex (38) and alkyne-RhCo complex (40) are identified as the key catalytic species for silylformylation of 1-alkyne.

294


Although the insertion of carbonyl compounds or imines into silicon-transition metal bonds is known in rhodium complex-catalyzed hydrosilylations, that of alkynes and alkenes in catalysis has not been reported under thermal reaction conditions. Recently we found the selective insertion of 1-alkynes to the R h - S i bond in rhodium complex-catalyzed hydrosilylation (35). O u r results present the first examples of such a process in catalysis by taking advantage of the unique properties of the C o - R h mixed-bimetallic system. Also, the silylformylation of alkynes catalyzed by the C o - R h mixedmetal system is very likely to involve homogeneous bimetallic catalysis.

Acknowledgment This work has been supported by grants from National Science Foundation, National Institutes of Health, The Petroleum Research F u n d administered by American Chemical Society, and Mitsubishi Kasei Corporation.

19.

Carbonyhtions Catalyzed by Transition Metal Complexes 295

OJIMA E T AL.

2HSiR

(CO) (R3Si)2Rh-Co(CO)4

3

38 R'

Co Rh (CO)i2

• 1/2

n

2

2

1^ R'CaCH

H

>=
3

^co

J R Si

n

Cycle A

3

Y = R3Si m = l

OC-

Co—-Rh"

/

R h - SiR

3

/ X

,

„

>=

New Carbonylations Catalyzed by Transition Metal Complexes

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