oxaziridines: new perspectives and insights · 2009. 2. 16. · ogata,y.; sawaki, ,y. j. am. chem....

Oxaziridines: New Perspectives and Insights

Konstantinos RampalakosMichigan State University

11/26/2003

Oxaziridines

ON

R''

R'

RFirst discovered by Emmons in 1957

(Emmons, W. D. J. Am. Chem. Soc. 1957, 79, 5739)

Strained three-membered ringWeak N-O bond Unusual Reactivity

Summary of Oxaziridines’ Reactivity

O N

C

Nu Nu

Nu

hv

Photochemical Rearrangements

O-transfer N-transfer

Outline

• Introduction• Heteroatom Transfer Reactions

N vs O Transfer: Mechanistic ConsiderationsO Transfer Reactions N Transfer Reactions

• Photochemical Rearrangement Reactions• Conclusion

Oxaziridines: Common Methods of Preparation

R'

O

R R'

HNOH2NOSO3H / OH-

R 50-73 %

(method prefered for N-H oxaziridines)

N

O

mCPBA56 %

N

O

O

K2CO3

NSO2

NOS

O2

KHSO5

9O %K2CO3

Andreae, S.; Schmitz, E. Synthesis 1991, 327-341

Davis, F. A.; Towson, J. C.; Weismiller, M. C.; Lal, S.; Carroll, P. J. J. Am. Chem. Soc. 1988, 110, 8477-8482Widmer, J.; Schierlein, W. K. Helv. Chim.Acta 1974, 57, 657-664

Mechanism of Imine Oxidation by Peroxy Acids

NR

ONHO

O

Ar

R NO

R

mCPBA

Two-stage mechanism (Baeyer-Villiger type)

slow fast

NR

mCPBAN

RO

O

H O

Ar

NO

R

Concerted mechanism

The two-stage mechanism is generallyaccepted for the reaction

Ogata,Y.; Sawaki, ,Y. J. Am. Chem. Soc. 1973, 95, 4687

Outline




O-Transfer: Mechanistic Considerations

ρ = 0.55−1.03

NO

SO O X

Y

Nu "Nu O"

NuN

O

R R

RN

O

R R

R

R R

NR

"Nu O"

Mechanism for Oxygen Transfer to various Nucleophiles:

δ+

δ−

δ−Nu

Davis, F. A.; Billmers, J. M.; Gosciniak, D. J.; Towson, J. C. J. Org. Chem. 1986, 51, 4240-4245

N-Transfer: Mechanistic Considerations

ON

First stable N - transfer

O

OMe

ρ = 0.83

O N

O N NHMoc

oxaziridine

X

NuO

N ON

O"Nu NR"

Mechanism for Nitrogen Transfer to various Nucleophiles:

δ+

δ−

δ−Nu

R R

Vidal J.; Damestoy S.; Guy L.; Hanachi J,; Aubry A.; Collet A. Chem. Eur J. 1997, 3, 1691-1709

Rationalizing N vs O transfer: A Balance Between Electronics and Sterics

C

N O

Ph

CO

O

Me- 0.30

-0.24-O.26

+0.19-O.O6

C

N O

Ph

SOO

Me

+0.20

-0.25-0.02

-0.16

-0.16

Electronics: N is more e- defficient than O

Sterics: Bulky N-substituent can make the attack of Nu to N less favored

(O-transfer reagent)(N-transfer reagent)

Electronics: Nitrogen is more electron deficient than Oxygen

NNNNORNH2, RR’NH

O/NNOOOORSMe

00.61.21.82.52.52.54.8A value

HClCO2MeMeCF3Ph2POSO2MetBuN-substituent

Sterics: Nucleophilic attack to Nitrogen can be hindred

C

N O

Ph

CO

O

Me- 0.30

-0.24-0.26

+0.19-0.06

C

N O

Ph

SOO

Me

+0.20

-0.25-0.02

-0.16

-0.16

(O-transfer reagent)(N-transfer reagent)


N vs O Transfer: A suggested Model

O

N

R

R''

R'

smaller R'' groups

Nu"Nu-N"

N

O

R

R'

Nuδ+

δ−

δ−

R''

R, R'' = EWG

Nucleophilic Attack:Electronics Control

O

N

R

R''

R'

R" = bulky group

Nu

Nu"Nu-O"

O

N

R

R''

R'

Nuδ+

δ−

δ−

R, R', R'' = EWG

Nucleophilic Attack: Sterics Control


Outline


N vs O Transfer: Mechanistic ConsiderationsO Transfer ReactionsN Transfer Reactions


N-Sulfonyl (Davis) Oxaziridine

NO

SO

O

X

Y

N-Sulfonyl Oxaziridines (Davis Oxaziridines)

-Oxidation of Sulfides

NO Ph

SPhO

OPh2S Ph2SO

93 %

N O

OO NaHMDS-78 oC,THF

NO

S

PhO

OPh

N O

OO

OH

86 %, 98 % d.e.

-Hydroxylations of Enolates-Hydroxylation of Enolates

Davis, F. A.; Lal, S. G.; Durst, H. D. J. Org. Chem. 1988, 53, 5004.Evans, D. A.; Morrissay, M. M.; Dorow, R. L. J. Am. Chem. Soc. 1985, 107, 4346-4348

Davis Reagent: Epoxidation of Alkenes

60 oC

NO

S

PhNO2

O

OPh

PhPh

PhPh

O

3 h, 47 %

12 h, 95 %

Less Efficient than mCPBA

Davis, F. A.; Malik, N. F.; Award, S. B.; Harakal, M. E. Tetrahedron Lett. 1981, 917

Epoxidations with Oxaziridinium Salts

1

N

OCH3

BF4-

10 min, 84 %1h, 81 %

2h, 91 %10 h, 88 %

1h, 89 %3h, 92 %

1mCPBAAlkene

Ph Ph

C7H15

Ph

Ph

Ph

PhO

CO3H

Cl

NO

CH3

BF4-

CH2Cl2, 20 oC,

CH2Cl2, 20 oC,

0.5 h96 %

4 h95 %

O O

O86 %

no Bayer-Villiger product,O O

( )

1

Hanquet,G.; Lusinchi, X.; Milliet, P. Tetrahedron 1993, 22, 423-438Lusinchi, X.; Hanquet, G. Tetrahedron 1997, 53, 13727-13738

Epoxidation with Oxaziridinium Salts: Directing Effects

OO

HO

R

O

H

Epoxidation with peracid

substarate yield cis/trans selectivity

oxaziridinium salt mCPBA

OH

OH

85

95

60 / 40 92 / 8

60 / 40 95 / 5

OON

O

Epoxidation with oxaziridinium salt

δ+δ−

OAc

OAc

92

95

95 / 5 50 / 50

95 / 5 40 / 60

Lusinchi, X.; Hanquet, G. Tetrahedron 1997, 53, 13727-13738

The first Chiral Oxaziridinium Salt

H2N Ph

Me OH

NH

MePhOH

NH

MePh

N

MePh

N

MePh

BF4-

N

MePh

BF4-

O

1) MgSO4/CH2Cl22) NaBH4/EtOH

CF3COOH/ H2SO4

1) NaOCl2) NaOMe

Me3O+BF4-/

MeOHmCPBA/ CH2Cl2

20oC

single diastereomer23 % overall yield

Preparation

O

Ph

Ph

Ph

Ph

Ph

Ph

Ph O

Ph O

PhO

Ph O 63 %42 % ee

50 %23 % ee

10 %12 % ee

65 %5 % ee

Epoxidation reactions (Stoichiometric)

Bohe, L.; Hanquet, G.; Lusinchi, M.; Lusinchi, X. Tetrahedron Lett. 1993, 34, 7271-7274 Bohe, L.; Lusinchi, M.; Lusinchi, X. Tetrahedron 1999, 55, 141-154

Catalytic Asymmetric Epoxidation using a ChiralIminium Salt

Ph

Ph Ph

Ph O

0.05 eq 1, 2 eq KHSO5, 4 eq NaHCO3

MeCN / H2O (3%), ~10 h, rt

Ph

Ph O

Ph

Ph

KHSO5

KHSO4

N

MePh

BF4-

N

MePh

BF4-O

35 % e.e. 1

Bohe, L.; Lusinchi, M.; Lusinchi, X. Tetrahedron 1999, 55, 141-154

Iminium Salts with “Exocyclic Chirality”

Preparation:

O O

Br

Br2, CCl4

1 h

HBr (conc.)

10 min, 65 %

Br

O

R*NH2, NaBPh4,EtOH/MeCN2 h, 0oC to rt

N

BPh4-

70 %

1

Ph

Ph

PhPh

PhPh

Alkenecat..load. (mol %) yield (%) ee (%)

majorenantiomer

0.55

5

510

5

6868

73

7875

72

2740

63

6873

15

(R,R)(R,R)

(S,S)

(R,R)(R,R)

(R,R)

RR''

R'

RR''

R'

O

Oxone, Na2CO3,CH3CN, H2O, 0oC

Epoxidation Reactions:

1

Page, P. B.; Rassias, G.; Bethel, D. Schilling, M. B. J. Org.Chem. 1998, 63, 2774-2777

Stereocontrol of Oxaziridination is a possible problem

N RO

*

N R *

X-

N R *

OOSO3-

N R *

OOSO3-

N R *O

- SO42-

- SO42-

KHSO5

R3 R2

R1R3 R2

R1O

R3 R2

R1

R3 R2

R1OR3 R2

R1

O R3 R2

R1O

Page, P. B.; Rassias, G.; Barros, D. Ardakani, A.; Slawin A. J. Org. Chem. 2001, 66, 6926-6931

Can Functionalized Iminium Salts Solve the Problem of Stereocontrol ?

N

O

O

Ph

BPh4-

N OPh

OO

N

OO

N

OO

N

O

O

major

minor

O

ring flip

( a )

( b )

A B(X-Ray

Supported)


Catalytic Asymmetric Epoxidation Using a Functionalized Iminium Salt

64%, 20% ee (R)68%, 8% ee (R)

64%, 49% ee (1S,2R)73%, 20% ee (1S,2R)

52%, 17% ee (1S,2R)34%, 3% ee (1S,2R)

54%, 59% ee (S)43%, 5% ee (S)

52%, 52% ee (1S,2S)72%, 15% ee (R,R)

Catalyst 2Catalyst 1Epoxide

Me

PhO

OPh

Me

Ph

OPh

Ph

Ph

O

OPh

N

BPh4-

1

N

O

O

Ph

BPh4-

2

(Conditions: Oxone (2eq), Na2CO3 (4eq), H2O/MeCN (1:1), 0oC, 5 mol% catalyst)


Intramolecular Epoxidation of UnsaturatedOxaziridines

R1 R3

Nn

BnO

1. MeOTf, CH2Cl2, 0oC

R1

R2

R3

On

O2. aq. NaHCO3

R2

39%1HHnBu

41%2MeMeH

39%1MeMeH

Yield nR3R2R1

O O

OHmCPBACH2Cl2 OH

O

1. BnNH2, CH2Cl22. Oxone, NaHCO3, CH3CN / H2O

1. MeOTf, CH2Cl22. Aq. NaHCO3

NO

Bn

O

47 % 48 %

Regioselectivity can be complementaryto that of mCPBA

Armstrong, A.; Draffan, A. G. J. Chem. Soc., Perkin Trans. I 2001, 2861-2873

Stereoselective Intramolecular Epoxidation in Unsaturated Oxaziridines

O

nPrO

NPh

nPrO

1. MeOTf, CH2Cl2, 0oC2. Aq. NaHCO3

H

90 % de93 % ee

O H

OnPr

Ph

nPr

NO

1. MeOTf, CH2Cl2, 0oC2. Aq. NaHCO3

H

90 % de92 % ee

Armstrong, A.; Draffan, A. G. J. Chem. Soc., Perkin Trans. I 2001, 2861-2873

1

Epoxidations with Perfluorinated

Oxaziridines

NO

Fn-C4F9

C3F7N(C4F9)3

10 mol % SbF5

24h, 120 oC85 %

NC4F9F

C3F7 mCPBA

68 %

Preparation:Preparation:

O86 %

OAcO

OAc

85 %

CFCl3/CHCl3

CFCl3/CHCl3

CFCl3/CHCl3

-40oC 4 min

-40oC 4 min

-40oC 4 min

Epoxidation reactions :

OAc OAcO

88 %

2

3

4

Epoxidation reactions:

1

Arnone, A.; Marteau, D.; Novo, B.; Petrov, V.; Pregnolato, M.; Resnati, G.; J.Org. Chem. 1996, 61, 8805

Epoxidations of Electron Poor Olefins

F F

F X

F F

F XO

NO

F

n-C4F9

n-C3F7

1N

O

F

n-C5F11

n-C6F13

2

X Oxaziridine Conditons Yield(CH3)3Si neat / rt / 20 min 82 %

neat / 100oC / 16 h 60 %21Cl

Arnone, A.; Marteau, D.; Novo, B.; Petrov, V.; Pregnolato, M.; Resnati, G.; J.Org. Chem. 1996, 61, 8805

Oxyfunctionalization of Unactivated C-H Bonds by Perfluorinated Oxaziridines

• Enantiospecific• 3o C-H > 2o C-H > 1o C-H• Equatorial C-H > Axial C-H• Oxenoid Atom Insertion (?)

H

H

H

H

H

H

H

HHO

H

H

H

H

70 %

NO

C4F9 FC3F7

CFCl3,rt

H

BrCl

N

O

n-C4F9 F

n-C3F7

BrCl

OH

CF3Cl, rt, 30 he.e = 96 %e.e = 99%

Resnati, G.; Arnone, A.; Cavicchioli, M.; J. Org. Chem. 1994, 59, 5511-5513Arnone, A.; Foletto, S.;Metrangolo, P.; Pregnolato, M. Resnati, G. J.Org. Chem. 1999, 1, 281-284

Summary for O-Transfer Reactions

Oxaziridinium Salts:

•Very reactive for epoxidation of alkenes.•Syn selectivity for epoxidation of allylic acetates.•Capabe for catalytic asymmetric epoxidation.

Perfluorinated Oxaziridines :

• Very reactive O-donors.•Epoxidation of e- poor alkenesefficient.•Capable for C-H activation.

N

O

F

n-C4F9

n-C3F7

N

OCH3

BF4-

Outline

• Introduction• HeteroatomTransfer Reactions


• Photocemical Rearrangement Reactions• Conclusion

Amination of Ammonia:A large Scale Industrial Process

NH2Cl OH-O O NH

NN HN NH

NH3

The First Stable N-Transfer Oxaziridines: N-N Bond Formation

NHR1

R2 Ar

PGN

ON

R1

R2NH

PGArCHO O

N

O

OtBu

NC

ON

O

OMe

58 %76%PG = Moc, 91 %PG = Boc, 92%

O N NHPG

NN NH

Moc

N CONH2

HNMoc

Ph

OH

NN

PG

H

PG = Moc, 77%PG = Boc, 70%

Ph

OH

NNH

Moc

76 %

H2NCOOH

HNNH

Moc

85 %

HO HNNH

COOH

Moc

67 %

Vidal J.; Damestoy S.; Guy L.; Hanachi J,; Aubry A.; Collet A. Chem. Eur J. 1997, 3, 1691-1709Vidal, J.; Guy, L.; Sterin, S.; Collet, A. J. Org. Chem. 1993, 58, 4791-4793

A More Powerful Reagent For Electrophilic Amination

Cl3CCHON

CCl3

Boc Oxone,K2CO3

H2O / CHCl3

Cl3C

Boc

86%

NO

PPh3 NBoc

O NCDCl3, 20 oC

quantitative

not detected

Cl3CNO

Boc

NO

Moc

O N NHBoc

O N NHMoc

Vidal, J; Hannachi, J. C.; Hourdin, G.; Mulatier, J. C.; Collet, A. Tetrahedron Lett. 1998, 39, 8845-8848

O-Amination of Alcohols: Synthesis ofHydroxylamines

OH1) LiHMDS, -78oC, THF

2 )O

NHBoc

> 95 %N

O

Boc

Cl3C

ONHBoc

80 %

O

NHBoc

85 %

O

NHBoc

50 %

ONHBoc

96 %

ONHBoc

80 %

MeO

ONHBoc

87 %

O

ONHBoc

80 %

Foot, O. F.; Knight, W. Chem. Comm. 2000, 975-976

O-Amination of Alcohols: One-pot Preparation ofOximes

ONH2

86 %

ONH2

70 %

ONH2

10 %

O

N

ON

51 %

78 %

O

HO

NO

56 %

NH m-CPBA

CH2Cl2, 0oC, 1h 49 %

HN O

R OH1) KH, DMPU, 0oC

2)R ONH2

O

O

HAcOH, rt,10 min

H

RON

HN O

Choong, I. C.; Ellman, J. A. J. Org. Chem. 1999, 64, 6528-6529

S-Amination : O-Transfer is a Problem

Ph

MocNH

< 15 min

S SNMoc

SO

O

Solvent T (oC) Amination/Oxidation

CDCl3

CDCl3

CDCl3

CH3CN

CH3CN

CH3CN

19

0

-34

19

0

-35

34 / 66

45 / 55

52 / 48

48 / 52

58 / 42

67 / 33


Improved Control of N vs O Transfer to S-Nucleophiles

SN

O

NEt2SO

S Cl

NO

O

NEt2

>10/1-40CF3CH2OH

10/119CF3CH2OH

7/1-30MeOH

3/119MeOH

2/3-40THF

1/419THF

1/3-40Toluene

<1/1019Toluene

N/OTemp. oC Solvent

Ar

ON

COOR

O N

Ph

COORvsPh S

Me

PhSMe

δ−

δ+ δ−

δ+

(more polar TS)

Armstrong, A.; Edmonds, I. D.; Swarbrick, M. E. Tetrahedron Lett. 2003, 44, 5335-5338

High Degree of N-Transfer with a Novel Oxaziridine

1. BocN=PPh3, THF

2. mCPBA, BuLi CH2Cl2

50 %

EtOOC COOEt

O

EtOOC COOEt

O NBoc

>98/2-40CDCl3

95/50CDCl3

90/1010CDCl3

N/OT (oC)Solvent

S SNMoc

SO

EtOOC COOEt

O NBoc

EtOOC COOEt

N BocO

Nu

H

N BocO

Nu

NC

(less favored)

Transition States for Byproduct’s Formation:

Armstrong, A.; Cooke, R. S. Chem. Comm. 2002, 904-905

Tandem Amination of Sulfides / [2,3]-sigmatropic rearrangement

SPh R1

R2

NBocR1

R2

PhSEtOOC COOEt

1.05 eq

O NHBoc

CH2Cl2-40 oC

R1

SX

R2

R2

BocNS X

R1

EtOOC COOEt1.05 eq

O NHBoc

CH2Cl2-40 to rt 1h

13MeMeN-hexyl

88PhHPh

66COOtBuHPh

85IHPh

Yield (%)R2R1X

85COOMeH

73PhH

93MeMe

94MeH

Yield (%)R2R1

Armstrong, A.; Cooke, R. S. Chem. Comm. 2002, 904-905

Armstrong, A.; Cooke, R. S.; Shanahan, S. E. Org. Biomol. Chem. 2003, 1, 3142-3143

C-Amination of Enolates with N-HOxaziridines

HN NH

O

O O

HN NH

O

O ONH2

HN NH

S

O O

HN NH

S

O ONH2

78 %

82 %

HN NBn

OO

O O

OO

O O

OON

N NBn

HONH2

O90 %

79 %

ONH

aq. NaOH

aq. NaOH

aq. NaOH

aq. NaOH

Andreae, S.; Schmitz, E. Synthesis 1991, 327-341

Amination of Enolates: Aldol Addition is Competing

NO

NC

Boc

OLI

Ph

O

PhNHBoc

OLi

BuO

O

BuONHBoc

O N

OLi

Ph

O

O N

O

Ph

O

NHBoc

35 %

33 %

38 %

O

Ph

major problem: aldol addition

O

BuO

OH

CN

O N

O

Ph

O

HO

CN

HO

CN

“significant amounts”

Vidal, J.; Guy, L.; Sterin, S.; Collet, A. J. Org. Chem. 1993, 58, 4791-4793

Can ortho-substituted Oxaziridines Slow Down the Aldol Addition ?

O

X H2N

O

NEt2

p-TsOH (cat)

PhMe, heat

N

X

O

NEt2mCPBA, nBuLi

CH2Cl2,-78 to rt

N

X

O

NEt2O

> 92 % trans

7552-CN

10314-Cl

--2,6-diCl

--2-Cl

2039 4-CN

21X

ButO

O 1. LDA, THF

2. oxaziridine

ON

O

NEt2H

tBuO

O

tBuO

OH

X1 2

Armstrong, A.; Atkin, M. A.; Swallow, S. Tetrahedron Lett. 2000, 41, 2247-2251

Asymmetric Amination of Carbanions

R CN1. LHMDS, THF, -78oC

2. oxaziridine NNH2

N

0NH2

R

(R) (S)O

R

OHN

23577CN

33484 1-naphthyl

523192-naphthyl

50555Ph

de (%)Yield (%)Time (h)R

Page, P. B.; Limousin, C.; Murrell, V. L. J. Org. Chem. 2002, 67, 7787-7796

Summary for N-Transfer

ON

O

OtBu

NC

Cl3C

BocN

OO NH HN O

N-Amination,O-Amination

Unstable N-Amination O-Amination

NO

O

NEt2

Cl

COOEtEtOOC

O NBoc

S-AminationS-Amination

NO

O

NEt2

CNO

HN

Enolate Amination Carbanion Amination

Outline




Photochemical Rearrangements:General Pattern

R ON

R'R

hν RN

R'R

O Alkyl migrationN

O

R R'

R

Photochemical Rearrangement ofOxaziridines: Puzzling selectivity !

NROMe hv N

Me

O RN

O

Me

R

95 5

NR

MeO N

R

MeO

NR

MeO

( less stable radical predominates )

( b )( a )

:

NO

Me

hvMe

O

N not objerved : N

O

Me

(more strained product predominates)

b)

2)

1)

Oliveros, E.; Riviere, M.; Lattes, A. Nouv. J. Chim. 1979, 3, 739-753

Rarello, J.; Riviere, M.; Desherces, E.; Lattes, A. C. R. Hebd. Seances Acad. Sci., Ser. C 1971, 273, 1097-1100

Theoretical Studies Gave the Answer

Stepwise mechanism:a) Photochemical breaking of the N-O bondb) H (or R) migration to N

NO

HHAHS

A

NO

HSHA H

NO

HS HAH

NO

HSHHA N

O

HS

H

HA

NO

HA

H

HS

TSS

TSA

E

- 5 kcal/mol0 kcal/mol

19.8 kcal/molTSS

TSA

A

The bond anti to the N-lone pair is cleaved more easily

Oliveros, E.; Riviere, M.; Malrieu, J. P.; Teichteil, C J. Am. Chem. Soc. 1979, 101, 318-322

Basic Concepts that can be Exploited

NO

R'

hvR NR

OR'

a)

Migration of less substituted group

Asymmetric Ring Expansion

N Ph

Me

O

tBu

hv NO

tBu

Ph

Me

NO

tBu

PhMe

13 : 1

b)

Lattes, A.; Oliveros, E. J. Am. Chem. Soc. 1982, 104, 3929-3934

Applications of Photochemical rearrangement of Oxaziridines

N O

Me R

Ph Ph

OBnO2C N

O hv

benzene

N O

Me R

Ph Ph

OBnO2C N

O

H

H

N

H

O

N

O

48 %

hv (rarely synthesized bicyclic skeleton)

NN

OCOOCH3

H HH

hv, CH3CN

68 % N

COOCH3

HNH H

O N

COOCH3

HNH H

H

(+)-Alloyohimbane(d.r. = 2.2 : 1)

Wenglowsky, S.; Hegedus, L. J. Am. Chem. Soc. 1998, 120, 12468-12473Bourguet. E.; Baneres, J. L.; Girard, J. P.; Parello, J.; Vidal, J. P.; Lusinchi, S.; Declercq, J. P. Org. Lett. 2001, 3, 3067-3070

Aube, J.; Ghosh, S.; Tanol, M. J. Am. Chem. Soc. 1994, 116, 9009-9018

ConclusionsOxaziridines show a diversity in reactivity

that can be very useful in Organic Synthesis:

•Oxaziridinium salts are systems that can be furtherdevelopped in catalytic asymmetric epoxidations.

•Perfluorinated oxaziridines’ reactivity should be explored more, especially in C-H activation reactions.

•N-transfer oxaziridines are very useful for electrophilic amination processes.

•Oxaziridines’ photochemical rearrangement is a valuable method for lactam synthesis.

Thanks-Giving To:

Proffesor W. D. Wulff

Wulff’s Group:

ThaliaKyoungsoo ChrysoulaSoong-hyun

ManishVijayYuZhenjieGangDingKeith

LianJieGlennVictorCoryChunruiReddyYongheng

oxaziridines: new perspectives and insights · 2009. 2. 16. · ogata,y.; sawaki, ,y. j. am. chem....

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