Chiral Rhodium(II) Carboxamides - ACS Symposium Series (ACS

Chapter 4

Chiral Rhodium(II) Carboxamides Remarkably Effective Catalysts for Enantioselective Metal Carbene Transformations

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch004

Michael P. Doyle Department of Chemistry, Trinity University, San Antonio, TX 78212

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrrolidone-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene trans formations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubstituted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro gen insertion reactions of diazoacetate esters that result in substituted γ -butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabiliza tion of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement.

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S:) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B:) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. Transition metal catalysts that are effective for carbenoid transformations include those of copper(I), palladium(II) or platinum(II), cobalt(II), and rhodium(II) (7-5, 6-8), but only copper and rhodium catalysts have been routinely employed.

0097-6156/93/0517-0040$06.00/0 © 1993 American Chemical Society

In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

4. DOYLE

Chiral Rhodium (II) Carboxamides

41

Copper catalysts have historically been the most widely used, but many of their applications are limited by competing reactions (7). With rhodium catalysts, specific ally dirhodium(II) tetrakis(carboxylates), carbenoid transformations have undergone a renaissance in synthetic applications because of the high yields and selectivities that characterize their uses. Rhodium(II) carboxylates (1) are structurally well defined, having D2 symmetry (9-77), with axial coordination sites at which metal carbene formation occurs in reactions with diazo compounds (7). Two limiting structures portray the primary functional characteristics of these dirhodium(II) carbenes (Scheme Π): metal carbene 3 and the metal-stabilized carbocation 2, whose inherent stability arises from electron donation through the dirhodium(II) framework (7, 2). In fact, the metal-stabilized carbocation concept for these carbenes explains a broad selection of observations concerning their highly electrophilic character. A major advantage of dirhodium(II) catalysts for use in carbenoid reactions results from the electronic influences of their bridging ligands on reactivity and selectivity. In the series, Rh (pfb)4 (pfb = perfluorobutyrate), Rh2(OAc)4 (OAc = acetate), and Rh2(acam)4 (acam = acetamide), where electron withdrawal by the ligand increases from Rh2(acam)4 to Rh2(pfb)4, Rh2(pfb)4 exhibits the highest reactivity for both metal carbene formation from diazo compounds and metal carbene reactions, and Rh2(acam)4 has the highest selectivity (12,13). These electronic influences, which are consistent with the description of dirhodium(II) carbene complexes as metal stabilized carbocations, expand the utility of dirhodium(II) catalysts beyond that possible with other transition metal catalysts. Their applications for carbenoid reac tions are too numerous to list exhaustively, but several general processes are described in Figure 1. This chapter will focus on the development of chiral dirhodium(II) catalysts that are effective for three of the four transformations.


n

2

Chiral Catalysts The design and development of chiral catalysts for asymmetric carbenoid transforma tions have been actively pursued since the first reports by Nozaki of enantioselective cyclopropanation reactions with a copper(II) chelate complex possessing a chiral salicylaldimine ligand (14,15). Although optical yields of cyclopropane products using the Nozaki catalyst were low (6% ee in reactions of ethyl diazoacetate with styrene), elaboration of the design for this catalytic system by Aratini and coworkers (16-19) provided a copper(II) complex (A-Cu) with which enantioselectivities exceeding 90% ee could be achieved (Figure 2) in selected cases (syntheses of permethrin, cilastatin, and ira/w-chrysanthemic acid) (19-22). More recently, Pfaltz has reported high enantioselectivities for the cyclopro panation of monosubstituted alkenes and dienes with diazo carbonyl compounds using chiral (semicorrinato)copper complexes (P-Cu) (23-25), and Evans, Masamune, and Pfaltz subsequently discovered exceptional enantioselectivities in intermolecular cyclopropanation reactions with the analogous bis-oxazoline copper complexes (2628). With the exception of the chiral (camphorquinone dioximato)cobalt(II) catalysts (N-Co) reported by Nakamura and coworkers (29, 30), whose reactivities and selectivities differ considerably from copper catalysts, chiral complexes of metals other than copper have not exhibited similar promise for high optical yields in cyclopro panation reactions (57). The approach that we have taken for the design of chiral rhodium(II) catalysts is based on the selectivity obtained in the preparation of geometric isomers with a limited number of rhodium(II) carboxamides. Although four different orientations of amide


SELECTIVITY IN CATALYSIS

SCR


UM-B ^

LJVI

UM=CR

k

Scheme I:

2

. A

UM-CR

2

N

2

Catalytic Cycle for Metal Carbene Transformations

H

Ç 3 O N Rh^-Rh^

Rh (acam)4 2

2

CH O

3

O

Rh^Rh^

Rh2(OAc)4

C3F7 O

O

Rh-Rh

Rh2(pfb)4


4. DOYLE

43

Chiral Rhodium(II) Carboxamides

INSERTION ZCHXOY CYCLOPROPANATION ZH

U M + N =CHCOY


2

τ

L M=CHCOY N

RZ:

+

"COY

_

RZ-CHCOY

CYCLOPROPENATION

YLIDE G E N E R A T I O N

Figure 1. Catalytic Metal Carr^ne Transformations

H

ÇH

3

R =

*-Bu

CN

CN

P-Cu

N-Co



Me

Me

Me

+

+

COOR

N^^COOEt

^ \

NÎ^COOEt

N,

(R)-4

(S)-4

•

Me

M

. / - —

92% ce

λ Me

Me

Me

Me

COOEt

^COOEt

-COOR

EtOH *

KOH

Cl

Cl

COOH pcnncthrinic acid

Me

Me

R = Et : 68% ce (trans), 62% ec (cis) R = f-Bu: 75% ee (trans), 46% ec (cis) R = /-Menthyl: 94% ce (trans), 46% ce (cis)

Figure 2. Representative Applications of the Aratani Catalysts for Intermolecular Cyclopropanation

Me

Me

Me

+

(R)-4



4. DOYLE

45

Chiral Rhodium (II) Carboxamides

ligands on one face of the dirhodium tetrakis(carboxamide) are conceptually possible, only one (4) has been isolated with acetamide or trifluoroacetamide as the bridging ligand (32,33). With chiral amides whose asymmetric center is adjacent to nitrogen, the construction of rhodium(II) carboxamides allows placement of a chiral attachment at nitrogen close to the reactive carbene center. When the bridging amide ligands are positioned such that each rhodium has a pair of nitrogen donor atoms in a cis arrangement, the resulting rhodium(II) carboxamide presents an asymmetric environment for the metal stabilized carbene that can be expected to provide substantial facial selectivity in carbenoid transformations. Chiral catalysts of copper and cobalt have been constructed to effect enantiocontrol in cyclopropanation reactions by limiting access to the carbenoid carbon through steric interactions enforced by substituents of the chiral ligand in close proximity to the reacting carbene-alkene (19,25,30). Based on this rationale we prepared a series of chiral dirhodium(II) 4-alkyloxazolidinones, 5-8 (IPOX = isopropyloxazolidinone, B N O X = benzyloxazolidinone [6 = 4S and 7 = 4R], M P O X = methyl-5-phenyloxazolidinone) in which the chiral center is adjacent to the rhodiumbound nitrogen, and with which only steric interactions emanating from the protruding alkyl substituent should control enantioselectivity. The capabilities of 5-8 for enantioselective cyclopropanation were determined (34) from reactions at room temperature of d~ and/or /-menthyl diazoacetate (MDA) with styrene (Table 1), which allows direct comparison with results from both the Aratani (A-Cu) and Pfaltz (P-Cu) catalysts (19,24). Cyclopropane product yields ranged from 50 to 75%, which were comparable to those obtained with chiral copper catalysts, but enantiomeric excesses were considerably less than those reported from use of either P - C u or A - C u . Furthermore, these reactions were subject to exceptional double diastereoselectivity not previously seen to the same degree with the chiral copper catalysts. Although chiral oxazolidinone ligands proved to be promising, the data in Table 1 suggested that steric interactions alone would not sufficiently enhance enantioselectivities to advance RI12L4 as an alternative to A - C u or P-Cu. An alternative approach to the design of RI12L4 presented itself in the form of readily accessible chiral 2-pyrrolidone-5-carboxylate ligands (9) in which the carboxylate carbonyl group can be viewed to reside above or perpendicular to the "empty" ρ orbital of the electrophilic carbene. Electronic interaction of the ligand's carboxylate carbonyl group with the carbene's ρ orbital could be expected to stabilize the bound carbene, orient the carbene sub stituents into two limiting configurations (10a and 10b), of which one (10a) is more stable, and direct incoming nucleophiles such as alkenes to backside attack on the side of the carbene opposite to the stabilizing carbonyl. An indication of the success of this approach to the design of RI12L4 can be seen from the enantioselectivities achieved with the use of Rh2(5S-MEPY)4 at 1.0 mol % in refluxing C H C l 2 (Table 2). Product yields were similar to those with the chiral dirhodium(II) oxazolidinones, and optical purities similar to those of the Aratani catalysts were obtained (34). The enantiomeric R h 2 ( 5 R - M E P Y ) 4 , which is also readily accessible, gave identical % ee's but opposite chirality in cyclopropanation reactions, and reversed diastereomeric preferences were obtained from reactions with M D A . Since the trans isomer has C 2 symmetry, the geometry of the R h ( M E P Y ) 4 catalysts could be established by N M R spectroscopy to be that of the cw-isomer 9. 2

2

Intramolecular Cyclopropanation with C h i r a l Rhodium(II) 2-Pyrrolidone-5-carboxylates. Applications of chiral copper and cobalt catalysts, including



46 SELECTIVITY IN CATALYSIS


4.

Table

47

Chiral Rhodium(II) Carboxamides

DOYLE

1. Diastereoselectivities for the Cyclopropanation of Styrene by Menthyl


Diazoacetate with Chiral Rhodium(II) Oxazolidinone Catalysts

MDA catalyst

MDA

P-Cu

trans :cis

% de trans

% de cis

/

85:15

91 (1S.2S)

90 (1S.2R)

d

82:18

97 (1S.2S)

95 (1S.2R)

Rh (OAc)

4

1

68:32

5 (1R.2R)

13 (1R.2S)

Rh (OAc)

4

d

55:45

9 (1S.2S)

13 (1S.2R)

34 (1R.2R)

56 (1R.2S)

P-Cu 2

2

Rh (4S-IPOX) 2

Rh (4S-IPOX) 2

4

1

70:30

4

d

75:25

Rh (4S-BNOX)

4

1

61:39

Rh (4S-BNOX)

4

d

63:37

2

2

2 (1 R.2R)

4 (1R.2S)

34 (1R.2R)

63 (1R.2S)

4 (1 R.2R)

24 (1R.2S)

Rh (4R-BNOX)

4

1

62:38

4 (1S.2S)

25 (1S.2R)

Rh (4R-BNOX)

4

d

67:33

30 (1S.2S)

60 (1S.2R)

Rh (4R-MPOX)

4

1

71:29

Rh (4R-MPOX)

4

d

77:23

2

2

2

2

Rh (4S-IPOX) 2

4

Rh (4S-BNOX) 2

4 (1 R.2R)

4 (1R.2S)

23 (1R.2R)

4

Rh (4R-MPOX) 2

20 (1S.2R)

4


SELECTIVITY IN CATALYSIS


48

A - C u , N - C o , and P - C u to carbenoid transformations have been limited to intermolecular reactions, for which they remain superior to chiral dirhodium(II) catalysts for intermolecular cyclopropanation reactions. Few examples other than those recently reported by Dauben and coworkers (eq 1) (35) portray the effectiveness of these chiral catalysts for enantioselective intramolecular cyclopropanation reactions, and these examples demonstrate their limitations. However, with Rh2(5S-MEPY)4 intramolecular cyclopropanation of 3-methyl-2-buten-l-yl diazoacetate (eq 2) occurs in high yield and with 92% enantiomeric excess (36). Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioselective intramolecular cyclopropanation, even though the steric bulk of their chiral ligand attachments (COOMe versus /-Pr or C l ^ P h ) are similar. Signifi cantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either R h ( 4 S - I P O X ) , R h ( 4 S - B N O X ) , or R h ( 4 R - B N O X ) (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme Π), was of considerable importance in reactions performed with 5-7 but was only a minor constituent (< 1%) from reactions catalyzed by Rh (5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the carbocation form of the intermediate metal carbene. The directional orientation of chiral ligand substituents on Rh2(5S-MEPY)4 establishes relatively unimpeded pathways for intramolecular cyclization whose preferred route is determined from the relative stability of two limiting configurations (13a and 13b). According to this model Z-olefins should afford higher enantioselec tivities than do Ε-olefins. The exceptional capabilities of R h 2 ( 5 S - M E P Y ) 4 and R h 2 ( 5 R - M E P Y ) for asymmetric induction in intramolecular cyclopropanation reactions are evident in the results obtained with a series of allyl diazoacetates (eq 3). The % ee's determined for 14b-e by the use of chiral N M R shift reagent showed none of the corresponding enantiomers; since the limit of detection by this method is generally considered to be 97:3, the % ee's are denoted as > 94 even though only one enantiomer was detected. Identical yields and % ee's of these products were obtained using Rh2(5R-MEPY)4 but, of course, having opposite product chirality. 2

4

2

4

2

4

2

4

Intermolecular Cyclopropenation of Alkynes. Functionalized cyclopropenes are viable synthetic intermediates whose applications (37,38), which extend to a wide variety of carbocyclic and heterocyclic systems, have been largely ignored because of the relative inaccessibility of these strained compounds. However, recent advances in the synthesis of cyclopropenes, particularly from rhodium(II) carboxylate catalyzed decomposition of diazo esters in the presence of alkynes (39-42), has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes (43,44), but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (45-48). Potential uses of the cyclopropene ring as a template in enantiocontrolled synthesis have been recognized, but until now synthetic chiral cyclopropene derivatives have been accessible only through resolution (49). Chiral rhodium(II) carboxamides are exceptional catalysts for highly enantio selective intermolecular cyclopropenation reactions (50). With ethyl diazoacetate and a series of alkynes, use of dirhodium(II) tetrakis[methyl 2-pyrrolidone-5-(/?)carboxylate], Rh (5/?-MEPY)4, in catalytic amounts (

Chiral Rhodium(II) Carboxamides - ACS Symposium Series (ACS

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