4.16 Enantioselective Cyanation of Carbonyls and Imines

4.16 Enantioselective Cyanation of Carbonyls and Imines M North, University of Newcastle, Newcastle upon Tyne, UK r 2012 Elsevier Ltd. All rights reserved.

4.16.1 4.16.2 4.16.3 4.16.3.1 4.16.3.2 4.16.3.2.1 4.16.3.2.2 4.16.3.3 4.16.3.3.1 4.16.3.3.2 4.16.4 4.16.4.1 4.16.4.2 4.16.4.3 4.16.4.4 4.16.4.5 References

Introduction Background Asymmetric Addition of Cyanide to Aldehydes and Ketones Introduction Use of Bimetallic Catalysts Titanium based catalysts Vanadium based catalysts Use of Combined Lewis Acid/Lewis Base Catalysts Binol based catalysts Shibasaki’s sugar derived ligand Asymmetric Addition of Cyanide to Imines and Related Species Introduction Use of Binol and Related Catalysts Use of Salen Derived Catalysts Use of C1-symmetrical Phenol Containing Catalysts Use of Pybox Based Catalysts

Glossary Chiral pool Naturally occurring enantiomerically pure compounds which can be used as starting materials for the synthesis of other enantiomerically pure compounds. Cyanohydrin A compound which contains an alcohol and nitrile group attached to the same carbon atom. Enantioselective A reaction which produces more of one enantiomer of a product than the other from an achiral starting material. Nonlinear effect The enantiomeric excess of the product of a catalyzed reaction is not proportional to the enantiomeric excess of the catalyst. A positive nonlinear effect is when the enantiomeric excess of the product is higher than expected and a negative nonlinear effect is

4.16.1

315 316 316 316 317 317 319 320 320 321 322 322 322 324 325 326 326

when the enantiomeric excess of the product is lower than expected. Reissert reaction The addition of cyanide to an acyl iminium ion which may be generated in situ from an imine and acid chloride. Salen ligand A ligand derived from a diamine and two equivalents of a 2-hydroxybenzaldehyde. Strecker reaction The synthesis of a-aminonitriles by the three component condensation of a carbonyl compound, an amine, and cyanide. Often also used to refer to the addition of cyanide to a preformed imine. Taddol trans-a,a0 -(dimethyl-1,3-dioxolane-4,5-diyl)bis(diphenylmethanol).

Introduction

Caution: All of the reactions described in this chapter involve the use of highly toxic and in many cases volatile cyanide derivatives. These reactions should be carried out by appropriately trained personnel in a well-ventilated fume cupboard and in full compliance with all local safety regulations regarding the use of cyanides. This chapter covers the asymmetric addition of cyanide to aldehydes and ketones forming cyanohydrins (Scheme 1) and to imines and other compounds which contain a carbon–nitrogen double bond, forming a-aminonitriles (Scheme 2). Only those methods which rely on the use of a metal-based catalyst will be discussed in this chapter. For related work on enzyme catalyzed cyanohydrin synthesis and use of organocatalysts to induce the formation of chiral cyanohydrins and a-aminonitriles, the reader is referred to Chapters 7.16 and 6.7, respectively. Chiral auxiliaries are widely used to control Strecker reactions and these will be discussed in volume 3. The intention of this chapter is to guide the reader through the most useful and versatile methods for

OX

O + XCN R1

chiral catalyst

R2

R2

R1

CN

Scheme 1

Comprehensive Chirality, Volume 4

http://dx.doi.org/10.1016/B978-0-08-095167-6.00415-8

315

316

Enantioselective Cyanation of Carbonyls and Imines NHP

NP + XCN R1

chiral catalyst

R2

R2

R1

CN

Scheme 2

the synthesis of enantiomerically enriched cyanohydrins and a-aminonitriles. It is not intended that the coverage should be comprehensive and for more information the reader is referred to recent reviews of asymmetric cyanohydrin synthesis1 and aaminonitrile synthesis.2 Many different cyanide sources are used in asymmetric additions to carbon–oxygen and carbon–nitrogen double bonds. By far the most popular is trimethylsilyl cyanide (TMSCN 1), though cyanoformates 2, acyl cyanides 3 cyanophosphonates 4 and metal cyanides have also been used. Hydrogen cyanide itself is rarely used with Lewis acid catalysts due to its volatile and hazardous nature (but see Chapters 6.7 and 7.16), though it may be generated in situ from any of reagents 1–4 through reaction with protic groups within the catalyst, by reaction with adventitious moisture or through deliberate addition of a protic additive to the reaction mixture. Thus, the X group protecting the cyanohydrin in Scheme 1 may have a wide range of different structures and may not always directly correspond to the group present in the cyanide source. For additions to imines, the X group is generally not incorporated into the aminonitrile product, but is instead replaced by a proton. In asymmetric cyanohydrin synthesis, the X group plays an important role in preventing the racemization of the cyanohydrin product through its equilibration with the achiral starting materials, though for asymmetric cyanohydrin synthesis from aldehydes (R1 ¼ H in Scheme 1), care must be taken to avoid racemization due to deprotonation of the acidic proton adjacent to the nitrile in the cyanohydrin. O

O

O Me3Si CN RO 1

4.16.2

CN 2

R

CN 3

RO P CN RO 4

Background

The synthesis of cyanohydrins and a-aminonitriles by the addition of cyanide to carbon–oxygen or carbon–nitrogen double bonds are two of the fundamental carbon–carbon bond forming reactions in organic chemistry. Both of these reactions have been known for well over 150 years, racemic cyanohydrin synthesis having been first reported in 18323 and the racemic Strecker reaction (the three-component condensation of an aldehyde, an amine and cyanide) in 1850.4 Asymmetric cyanohydrin synthesis also has a long history. The first enzymatic cyanohydrin synthesis was reported in 1908,5 however, the first reports of chiral Lewis acid catalyzed asymmetric cyanohydrin synthesis did not appear until 1986.6 Chiral auxiliary based asymmetric Strecker reactions were first reported in 1970,7 though Lewis acid catalyzed asymmetric Strecker reactions have only been developed since 1998. Many of these early methods are now of only historical interest as they gave low levels of asymmetric induction. However, there have been major advances in the development of asymmetric catalysts for both cyanohydrin and a-aminonitrile synthesis over the last 15 years and these will be the focus of this chapter.

4.16.3 4.16.3.1

Asymmetric Addition of Cyanide to Aldehydes and Ketones Introduction

Table 1 lists a selection of ‘first generation catalysts’ for asymmetric cyanohydrin synthesis. These catalysts were all based around the use of C1-symmetrical ligands and gave levels of asymmetric induction that depended strongly on the structure of the aldehyde substrate after long reaction times at low temperatures, using relatively large amounts of catalyst. One conclusion which could be drawn from these early results was that titanium based catalysts were likely to give the best results, and titanium complexes of phenolic Schiff’s bases appeared particularly promising. A general approach to increasing asymmetric induction in asymmetric catalysis is to use C2-symmetric catalysts to reduce the number of possible transition states. Table 2 shows the results that were subsequently obtained using the titanium complexes of four common C2-symmetric ligands. With the exception of the trans-a,a’-(dimethyl-1,3-dioxolane-4,5-diyl)bis(diphenylmethanol) (taddol)-based catalyst, the results were a significant improvement on the use of C1-symmetrical ligands in terms of reaction temperature, amount of catalyst required and substrate scope. The use of salen ligands appeared particularly promising, a result which was consistent with the analysis of first generation catalysts. However, the reason why Schiff-base ligands should be so effective was not immediately apparent. After extensive mechanistic investigation,13 this was eventually traced to the ability of the complexes of these ligands to simultaneously activate both the aldehyde and TMSCN during the key transition state of the reaction. This appreciation of the need for bifunctional catalysis turned out to be critical for future developments of asymmetric

Enantioselective Cyanation of Carbonyls and Imines

Table 1

First generation catalysts for the asymmetric addition of TMSCN to aldehydes

Ligand (or complex)

Ph

317

Metal

Catalyst (mol%)

Temp. (1C)

Time (h)

Product ee (%)

Ref.

B

10

 78

140

12–16

6

Al

10–100

 78

0.5–24

37–71

8

Sn

100

 78

14

72–96

9

Ti

20

 78

12–40

20–96

10

Ti

20

 78

60

48–92

11

Ti

100

 50

20–24

37–91

12

Ph B OMe OH

O

N

NHCy

H HO N H N

tBu

N

OH

OH

Ph

Ph

HO

N tBu

HO t Bu

Me

NH S

OH

O

catalysts for asymmetric cyanohydrin synthesis and will be discussed in detail throughout this chapter. The bifunctional catalysis can be achieved in two ways: use of a bimetallic complex or use of a monometallic complex with an additional Lewis basic group capable of activating the TMSCN.

4.16.3.2 4.16.3.2.1

Use of Bimetallic Catalysts Titanium based catalysts

In a series of papers starting in 1996, Belokon’ and North developed and extensively studied asymmetric cyanohydrin synthesis using titanium(salen) complexes. Early work using TMSCN as the cyanide source formed the catalyst in situ simply by mixing the salen ligand and titanium tetraisopropoxide,18 though it was subsequently shown that better substrate to catalyst ratios could be obtained by employing the preformed complex of a salen ligand and titanium tetrachloride.19 Eventually however, it was shown that both of these systems actually generated bimetallic complex 5 in situ and the best results (substrate to catalyst ratio 1000:1 and ee 80–90% for aromatic aldehydes and 460% for aliphatic aldehydes at room temperature) were obtained by preforming complex 5.17 The bridging oxygens needed for the formation of complex 5 come from adventitious or added moisture. Detailed kinetic studies showed that the bimetallic structure of complex 5 was critical for its exceptional catalytic activity, with both

318

Enantioselective Cyanation of Carbonyls and Imines

Table 2

C2-Symmetrical ligands used in titanium complexes for the asymmetric addition of TMSCN to aldehydes

Ligand

Catalyst (mol%) iPr

HO

CO2

HO

CO2iPr

20

Ph Ph Me Ph

O

OH

O

OH

Temp. (1C)

Time (h)

Product ee (%)

Ref.

18

60–91

14

12–48

61–96

15

0

18

33–75

16

20

1

52–92

17

0

100

 78 to  65

Ph Ph

20

OH OH

0.1

N tBu

N tBu

OH HO tBu

tBu

titanium atoms playing a role in the key transition state (Figure 1). One titanium atom acts as a Lewis acid to coordinate and activate the aldehyde whilst the other formed a titanium-cyanide species, thus allowing intramolecular addition of cyanide to the aldehyde to occur within the asymmetric environment of the two salen ligands.13 It was subsequently shown that complex 5 would also catalyze the asymmetric addition of trimethylsilyl cyanide to methyl ketones.20,21 The transition state shown in Figure 1 illustrates that there is sufficient space to accommodate a methyl ketone, but not a ketone with a larger substituent in the key transition state. Asymmetric cyanohydrin synthesis catalyzed by complex 5 has been commercialized by Avecia.22

tBu

tBu

N N R

tBu tBu

N

O Ti

N

O

O O

R

N

tBu

R R

O O

R

R 5

N C

N

tBu tBu

tBu

O

Ti

O O

O

Ti

R O N

Ti

H O

R

N

Figure 1 Stereodetermining transition state for asymmetric cyanohydrin synthesis using catalyst 5 (R ¼ tert-butyl).

Although TMSCN is the most commonly used cyanide source in asymmetric cyanohydrin synthesis, its relatively high cost and volatility mean that it is not the ideal cyanide source. One of the advantages of complex 5 is that it has been shown to catalyze the asymmetric addition of other cyanide sources to aldehydes. Thus, reaction with cyanoformates leads directly to cyanohydrin carbonates23,24 and reaction with acyl cyanides gives cyanohydrin esters.25 Both of these reactions are cocatalyzed by achiral nitrogen bases25–27 and the synthesis of cyanohydrin carbonates has been shown to be cocatalyzed by cyanide.28,29 However,

Enantioselective Cyanation of Carbonyls and Imines

319

probably the ultimate cyanide source in terms of both cost and lack of volatility is an alkali-metal cyanide and complex 5 has also been shown to catalyze the asymmetric addition of potassium cyanide to aldehydes. The reaction is carried out in the presence of an anhydride, so that the isolated product is again a cyanohydrin ester (Scheme 3).24,30–32

5 or 6a (1 mol%)/

O + KCN + (R′CO)2O R

OCOR′

tBuOH/H O/CH Cl 2 2 2

−40 °C

H

R

CN

Scheme 3

4.16.3.2.2

Vanadium based catalysts

Whilst the titanium(salen) complexes discussed in Section 4.16.3.2.1 are highly active and versatile catalysts for asymmetric cyanohydrin synthesis, their enantioselectivity in reactions carried out at room temperature with TMSCN is typically less than 90%. Belokon’ and North reasoned that if the titanium was changed to a metal in the þ 5 oxidation state, then this would bind the aldehyde more strongly, resulting in a higher level of asymmetric induction. Thus, vanadium complex 6a was prepared and used as a catalyst for the asymmetric addition of TMSCN to aldehydes. Reactions catalyzed by complex 6a were much slower than those catalyzed by complex 5 (typically requiring reaction times of 24 h), but did display enhanced enantioselectivities of up to 95% for aromatic aldehydes and up to 77% for aliphatic aldehydes.21,33 The structure of the catalyst in these reactions was initially thought to be vanadium(IV)(salen) complex 7, but was later shown to be vanadium(V) complex 6a by X-ray crystallography.31 Complex 6a was also found to catalyze the synthesis of cyanohydrin esters from aldehydes, potassium cyanide and acetic anhydride (Scheme 3) and did so with a similar enantioselectivity to complex 5.31

tBu

N O N V O O tBu X tBu 6a: X = EtOSO3 6b: X = NCS

tBu tBu

N O N V O O tBu

tBu

tBu

7

Although complex 6a was found to be monometallic in the solid state, kinetic studies33,34 showed that it formed a bimetallic species in solution, thus the asymmetric induction is thought to originate from the same type of transition state as for complex 5 (Figure 1). To enhance the rate of reactions catalyzed by vanadium (V)(salen) complexes, the effect of the counterion on the catalytic activity was investigated. A series of complexes with different counterions were prepared and it was shown that the counterions had a dramatic effect on the rate of reaction, but had no influence on the enantioselectivity of the reaction.34,35 The most effective catalyst was complex 6b with an isothiocyanate counterion, which allowed the synthesis of cyanohydrin trimethylsilyl ethers with similar enantioselectivities to those obtained using complex 6a, but after a reaction time of just 2 h at room temperature. The enhanced activity of complex 6b also allowed asymmetric cyanohydrin synthesis to be carried out in a green solvent, propylene carbonate, rather than the more common dichloromethane.36 Complexes 6 have been used commercially for the synthesis of cyanohydrins,22 have also been utilized by other academic research groups,37,38 and have been immobilized on various solid supports.1 The lower intrinsic activity of complexes 6 compared to titanium complex 5 means that, in the absence of a cocatalyst, the vanadium based catalysts will not catalyze the asymmetric addition of ethyl cyanoformate to aldehydes. However, it has been shown that in the presence of imidazole39 or solid bases such as sodium hydroxide, alumia or hydrotalcite,40 complex 6a will catalyze the formation of cyanohydrin ethyl carbonates with enantioselectivities of 70–95%. Acetone cyanohydrin is not often used as a cyanide source, though the equilibria are such that this ketone derived cyanohydrin will transfer cyanide to aldehydes. Katsuki has shown that vanadium(salen) complex 8 (with an added base such as 2,4,6-collidine) or vanadium(salalen) complex 9 will catalyze the transfer of cyanide from acetone cyanohydrin to aldehydes (Scheme 4).41,42 Complex 9 gave the best results,42 94–95% ee for aliphatic aldehydes, and for both catalysts, aliphatic aldehydes were much better substrates than benzaldehyde. Reactions catalyzed by complex 9 require an oxygen containing atmosphere, presumably to oxidize the vanadium(IV) complex to the catalytically active vanadium(V) complex and to ensure that the vanadium remains in the þ 5 oxidation state throughout the reaction.43

320

Enantioselective Cyanation of Carbonyls and Imines

Et N O N V O O PhPh

N O N V O O

t Hex

t Hex

t Hex

t Hex

9 ClO4 8 + R

8 (5 mol%) + 2,4,6-collidine (5 mol%), 10 °C,48 h; or CN 9 (10 mol%), 0 °C, 36 h, O 2

OH

O H

Me Me

O

OH + R

CN

Me

Me

Scheme 4

4.16.3.3 4.16.3.3.1

Use of Combined Lewis Acid/Lewis Base Catalysts Binol based catalysts

In 1999, Shibasaki reported the development of the aluminum complex of ligand 10 as a catalyst for the asymmetric addition of TMSCN to aldehydes.44,45 Reactions had to be carried out at  40 1C and required 9 mol% of the catalyst along with 36 mol% of an achiral phosphine oxide cocatalyst, but gave cyanohydrin trimethylsilyl ethers with very high enantioselectivities (83–98%) for a wide range of aromatic and aliphatic aldehyde substrates. The key stereodetermining transition state for reactions employing ligand 10 is shown in Figure 2. The key features are the activation of the aldehyde and TMSCN by the metal and phosphine oxide units within the complex respectively. The role of the achiral phosphine oxide cocatalyst is to change the geometry of the coordinated aluminum from tetrahedral to trigonal bipyramidal to facilitate the intramolecular transfer of cyanide to the coordinated aldehyde. The utility of the aluminum complex of 10 has since been demonstrated by its use in syntheses of Epothilones A and B,46,47 antihyperglycemic agents,48 and HIV protease inhibitors.49

Me3 R Si

Ph Ph P O HO HO O

C N

H

Ph P Ph O O

Cl Al O O R3P O

Ph P Ph 10

O

P Ph Ph

Figure 2 Stereodetermining transition state for asymmetric cyanohydrin synthesis using ligand 10.

Najera50,51 and Pu,52,53 both modified Shibasaki’s ligand 10, replacing the Lewis basic phosphine oxide groups with tertiary amines possessing Brønsted basicity to give ligands 11 and 12, respectively. The Pu ligand 12 has the advantage of being preparable in a single step from 2,20 -binaphthol (binol), but far more work has been done on the applications of Najera’s ligand 11. Thus, whilst the aluminum complexes of both 11 and 12 have been used to catalyze the asymmetric addition of TMSCN to aldehydes,50–53 the aluminum complex of ligand 11 has also been shown to catalyze the asymmetric addition of various other cyanide sources to aldehydes.54–60 For ligand 12, the optimal conditions require the use of 10 mol% of the aluminum complex, 40 mol% of hexamethyl phosphoramide (HMPA) as an additive, 4 A˚ molecular sieves, and three equivalents of TMSCN at  20 1C in ether. Under these conditions, enantioselectivities of 74–98% could be achieved for a range of aromatic and aliphatic aldehydes and unusually, the best asymmetric induction was obtained with the aliphatic substrates.52,53 The reaction displayed a very large positive nonlinear effect, which is significant since binol needs to be resolved, and this allowed high enantiomeric excesses (ee’s) to be obtained even when ligand 12 had low enantiopurity. The optimal conditions for the addition of TMSCN to aldehydes catalyzed by the aluminum complex of ligand 11 also involved the use of 10 mol% of the catalyst along with 4 A˚ molecular sieves and three equivalents of TMSCN at  20 1C. However, the optimal additive was found to be triphenylphosphine oxide and the highest enantioselectivities (499%) were obtained from reactions carried out in toluene with aromatic aldehydes as substrates.50,51 Aliphatic aldehydes gave much lower levels of

Enantioselective Cyanation of Carbonyls and Imines

321

asymmetric induction (20–88%) even at a lower temperature (  40 1C). Another difference between reactions catalyzed by the aluminum complexes of ligands 11 and 12 is that reactions involving ligand 11 do not show a nonlinear effect. The role of the 4 A˚ molecular sieves is to supply a controlled amount of water to the reactions as the actual cyanating agent is hydrogen cyanide generated in situ by hydrolysis of TMSCN. The transition state shown in Figure 3 has been proposed to explain the origin of the asymmetric induction in reactions involving ligand 11.51

NEt2 PPh3

NEt2

O

N

OH OH

OH OH

NEt2

O O

O

O

O

N

11

Al

Cl R H

Et2N H NC

12

Figure 3 Stereodetermining transition state for asymmetric cyanohydrin synthesis using ligand 11.

In addition to TMSCN, Najera has shown that the aluminum complex of ligand 11 will catalyze the asymmetric addition of methyl cyanoformate54–56 (2: R ¼ Me) and diethyl cyanophosphonate56–60 (4: R ¼ Et) to aldehydes. The reaction with methyl cyanoformate could be carried out at 25 1C in toluene using 10 mol% of the aluminum complex of 11 in the presence of 4 A˚ molecular sieves and gave cyanohydrin methyl carbonates with 4–82% ee.55 In contrast to the trimethylsilylation reaction, cyanohydrin carbonate formation did show a positive nonlinear effect. The structure of the key stereodetermining transition state is less clear in this case as two different structures have been proposed,54,55 which differ in the coordination number of the aluminum ion (4 or 5) and the nature of the cyanating agent (MeOCOCN or HCN generated in situ). Under identical reaction conditions to those used with methyl cyanoformates, the aluminum complex of ligand 11 would also catalyze the asymmetric addition of diethyl cyanophosphonate to aldehydes, giving cyanohydrin phosphonates with 4–98% ee.57–59 This reaction also showed a positive nonlinear effect. When the cyanohydrin carbonate or phosphonate is derived from an a,b-unsaturated aldehyde, the cyanohydrin derivative can be used as a substrate for palladium or iridium catalyzed allylic displacement reactions, thus allowing the synthesis of a wide range of a,b-unsaturated nitriles with a stereocenter at the g-position (Scheme 5).56–59 The cyanohydrin phosphonates similarly undergo SN20 reactions when treated with organocuprates (Scheme 5).60

R′

CN

R′ +

R

R

OP

R′MgCl/CuCN CN

P = PO(OEt)2

R

CN

Pd or Ir catalyst/ nucleophile (Nuc) P = CO2Me or PO(OEt)2

Nuc R

CN

Scheme 5

4.16.3.3.2

Shibasaki’s sugar derived ligand

The catalysts discussed in the above sections can give excellent results for the asymmetric addition of various cyanide sources to aldehydes. However, none of them are particularly effective catalysts for the asymmetric addition of cyanide to ketones. A major breakthrough in this area was made in 2000 when Shibasaki reported the synthesis of D-glucose-derived ligand 13.61 The complex formed from 10 mol% of ligand 13 and 10 mol% of titanium tetraisopropoxide was found to catalyze the asymmetric addition of TMSCN to a range of ketones with 76–95% ee. The proposed stereodetermining transition state for this reaction is shown in Figure 4 and is similar to that shown in Figure 2 for reactions catalyzed by the aluminum complex of ligand 10. Shibasaki subsequently showed that the titanium complex of ligand 14 was a more effective catalyst and this allowed the catalyst loading to be reduced to 1 mol%.62 The synthetic utility of the titanium complexes of ligands 13 and 14 was demonstrated by their use in syntheses of Fostriecin,63,64 8-epi-Fostriecin64 and Neurokinin receptor antagonists.65 The titanium complexes of ligands 13 and 14 add cyanide to the si-face of ketones. One potential disadvantage of this class of catalysts is that since the ligands are derived from the chiral pool, the enantiomeric ligand is not readily available. However, in 2001, as part of a total synthesis of (20S)-Camptothecin, Shibasaki showed that the gadolinium complex of ligand 13 would catalyze the asymmetric addition of TMSCN to the re-face of ketones, thus allowing either enantiomer of a ketone derived cyanohydrin trimethylsilyl ether to be prepared.66 The reason for the change in stereochemical induction was traced to the formation of a 3:2 ligand to metal complex in which the cyanide is now transferred intramolecularly from the lanthanide metal (Figure 5) rather than from a phosphine oxide bound TMSCN group as in Figure 4. The gadolinium complex of ligand 13 was also used in a total synthesis of (S)-oxybutynin.67,68 In subsequent work, Shibasaki showed that by optimizing the substituent

322

Enantioselective Cyanation of Carbonyls and Imines X

Y

Me CN Me Me Si RL Ph O Ph RS O O O O Ti O O CN

O O OH Ph Ph P OH O 13: X = Y = H 14: X = COPh, Y = H 15: X = Y = F

Figure 4 Stereodetermining transition state for asymmetric cyanohydrin synthesis using the titanium complex of ligand 13.

O Ph

P Ph O O O O

O

O

NC

Ln

CN

O

Ln O

O O R1

O PPh2

O R2 Ph2P

O

O

SiMe3

SiMe3

O

Figure 5 Stereodetermining transition state for asymmetric cyanohydrin synthesis using lanthanide complexes of ligand 13.

pattern on the catechol (ligand 15 often gives excellent results) and the lanthanide metal (gadolinium or samarium), the efficiency of the catalyst system could again be improved69,70 and used to synthesize triazole containing antifungal agents.71

4.16.4 4.16.4.1

Asymmetric Addition of Cyanide to Imines and Related Species Introduction

The addition of cyanide to imines may appear to be an almost identical reaction to the synthesis of cyanohydrins and in some cases the same catalysts can be used to accomplish both reactions. However, there are significant differences associated with the lower polarity of the C¼N bond and the presence of a protecting group on the nitrogen atom. For this reason, not all of the systems discussed in Section 4.16.3 will accept imines as substrates and some systems which do make good catalysts for asymmetric Strecker reactions are not effective for asymmetric cyanohydrin synthesis. The following sections are divided according to the nature of the chiral ligand employed in a catalyst system.

4.16.4.2

Use of Binol and Related Catalysts

In 2000, just a year after showing that the aluminum complex of ligand 10 would catalyze asymmetric cyanohydrin synthesis, Shibasaki showed that the same catalyst would catalyze the asymmetric addition of TMSCN to N-fluorenyl aldimines, giving aaminonitriles with 70–96% ee (Scheme 6).72,73 Reactions had to be carried out at  40 1C using 9 mol% of the aluminum complex of ligand 10 and with reaction times of 24–68 h. The authors showed that a protic additive was essential to obtain optimal results, but that TMSCN was a more reactive cyanating agent than HCN. On the basis of this data, the authors developed two experimental protocols. The first involved the use of stoichiometric amounts of TMSCN along with the slow addition of

10/Et2AlCl (9 mol%) TMSCN/PhOH or HCN −40 °C, CH2Cl2

N R Scheme 6

H

H HN R

CN

Enantioselective Cyanation of Carbonyls and Imines

323

phenol over the first 17 h of reaction. The second protocol allowed the use of just 20 mol% of TMSCN in conjunction with 120 mol% of HCN. The authors proposed a catalytic cycle for the asymmetric Strecker reaction, with a key stereodetermining transition state which was essentially identical to that shown in Figure 2 for asymmetric cyanohydrin synthesis. Shibasaki subsequently developed a polymer supported, recyclable version of ligand 10, the aluminum complex of which could also be used to catalyze the reaction shown in Scheme 6 with 83–87% enantioselectivity.74 If the C¼N bond is part of a quinoline or isoquinoline ring, then the addition of cyanide is known as the Reissert reaction. Shibasaki and coworkers showed that the aluminum complex of ligand 1075,76 or its polymer supported analogue76 would also catalyze this reaction (Scheme 7). In this case, the addition of an acid chloride was necessary to acylate the nitrogen atom prior to cyanide addition. By introducing bromo substituents onto ligand 10, Shibasaki and coworkers were able to increase the reactivity of the catalyst system to allow the synthesis of quaternary stereocenters using the asymmetric Reissert reaction.77 Further development of ligand 10 to phosphine sulfide or sulfoxide analogues allowed asymmetric Reissert reactions to be carried out on pyridine derivatives.78 10/Et2AlCl (9 mol%) TMSCN/RCOCl −40 °C, CH2Cl2

X Y

X Y

N

CN

N O

R

Scheme 7

Whereas the Shibasaki system discussed above exploits cooperative Lewis acid and Lewis base catalysis, Kobayashi showed that bimetallic complexes based around binol ligands could also be used as catalysts for asymmetric Strecker reactions. Thus, zirconium complex 16 derived from two different brominated binol ligands was found to catalyze the asymmetric addition of tributyltin cyanide to N-2-hydroxyphenyl aldimines. The optimal conditions involved the use of 10 mol% of complex 16 at  65 to 0 1C in a toluene/benzene mixture for 12 h and gave a-aminonitriles with 74–92% ee.79 It was subsequently shown that catalyst 16 could also be used with hydrogen cyanide as the cyanide source and that the aldimine could be generated in situ from an aldehyde and 2-amino-3-methylphenol.80,81 Another bimetallic catalyst system was developed by Valle´e and coworkers.82 Thus, the aluminum (or scandium) lithium complexes 17a,b (10 mol%) were found to catalyze the asymmetric addition of TMSCN or hydrogen cyanide to both N-benzyl aldimines and N-benzyl ketimines with up to 95% ee, though only three substrates were examined. Br

Br

Br NMI

OtBu O O

Zr

O O

NMI Br

Zr

O

O OtBu

Br NMI = N-methylimidazole

Br

16

Feng and coworkers showed that the sodium salt of binol phosphonic acid 18 would catalyze the asymmetric addition of TMSCN to N-diphenylphosphinyl ketimines.83 A protic additive (optimally para-tert-butyl-ortho-adamantylphenol) was required and the optimal conditions required the use of 10 mol% of both the catalyst and additive at  20 1C in toluene for 1.5–3.5 days. Under these conditions, a wide range of ketimines were converted into a-aminonitriles with enantioselectivities of 79–95%. The lanthanum complex of binaphthyl bis-sulfonic acid 19 was shown by Ishihara et al. to catalyze the asymmetric addition of TMSCN to N-diphenylmethyl aldimines.84 The optimal reaction conditions involved the use of 10 mol% of 19, 10 mol% of lanthanum(III)phenoxide and 50 mol% of a carboxylic acid additive in propionitrile at  20 oC for up to four days and gave enantioselectivities of 41–92%.

O M

Li O O

17a: M = Al; b: M = Sc

O O

18

O

SO3H

ONa

SO3H

P

19

Aluminum complex 20 can be considered as a cross between a binol and a salen complex, and Abell and Yamamoto showed that it would catalyze asymmetric Strecker reactions of N-diarylphosphonyl aldimines and ketimines.85 The reaction required the

324

Enantioselective Cyanation of Carbonyls and Imines

presence of an achiral Lewis base and triethylamine (10 mol%) was found to give good results. The reaction is unusual in that ethyl cyanoformate (2, R ¼ Et) is used as the cyanide source, and 1.5 equivalents of isopropanol were also required. Under optimized conditions (room temperature, toluene, 6 h), 10 mol% of catalyst 20 gave a-aminonitriles from aldimines with 90–98% ee (Scheme 8). Ketimines were slightly less reactive, requiring reaction times of 12 h and 2.5 equivalents of both ethyl cyanoformate and isopropanol, but still gave quaternary a-aminonitriles with 82–96% ee. The initial product of these reactions is the carbamate derivative, but this is hydrolyzed on work-up to give the N-diarylphosphonyl amines.

O P Ar + N Ar R

20 (10 mol%)/ O Et3N (10 mol%)/ i EtO CN PrOH (1.5 equivalent)/ toluene, r.t., 6 h

Br

O EtO

O N

R

P Ar Ar CN

N Cl Al

Me O O

N O P Ar HN Ar R

CN

20

Me

Br

Scheme 8

Whilst binol is configurationally stable at room temperature, Feng et al. showed that even configurationally unstable biphenyls could be used as catalysts for asymmetric Strecker reactions provided a second chiral species was present. Thus, in 2007 they showed that in the presence of 5–10 mol% of cinchonine 21, 6–12 mol% of titanium tetraisopropoxide and 6–12 mol% of biphenyl 22 would catalyze the asymmetric addition of TMSCN to N-tosyl aldimines and ketimines with 69 to 499% enantioselectivity.86,87 The system is also effective for asymmetric cyanohydrin synthesis from both aldehydes and ketones.87

N HO H

OH OH

N 21

4.16.4.3

22

Use of Salen Derived Catalysts

Despite their preeminent position in asymmetric cyanohydrin synthesis (Sections 4.16.3.2.1 and 4.16.3.2.2), metal(salen) complexes have not achieved the same level of success in asymmetric Strecker reactions. Never the less, in 1998 Sigman and Jacobsen showed that 5 mol% of aluminum(salen) complex 23 would catalyze the asymmetric addition of hydrogen cyanide to N-allyl aldimines at  70 1C in toluene, giving a-aminonitriles with 37–95% ee.88 North and coworkers investigated the use of titanium and vanadium salen complexes in asymmetric Strecker reactions of N-benzyl imines. The best results (16–75% ee) were obtained using vanadium (V) complex 6a with TMSCN as the cyanide source, though a protic additive was necessary, suggesting that the real cyanating agent may be the hydrogen cyanide generated in situ.89 Kahn et al. prepared a bimetallic analogue (24) of complex 6a and showed that it would catalyze the asymmetric addition of TMSCN to aldimines with 22–94% enantioselectivity at  30 1C.90 Again, water had to be present, suggesting that the real cyanating agent may be hydrogen cyanide.

Enantioselective Cyanation of Carbonyls and Imines

O P Ar + N Ar R

20 (10 mol%)/ O Et3N (10 mol%)/ i EtO CN PrOH (1.5 equivalent)/ toluene, r.t., 6 h

Br

O EtO

O N

R

P Ar Ar

N

CN

Cl Al

Me O O

N O P Ar HN Ar R

4.16.4.4

325

20

CN

Me

Br

Use of C1-symmetrical Phenol Containing Catalysts

In 1999, Snapper and Hoveyda reported that peptide derived Schiff bases 25 could be complexed to titanium tetraisopropoxide and used to catalyze the asymmetric addition of TMSCN to N-benzhydryl aldimines. Optimal results were obtained using 10 mol% of the catalyst at 4 1C in toluene for 20 h with isopropanol as a protic additive.91 Under these conditions, enantioselectivities of 85 to 499% could be achieved, though it was necessary to optimize the structure of the Schiff base component of the catalyst for each substrate. This optimization of catalyst structure could be achieved through the screening of a parallel library of catalysts.92 Mechanistic studies indicated that HCN was the true cyanating agent, and that whilst the Schiff base coordinated titanium ion activated the aldimine, the peptide chain had a role in activating and preorganizing the cyanide.93 The utility of this catalyst system was shown by its use in a total synthesis of the anti-HIV agent chloropeptin 1.94

tBu

X

H N

N OH

O 25

O N H OtBu

R CO2Me

N H OH 26: R = CH2Ph 27: R = tBu

OH

Vilaivan found that the titanium complexes of amino alcohol derived Schiff bases did not form good catalysts for asymmetric Strecker reactions. However, by reducing the imine to the corresponding amine, it was found that the resulting titanium b-amino alcohol complexes were effective catalysts.95 The best results were obtained using the titanium complex of ligand 26, which, at a 10 mol% catalyst loading at 0 1C in toluene catalyzed the asymmetric addition of TMSCN to N-benzyl aldimines with 39–80% enantioselectivity. It was subsequently shown that N-benzhydryl aldimines were better substrates (giving enantioselectivities of 91 to 498%) and that protic additives (water or isopropanol) accelerated the rate of reaction and were found to be essential when the reactions were scaled up.96 In the presence of isopropanol, the amount of catalyst required could be reduced to 2.5 mol% whilst still giving complete conversions after 72 h and enantioselectivities of 96–98% when N-benzhydryl aromatic aldimines were used as substrates.97 Recently, Seayad et al. showed that 5 mol% of the titanium complex of ligand 27 would catalyze the asymmetric addition of TMSCN to a range of aldimines bearing different protecting groups in 15–60 min at room temperature with 61–98% enantioselectivity. The presence of an additive (butanol) was again essential to obtain high levels of catalytic activity and enantioselectivity, and the authors attributed this to partial hydrolysis of the initially formed oligomeric titanium complex to catalytically active monomeric species.98 Shibasaki developed glucose derived ligands 13–15 for use in asymmetric cyanohydrin synthesis from ketones (see Section 4.16.3.3.2). In 2003, Shibasaki and coworkers extended this work to ketimines and showed that the gadolinium complex of ligand 15 would catalyze the asymmetric addition of TMSCN to N-diphenylphosphinyl ketimines with enantioselectivities of 51–98% at  40 1C.99 The optimal conditions required the use of 2.5 mol% of gadolinium(III) isopropoxide and 5 mol% of ligand 15. The addition of a protic additive (optimally 2,6-dimethylphenol) was again found to increase the reaction rate and allowed a wider range of ketimines to be used as substrates including heteroaromatic, aliphatic and cyclic ketimines. The utility of this catalyst system was then demonstrated by an asymmetric synthesis of sorbinil.100 Even lower catalyst loadings (0.1 mol%) could be used by using only a catalytic amount of TMSCN (2.5–5 mol%) along with a slight excess of hydrogen cyanide as cyanating agent.101,102 Extensive mechanistic studies including electrospray ionization mass spectrometry (ESI-MS) and X-ray analyses indicated that a 3:2 ligand to gadolinium complex, analogous to that shown in Figure 5 for asymmetric cyanohydrin synthesis is responsible for the catalytic activity of this system.103

326

Enantioselective Cyanation of Carbonyls and Imines

4.16.4.5

Use of Pybox Based Catalysts

There has been almost no work on the asymmetric catalysis of the addition of cyanide to other C¼N containing species. However, in 2004, Keith and Jacobsen showed that in the presence of methanol, 5 mol% of the erbium complex of pybox ligand 28a would catalyze the asymmetric addition of TMSCN to N-benzoyl hydrazones at 0 1C in chloroform. The product a-hydrazinonitriles were obtained with 31–97% ee after reaction times of 2–4 days.104 In 2009, Karimi and Maleki showed that the ytterbium(III) complex of closely related ligand 28b would catalyze the asymmetric addition of TMSCN to N-benzhydryl aldimines. The best results (45–98% ee) were obtained from reactions carried out at  78 1C in dichloromethane in the presence of methanol for 24–72 h.105 X

O

O

N N

N

Ph

Ph 28a: X = H 28b: X = Br

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

North, M.; Usanov, D. L.; Young, C. Chem. Rev. 2008, 108(12), 5146–5226. Gro¨ger, H. Chem. Rev. 2003, 103(8), 2795–2827. Winkler, F. W. Ann. Chem. 1832, 4, 246–249. Strecker, A. Ann. Chem. Pharm. 1850, 75, 27–45. Rosenthaler, L. Biochem. Z. 1908, 14, 238–253. Reetz, M. T.; Kunisch, F.; Heitmann, P. Tetrahedron Lett. 1986, 27(39), 4721–4724. Patel, M. S.; Worsley, M. Can. J. Chem. 1970, 48(12), 1881–1884. Ohno, H.; Nitta, H.; Tanaka, K.; Mori, A.; Inoue, S. J. Org. Chem. 1992, 57(25), 6778–6783. Kobayashi, S.; Tsuchiya, Y.; Mukaiyama, T. Chem. Lett. 1991, (4), 541–544. Hayashi, M.; Inoue, T.; Miyamoto, Y.; Oguni, N. Tetrahedron 1994, 50(15), 4385–4398. Jiang, Y.; Zhou, Z.; Hu, W.; Li, Z.; Mi, A. Tetrahedron: Asymmetry 1995, 6(12), 2915–2916. Bolm, C.; Mu¨ller, P.; Harms, K. Acta Chem. Scand. 1996, 50(4), 305–315. Belokon’, Y. N.; Green, B.; Ikonnikov, N. S.; et al. Eur. J. Org. Chem. 2000, (14), 2655–2661. Hayashi, M.; Matsuda, T.; Oguni, N. J. Chem. Soc. Perkin Trans. 1992, 1(22), 3135–3140. Minamikawa, H.; Hayakawa, S.; Yamada, T.; Iwasawa, N.; Narasaka, K. Bull. Chem. Soc. Jpn. 1988, 61(12), 4379–4383. Mori, M.; Imma, H.; Nakai, T. Tetrahedron Lett. 1997, 38(35), 6229–6232. Belokon’, Y. N.; Caveda-Cepas, S.; Green, B.; et al. J. Am. Chem. Soc. 1999, 121(16), 3968–3973. Belokon’, Y.; Ikonnikov, N.; Moscalenko, M.; et al. Tetrahedron: Asymmetry 1996, 7(3), 851–855. Tararov, V. I.; Hibbs, D. E.; Hursthouse, M. B.; et al. Chem. Commun. 1998, 3, 387–388. Belokon’, Y. N.; Green, B.; Ikonnikov, N. S.; North, M.; Tararov, V. I. Tetrahedron Lett. 1999, 40(46), 8147–8150. Belokon’, Y. N.; Green, B.; Ikonnikov, N. S.; et al. Tetrahedron 2001, 57(4), 771–779. Blacker, A. J.; North, M.; Belokon’, Y. N. Chem. Today 2004, 22(June, chiral catalysis supplement), 30–32. Belokon’, Y. N.; Blacker, A. J.; Clutterbuck, L. A.; North, M. Org. Lett. 2003, 5(23), 4505–4507. Belokon’, Y. N.; Blacker, A. J.; Carta, P.; Clutterbuck, L. A.; North, M. Tetrahedron 2004, 60(46), 10433–10447. Lundgren, S.; Wingstrand, E.; Moberg, C. Adv. Synth. Catal. 2007, 349(3), 364–372. Lundgren, S.; Wingstrand, E.; Penhoat, M.; Moberg, C. J. Am. Chem. Soc. 2005, 127(33), 11592–11593. Wingstrand, E.; Lundgren, S.; Penhoat, M.; Moberg, C. Pure Appl. Chem. 2006, 78(2), 409–414. Belokon’, Y. N.; Ishibashi, E.; Nomura, H.; North, M. Chem. Commun. 2006, (16), 1775–1777. Belokon’, Y. N.; Clegg, W.; Harrington, R. W.; et al. Tetrahedron 2007, 63(39), 9724–9740. Belokon’, Y. N.; Gutnov, A. V.; Moskalenko, M. A.; et al. Chem. Commun. 2002, (3), 244–245. Belokon’, Y. N.; Carta, P.; Gutnov, A. V.; et al. Helvetica Chim. Acta 2002, 85(10), 3301–3312. Belokon’, Y. N.; Carta, P.; North, M. Lett. Org. Chem. 2004, 1(1), 81–83. Belokon’, Y. N.; North, M.; Parsons, T. Org. Lett. 2000, 2(11), 1617–1619. Belokon’, Y. N.; Clegg, W.; Harrington, R. W.; et al. Chem. Eur. J. 2009, 15(9), 2148–2165. Belokon’, Y. N.; Maleev, V. I.; North, M.; Usanov, D. L. Chem. Commun. 2006, (44), 4614–4616. North, M.; Omedes-Pujol, M. Tetrahedron Lett. 2009, 50(31), 4452–4454. Hanessian, S.; Reddy, G. J.; Chahal, N. Org. Lett. 2006, 8(24), 5477–5480. Lloyd-Jones, G. C.; Wall, P. D.; Slaughter, J. L.; Parker, A. J.; Laffan, D. P. Tetrahedron 2006, 62(49), 11402–11412. Khan, N. H.; Agrawal, S.; Kureshy, R. I.; et al. Eur. J. Org. Chem. 2008, (26), 4511–4515. Khan, N. H.; Agrawal, S.; Kureshy, R. I.; et al. Chirality 2010, 22(1), 153–158. Watanabe, A.; Matsumoto, K.; Shimada, Y.; Katsuki, T. Tetrahedron Lett. 2004, 45(33), 6229–6233. Takaki, J.; Egami, H.; Matsumoto, K.; Saito, B.; Katsuki, T. Chem. Lett. 2008, 37(5), 502–503. Chechik, V.; Conte, M.; Dransfield, T.; North, M.; Omedes-Pujol, M. Chem. Commun. 2010, 46(19), 3372–3374. Hamashima, Y.; Sawada, D.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 1999, 121(11), 2641–2642. Hamashima, Y.; Sawada, D.; Nogami, H.; Kanai, M.; Shibasaki, M. Tetrahedron 2001, 57(5), 805–814.

Enantioselective Cyanation of Carbonyls and Imines

46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105.

Sawada, D.; Shibasaki, M. Ang. Chem. Int. Ed. 2000, 39(1), 209–213. Sawada, D.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122(43), 10521–10532. Takamura, M.; Yanagisawa, H.; Kanai, M.; Shibasaki, M. Chem. Pharm. Bull. 2002, 50(8), 1118–1121. Nogami, H.; Kanai, M.; Shibasaki, M. Chem. Pharm. Bull. 2003, 51(6), 702–709. Casas, J.; Na´jera, C.; Sansano, J. M.; Saa´, J. M. Org. Lett. 2002, 4(15), 2589–2592. Casas, J.; Na´jera, C.; Sansano, J. M.; Saa´, J. M. Tetrahedron 2004, 60(46), 10487–10496. Qin, Y. C.; Liu, L.; Pu, L. Org. Lett. 2005, 7(12), 2381–2383. Qin, Y. C.; Liu, L.; sabat, M.; Pu, L. Tetrahedron 2006, 62(40), 9335–9348. Casas, J.; Baeza, A.; Sansano, J. M.; Na´jera, C.; Saa´, J. M. Tetrahedron: Asymmetry 2003, 14(2), 197–200. Baeza, A.; Casas, J.; Na´jera, C.; Sansano, J. M.; Saa´, J. M. Eur. J. Org. Chem. 2006, (8), 1949–1958. Baeza, A.; Casas, J.; Na´jera, C.; Sansano, J. M. J. Org. Chem. 2006, 71(10), 3837–3848. Baeza, A.; Casas, J.; Na´jera, C.; Sansano, J. M.; Saa´, J. M. Ang. Chem. Int. Ed. 2003, 42(27), 3143–3146. Baeza, A.; Na´jera, C.; Sansano, J. M.; Saa´, J. M. Chem. Eur. J. 2005, 11(13), 3849–3862. Baeza, A.; Sansano, J. M.; Saa´, J. M.; Na´jera, C. Pure Appl. Chem. 2007, 79(2), 213–221. Baeza, A.; Na´jera, C.; Sansano, J. M. Eur. J. Org. Chem. 2007, (7), 1101–1112. Hamashima, Y.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122(30), 7412–7413. Hamashima, Y.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2001, 42(4), 691–694. Fujii, K.; Maki, K.; Kanai, M.; Shibasaki, M. Org. Lett. 2003, 5(5), 733–736. Maki, K.; Motoki, R.; Fujii, K.; et al. J. Am. Chem. Soc. 2005, 127(48), 17111–17117. Takamura, M.; Yabu, K.; Nishi, T.; et al. Synlett. 2003, (3), 353–356. Yabu, K.; Masumoto, S.; Yamasaki, S.; et al. J. Am. Chem. Soc. 2001, 123(40), 9908–9909. Masumoto, S.; Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2002, 43(48), 8647–8651. Masumoto, S.; Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron 2004, 60(46), 10497–10504. Yabu, K.; Masumoto, S.; Kanai, M.; Curran, D. P.; Shibasaki, M. Tetrahedron Lett. 2002, 43(16), 2923–2926. Yabu, K.; Masumoto, S.; Kanai, M.; et al. Heterocycles 2003, 59(1), 369–375. Suzuki, M.; Kato, N.; Kanai, M.; Shibasaki, M. Org. Lett. 2005, 7(13), 2527–2530. Takamura, M.; Hamashima, Y.; Usuda, H.; Kanai, M.; Shibasaki, M. Ang. Chem. Int. Ed. 2000, 39(9), 1650–1652. Takamura, M.; Hamashima, Y.; Usuda, H.; Kanai, M.; Shibasaki, M. Chem. Pharm. Bull. 2000, 48(10), 1586–1592. Nogami, H.; Matsunaga, S.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2001, 42(2), 279–283. Takamura, M.; Funabashi, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122(26), 6327–6328. Takamura, M.; Funabashi, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2001, 123(28), 6801–6808. Funabashi, K.; Ratni, H.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2001, 123(43), 10784–10785. Ichikawa, E.; Suzuki, M.; Yabu, K.; et al. J. Am. Chem. Soc. 2004, 126(38), 11808–11809. Ishitani, H.; Komiyama, S.; Kobayashi, S. Ang. Chem. Int. Ed. 1998, 37(22), 3186–3188. Ishitani, H.; Komiyama, S.; Hasegawa, Y.; Kobayashi, S. J. Am. Chem. Soc. 2000, 122(5), 762–766. Kobayashi, S.; Ishitani, H. Chirality 2000, 12(5–6), 540–543. Chavarot, M.; Byrne, J. J.; Chavant, P. Y.; Valle´e, Y. Tetrahedron: Asymmetry 2001, 12(8), 1147–1150. Shen, K.; Liu, X.; Cai, Y.; Lin, L.; Feng, X. Chem. Eur. J. 2009, 15(24), 6008–6014. Ishihara, K.; Furuya, Y.; Hattori, Y.; Hatano, M. Org. Lett. 2009, 11(11), 2321–2324. Abell, J. P.; Yamamoto, H. J. Am. Chem. Soc. 2009, 131(42), 15118–15119. Wang, J.; Hu, X.; Jiang, J.; et al. Ang. Chem. Int. Ed. 2007, 46(44), 8468–8470. Wang, J.; Wang, W.; Li, W.; et al. Chem. Eur. J. 2009, 15(43), 11642–11659. Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120(21), 5315–5316. Blacker, J.; Clutterbuck, L. A.; Crampton, M. R.; Grosjean, C.; North, M. Tetrahedron: Asymmetry 2006, 17(9), 1449–1456. Khan, N. H.; Saravanan, S.; Kureshy, R. I.; et al. J. Organomet. Chem. 2010, 695(8), 1133–1137. Krueger, C. A.; Kuntz, K. W.; Dzierba, C. D.; et al. J. Am. Chem. Soc. 1999, 121(17), 4284–4285. Porter, J. R.; Wirschun, W. G.; Kuntz, K. W.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122(11), 2657–2658. Josephsohn, N. S.; Kuntz, K. W.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123(47), 11594–11599. Deng, H.; Jung, J.-K.; Liu, T.; et al. J. Am. Chem. Soc. 2003, 125(30), 9032–9034. Mansawat, W.; Bhanthumnavin, W.; Vilaivan, T. Tetrahedron Lett. 2003, 44(19), 3805–3808. Banphavichit, V.; Mansawat, W.; Bhanthumnavin, W.; Vilaivan, T. Tetrahedron 2004, 60(46), 10559–10568. Banphavichit, V.; Mansawat, W.; Bhanthumnavin, W.; Vilaivan, T. Tetrahedron 2009, 65(29–30), 5849–5854. Seayad, A. M.; Ramalingam, B.; Yoshinaga, K.; Nagata, T.; Chai, C. L. L. Org. Lett. 2010, 12(2), 264–267. Masumoto, S.; Usuda, H.; Suzuki, M.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125(19), 5634–5635. Kato, N.; Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2004, 45(15), 3147–3151. Kato, N.; Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2004, 45(15), 3153–3155. Kanai, M.; Kato, N.; Ichikawa, E.; Shibasaki, M. Pure Appl. Chem. 2005, 77(12), 2047–2052. Kato, N.; Mita, T.; Kanai, M.; et al. J. Am. Chem. Soc. 2006, 128(21), 6768–6769. Keith, J. M.; Jacobsen, E. N. Org. Lett. 2004, 6(2), 153–155. Karimi, B.; Maleki, A. Chem. Commun. 2009, 34, 5180–5182.

327