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4 Group 11 Metals (Au) and Early Transition Metals

4 Group 11 Metals (Au) and Early Transition Metals

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252



J.-i. Ito and H. Nishiyama



Ph2P

O



AuCl



O



iPr



N



O

N



Li



+



N



iPr



Ph2P



Au

N



iPr



O



iPr

ClAu



32



31

iPr iPr



O

N



2



PPh2



MCl3X



O



MCl3X



N

X



MCl2



X



Cl2M



N

O



O



N

iPr iPr



O

N



O



iPr



34a: M = Zr X = Cl (82%)

34b: M = Hf, X = Cl (83%)

34c: M = Nb, X = O (76%)



M

Cl2X



N

iPr



33a: M = Ti, X = Cl (73%)

33b:M = V, X = Cl (82%)

33c:M = Cr, X = Py (75%)



Scheme 11 Synthesis of Au and early transition metal complexes



˚ ) and angles

Table 1 Bond distances (A

1

2

3

1

R , R2, R3

M-C

M

L,L,L

Ni

I

Me, Me, H

1.859(4)

Ni

Br

iPr, H, H

1.8491(19)

Pd

Cl

iPr, H, H

1.92(1)

Pt

Cl

iPr, H, H

1.928(10)

Pt

Cl, Cl, Cl

tBu, H, H

1.96(1)

Rh

H2O, Cl, Cl

Bn, H, H

1.921(7)

Me, Me, Me

1.930(3)

Ir

H2O, Cl, Cl

Fe

CO, Br,CO

Me, Me, H

1.930(2)

Ru

acac, CO

Ph, H, Me

1.9635(18)

iPr, H, H

2.060(2)

Au

PPh3

Ti

Cl, Cl, OiPr

iPr, H, H

2.141(3)

V

Cl, Cl, Cl

iPr, H, H

2.0906(17)

Cr

py, Cl, Cl

iPr, H, H

1.9973(19)

R3

O



M-N (av.)

1.975

1.909

2.053

2.034

2.065

2.079

2.062

2.009

2.095



2.184

2.093

2.109

R3



C



N

M

R2

R1 (L3)

L1



O

(L2)

N



R2

R1



N-M-N

161.04(13)

162.20(8)

159.6(5)

158.6(5)

159.3(3)

158.7(2)

158.52(12)

157.42(7)

156.37(6)



145.73(10)

148.26(6)

154.54(6)



References

[37]

[38]

[35]

[35]

[35]

[26]

[30]

[44]

[40]

[46]

[46]

[46]

[46]



Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts



a



b



c



O



Cl



O



C

C



N



Ir



O



N

O



N



C

C

Ru N

O



O



Cl



253



N

C



Fe



N



Br



Fig. 4 Molecular structures of (phebox-dm)IrCl2(H2O) (a), (phebox-ip)Ru(acac)(CO) (b), and

(phebox-dm)FeBr(CO)2 (c)



O



O

N



N

Li



N



Li



O



O

N



N



O



O



N

SnMe3

36



35



Fig. 5 (Phebox)Li and (phebox)SnMe3



˚ and 1.90–2.18 A

˚ , respectively,

The M–C and M–N bond distances are 1.84–2.14 A



and the N–M–N bond angles are 145.7–162.2 (Table 1). The Ni and Fe complexes

have the tendency to have shorter M–C and M–N bond lengths. On the other hand,

early transition metal complexes, such as Ti and V, exhibit longer M–C bond

lengths.

The molecular structures of (phebox)Li 35 and (phebox)SnMe3 36 were also

determined by X-ray analysis (Fig. 5) [47]. The (phebox)Li complex 35 has a dimer

˚ . In the (phebox)Sn

structure, with Li–C bond distances of 2.268(3)–2.299(3) A

complex 36, there is no interaction between the Sn atom and the N atoms. The Sn–C

˚ is longer than those of other transition metal

bond distance of 2.1810(18) A

complexes.



3 Organometallic Reactions of (Phebox)Rh Complexes

3.1



Reactions with Arenes



The (phebox)Rh(III) complexes exhibited reactivity for the C–H and C–Cl bond

cleavage of arenes. The reaction of the acetate complex 37 with aromatic

compounds, such as benzene, toluene, chlorobenzene, acetophenone, and anisole,



254



J.-i. Ito and H. Nishiyama

Cl

O



O

N



X

Rh



O



N



X

OH2

37 (X = OAc)

38 (X = Cl)



O

N

Rh

O

O



120 °C, 50 h

80%



O

+



N



O

N

Rh

O

O



N

Cl



(X = OAc)

Cl



39a



tBu



39a:39b = 1:2.2



39b



tBu

Cl



O



O

N



Cl

Rh

Cl

OH2

40



N



R

NH(iPr)2

80 °C, MS 4Å



O



O

N



Cl

Rh



N



H2O

R

41 (R = H, 90%)

42 (R = Me, 80%)



Scheme 12 C–H and C–Cl bond cleavage by (phebox)Rh complexes



afforded the corresponding aryl complexes via C–H bond cleavage (Scheme 12)

[48]. For example, C–H bond activation of chlorobenzene resulted in the formation

of the p- and m-substituted chlorophenyl complexes 39a and 39b in the ratio of 1:2.2.

Although there are several mechanisms for C–H bond activation, the reaction with

the Rh acetate complex 37 was proposed as a base promoted C–H bond activation. In

contrast, the chloro analogue 38 did not react with chlorobenzene. This result

supported the premise that the acetate ligand was required for C–H bond cleavage.

The (phebox)Rh complex was also found to undergo oxidative addition to the

C–Cl bond. When the chloro complex 40 was treated with chlorobenzene in the

presence of NH(iPr)2 at 80  C, phenyl complex 41 was obtained in 90 % yield

accompanied by N(¼CMe2)(iPr) [49]. Generally, oxidative addition proceeds via a

Rh(I) intermediate. The inactive (phebox)Rh(III) complex may be reduced by NH

(iPr)2, which serves as a reductant. The preference between C–H and C–Cl bond

cleavage of chlorobenzene can be tuned using different reaction conditions.



3.2



C–C Bond Formation Reactions



The (phebox)Rh complex can undergo a unique C–C bond formation reaction

(Scheme 13) [50]. The reaction of 43 with dichloromethane and diisopropylamine

at 50  C resulted in formation of an azarhodacyclopentene complex 44,

accompanied by a small amount of the diisopropylamine Rh complex 45 in the

ratio of 96:4. This reaction is formally described as a C–C bond formation via C–Cl

bond cleavage of CH2Cl2 and C–H bond activation at the b-carbon of



Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts

tBu



tBu



O



O

N



Cl



Rh



N



tBu



O



CH2Cl2



O

N



NH(iPr)2



Cl

OH 2

43



Cl



O

+



N



Rh



255



O

N



Cl



Rh



N



Cl NH2iPr

45



iPr N

44



Scheme 13 C–C bond formation on the (phebox)Rh complex



O



O

AcO



N

iPr



Rh



PhCCH



O



N



OAc

OH2

8



iPr



iPr



O

N

O Rh



ECCE



O



N

iPr



O

46



iPr

E = CO2Me



O

N

O Rh



N

E



O



Ph



iPr



E

Ph



47



PhCCH

E

E

48



Ph



Scheme 14 Reaction of the (phebox)Rh complex with alkynes



isopropylamine. The proposed reaction mechanism involves (1) reduction to a Rh

(I) intermediate induced by oxidation of diisopropylamine to N-isopropylpropylideneamine, (2) oxidative addition of CH2Cl2 to the Rh(I) intermediate, (3)

tautomerization to the enamine form, and (4) cyclization.

Activation of a terminal alkyne with a transition metal complex gives an

acetylide complex, which serves as a key intermediate for alkynylation reactions

such as dimerization. The acetate complex 8 reacted with phenylacetylene at 60  C

to give the acetylide complex 46 (Scheme 14) [51]. The subsequent reaction of 46

with dimethyl acetylenedicarboxylate at room temperature readily afforded the

corresponding vinyl complex 47 via insertion of the second alkyne into

the Rh–acetylide bond. Finally, the reaction of 47 with phenylacetylene afforded

the enyne 48 with regeneration of the acetylide complex 46.

The formation of the enyne 48 suggests that the cross-coupling reaction of two

different alkynes may be achieved. Such a catalytic cross-coupling reaction is a

versatile method for the preparation of a variety of substituted enynes. Reaction of

phenylacetylene with dimethyl acetylenedicarboxylate was catalyzed by 1 mol% of

the acetate complex 8-ip at 100  C to give the enyne 48 in 85 % yield with high Zselectivity (Scheme 15). Interestingly, this catalytic transformation was accelerated

under 1 atm of H2 atmosphere.



256



J.-i. Ito and H. Nishiyama

(phebox-iPr)Rh (8-ip)

(1 mol%)

H2 (1 atm)

Ph



H



+



MeO2C



CO 2Me



CO 2Me

Ph



toluene

100 °C, 4 h



CO2Me

48

85%

(Z :E =

98:2)



Scheme 15 Cross-coupling of alkynes catalyzed by (phebox)Rh acetate complex

C

N

O



O

N



Bn



Cl



Rh

Cl

OH2

1



O

CO 2Me



O



N



O

N



Bn



Cl



N



Rh



Bn

C

N



Cl



H



O



O

N



tBuOK

Bn



Cl



Rh



Bn



N

Cl

NH



O

5S



CO 2Me

49



t Bu



Bn



4R



tBu



CO2Me

50



Scheme 16 Reaction of (phebox)Rh complex with isocyanoacetate and aldehydes



The (phebox)Rh complex reacted with isocyanoacetate to afford the isocyanide

complex 49 in 99 % yield (Scheme 16) [52]. The complex 49 could then undergo an

aldol type reaction with an aldehyde in the presence of a base to give the chiral

Fisher carbene complex 50 with high dr and trans-selectivity. X-ray analysis of the

major isomer revealed the (4R,5S) absolute configuration of the carbene fragment.

The proposed mechanism of the aldol reaction involves an enolate intermediate,

generated by deprotonation of 49, which reacts with an aldehyde, followed by

successive cyclization and protonation to form the carbene complex 50. The re-face

attack of the enolate constructs the observed (4R,5S) absolute configuration.



3.3



Molecular Recognition



The C2-symmetric cavity of the (phebox)Rh complex can provide a suitable

environment for molecular recognition of organic substrates. The acetate ligand

of the (phebox)Rh complex 4 underwent ligand-exchange reactions with 2,20 biphenol and 1,10 -bi(2-naphthol) derivatives (Scheme 17) [53]. When 2,20 -biphenol

was subjected to the ligand-exchange reaction, the corresponding biphenolate

complex was formed. X-ray analysis showed that the absolute configuration was

S. Furthermore, complex 4 could be used in the kinetic resolution of racemic 1,10 -bi

(2-naphthol) (rac-52). Reaction of 4 (1.1 eq) with rac-52 (2 eq) produced a

binaphtholate complex 53 in 91 % yield along with (R)-52 (73 % ee). Finally, the

complex 53 afforded (S)-52 with 93 % ee.



Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts



O



4



+



HO

HO



CH2Cl2

40 °C, 24 h



257



i Pr



N

AcO H

Rh O

O

N

O

iPr



S



51

44%

O

4

+

1.1 equiv



HO

HO



K2CO3

toluene

50 °C, 24 h



r ac -52

2.0 equiv



iPr



N

AcO H

Rh O

O

N

O

iPr



S



+



(R)-52

50%, 73% ee



53

91%



(S )-52

93% ee



Scheme 17 Reactions with biphenol and binaphthol



4 Asymmetric Catalytic Reactions with (Phebox)Rh Complexes

4.1



Lewis Acid Catalysis



Dissociation of the H2O ligand in the (phebox)Rh complex 1 generates a vacant site

on the Rh center, which can accept a Lewis base such as pyridine or acetone.

Complex 1 was used as a traditional Lewis acid catalyst in asymmetric C–C bond

formation reactions. Complex 1 was found to be an efficient catalyst for allylation

of aromatic aldehydes with allyltributyltin, giving the corresponding homoallylic

alcohols in good yields with 61–80 % ee (Scheme 18) [26, 27]. The reaction with

methallyltributyltin furnished the desired products with high enantiomeric excesses

(up to 93 % ee) [54]. Complex 1 also serves as a mild Lewis acid catalyst for

asymmetric hetero Diels–Alder reactions between Danishefsky’s dienes and

glyoxylate derivatives (Scheme 19) [29]. In this reaction, high enantioselectivity

and cis-(endo)-diastereoselectivity were observed. It was proposed that the catalytic

reaction could proceed via a concerted [4 + 2] mechanism rather than the stepwise

Mukaiyama-aldol mechanism. The (phebox)Rh system was also applied to the

asymmetric Michael addition of a-cyanopropionates to acrolein (Scheme 20)

[55]. The (phebox)Rh catalyst was prepared in situ by treatment of [RhCl(coe)2]2

with (phebox)SnMe3 (2). In this catalytic system, (phebox)RhCl(SnMe3) was

considered to be the active Lewis acid catalyst.



258



J.-i. Ito and H. Nishiyama

O

H



+



OH R2



phebox Rh (1)

(5 mol%)



R2

SnBu3



R1



CH2Cl2, rt

7 - 12 h

MS 4Å



R1



53a: R1 = OMe, R2 = H; 99%, 80% ee (cat. 1-bn)

53b: R1 = H, R2 = Me; 95%, 93% ee (cat. 1-ip)



Scheme 18 Asymmetric allylation of aldehydes



OMe

R1



O

+



H



TBSO



(i) phebox Rh (1-bn)

(2 mol%)

toluene, 1 h, MS 4Å



R1



(ii) TFA



O



CO2Bu



R2



O

CO 2Bu

R2



54a: R1 = R2 = H; 90%, 80% ee

54b: R1 = R2 = Me; 67%, 83% ee, dr = 93:7



Scheme 19 Asymmetric hetero Diels–Alder reaction of Danishefsky’s diene



Me

H



+



NC



(phebox)SnMe3 (2-tb)

(2 mol%)

[RhCl(coe)2]2 (1 mol%)



CO2CHtBu2



NC Me

H



toluene, 25 °C



O



CO2CHtBu2

O



55

82%, 86% ee



Scheme 20 Asymmetric Michael reaction



phebox Rh (1-ip) (1 mol%)

AgBF4 (2 mol%)

Ph



+ (EtO)2SiMeH



Si

+



Ph

toluene, 50 °C, 12 h



56



Si



Ph

57



83%, 56:57 =

55:45, 94% ee



Scheme 21 Asymmetric hydrosilylation of alkenes



4.2



Hydrosilylation and Conjugate Reduction



The (phebox)Rh complex acts as an efficient catalyst for activation of hydrosilanes.

Asymmetric hydrosilylation of aromatic alkenes proceeded in the presence of the

(phebox)RhCl2(H2O) complex 1 (Scheme 21) [56]. Although diastereoselectivity

between the a- and b-positions was moderate, high enantioselectivity of a-silylated

compound 56 was obtained. The proposed mechanism was suggested to be the

conventional Chalk–Harrod mechanism. This (phebox)Rh catalytic system can also

be applied to the conjugate reduction of a,b-unsaturated carbonyl compounds



Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts

R

CO 2Et



Ph



phebox Rh (4-ip) (1 mol%)

(EtO) 2MeSiH (1.5 eq)



R

COMe



Ph



Ph



CHO



CO 2Et



R

COMe



Ph



(2)



59

R = Me; 97%, 95% ee

R = iPr; 97%, 98% ee (60 °C)



phebox Rh (4) (1 mol%)

(EtO) 2MeSiH (1.5 eq)



R



R

CHO



Ph

toluene, 30-60 °C, 1 h



(1)



58

R = Me; 96%, 96% ee

R = iPr; 97%, 98% ee



phebox Rh (4-ip) (1 mol%)

(EtO) 2MeSiH (1.5 eq)

toluene, rt, 1 h



R



R

Ph



toluene, 60 °C, 1 h



259



+ Ph



60



R = H (cat. 4-tb); 98%

R = Me (cat. 4-ip); 51%, 91% ee



OH



(3)



61

0%

47%



Scheme 22 Asymmetric conjugate reduction of a,b-unsaturated carbonyl compounds



(Scheme 22). In this reaction, the acetate complexes, (phebox)Rh(OAc)2(H2O) (4),

were found to be efficient catalysts. The conjugate reduction of a,b-unsaturated

esters in the presence of 4 and hydrosilane (EtO)2MeSiH proceeded smoothly at

30–60  C for 1 h to give dihydrocinnamate derivatives 58 in high yields with

excellent enantiomeric excesses [Scheme 22 (1)] [28, 57]. b,b-Alkyl substituted

a,b-unsaturated esters were also reduced by the (phebox)Rh complex 4 and

hydrosilane in high yield with high ee. Similarly, complex 4 effectively catalyzed

conjugate reduction of a,b-unsaturated ketones [Scheme 22 (2)]. Use of alkoxyhydrosilanes, such as (EtO)2MeSiH, showed excellent 1,4-selectivity, while use of

other hydrosilanes, such as Et2MeSiH, Me2PhSiH, and Ph2SiH2, caused side

reactions such as the 1,2-reduction of the carbonyl group to give an allylic alcohol.

The conjugate reduction of a,b-unsaturated aldehydes has an inherent problem

in selectivity between 1,2- and 1,4-reduction. When complex 4-tb was subjected to

the conjugate reduction of cinnamaldehyde, 1,4-reduction proceeded exclusively to

afford dihydrocinnamaldehyde in 98 % yield [Scheme 22 (3)] [58]. The formation

of cinnamyl alcohol arising from 1,2-reduction was not detected. In this reaction,

selectivity between 1,2- and 1,4-reduction was significantly influenced by the

substituents on the oxazoline rings and the hydrosilanes. Furthermore, asymmetric

reaction was investigated. Complex 4-ip was used in the asymmetric conjugate

reduction of (E)-b-methylcinnamaldehyde to give the desired product with 91 % ee.

However, there is still room for improvement of the selectivity in 1,4-reductions.



4.3



Reductive Aldol Reactions



As described earlier, the (phebox)Rh acetate complex 4 acts as a highly efficient

catalyst for asymmetric conjugate reduction of b,b-disubstituted a,b-unsaturated



260



J.-i. Ito and H. Nishiyama



carbonyl compounds with hydrosilane as a reducing agent, giving a variety of

optically active carbonyl compounds containing a chiral center at the b-position

[28, 57, 58]. These results implied that the Rh enolate species formed via 1,4reduction with a Rh-H species was a key intermediate (Scheme 23) [59]. Consequently, trapping the Rh-enolate by an electrophile, such as an aldehyde or ketone,

could lead to a reductive aldol type coupling reaction.

The catalytic reductive aldol coupling of benzaldehyde and tert-butyl acrylate

with hydrosilane was successfully catalyzed by 4-bn (1 mol%) to furnish the

anti-coupling product 62 in high yield with high ee (Scheme 24) [60]. Both

alkoxyhydrosilanes and alkylhydrosilanes appeared to be suitable hydride sources.

Aromatic and aliphatic aldehydes could be used as electrophiles. Construction of the

2R,3S-absolute configuration of 62 was explained by the chair-like

Zimmerman–Traxler-type transition state (Scheme 25). The attack of the si-face of

the Rh (E)-enolate to the si-face of the coordinated aldehyde predominantly afforded

the major product 62. This proposed transition state was supported by density

functional theory calculations (B3LPY) conducted by Wu and co-workers [61].

The complex 4-ph also catalyzed the coupling reaction of cyclopent-2-enone

and benzaldehyde with hydrosilane to give the corresponding anti-bhydroxyketone 64 with high ee (Scheme 26) [62]. Reductive aldol reaction of

a,b-unsaturated esters with ketones as acceptors gave ester derivatives bearing btertiary alcohols. Furthermore, complex 4-ip was used in the coupling reaction of

ethyl cinnamate and acetone with MePh2SiH to yield the aldol product 65 in 83 %

yield with 97 % ee (Scheme 27) [63]. Such reductive aldol reactions offer a

convenient way to construct a,b,g-stereotriads (Scheme 28) [64]. The coupling

reactions of (S)-2-phenylpropanal (S)-66 with tert-butyl acrylate in the presence of

the achiral (phebox)Rh acetate complex 37 predominantly afforded the Felkin–Anh

product 67 with (2R,3R,4S)-configuration accompanied by the anti-Felkin–Anh

product 68 as a minor diastereomer. The use of the chiral (S,S)-(phebox-iPr)Rh

complex 4-ip for the reaction with (S)-2-phenylpropanal (S)-66 furnished the

Felkin–Anh product 67 with high ee and dr, whereas the reaction of (R)-2phenylpropanal (R)-66 afforded the anti-Felkin–Anh product 680 (enantiomer) as

the major diastereomer with moderate enantioselectivity. These results indicated

that a combination of (S)-2-phenylpropanal (S)-66 with 4-ip was a matched pair.

a,b-Stereochemistry in the aldol product was constructed by the catalyst-controlled

reaction via a transition state similar to that described in Scheme 25.



4.4



Direct Aldol Reactions



The (phebox)Rh complex serves as an acid–base bifunctional catalyst, in which the

Rh center acts as a Lewis acid site and the acetate ligand acts as a Brønsted basic

site. This synergistic interaction might promote a direct asymmetric aldol reaction

[65]. Indeed, the acetate complex 4-ip catalyzed the direct asymmetric aldol

reaction of cyclic and acyclic ketones with benzaldehyde derivatives, giving

anti-b-hydroxyketone derivatives 69–70 (Scheme 29) [66]. In this catalytic system,



Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multipotent Catalysts

O



Rh

O



O



Rh-H

X



R



Rh

R



OH O



O



O



H



X

CH3

Rh-enolate



1,4-reduction



261



R



X



H



X

CH3



CH3



Scheme 23 Reductive aldol reaction



Ph



(phebox-Bn)Rh (4-bn)

(1 mol%)

(EtO)2MeSiH (1.6 eq)



O



O

H



+



OtBu



OH O

3S



toluene, 60 °C, 1 h

then H3O+



OH O



2R



Ph



Ot Bu



3R



+



Ot Bu



Me

62

ant i



98%; anti:syn = 98:2

94% ee (anti); 57% ee (syn)



2R



Ph

Me

63

syn



Scheme 24 Asymmetric reductive aldol reaction of aldehydes

Ot Bu

Si



OO

N

Rh



O



N

R



Me



H

O



SiR 3



R

H

Si



OH O

3S



2R



Ph



OtBu

Me

62



Ph



Scheme 25 Proposed mechanism



O



(phebox-Ph)Rh (4-ph) (1 mol%)

MePh2SiH (1.7 eq)



O

+

Ph



toluene, 50 °C, 0.5 h

then H 3O +



H



OH

1'R



Ph



O



2S



64

80%, anti:syn = 95:5

85% ee (ant i)



Scheme 26 Asymmetric reductive aldol reaction of aldehydes



Me



(phebox-iPr)Rh (4-i p) (1 mol%)

Ph2MeSiH (1.3 eq)



O



O

Me



+ Ph



OEt



50 °C, 0.5 h

then H 3O +



Me

Me



OH O

2R



65

83% yield,

97% ee



Scheme 27 Asymmetric reductive aldol reaction of ketones



O

E

Pht



262



J.-i. Ito and H. Nishiyama



O

Ph



(phebox)Rh

(1 mol%)

MePh2SiH



O



H

Me

66

r ac, S, R



+



OtBu



OH O

Ph



toluene

50 °C, 0.5 h

then H3O+



OH O

OtBu



Me



+



Ph



Me



OtBu

Me



67

Felkin-Anh



Me



68

anti-Felkin-Anh

+ other diastereomers



Aldehyde (66)



Cat.



S (92% ee)

r ac

S (92% ee)

R (95% ee)



achiral (37)

S,S (4-ip)

S,S (4-ip)

S,S (4-ip)

a



Yield (%)

84

70

83

75



dr

a



ee (%)



b



93

76

99

71



68 :20 :10:2

55a:35b: 8:2

88a: 4b: 7:1

20a:65b:14:1



(2R,3R,4S )a

(2R,3R,4S )a

(2R,3R,4S )a

(2R,3R,4R )b



Felkin-Anh product 67, b anti-Felkin-Anh product 68



Scheme 28 Asymmetric reductive aldol reactions for construction of a,b,g-stereotriads

(phebox)Rh (4-ip) (5 mol%)

AgOTf (5-10 mol%)



O



O

+ H

n = 0, 1



O



OH



toluene, 60 °C, 72 h

NO2



n = 0, 1



NO2



69: n=1; 79%; dr = 87:13

87% ee (anti)

70: n=0; 75%, dr = 80:20

91% ee (anti)



Scheme 29 Asymmetric direct aldol reaction of ketones



addition of AgOTf significantly improved both diastereoselectivity and

enantioselectivity.

Direct aldol reactions with enone derivatives in place of ketones provide

vinylogous unsaturated aldol products, which are useful components for organic

synthesis. The acetate complex 4-ip was used in the aldol reaction of 2cyclohexenone with p-nitrobenzaldehyde, giving the coupling product 71

(Scheme 30) [67]. Addition of AgOTf was required for achievement of high

diastereo- and enantioselectivity. However, the product yield was not sufficient,

likely due to a competing retro-aldol reaction. To solve this problem, in situ

trapping of the aldol product by addition of acetic anhydride was conducted.

Gratifyingly, this reaction proceeded to furnish the acetylated product 72 in 80 %

yield without loss of ee and dr. It is noted that the reaction of 71 with Ac2O in the

presence of 4-ip and AgOTf gave only 10 % of the acetylated compound 72 and

90 % of recovered 71. This control experiment indicated that most of the acetylation proceeded via the in situ formed Rh-aldolate species.



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4 Group 11 Metals (Au) and Early Transition Metals

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