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Topics in Organic Chemistry > Cyclic compounds > Flashcards

Cyclic compounds Flashcards

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1
Q

acyclic compounds

A

high flexible (bond rotation)

lots of low energy conformations

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2
Q

cyclic compounds

A

less flexible

one low energy conformation

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3
Q

what does axial attack result in?

A

eq. addition of group

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4
Q

eq. addition of group

A

axial addition of group

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5
Q

stereoelectronics for large nucleophiles

A

[prefer eq attack]

suffer steric interactions with diaxial H during axial attack

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6
Q

stereoelectronics for small nucleophiles

A

[prefer axial attack]

rehybridisation (sp2 -> sp3) during axial attack pushes C=O down away from neighbouring C-H

reduces torsional strain (= favoured)

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7
Q

why are early transition states less selective?

A

[exothermic reactions]

resemble reactants - doesn’t feel developing strain with twist-boat product

less differentiation between competing TS = less selectivity

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8
Q

solution to reduced selectivity

A

use enamines as Nu

less reactive - later TS

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9
Q

issue with using enamines

A

competing N-alkylation

lower yield

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10
Q

how would you access cis-isomer?

A

rely on thermodynamic control

LDA, THF @-78 then MeI

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11
Q

why is the attack during the alkylation of cyclohexanone always axial?

A

when electrophile attacks π system, it can only interact with p-orbital if it’s above or below

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12
Q

regiochemical control in cyclohexene epoxides

A

-OH and LG must be trans

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13
Q

types of bicyclic compounds

A

fused = 1 bond in common

spiro = 1 atom in common

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14
Q

decalin

A

preferred = trans

H at both ring junctions = axial alkylation

any other sub. = bad 1,3-diaxial interaction; switch to other face

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15
Q

cis-fused rings - epoxidation

A

cis + stereospecific

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16
Q

cis-fused rings - bromonium ion reactions

A

trans + stereospecific

bromonium ion can be formed on outside of rigid structure

water forced to add from inside + attacks less hindered end of bromonium ion

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17
Q

how to force epoxidation to be stereoselective?

A

block 1 face of ring with sub.

approaches less hindered face of ring

EXCEPTION = sub. is OH
- attacks face of alkene syn to OH (can H bond - stabilise transition state)

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18
Q

cyclobutene structure

A

planar

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18
Q

cyclobutane structure

A

puckered

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19
Q

Nu addition to cyclobutanones

A

selective for attack on C=O opposite C3 (regardless of Nu size)

axial attack is preferred

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20
Q

electrophilic attack on cyclic enolates

A

on face opposite large sub. (due to sterics)

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21
Q

what are the 2 low energy conformations for 5-membered ring systems?

A

envelope and half-chair

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22
Q

5-membered rings substituent preferences

A
  1. single sub. prefer eq in envelope
  2. carbonyl/exo C=C prefer flat position on half-chair
  3. endocyclic double bonds can only accommodate end of envelope
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23
Q

Nu addition to cyclopentanones - small vs large hydrides

A

small - axial

large - equatorial

24
Q

which are less reactive - cyclohexanones or cyclopentanones?

A

cyclopentanones

due to torsional strain during sp2 -> sp3 hybridisation

25
Q

electrophilic addition to cyclopentenes (4 sub.)

A

selectivity occurs on opposite face to sub.

not sterics - eq groups don’t block either C=C face

torsional control -> eq and electrophile add to top face to relieve torsional strain in TS

26
Q

electrophilic addition to cyclopentenes (3 sub.)

A

electrophiles tend to add to same face as sub.

goes against sterics - linked to torsional strain of TS

27
Q

SRS strategy

A

[self-regeneration of stereocentres]

involves generation of temporary stereogenic centre, allowing original to be desteroyed + then reset diastereoselectivity

28
Q

substrate directed reactions

A

reagents preassociate with polar functional groups

interactions = attractive

provide stereochemical outcome opposite to steric predictions

involve H bonding, covalent or Lewis acid-base interactions

29
Q

examples of substrate directed reactions

A
  1. hydroxyl-directed cyclopropanation/epoxidation (same face)
  2. cyclic allylic amines - H bond-directed epoxidation if they are first protonated on nitrogen (same face)
  3. homogeneous hydrogenation of alkenes using cationic iridium/rhodium (same face)
30
Q

bridged bicycle

A

“monocycle with bridge over it”

31
Q

can trans-fused ring systems flip?

A

no - structure too rigid

32
Q

additions to trans-enolates - axial sub. on ring fusion = H

A

axial attack of E = kinetically favoured

due to slight chair-like character in TS

33
Q

additions to trans-enolates - axial sub. on ring fusion = NOT H

A

sterics = more important

enolates have early TS - energy penalty for twist-boat is very small

34
Q

cis-fused bicyclic systems

A

attack = less hindered face

35
Q

cis-fused bicyclic systems - concave vs convex

A

concave = inside face

convex = outside face

36
Q

bridged ring systems

A

steric constraint = additions on less hindered face

37
Q

bridged ring systems - endo vs exo

A

endo = sub. syn to longest bridge

exo = sub. anti to longest bridge

38
Q

selectivity of bicyclic systems

A

highly selective exo attack due to release of torsional strain in TS

H’s pushed downwards to avoid eclipsing interactions with back H’s

39
Q

why are temporary rings inserted into mechanisms?

A

solve problems with acyclic system selectivity (sets stereocentres)

40
Q

carbonyl compounds - ground state conformation

A

C-H bond eclipsed C=O group

= best orbital alignment for stabilising σC-H -> π*C-O

= hyperconjugation

41
Q

why are ketones less reactive than aldehydes?

A

ketones have more hyperconjugation interactions to stabilise C=O group

42
Q

approach trajectory of Nu for C=O

A

BD angle 107

minimises antibonding and maximizes bonding

43
Q

conformation of acetalaldehyde undergoing Nu attack

A

NOT GROUND STATE but staggered conformation

enables σC-H -> π*C-O LUMO-lowering interaction

makes C=O better e- density acceptor

lower LUMO = more reactive

44
Q

polar Felkin-Anh model

A

for aldehydes/ketones with heteroatom alpha-sub. (O, N, S, halogen)

most reactive conformation is when C-X is perpendicular to C=O

C-X = better alpha-acceptor than C-H/C-C + offers > hyperconjugative stabilization of TS

45
Q

polar Felkin-Anh model - effect of elements further down group

A

= lower energy of σ orbitals

46
Q

Cram chelate model

A

for aldehydes/ketones with heteroatom alpha-sub. capable of chelation to metal ion

47
Q

good chelators

A

OR/OH/SH/NR2

Mg2+/Zn2+/Al3+ (high charge density)

48
Q

bad chelators

A

OSiR3 (too bulky)

Na+ / K+ (low charge density)

49
Q

products for polar F-A model and Cram chelate model

A

predict opposite products

50
Q

differentiating between Cram chelate and polar F-A

A

O-protecting group

solvent - strongly Lewis basic solvents can suppress chelation by competing for Lewis acidic metal

also look if chelation is possible

51
Q

Fráter-Seebach alkylation of beta-hydroxyl C=O compounds

A

involves diastereoselective alkylation of chelated dianion

steric effects override torsional strain

rxn occurs on opposite side of Me group despite twist-boat like character early TS

52
Q

1,3-induction in C=O additions

A

stereogenic centre @beta-position = electronegative sub. (OH / OR)

sterics have minor role

53
Q

ketone and H- Nu

A

1,3-syn product

54
Q

Narasaka-Prasad reduction

A

diastereoselective reduction of beta-hydroxy ketones - syn-1,3-diols

must have OH in beta position

via intermolecular delivery of H-
-> chelation with B can only occur if ligand exchange happens with substrate first

55
Q

Evans-Saksena reaction

A

1,3-induction in beta-hydroxy ketone reduction - anti-1,3-diol

CONDITIONS:
Me4N+BH(OAc)3-
AcOH, -20

56
Q

reason for development of Evans-Tishchenko reduction

A

difficult to differentiate -OH groups if R groups were the same

57
Q

Evans-Tishchenko reduction

A

uses aldehyde as reducing agent and SmI2 as Lewis acid catalyst

able to differentiate OH groups as one is selectively protected in process

Topics in Organic Chemistry (3 decks)

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