Silicon 2 Flashcards
What affect does silicated alkenes have on the reaction with electrophiles and why?
Silicated alkenes increase the rate of reaction through hyperconjugation
(long C-Si bonds increase orbital overlap of C=C bond by hyperconjugation)
How do silicated alkenes react with electrophiles?
- Electrophilic attack from C=C bond to electrophile
- Carbocation intermediate is stabilised by silicon group
- silicon group drops off and C=C reforms
how to the following two species react
- E- come down off the carbonyl oxygen
- Causing C-Cl bond to break and a new bond between Zn-Cl to form
- Results in an acylium ion forming
How does the newly formed acylium ion then react with an alkene
- E- from the C=C bond attack the δ⁺ C of the acylium
- Causes the C-O triple bond to break
- Forming a tertiary carbocation
From the formed tertiary carbocation, how do we form the minor product of this reaction?
- Terminal hydrogen is lost
- Reforming C=C bond
From the formed tertiary carbon cation, how is the major product then formed?
- E- from the Cl-Zn bond attack the carbocation
If we add a silicon group to the alkene starting product, how does this affect the regiochemistry of the reaction?
You only form the alkene product, rather than the chlorine product
If there is a silicon group attached to the alkene reacting with the acylium ion
How does this affect the formation of the carbocation
- Carbocation forms in a different place due to the effect of stabilisation from the silicon group
Once the stable carbocation is formed, how is the final product form, including the removal of the silicon group
- E- from Cl-Al bond attack silicon, breaking C-Si bond
- And reforming the double bond
What is the first step of this reaction
- Lewis acid coordinates to the chloride
- E- from the carbonyl oxygen form C-O triple bond and C-Cl bond is broken
- formation of new Cl-Al bond
- Results in formation of an Acylium ion
How does the formed acylium ion react with ethyl-substituted benzene
- Et is electron donating and hence o/p directing (p due to sterics)
- E- from C=C bond attack the δ⁺C of the acylium breaking the triple C-O bond
- C-H bond breaks, reforming C=C bond
What affect does adding a silico group onto a benzene ring for Friedel craft acylation
- The acylium ion will react ipso to the silico group
- The silicon will then leave as a cationic species, and the C=C bond will reform (can be enabled by AlCl₄)
What is Peterson Olefination?
- Alkene synthesis where the position (cis/trans) of the C=C is tunable
- (silicon group is alpha to nucleophilic position which stabilises it)
How is this stabilised alpha anion formed?
- React a halo-silico compound with grignards
- loss of MgCl group
How to the following two synthons react to forms a C-C bond
- The E- from the anion, attack the carbonyl carbon
- Causing the C=O bond to break
Relative sterochemistry of silicon and hydroxyl groups can affect the alkene product of this reaction (Silico and oxygen need to be on the same side in basic conditions)
What is the initiating steps of this reaction?
- Hydride base deprotonates the hydroxyl hydrogen
- Rotation around the central bond allows the O- to be adjacent to the silico group
- E- from the oxygen attack the silicon
From this silico-oxgyen intermediate how is the alkene formed
- Si-C bond breaks and E- used to reform C=C bond
- C-O bond breaks
- Cis isomer forms in basic conditions
Relative sterochemistry of silicon and hydroxyl groups can affect the alkene product of this reaction (Silico and oxygen need to be on opposite sides in acidic conditions)
What are the initiating steps of this reactions?
- LP on oxygen attacks the boron
- the β-cation is stabilised by the silicon
How does this silicon intermediate (from the peterson reaction) form the alkene final product?
- C-O bond breaks forming secondary carbocation which is stabilised by by the silico group
- C-Si bond breaks, reforming C=C bond
The elimination in a peterson acidic reaction is…
Anti-periplanar
Starting relative serochemistry of silicon and hydroxyl groups and conditions combine to dictate product alkene sterochemistry
Is a cis or trans alkene formed in the following reaction?
trans alkene
Relative sterochemistry of silicon and hydroxyl groups and conditions combine to dictate prodct alkene sterochemistry
Is a cis or trans alkene formed in the following reaction?
- rotation around the central double bond is needed for the hydroxyl and silico groups to be antiperiplanar
- Produce the cis isomer
What other starting material is required to produce this product?
hydroxyl and silicon need to be on the same side to produce the trans alkene
What other starting material is required to produce this product?
hydroxyl and silico groups are on opposite sides producing the antiperiplanar conformation
Produces the cis alkene
