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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)

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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

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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

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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

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5
Q

How does substitution α- to Si occur?

A
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6
Q

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

A

The group most able to stabilise a -ve charge migrates

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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

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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

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9
Q

What is a common reaction between vinyl silanes and electrophiles?

A

Direct electrophilic substitution occurs

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10
Q

How do allyl silanes react with electrophiles?

A

Electrophilic substitution with allylic transposition

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11
Q

How do Si stablise α-carbanions?

A

Electropositivity and hyperconjugation

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

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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

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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

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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

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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

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16
Q

What groups can TBS protect?

A

1º alcohol

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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

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18
Q

What is the Si-modified acyloin reaction used for?

A

Generating small ring compounds with no side products

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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

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20
Q

How can acylsilanes undergo the Brook rearrangement?

A

Acylsilane + NaCN → R-C(OTMS)-CN

Still goes via 1,2-rearrangement

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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

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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

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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

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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

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25
Q

How can you convert a hydrogen α to a ketone be replaced for an R group?

A

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

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26
Q

How are sinyl enol ethers synthesised from enones?

A

Either: R2CuLi followed by TMSCl

Or: Li in NH3 (l) then t-BuOH followed by TMSCl

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27
Q

What can be added to a silyl enol ether to make an enolate?

A

MeLi

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28
Q

What is the reactivity of silyl enol ethers and enolates with 3º-alkyl halides?

A

Enolates don’t react with 3º-alkyl halides

TMS enol ethers react with 3º-alkyl halides via a SN1-like mechanism

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29
Q

How do TMS enol ethers react with 3º-alkyl halides?

A

Use TiCl4 as a catalyst

Forms R3C+-TiCl5

30
Q

How can you add a bulkyl tertiary R group to a ketone?

A

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
Q

How do silyl enol ethers react with π-electrophiles?

A

Adds electrophile to alkene

E.g. Eschenmoser’s salt, Chloroalkyl ether & TiCl4

32
Q

How can you add -CH2OH to a C α to a carbonyl?

A

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
Q

How can you add -CH2NMe2 to a C α to a ketone?

A

Convert to a silyl enol ether

Then add Eschenmoser’s salt CH2=N+Me2 I-

34
Q

How can you add =CH2 to a C α to a ketone?

A

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
Q

How can a silyl enol ether be used to add a -OH to the C α to a ketone?

A

Make into a silyl enol ether

Then add m-CPBA which can then be hydrolysed

36
Q

How can a halide atom be added to a C atom α to a ketone?

A

Make a silyl enol ether

Then add X2

37
Q

How are silyl enol ethers used as EWG for diels-alder reactions?

A

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
Q

What is Danishefsky’s diene and how does it react?

39
Q

What is a silyl ketene acetal?

A

Equivalent of silyl enol ethers but for esters

40
Q

What is the Ireland-Clasien rearrangement?

A

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
Q

What is the selectivity of the Ireland-Clasien rearrangment?

A

High-stereoselectivity as 6-membered chair TS

E- only gives anti product

Z - only gives syn product

42
Q

What is the conformation for the Ireland-Clasien rearrangment?

43
Q

What routes can be used to make vinyl silanes?

A

Silyation of organometallics - from alkynes or alkenes

Shapiro reaction - from ketones

44
Q

How can vinyl silanes be made from silylation of organometallics?

A

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
Q

What is the shapiro reaction?

A

Ketone to vinyl silane

Reagents: ArSO2NHNH2 then 2BuLi followed by TMSCl

46
Q

How do vinyl silanes react with electrophiles?

A

Via β-carbocation intermediates

Stereospecific due to orbital overlap

47
Q

What is the orbital overlap for vinyl silane reaction with electrophiles?

48
Q

What is the stereospecificity of a desilyation reaction?

A

E alkene to E

Z to Z

As requires sufficient overlap in elimination step

49
Q

How do vinyl silanes react with Cl2 or Br2?

A

Anti addition of the halide then the alkoxide/F- induced anti elimination

So reverses stereochem

50
Q

How can you reverse the E/Z of an alkene with a halide?

A

Anti addition of Cl2/Br2 to vinyl silane, then alkoxide/F- anti elimination

Gives a vinyl halide

51
Q

How do vinyl silanes react with C-electrophiles?

A

Si-accelerated Friedel-Crafts reaction

52
Q

How can you add a carbonyl (ketone or acyl) to a vinyl group?

A

Make a vinyl silane and then add RCOCl and AlCl3

Si-accelerated Friedel-Crafts reaction

53
Q

How can an aryl silane be prepared?

A

Organometallic reacted with chlorosilane (R3Si-Cl)

Can either use bromoaryl with grignard or 2Li

54
Q

How do arylsilanes react with electrophiles?

A

Ipso substitution of silane (electrophile replaces silane)

55
Q

How are epoxy silanes prepared?

A

Either: Si varient of Darzens - uses s-BuLi then aldehyde

Epoxidation of vinyl silanes - uses mCPBA

Epoxide deprotonation - silylation using a base + TMSCl

56
Q

What is the difference between n and s-BuLi?

A

n-BuLi is more nucleophillic but less basic, less regioselective

sec-BuLi is more basic and less nucleophillic, more regioselective

57
Q

How are epoxy silanes used to mask carbonyls?

A

Can be cleaved easily with acidic hydrolysis to form enone which is in eqm with the carbonyl

58
Q

What is an allyl silane?

59
Q

How are allyl silanes prepared?

A

Silyation of organometallics (bromo allyl then Mg/Li and TMSCl)

Wittig olefination - using ketone with select R groups, and phosphonium with -TMS

60
Q

What is the Wittig olefination?

A

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
Q

Why is the Wittig olefination useful for creating allyl silanes?

A

Carbonyl can have different R-groups to add different groups

62
Q

How do allyl silanes react?

A

React with electrophiles via β-carbocation to Si stabilisation

Results in transposition of double bond and loss of TMSCl

63
Q

How do acetal alkenes react with lewis acids?

64
Q

How can allyl silanes be used in diels-alder reactions?

A

Attached to diene - can then transpose the double bond formed in the DA reaction

Can use acid or TBAF and H2O

65
Q

How are olefins formed from hydroxy silanes?

A

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
Q

What types of olefins formed from hydroxy silanes?

A

Acidic - goes via anti elimination

Base - goes via syn elimination

67
Q

What is the peterson olefination?

A

α-silylcarbanions (from organometallic) + ketone/aldehyde → β-hydroxy silane → alkene

68
Q

How do epoxysilanes react with Bu2CuLi and H2O?

69
Q

When reacting an aldehyde + amine + allyl silane react what occurs first?

A

imine forms and then the allyl silane attacks the reactive C

70
Q

How do -SiCl3 compounds react with KF, H2O2 with KHCO3?

A

-SiCl3 → -OH (retention of configuration)

-O-OH attacks Si, puts -ve on Si

Year 2 - Chem (23 decks)

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