Organoboron

Question 1

What is the electronegativity of boron?

Answer 1

Boron is χ2 on Pauling scale

For reference: C is χ2.5

Question 2

What is the structure of a borane?

Answer 2

BR₃

Isoelectronic with carbocations & trigonal planar

Empty p-orbitals so lewis acid

Question 3

What is a boronate complex?

Answer 3

^-BR₃Nu⁺

8 VE species with a formal -ve charge on B

e- rich, engage in ligand transfer

Question 4

What is hydroboration?

Answer 4

Reaction of BH₃ with alkenes

Concerted & asynchronous cis-addition

π-complex donates to electrophillic B centre

Question 5

What is the asynchronicity of hydroboration?

Answer 5

B-C forms before H-C

Leads to predictable regiochemistry

Question 6

What are the regio- and stereochem of hydroboration goverened by?

Answer 6

At room temp, regio- and stereochem kinetically governed

Question 7

What is the stereoselectivity of hydroboration?

Answer 7

Approaches alkene from least hindered side

Question 8

What is the regiochemical preference of hydroboration?

Answer 8

Adds to side which places +ve charge alpha to where the C the boron adds to

Question 9

Explain the stereochem preference of this hydroboration

Answer 9

Boron compound can approach either large or medium face for the overlap

Therefore approaches medium

Question 10

Name some hydroboration reagents and their selectivity

Answer 10

Thexyl borane (Me₂CHCMe₂BH₂) - only adds once (won’t add further to tetrasub alkenes)

Disiamyl borane - adds twice (wont add further to trisub alkenes)

9-BBN - solid, v sterically demanding & selective for terminal position

Catechol borane - has O-B so v reactive as O are EWG

Question 11

How does hydroboration occur at high T (~150ºC)?

Answer 11

Reversible and can isomerise R₂B to most stable site

Forms thermo product which is least hindered

Question 12

What occurs when boranes react with RCOOH?

Answer 12

RCOOH + R*Me₂B → R*H + RCOBMe₂

Acyl borane and R-H forms

Stereospecific

Can use RCOOD to deuterate

Question 13

What is the mechanism for borane protonation with RCOOH?

Answer 13

Question 14

What occurs in borane oxidation?

Answer 14

Borane (BR₃) + H₂O₂ and NaOH → BR₂OR → R-OH

Alternatively can use N-oxide

Question 15

What can be used to synthesise a carbonyl compound from an alkene?

Answer 15

Borane reagent (e.g. Sia₂BH, 9-BBN, BH₃•SMe₂)

Then NMO-TPAP (chromic acid alternative)

Question 16

What can the hydroboration and oxidation of alkenes / alkynes give?

Answer 16

Alkene → Carboxylic acid

Alkyne → Carbonyl (ketone or aldehyde)

Question 17

How can you prepare phenols from a grignard/aryl?

Answer 17

Trialkyl borate (B(OR)₃) + Aryl grignard → Ar-B(OR)₂ → Ar-OH

Trialkyl borate (B(OR)₃) + Aryl + LDA→ Ar-B(OR)₂ → Ar-OH

2nd step reagents: H₂O₂ and KOH

Question 18

How can you add an amino group to an alkene?

Answer 18

BR₃ + H₂N-Cl or H₂N-OTs → R₂B-NHR → R-NH₂

Question 19

How can you add a halogen to an alkene?

Answer 19

BH₃ then NaOMe with Br₂/I₂

Borone forms, then base forms bromate

B- attacks X₂

Question 20

How can you form Z-alkenes from a specific alkyne?

Answer 20

R-≡+ R’₂BH → RHC=CHBR’₂ (anti) + NaOH + X₂ → RXHC-CR’B^-R’(OH)₂ → RC=CR’

Question 21

How can dienes be formed from an alkyne bromide?

Answer 21

Question 22

How can you make a carbonyl compound or 3º alcohol from a borane?

Answer 22

BR₃ + CO → acyl borane + reagent then NaOH + H₂O₂

Reagent:

acyl borane + LiAlH(OMe)₃ → aldehyde

acyl borane + H₂O → ketone

acyl borane + ethylene glycol → 3º alcohol

Question 23

How does a borane react with CO?

Answer 23

^-C≡O⁺ , -ve charge on C attacks the B

B-R bond migrates to form R₂B-COR

Question 24

What is a cyanoborate process?

Answer 24

BR₃ → R-C(O)-R + RB(OH)₂ + CF₃CONH₂

Reagents: LiCN, then TFAA

Followed by H₂O₂ and NaOH, then hydrolysis

Alternatively can heat and XS TFAA to produce 3º alcohol

Question 25

What attacks TFAA in the cyanoborate process?

Answer 25

N from the CN

Not usually nucleophillic, is activated by bonding to the B

Question 26

What is the mechanism for the cyanoborate process?

Answer 26

Question 27

How can α-bromoester enolates be used with boranes?

Answer 27

α-halo ester + Base → α-bromoester enolates (e.g. BrCH₂CO₂Et)

α-bromoester enolates + BR₃ → R-CH₂CO₂Et

Question 28

What is the mechansim for the reaction between boranes and α-bromoester enolates?

Answer 28

Question 29

What is the Matteson homologation?

Answer 29

Grignard reagent puts R- onto boronate ester

R group then migrates to displace Br

Question 30

What can you treat an aldehyde with to get a stereospecific alcohol with an allyl added?

Answer 30

Use an allyl boronate reagent

O from carbonyl attacks empty p-orbital on the B

Then via a 6-membered TS (chair configuration) reacts to add to the carbonyl

Question 31

What is the Crotyl transfer?

Answer 31

Crotyl is RCH₂CH=CHCH₃

Trasnfer of crotyl with specified E/Z-

E gives anti product (OH and Me one up one down)

Z gives syn product (OH and Me both up/down)

Question 32

Why is the crotyl transfer important?

Answer 32

OTher common allylic organometallics have poor regio and/or stereo control

As they dont follow the Zimmerman-Traxler TS

Question 33

How can boron be used in diels-alder reactions?

Answer 33

Diene reacts with boronate with very ewg bonded to it

Question 34

How can a diene be formed from sulfolene?

Answer 34

3-Sulfolene + Heat → Diene + SO₂