diastereoselective reactions - lmu

82
3 Diastereoselective reactions 3.3 Chiral substrates Important reviews Chem. Rev. 1993, 1307 (Hoveyda, Evans) Chem. Rev. 1999, 1191 (Reiser) Science 1986, 231, 1108 (Houk) Chem. Rev. 1999, 1265 (Cieplak) Chem. Rev. 1999, 1437 (Mehta) This chapter gives a general overview on selected stereoselective reactions. Exceptions to the models that are proposed here cannot be excluded. 1

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Page 1: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substrates

Important reviews

Chem. Rev. 1993, 1307 (Hoveyda, Evans)

Chem. Rev. 1999, 1191 (Reiser)

Science 1986, 231, 1108 (Houk)

Chem. Rev. 1999, 1265 (Cieplak)

Chem. Rev. 1999, 1437 (Mehta)

This chapter gives a general overview on selected stereoselective reactions.Exceptions to the models that are proposed here cannot be excluded.

1

Page 2: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Cyclic systems

models relie on configurational analysis models relie on conformational analysis

Acyclic systems

steric hindrance coordination steric hindrance coordination

different models, different ways of interpreting the stereoselectivity

and can induce - steric constraints- electronic effects- stereoelectronic effects

2

Page 3: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

seen in Chapter 1

Conformational analysis of acyclic olefins

most stable conformationsfor different acyclic olefins

in following acyclic models, Newman and Sägebock

projectionswill be employed Newman Sägebock

RL is the largest

substituent

3

Page 4: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Heterogeneous hydrogenation of C=C bonds

steric interactions disfavor the approach of H2 from the top face

H2 adds from the least hindered faces of the double bond (bottom)

ChemCatChem 2019, 1518 (Xie, Yu)

4

Page 5: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Heterogeneous hydrogenation of C=C bonds

JOC 1975, 3073 (Sehgal)

JOC 1985, 4270 (Thompson)5

Page 6: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Homogeneous hydrogenation of C=C bonds

J. Organomet. Chem. 1977, 141, 205 (Crabtree)

J. Organomet. Chem. 1981, 216, 263 (Sidebottom)

coordinationligand exchange

oxidativeaddition

migratoryinsertion

reductiveelimination

6

Page 7: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Homogeneous hydrogenation of C=C bonds – cyclic systems

JACS 1974, 6232 (Thompson)

closer the coordination site, higher the selectivity

TL 1984, 4637 (Evans) 7

Page 8: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Homogeneous hydrogenation of C=C bonds – cyclic systems

coordination can circumvent steric effects JACS 1984, 3866 (Evans)

JOC 1986, 2655 (Crabtree)J. Organomet. Chem. 1985, 285, 333 (Hall)8

Page 9: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydrogenation – C=C bonds – acyclic stereocontrol – allylic alcohols

steric constraint

diastereoselectivity rationale

disfavoredfavored

syn (minor)anti (major)

9

Page 10: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydrogenation – C=C bonds – acyclic stereocontrol – allylic alcohols

steric constraint

diastereoselectivity rationale

disfavoredfavored

anti (minor)syn (major)

10

Page 11: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydrogenation – C=C bonds – acyclic stereocontrol – allylic alcohols

JACS 1984, 3866 (Evans)

anti (major)

syn (major)

11

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydrogenation – C=C bonds – acyclic stereocontrol – homoallylic alcohols

steric constraint

diastereoselectivity rationale

anti (minor) syn (major)

disfavored favored

12

Page 13: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydrogenation – C=C bonds – acyclic stereocontrol – allylic alcohols

JACS 1990, 5290 (Evans)

anti (major)

syn (major)

13

Page 14: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – cyclic series

most hindereddiastereotopic face

least hindereddiastereotopic face

about stereoselectivity about regioselectivity

most hinderedolefin site

most d + due tohyperconjugation

least substitutedmost stable

C-B bondd - d +

14

Page 15: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series

Intermolecular hydroboration reactions proceed without coordination ofoxygen-containing groups to the boron atom

RL = large size substituent

RM = medium size substituent

most destabilizinginteraction

15

Page 16: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series – reaction with BH3

most destabilizinginteraction

„steric shield“

rationale onstereoselectivity

Tet. 1984, 2257 (Houk)

16

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series – reaction with BH3

JACS 1979, 259 (Kishi)

TL 1984, 243 (Heathcock)

RL

R

17

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

diastereotopic faces

Hydroboration of chiral olefins – acyclic series

with BH3 with R2BH

18

Page 19: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series

with BH3

RL

RM

19

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series

with R2BH

9-BBN

20

Page 21: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Hydroboration of chiral olefins – acyclic series

with thexylBH2

thexylBH2

21

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Fürst-Plattner rule – cyclic olefins

pseudo-axial

pseudo-equatorial

controls the conformation

of the half-chair

disfavored(twisted boat)

favored(chair)

majordiastereoisomer 22

Page 23: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Bromination – cyclic series (adaptation of the Fürst-Plattner rule)

Explain the outcome of the following reactions

case 1

case 2

23

Page 24: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Bromination – cyclic series (adaptation of the Fürst-Plattner rule)

case 1

controls the conformation

of the half-chair-

not close enoughto direct the

addition of Br+

attacks at these positions lead to a twisted boat transition state (Fürst-Plattner)

„almost statisticaldistribution“

24

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Bromination – cyclic series (adaptation of the Fürst-Plattner rule)

case 2

electronically disfavored

follows the Fürst-Plattner rule (adaptation)

electronically favored

stabilized notstabilized

exclusiveproduct 25

Page 26: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Bromination – cyclic series (adaptation of the Fürst-Plattner rule)

exclusiveproduct

least hindereddiastereotopic face

26

Page 27: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Bromination – cyclic series (adaptation of the Fürst-Plattner rule)

least hindereddiastereotopic face

exclusive product

27

Page 28: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Oxy-mercuration – cyclic series (adaptation of the Fürst-Plattner rule)

least stabilized carbocation

electrophilic position leadingto a chair conformation

28

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

„Onium“ formation in acyclic seriespC-C

s*C-O

less stable(more reactive)

more stable(less reactive)

faster complexation

more stable productdestabilizing

interaction

slower complexationreaction with „X+“

29

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favored

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Iodonium formation - acyclic series

30

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favored

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Iodine-promoted lactonization – R is a nucleophile

less stable(more reactive)

more stable(less reactive)

faster complexation

slower complexation31

Page 32: Diastereoselective reactions - LMU

diastereoselective TS

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – endocyclic allylic alcohols and peracids

diastereoselective TS

32

Page 33: Diastereoselective reactions - LMU

diastereoselective TS

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – endocyclic allylic ethers and peracids

diastereoselective TS„H“ is much more acidic

33

Page 34: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – endo/exocyclic homoallylic alcohols and peracids

diastereoselective TSACIE 2015, 15884 (Didier)

JCS 1965, 2054 (Meakins)

34

Page 35: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic allylic acohols – mCPBA vs. VO(acac)2/TBHP

the modelization and stereochemical outcome depend on the method employed

with mCPBA with „[V]-O“

ca. 120 °ca. 40 °

35

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic allylic acohols with mCPBA

stereochemical considerations

favoreddisfavored

36

Page 37: Diastereoselective reactions - LMU

favored

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic allylic acohols VO(acac)2/TBHP

stereochemical considerations

disfavored

37

Page 38: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic allylic acohols VO(acac)2/TBHP

stereochemical considerations

destabilizing 1,2-interactions

38

Page 39: Diastereoselective reactions - LMU

favored

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

stereochemical considerations

JACS 1981, 7690 (Mihelich)

control element: 1,3-strain

39

Page 40: Diastereoselective reactions - LMU

favored

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

stereochemical considerations

JACS 1981, 7690 (Mihelich) 40

Page 41: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

anti-diastereoisomer syn-diastereoisomer

What diastereoisomer supposedly gives the highest diastereoselectionunder oxidative conditions with VO(acac)2/TBHP?

41

Page 42: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

anti-diastereoisomer

ax-ax

eq-eq

42

Page 43: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

syn-diastereoisomer

ax-eq

eq-ax

most destabilizing1,3-interactions

least destabilizing1,3-interactions

43

Page 44: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic homoallylic acohols VO(acac)2/TBHP

anti-diastereoisomer syn-diastereoisomer

What diastereoisomer supposedly gives the highest diastereoselectionunder oxidative conditions with VO(acac)2/TBHP?

full minimization of 1,3 strain=

better diastereoselectivity

one of the Me group has to be in a pseudo-axial position

gives > 400:1 dr gives a 85:1 dr

44

Page 45: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Epoxidation – acyclic bishomoallylic acohols VO(acac)2/TBHP

stereochemical considerations

TL 1978, 2741 (Kishi) 45

Page 46: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Cyclopropanation – zinc and samarium carbenoids

pC=CHOMO

s*C-I

LUMO

p*C=C

LUMO

sC-ZnHOMO

s*C-O

pC=C stabilization through orbital overlap

=less reactive

46

Page 47: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Cyclopropanation – cyclic olefins – reaction rates

which of the two following substrates reacts faster?

47

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Cyclopropanation – cyclic olefins – reaction rates

krel > 3 krel = 1

48

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3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

Cyclopropanation – acyclic olefins

stereochemical considerations

conformation of lowest energyfavored TS

49

Page 50: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

[3,3]-sigmatropic rearrangement – Claisen / Ireland-Claisen – cyclic substrates

reactiveconformation

unreactiveconformation

50

Page 51: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

[3,3]-sigmatropic rearrangement – Claisen / Ireland-Claisen – cyclic substrates

reactiveconformation

reactiveconformation

reaction should be slower for the trans isomer

51

Page 52: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesAlkenes

[3,3]-sigmatropic rearrangement – Ireland-Claisen – acyclic substrates

(Z)minor

(E)major

JOC 1991, 650 (Ireland)52

Page 53: Diastereoselective reactions - LMU

favored

3 Diastereoselective reactions

3.3 Chiral substratesDiels-Alder

[4+2]-cycloaddition – chiral cyclic diene

stereochemical considerations

OL 2017, 2114 (Didier)

53

Page 54: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesDiels-Alder

[4+2]-cycloaddition – chiral cyclic dienophile

stereochemical considerations

OL 2000, 2711 (Rawal)

favored54

Page 55: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesDiels-Alder

[4+2]-cycloaddition – chiral acyclic diene

stereochemical considerations

JOC 2018, 783 (Didier)

favored TS steric clash 55

Page 56: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesDiels-Alder

[4+2]-cycloaddition – example of chiral acyclic diene/dienophile (intramolecular)

stereochemical considerations

JOC 1987, 1236 (Marshall)

56

Page 57: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesSN2 vs. SN2‘

SN2 reactions

SN2‘ reactions

SN2 is stereospecific

SN2‘ is stereoselective

achiral achiral

achiral chiral

X = leaving group

a-substitution

g-substitution

57

Page 58: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesElectrophilic allylation

SN2‘ reaction – with organocopper reagents

substrate/reagent interactions stereochemical considerations

ACIE 2019, 1509 (Knochel)

dxz

HOMO

p*C=C

LUMO

anti-substitution

58

Page 59: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesElectrophilic allenylation

SN2‘ reaction – with organocopper reagents and propargylic substrates

formation of the nucleophile stereochemical considerations

OL 2011, 4462 (Oestreich)

activenucleophile

species

syn-carbometalation

anti-elimination

59

Page 60: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesElectrophilic allenylation

SN2‘ reaction – with organocopper reagents and propargylic substrates

stereochemical considerations

Chem. Sci. 2020, xxx (Knochel)

syn-carbometalation

anti-elimination

60

Page 61: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbometalation

Beilstein JOC 2013, 278 (Yorimitsu)

Chem. Soc. Rev. 2016, 4552 (Marek)

Carbometalation reactions

or carbometallation:addition of a C-[M] bond across a C-C unsaturated system leading to a new organometallic species

carbometalation reactions are syn-stereospecific (most cases)

61

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3 Diastereoselective reactions

3.3 Chiral substratesCarbometalation

Carbometalation – generalities

Alkynes Cyclopropenes

X = C, Si, S, O, N, P...Y = NR2 or OR

= coordinating group= bulky group

[Cu] =organocuprate

reagent

[M] =[Cu], [Mg] or [Zn]

62

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3 Diastereoselective reactions

3.3 Chiral substratesCarbometalation

Carbometalation – chiral cyclopropenes

JACS 2002, 14322 (Fox)

CEJ 2014, 1038 (Marek)

63

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3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – cyclic substrates

ACIE 2019, 1188 (Didier)

64

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3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – cyclic oxocarbenium ions

65

stereochemical considerations

JACS 2000, 168 (Woerpel)

favored

Page 66: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – cyclic oxocarbenium ions

66

stereochemical considerations

JACS 2000, 168 (Woerpel)

favored

stabilizedconformation

reactiveoxocarbenium ion

Page 67: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – acyclic substrates

Chem. Rev. 1999, 1191 (Reiser)

Important review on directed 1,2- carbonyl additions

Inspired by D. Evans and A. Myers lecture notes:https://www.pdfdrive.com/evans-and-myers-organic-chemistry-lecture-

notes-chem-206-and-215-e183957509.html 67

Fischer Cram Conforth FelkinAhn-

EisensteinCieplak Tomoda

Page 68: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

68

Cram chelate model

If chelation between the carbonyl group and one of the substituents of the a-stereocenter facilitated by a metal cation can occur, the substrate will be locked into a defined conformation

JACS 1952, 5828(Cram)

1987

Page 69: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

69

stereochemical considerations

TL 1992, 1817 (Grieco)

Page 70: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

70

stereochemical considerations

TL 1980, 1031 (Still)

Page 71: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

71

Cram model

If chelation cannot occur, steric effects have to be considered. It was assumed (at the time) that the decisive interaction to be avoided is between RL and the carbonyl group.

Felkin-Ahn model RL is placed orthogonal to the carbonyl group.

favoredfavored

Crammodel

Felkin-Ahnmodel

Page 72: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

72

Cram model

If chelation cannot occur, steric effects have to be considered. It was assumed (at the time) that the decisive interaction to be avoided is between RL and the carbonyl group.

favored

“The Cram rule proved to be a reliable tool to explain the preferred diastereoselection in carbonyl addition if no polar substituents were present on the a-stereocenter”

Chem. Rev. 1999, 1191 (Reiser)

“[…] The steric bulk of the carbonyl group was overestimated, resulting in an unfavorable alignment of RL and R, especially in ketones (R ≠ H)”

Page 73: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Cram‘s model

73

Cram modelIn summary: good model for additions onto aldehydes with no polar groups at the a-position

JOC 1990, 4990 (Molander)

favored

unfavored

Page 74: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Felkin-Ahn‘s model

74

Felkin-Ahn modelIn summary: complements and generalizes the initial model proposed by Cram

JCS Perkin Trans. 2 1983, 1645 (Pérez-Ossorio)

unfavored

favored

Page 75: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Felkin-Ahn‘s model – Influence of reagents and substrate nature

75

unfavored

favored

Size of the nucleophile

Size of the conter ion

Size of R in substrate

MeMgBr

Page 76: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Felkin-Ahn model

76

Previous models rely on steric analysis and therefore, cannot fully explain the differences in following selectivities

Page 77: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Ahn-Eisenstein considerations

77

Ahn-Eisenstein considerationsBest acceptor s* orbital is oriented antiperiplanar to forming bond

sC-Ph

Csp3-Csp

3 Csp3-Csp

2

sC-c-hex

s*C-c-hex

s*C-Ph

Page 78: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Felkin-Ahn-Eisenstein model

78

stereochemical considerations

TL 1984, 265 (Keck)

major

Page 79: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

79

stereochemical considerations

TL 1983, 2653 (Oishi)

major

1,2-addition – Felkin-Ahn-Eisenstein model

Page 80: Diastereoselective reactions - LMU

favored

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

1,2-addition – Felkin-Ahn-Eisenstein and Conforth models

80

Felkin-Ahn-Eisenstein modelBest acceptor s* orbital is oriented antiperiplanar to forming bond

Conforth modelConformational analysis rely on minimization of the dipolemoment

Conforthmodel

Felkin-Ahn-Eisensteinmodel favored

Page 81: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

Allylboration – Combination of Felkin-Ahn and Zimmermann-Traxler models

81

stereochemical considerations

for steric reasons, RL points

preferentially outward

(Z)-allylboronspecies

destabilizing interactions:1: 1,3-diaxial

2: gauche

favored

(2,3-syn-3,4-anti)

Page 82: Diastereoselective reactions - LMU

3 Diastereoselective reactions

3.3 Chiral substratesCarbonyls

Allylboration – Combination of Felkin-Ahn and Zimmermann-Traxler models

82

stereochemical considerations

for steric reasons, RL points

preferentially outward

(E)-allylboronspecies

destabilizing interactions:1: 1,3-diaxial

2: gauche

favored

(2,3-anti-3,4-syn)