Conformation Analysis + Diastereoselectivity

Question 1

configuration

Answer 1

physical arrangement of same atoms that can only be interconverted by breaking bonds

Question 2

conformation

Answer 2

physical arrangement of same atoms that can only be interconverted by rotation of bonds

Question 3

diastereomers

Answer 3

e.g. R,S vs S,S

Question 4

prochirality

Answer 4

has potential to become a chiral centre after addition/substitution

Question 5

stereogenic/chiral centre

Answer 5

atom with 4 different substituents

Question 6

stereoisomers

Answer 6

different spatial arrangement

e.g. S,S vs R,R

Question 7

chiral

Answer 7

mirror images CAN’T be superimposed

Question 8

syn

Answer 8

either both facing forward or backwards

Question 9

why are eclipsed conformers so high in energy?

Answer 9

C-Cσ -> C-Cσ donation = v. repulsive (+ sterics of R groups)

as opposed to staggered - C-Cσ -> C-Cσ* (slightly stabilising) + R groups further apart

Question 10

rings with 3-carbons

Answer 10

planar

highly strained => high energy

Question 11

rings with 4-carbons

Answer 11

puckered/butterfly

lower in energy compared to planar

Question 12

rings with 5-carbons

Answer 12

envelope

lower in energy compared to planar + puckered

Question 13

rings with 6-carbons

Answer 13

chair

not all in same plane -> can adopt ideal angle (each CH2 maintains tetrahedral geometry -> minimises ring strain)

conformation with lowest energy

Question 14

where are larger substituents most the likely to place themselves in the ring?

Answer 14

equatorial - due to sterics

Question 15

A value

Answer 15

difference in Gibbs free energy between axial and equatorial conformers

higher A value = larger proportion in EQUATORIAL conformation (larger penalty for being in axial position)

Question 16

which group is a conformationally locking group?

Answer 16

t-Bu (always equatorial)

trans-decalin (ring-flipping not possible)

Question 17

decalin

Answer 17

2 cyclohexanes fused together

Question 18

what conformation do E2 eliminations required?

Answer 18

anti-periplanar of hydrogen and LG

• if >1 anti-periplanar conformer exists, E-alkene predominates
• ring-flipping may have to occur to give 1,2-trans-diaxial conformation

Question 19

Sn2 substitutions - cyclohexanes

Answer 19

require Nu to attack into σ* at 180 to LG

inversion of stereochemistry

Question 20

nucleophilic addition to cyclohexanones

Answer 20

small Nu end up axial

large Nu end up equatorial (minimises clashing)

Question 21

which conformation does cyclohexene adopt?

Answer 21

half-chair

larger sub. = equatorial

Question 22

which conformation does cyclohexene oxide adopt?

Answer 22

half-chair
larger sub. = equatorial

Question 23

where do lone pairs prefer to sit on cyclohexanes? e.g. sub. pyridine - NR group

Answer 23

N (heteroatom) has R group and and lone pair

R group = larger so would sit Eq

lone pair occupies less steric space, so would preferably sit axially

*** in other cases, conjugation/hyperconjugation play a role in where the lone pair would prefer to sit (want lone pairs to align)

Question 24

Zimmerman-traxler model

Answer 24

cis enolate -> SYN

trans enolate -> ANTI

Question 25

what is the angle for nucleophilic attack?

Answer 25

Burgi-Dunitz (107)

Question 26

Felkin-Anh - presence of charge dense metal

Answer 26

alpha heteroatom with lone pairs + C=O will chelate to lock conformation in eclipsed

Question 27

when is deprotonation most favoured for enolates?

Answer 27

when C-H bond is perpendicular to C=O

Question 28

relationship between % of cis enolate and R group

Answer 28

bulkier R group adjacent to carbonyl = larger proportion is cis (sterics)

Question 29

how to control enolate geometry

Answer 29

[boron reagents]

bulkier boron sub. hinders formation of cis enolates while less bulky/more restricted boron sub. will promote cis enolate

Question 30

E-crotyl leads to …

Answer 30

anti-product

Question 31

Z-crotyl leads to …

Answer 31

syn-product