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CHEM333 (Bower) > P-based Alkene Formation > Flashcards

P-based Alkene Formation Flashcards

1
Q

Give the name of the reaction shown

A

The Wittig Olefination

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2
Q

What alkenes are unstabilised ylides selective for?

A

Z alkenes

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3
Q

What alkenes are stabilised ylides selective for?

A

E alkenes

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4
Q

What is the driving force of the Wittig reaction?

A

P=O bond formation
- P=O bond is strong

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5
Q

What’re the drawbacks of the Wittig Olefination?

A

O=PPh3 byproduct is often difficult to remove

Stabilised ylide not very reactive
- reacts poorly with ketones

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6
Q

Why is PPh3 used instead of Et3N in the Wittig reaction?

A

Because the lone pair in PPh3 is in a more diffuse and polarisable orbital than Et3N

Hence, Ph3P is a better nucleophile despite being less basic

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7
Q

What is the name of the intermediate formed in the Wittig reaction?

A
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8
Q

Why do unstabilised ylides form Z alkenes?

A

Sterics, groups move as far away as possible to minimise repulsive forces

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9
Q

Why are stabilised ylides selective for E alkenes?

A

For stabilised ylides, electronic effects outweigh steric effects

Hence, electronegative groups are remote to each other to minimise repulsion

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10
Q

Why must the reagents approach each other in this orientation in the Wittig reaction?

A

To facilitate entry into the 4-membered (puckered) ring transition state

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11
Q

Give the mechanism of the given reaction

A
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12
Q

Give the name of the following reaction

A

The HWE Olefination

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13
Q

Give the mechanism for the HWE Olefination

A
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14
Q

Why is the HWE Olefination superior to the Wittig Olefination?

A

Stabilised ylide reacts well with ketones

Dialkyl phosphate by-product is water soluble
- easily removed

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15
Q

How is E/Z selectivity achieved in HWE Olefination?

A
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16
Q

How is the HWE ylide prepared?

A
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17
Q

Why are alpha-Haloketones unsuitable for HWE ylide preparation?

A

Because of the competing Perkow side reaction

Doesn’t occur when using esters - like in HWE - as Perkow reaction would break resonance of ester

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18
Q

What alkene is selectively made using HWE Olefination with an aldehyde?

A

HWE is highly selective for E-alkenes for aldehydes

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19
Q

What part of the HWE Olefination determines what alkene is produced preferentially?

A

E-selectivity is determined by the relative stability of the oxphosphetane intermediate
- not the 1,2-addition products like in Wittig

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20
Q

What factors affect E-selectivity in HWE Olefination?

A

E-selectivity is finely balanced and can depend on the structure of:

  • R1
  • the phosphonate ester
  • the ethyl ester
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21
Q

How can the HWE Olefination be improved? What impact does this have?

A

By modifying -OR to make P-centre more electrophilic

  • phosphate becomes a better LG, final step is faster
  • oxaphosphetane formation is faster and becomes less reversible
  • step 1 now determine product selectivity
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22
Q

Give the name of the reaction shown

A

Ando modification (of HWE)

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23
Q

What’s the role of Na in the Ando modification?

A

To chelate to the carbonyl groups, locking the enolate geometry

It also aligns the approaching carbonyl
- electrostatic attraction between carbonyl O and Na

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24
Q

What provides the selectivity in the Ando modification?

A

Due to electrostatic interaction between aldehyde O and Na, 2 possible orientations are available

The bottom approach is favoured because R1 is far away from the big -OPh groups

Hence, Z-alkene is formed preferentially

25
Give the name of the reaction shown
The Still-Gennari modification (of HWE)
26
Why are esters inert under HWE and other P-based olefinations?
Because they are not good enough electrophiles due to resonance
27
What additional benefit does HWE Olefination have?
28
Give the name of the reaction shown
The Corey-Fuchs alkynylation Converting aldehydes to terminal alkynes over a 2 step sequence
29
Give the mechanism for step 1
(Blue arrow route preferred/more accepted)
30
Give the mechanism of step 2, followed by the rearrangement
31
Give the Fukui orbital analysis for the migration of H
32
What key points about carbenes should be remembered?
Carbenes have 6 valence electrons, is sp2 hybridised and is a V good electrophile - bearing a free p-orbital They can behave as Nuc or Elec - with carbene lone pair or with free p-orbital respectively
33
Why is H the best migrating group? Can other groups migrate?
Because of its non-directional s-orbital Other groups can migrate so long as they can access the vinyl carbene
34
Give the name of the following reaction
The Seyferth-Gilbert alkynylation Converting aldehydes or ketones to alkynes over a 1 step sequence
35
Give the mechanism of the following reaction
36
What advantages does the Seyferth-Gilbert alkynylation have over Corey-Fuchs alkynylation?
Wider scope - aldehydes AND ketones Only 1 step - additional step in C-F alkynylation Mild conditions used - C-F uses strong bases (typically n-BuLi) whereas S-G uses potassium carbonate
37
What is the original Seyferth-Gilbert reagent used that can react with ketones?
38
How are alkynes reduced to alkenes cis-selectively?
Using Lindlar’s catalyst - producing Z-isomer ONLY
39
Why is the alkene product not reduced further when using Lindlar’s catalyst?
Pb(OAc)2 and quinoline poison the Pd and make it less active, thereby preventing reduction of the alkene
40
How do you reduce alkynes trans-selectively?
Birch type conditions Directed reduction using LiAlH4 (usually on propargylic alcohols)
41
Give the mechanism of alkyne reduction using LiAlH4
42
Give the names of these functional groups
43
Give the names of these functional groups
44
Give the name of this reaction
The Staudinger reaction Converts azide to amine
45
What is typically used to reduce azide? How does this differ to what’s used in the Staudinger reaction and what drives this process?
LiAlH4 or cat. Pd/C, H2 Staudinger reaction uses PPh3 and H2O which are much milder reagents - Process is driven by loss of N2 and formation of strong P=O bond
46
Give the mechanism of the Staudinger reaction
47
Give a mechanism for the following reaction
48
Predict the structure of the product of the reaction given
49
Predict the structure of the product of the following aza-Wittig reaction
50
What is the name of the following reaction
The Appel reaction Conversion of alcohols to alkyl halides - with clean inversion of stereochem
51
Give the mechanism of the Appel reaction
52
Why is the Appel reaction not stereospecific when using tertiary alcohols?
Because SN2 at tertiary centre is not feasible - too bulky Hence the process occurs via SN1
53
Predict the structure of both product of the following Appel reactions
54
What’s a limitation of the Appel reaction?
Azides can be made (useful) but the process is not general for other nucleophiles
55
What’s an alternative, more general appproach, to the Appel reaction?
The Mitsunobo reaction
56
Give the name of the following reaction
The Mitsunobo reaction
57
What must the pKa of NuH be below for the Mitsunobo reaction to work?
58
Give the mechanism of the Mitsunobo reaction
59
Predict the structure of the product following the Mitsunobo reaction shown
The Mitsunobo reaction offers a way to invert alcohols on a molecule

CHEM333 (Bower) (6 decks)

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