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Year 2 - Chem > Carbenes and Nitrenes > Flashcards

Carbenes and Nitrenes Flashcards

1
Q

Define a carbene

A

Neutral divalent carbon species

6 e- around the central atom

Highly reactive

Generally electrophillic

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2
Q

What are the two types of carbene?

A

Singlet and triplet

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3
Q

What is a singlet carbene?

A

3 e- pairs and an empty p orbital around central atom

Bent structure with bp and lp in sp2 orbitals

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4
Q

What is the bond angle of singlet carbenes?

A

100-110°

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5
Q

What is a triplet carbene?

A

Two unpaired electrons (paramagnetic)

Non-linear bent strucutre

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6
Q

What is the bond angles in triplet carbenes?

A

130-150°

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7
Q

When are singlet/triplet carbenes most common?

A

Singlet - more stable in aqueous media

Triplet - more stable in the gaseous state

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8
Q

Why are triplet carbenes bent?

A

If linear then unpaired e- have no s character so less stable

Bending allows for some s character

However repulsion energy prevents more bending

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9
Q

When are singlets or triplets found wrt orbital energy?

A

Small energy gap between σ and p-orbitals then triplet

If large then singlet

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10
Q

What is the state of a heteroatom carbene?

A

Singlet ground state

Due to delocalisation of lone pair to empty p-orbital

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11
Q

What is the ranking of carbene stability?

A

Most stable: N-heterocyclic carbenes (nucleophillic), then singlet ground states, triplet ground state

The more e- donation the more stable

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12
Q

What type of carbene is made due to irradiation?

A

Usually singletexcited state

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13
Q

How can singlet carbenes be made?

A

Heating diazo or diazirine compounds

Relaxation to triplet state can occur though

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14
Q

How can a triplet state carbene be made via irradiation?

A

Using a triplet sensitier

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15
Q

What is a diazirine and how are they made?

A

Made from ketones via :

oximation (to oxime)

tosylation

NH3 to diaziridines (with single N-N)

Jones oxidation (CrO3 and H2SO4)

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16
Q

What is the conditions for oximation of a ketone to an oxime?

A

NH2OH, HCl, and Pyridine

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17
Q

What are the conditions for tosylation of oximes?

A

TsCl and NEt3

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18
Q

How is a diazo compound synthesised?

A

Synthesised from corresponding amine with HNO2 (nitrous acid)

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19
Q

What are the general reactions of carbenes?

A

Insertion into C-H bonds

Skeletal rearrangements

Additions to double bonds

Reactivity depends on substituent - if donate then carbene is not electrophillic

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20
Q

What is the Bamford-Stevens reaction?

A

Tosylhydrazones to alkene upon treatement with a strong base

Intermediate diazo compound can be isolated, this then forms carbene after heat

Carbenes only formed in aprotic solvents

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21
Q

What is the Shapiro reaction?

A

Treatment of tosylhydrazones with 2 equivalents of alkyllithium to form alkenes

Electrophile e.g. H+, RCHO etc can be added to alkene

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22
Q

What determines if the bamford-stevens or shapiro reaction occurs?

A

Bamford-stevens uses only one equivalent of alkyllithium

Shaprio reaction uses two equivalents of alkyllithium

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23
Q

What is Schlosser’s base?

A

n-BuLi and t-BuOK

Very strong base which can be used to produce a carbenoid (CR2LiCl) from CR2HLi

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24
Q

What occurs to alkynes at high temperatures?

A

R-C≡C-H ⇔ (RH)C=C:

Eqm lies to the alkyne side

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25
What TM carbenoid is equivalent to a carbene?
Rhodium carbenoid (as it reduces reactivity)
26
What occurs in the Wolff rearrangment?
α-diazo ketone to a ketene Uses Ag(I) catalyst to aid N2 leaving Mechanism is concerted or stepwise dependent on substrate and reaction conditions
27
When is the wolff-rearrangment stepwise or concerted?
Thermal - concerted Photochemical - stepwise via carbenes
28
How do ketens react with NuH or A=B group?
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29
What is the orbital requirement for migration in the Wolff rearrangment?
Bond whihc migrates must be antiperiplanar to the C-N+ bond which is broken
30
What is the Doering-LaFlamme allene synthesis?
Alkene to allene Steps: CHBr3 & NaOH, then BuLi ![]()
31
What is the Skattebol rearrangmenet?
Geminal dihalo cyclopropane to an allene using an organolithium base Via a C-H insertion ![]()
32
What are the types of carbene addition reactions?
Stereospecific for singlet carbenes - concerted reaction Non-stereospecific for triplet carbenes - radical reaction Distinguishes between them
33
Why do triplet carbenes have non-stereospecific reactions?
Spin inversion (requiring molecular collision) may be slower than bond rotation leading to stereochem scrambling Bond rotation causes triplet to singlet state
34
What is the Reimer-Tiemann reaction?
Formylation of phenol to give ortho as major product with meta as minor Uses CHCl3 and NaOH Base deportonates CHCl3 which alpha-eliminates to give dichlorocarbene which is attacked by aromatic ring
35
How can carbenes add to benzene?
Very electrophilic carbenes - requires strong EWG like an ester Heat in benzene and causes 6π electrocyclic ring opening ![]()
36
What is the simmons smith reaction?
Alkene + CH2I2 with Zn to form a stereospecific cyclopropane Goes via a zinc carbenoid (IZnCH2I) which can be coordinated by a OH group (adds to same side)
37
How are alpha-diazo ketones made?
From carboxylic acids: Arndt-Eistert homogylation From acid anhydride: K2CO3 and TsN3
38
What is the Arndt-Eistert reaction?
Homologated carboxylic acids or their derivatives By reaction of acyl chloride with diazomethane XS diazomethane can be destroyed by addition of small amounts of acetic acid or vigorous stirring Then can do the Wolff-Rearrangement
39
How do carbenes react with nucleophiles?
Nucleophiles attack carbene and -ve charge then attacks an electrophile
40
How are nucleophillic carbenes used for the benzoin reaction?
Acts like a good nucleophile in resonance form, stabilises adjacent -ve charge, good LG
41
What is the benzoin reaction?
Coupling between two aldehydes to allow preparation of alpha-hydroxyketones Uses N-heterocyclic carbene salt and base
42
What is a nitrene?
R-N Neutral monovalent N species with 6e- around it Isoelectronic with carbenes, but more reactive as more electrophillic
43
What is the structure of nitrenes?
Linear although they do have triplet and singlet states Larger energy separation between states - triplet is usually ground states (unless very good π donor substitutent)
44
How are nitrenes formed?
Thermolysis or photolysis of azides Reduction of nitro compounds using PR3 Carbazole synthesis using pschorr reaction Generation from isocyanates using hv
45
What is the Pschorr reaction?
Preparation of biaryl tricyclics by intramolecular substitution of one arene by an aryl radical Radical is generated in situ from an aryl diazonium salt by copper catalysis
46
How do nitrenes add to alkenes?
High conc of olefin causes singlet to react stereospecifically and low olefin conc there is a ground state to a triplet then will react non-stereospecifically Competing reaction is 1,3-dipolar cycloaddition to form triazoline which breaks down under heat/hv to aziridine
47
What conditions do nitrenoids require for C-H insertion?
Better yields when milder conditions Uses oxidant PhI(OAc)2 and Rh catalyst
48
What occurs when alkyl azides are photolysed?
Imines are formed via a concerted 1,2-shift Nitrenes are most likely not the intermediate
49
What is the Curtis rearrangment?
Thermal decomposition of carboxylic azides to produce and isocyantes Retention of config in migrating group Isocynates + R'OH → RNHCOOR' Isocynates + H2O → RNH2 + CO2 Isocynates + R'NH2 → RNHCONHR'
50
What is the structure of isocyantes?
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51
What is the mechanism of the curtis rearrangment?
52
What is the Hoffman rearrangment?
Amide → Amine RCONH2 → RNHCOOH Reagents: Br2 & NaOH
53
What is the lossen rearrangement?
hydroxamate ester → isocyanate → amine Reagents: NaOH to iso, then water to amine Deprotonates amine group and causes rearrangement
54
What is the schmidt reaction?
Carbonyl + HN3→ amine/amide Via isocyanate intermediate Alt: use an alkene to form an imine
55
What are the different forms of the Schmidt reaction?
Carboxylic acid to amine Ketone to amide (via azidohydrin) Aldehydes to nitrile Alkene to imine Intramolecular to alkyl azides (can form rings)
56
What is the beckmann rearrangment?
Oxime to amide by NH insertion Reagents: H2SO4 conc and heat then water
57
Which group migrates in the beckmann rearrangement?
Group anti to the LG, and retains config Oximes can undergo E/Z isomerism in acidic cond
58
What is the Neber rearrangement?
Ketoxime is converted into an alpha-aminoketone Using NaOR then water ![]()
59
Which substituent migrates in the neber rearrangement?
Substituent possesing most acidic H migrates
60
What is the Steiglitz rearrangement?
1,2-rearrangement of trityl amine derivatives to triaryl imines Reagent is PCl5 or AgNO3 & MeOH ![]()
61
What is the Baeyer-Villiger oxidation?
Conversion of a ketone to an ester by O insertion mCPBA in DCM Via e- deficient transition state (EWG on the O) so group stabilising the +ve charge more migrates

Year 2 - Chem (23 decks)

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