Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Year 2 - Chem > Organosilicon > Flashcards

Organosilicon Flashcards

1
Q

What is the electronegativity of Si?

A

Si - χ1.8

Electropositive wrt C (χ2.5) & B (χ2)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are some general reactions organosilicon compounds undergo?

A

SN2 or via a silicon-ate intermediate (5-coordinate intermediate with -ve charge)

Substitution α- to the Si via prior attack at Si

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the C-Si bond size compared to C-C?

A

C-Si is longer by 25%

Si in Me3Si is less hindered

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the significance in the bond strength of Si bonds?

A

Si-F is a very strong bond

Si-O is a strong bond

These are driving forces

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How does substitution α- to Si occur?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What group migrates when there is substitution α- to Si?

A

The group most able to stabilise a -ve charge migrates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

How do Si atoms stabilise β-carbocations?

A

C-Si bond polarity and hyperconjugation

Polarity gives slight -ve charge to the C

Hyperconjugation with p-orbital lowers lp on Si

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What does Si stabilising β-carbocations drive?

A

Vinyl- and allylsilane chemistry

Vinyl silanes undergo direct electrophilic substitution

Allyl silanes undergo electrophillic substitution with allylic transposition

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is a common reaction between vinyl silanes and electrophiles?

A

Direct electrophilic substitution occurs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How do allyl silanes react with electrophiles?

A

Electrophilic substitution with allylic transposition

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How do Si stablise α-carbanions?

A

Electropositivity and hyperconjugation

Hyperconj has the -ve charge donated into Si σ* orbital as electropositive

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What happens when you react a silicon subsituted alkane/alkene with a strong base?

A

Deprotonates C which is α to the Si group

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How can TMS be used as a protecting group?

A

R-OH + TMSCl → R-OTMS + HCl

In pyridine

Not a good protecting group as easily cleaved

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How can TBS be used as a protecting group?

A

R-OH + imidazole + t-BuMe2Si-Cl → R-OTBDMS

Imidazole is nucleophllic catalyst - attacks Si to kick off the Cl, then O attacks the Si

Good protecting group - resistant to basic conditions, only cleavable with acid or fluoride & regioselective for 1º alcohol

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How can a protecting group be removed with F- or methanolysis?

A

Can be under basic or acidic cond

If acidic: protonates O, then MeOH attacks Si

If base: MeO- directly attacks the Si

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What groups can TBS protect?

A

1º alcohol

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is TMSCl used for?

A

In-situ trap for alkoxides

E.g. Si modified acyloin reaction - the chlorosilane removes basic alkoxides to prevent side reactions such as Clasien/Dieckmann

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the Si-modified acyloin reaction used for?

A

Generating small ring compounds with no side products

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the Brook rearrangment?

A

Silyl carbinols + Et2NH ⇒ R-OTMS

1,2-anionic migration of a silyl group from carbon to oxygen

Catalytic amount of a base

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

How can acylsilanes undergo the Brook rearrangement?

A

Acylsilane + NaCN → R-C(OTMS)-CN

Still goes via 1,2-rearrangement

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is the structure of sinyl enol ethers?

A

Used as a precursor for specific enolates, generated at a particular site not necessarily governed by the acidity of the enolisable α-protons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What routes can be used to synthesise silyl enol ethers?

A

From ketones - can use kinetic or thermo control

From enones - R2CuLi then TMSCl, alternatively Li in NH3(l) and t-BuOH then TMSCl

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

How can silyl enol ethers be synthesised from ketones under kinetic control?

A

LDA @ -78ºC, then TMSCl to make the sinyl enol ether

Then can add MeLi then MeI

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

How can thermodynamic silyl enol ethers be synthesised?

A

TMSCl and Et3N @ 130ºC to make the sinyl enol ether

Then can add MeLi and MeI to form ketone

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
How can you convert a hydrogen α to a ketone be replaced for an R group?
Convert to sinol ethyl ether - use thermo or kinetic conditions to get wanted product Thermo - adds R to least substitued side Kinetic - adds R to more substituted side Then add MeLi and R-I
26
How are sinyl enol ethers synthesised from enones?
Either: R2CuLi followed by TMSCl Or: Li in NH3 (l) then t-BuOH followed by TMSCl ![]()
27
What can be added to a silyl enol ether to make an enolate?
MeLi
28
What is the reactivity of silyl enol ethers and enolates with 3º-alkyl halides?
Enolates don't react with 3º-alkyl halides TMS enol ethers react with 3º-alkyl halides via a SN1-like mechanism
29
How do TMS enol ethers react with 3º-alkyl halides?
Use TiCl4 as a catalyst Forms R3C+-TiCl5
30
How can you add a bulkyl tertiary R group to a ketone?
Form a silyl enol ether using thermo or kinetic control then adding MeLi Followed by adding the 3º-alkyl halide version of the bulkyl group
31
How do silyl enol ethers react with π-electrophiles?
Adds electrophile to alkene E.g. Eschenmoser's salt, Chloroalkyl ether & TiCl4
32
How can you add -CH2OH to a C α to a carbonyl?
Make a silyl enol ether Then react with benzyl chloromethyl ether to make a protected alcohol Then H2 and Pd/C to remove -CH2Ph ![]()
33
How can you add -CH2NMe2 to a C α to a ketone?
Convert to a silyl enol ether Then add Eschenmoser's salt CH2=N+Me2 I- ![]()
34
How can you add =CH2 to a C α to a ketone?
Convert to a silyl enol ether Then add Eschenmoser's salt CH2=N+Me2 I- to add -CH2NMe2 Then react with m-CPBA/MeI to make alkene ![]()
35
How can a silyl enol ether be used to add a -OH to the C α to a ketone?
Make into a silyl enol ether Then add m-CPBA which can then be hydrolysed ![]()
36
How can a halide atom be added to a C atom α to a ketone?
Make a silyl enol ether Then add X2 ![]()
37
How are silyl enol ethers used as EWG for diels-alder reactions?
Enone reacted with TMSCl and Et3N Produces diene with protected -OTMS which cna then react in Diels-Alder reaction -OTMS can be removed using acid or F-
38
What is Danishefsky's diene and how does it react?
![]()
39
What is a silyl ketene acetal?
Equivalent of silyl enol ethers but for esters ## Footnote ![]()
40
What is the Ireland-Clasien rearrangement?
Allyl ester → silyl ketene acetal → hydrolysed to γ,δ-unsaturated acids (extended carboxylic acid with an alkene at opposite side) Uses LDA (-78ºC) then TMSCl ![]()
41
What is the selectivity of the Ireland-Clasien rearrangment?
High-stereoselectivity as 6-membered chair TS E- only gives anti product Z - only gives syn product
42
What is the conformation for the Ireland-Clasien rearrangment?
![]()
43
What routes can be used to make vinyl silanes?
Silyation of organometallics - from alkynes or alkenes Shapiro reaction - from ketones
44
How can vinyl silanes be made from silylation of organometallics?
Alkynes: BuLi then R3Si-Cl to R-≡-SiR3, followed by H2 Pd/C to made Z-alkene and UV to E-alkene Alkenes: vinyl bromide to grignard followed by TMS ![]()
45
What is the shapiro reaction?
Ketone to vinyl silane Reagents: ArSO2NHNH2 then 2BuLi followed by TMSCl ![]()
46
How do vinyl silanes react with electrophiles?
Via β-carbocation intermediates Stereospecific due to orbital overlap ![]()
47
What is the orbital overlap for vinyl silane reaction with electrophiles?
![]()
48
What is the stereospecificity of a desilyation reaction?
E alkene to E Z to Z As requires sufficient overlap in elimination step
49
How do vinyl silanes react with Cl2 or Br2?
Anti addition of the halide then the alkoxide/F- induced anti elimination So reverses stereochem
50
How can you reverse the E/Z of an alkene with a halide?
Anti addition of Cl2/Br2 to vinyl silane, then alkoxide/F- anti elimination Gives a vinyl halide ![]()
51
How do vinyl silanes react with C-electrophiles?
Si-accelerated Friedel-Crafts reaction ![]()
52
How can you add a carbonyl (ketone or acyl) to a vinyl group?
Make a vinyl silane and then add RCOCl and AlCl3 Si-accelerated Friedel-Crafts reaction
53
How can an aryl silane be prepared?
Organometallic reacted with chlorosilane (R3Si-Cl) Can either use bromoaryl with grignard or 2Li
54
How do arylsilanes react with electrophiles?
Ipso substitution of silane (electrophile replaces silane) ![]()
55
How are epoxy silanes prepared?
Either: Si varient of Darzens - uses s-BuLi then aldehyde Epoxidation of vinyl silanes - uses mCPBA Epoxide deprotonation - silylation using a base + TMSCl ![]()
56
What is the difference between n and s-BuLi?
n-BuLi is more nucleophillic but less basic, less regioselective sec-BuLi is more basic and less nucleophillic, more regioselective
57
How are epoxy silanes used to mask carbonyls?
Can be cleaved easily with acidic hydrolysis to form enone which is in eqm with the carbonyl ![]()
58
What is an allyl silane?
![]()
59
How are allyl silanes prepared?
Silyation of organometallics (bromo allyl then Mg/Li and TMSCl) Wittig olefination - using ketone with select R groups, and phosphonium with -TMS
60
What is the Wittig olefination?
Aldehyde/ketone + triphenyl phosphonium ylide (wittig reagent) → Alkene + Ph3P=O Wittig reagent preapred from 1º alkyl halide + PPh3 → phosphonium salt, then phosphonium salt + BuLi → wittig reagent ![]()
61
Why is the Wittig olefination useful for creating allyl silanes?
Carbonyl can have different R-groups to add different groups ## Footnote ![]()
62
How do allyl silanes react?
React with electrophiles via β-carbocation to Si stabilisation Results in transposition of double bond and loss of TMSCl ![]()
63
How do acetal alkenes react with lewis acids?
![]()
64
How can allyl silanes be used in diels-alder reactions?
Attached to diene - can then transpose the double bond formed in the DA reaction Can use acid or TBAF and H2O
65
How are olefins formed from hydroxy silanes?
Acidic or basic (NaH) conditions Driven by strong Si-O bond Under base: O- can attack Si, syn elimination produces olefin and NaO-SiMe3 Under acid: X- attacks Si and anti-eliminates H2O, produces olefin and H2O + XSiMe3
66
What types of olefins formed from hydroxy silanes?
Acidic - goes via anti elimination Base - goes via syn elimination
67
What is the peterson olefination?
α-silylcarbanions (from organometallic) + ketone/aldehyde → β-hydroxy silane → alkene ![]()
68
How do epoxysilanes react with Bu2CuLi and H2O?
![]()
69
When reacting an aldehyde + amine + allyl silane react what occurs first?
imine forms and then the allyl silane attacks the reactive C
70
How do -SiCl3 compounds react with KF, H2O2 with KHCO3?
-SiCl3 → -OH (retention of configuration) -O-OH attacks Si, puts -ve on Si ![]()

Year 2 - Chem (23 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions