Organoboron Flashcards
What is the electronegativity of boron?
Boron is χ2 on Pauling scale
For reference: C is χ2.5
What is the structure of a borane?
BR3
Isoelectronic with carbocations & trigonal planar
Empty p-orbitals so lewis acid
What is a boronate complex?
-BR3Nu+
8 VE species with a formal -ve charge on B
e- rich, engage in ligand transfer
What is hydroboration?
Reaction of BH3 with alkenes
Concerted & asynchronous cis-addition
π-complex donates to electrophillic B centre
What is the asynchronicity of hydroboration?
B-C forms before H-C
Leads to predictable regiochemistry
What are the regio- and stereochem of hydroboration goverened by?
At room temp, regio- and stereochem kinetically governed
What is the stereoselectivity of hydroboration?
Approaches alkene from least hindered side
What is the regiochemical preference of hydroboration?
Adds to side which places +ve charge alpha to where the C the boron adds to
Explain the stereochem preference of this hydroboration
Boron compound can approach either large or medium face for the overlap
Therefore approaches medium
Name some hydroboration reagents and their selectivity
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
How does hydroboration occur at high T (~150ºC)?
Reversible and can isomerise R2B to most stable site
Forms thermo product which is least hindered
What occurs when boranes react with RCOOH?
RCOOH + R*Me2B → R*H + RCOBMe2
Acyl borane and R-H forms
Stereospecific
Can use RCOOD to deuterate
What is the mechanism for borane protonation with RCOOH?
What occurs in borane oxidation?
Borane (BR3) + H2O2 and NaOH → BR2OR → R-OH
Alternatively can use N-oxide
What can be used to synthesise a carbonyl compound from an alkene?
Borane reagent (e.g. Sia2BH, 9-BBN, BH3•SMe2)
Then NMO-TPAP (chromic acid alternative)
What can the hydroboration and oxidation of alkenes / alkynes give?
Alkene → Carboxylic acid
Alkyne → Carbonyl (ketone or aldehyde)
How can you prepare phenols from a grignard/aryl?
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
How can you add an amino group to an alkene?
BR3 + H2N-Cl or H2N-OTs → R2B-NHR → R-NH2
How can you add a halogen to an alkene?
BH3 then NaOMe with Br2/I2
Borone forms, then base forms bromate
B- attacks X2
How can you form Z-alkenes from a specific alkyne?
R-≡+ R’2BH → RHC=CHBR’2 (anti) + NaOH + X2 → RXHC-CR’B-R’(OH)2 → RC=CR’
How can dienes be formed from an alkyne bromide?
How can you make a carbonyl compound or 3º alcohol from a borane?
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
How does a borane react with CO?
-C≡O+ , -ve charge on C attacks the B
B-R bond migrates to form R2B-COR
What is a cyanoborate process?
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
What attacks TFAA in the cyanoborate process?
N from the CN
Not usually nucleophillic, is activated by bonding to the B
What is the mechanism for the cyanoborate process?
How can α-bromoester enolates be used with boranes?
α-halo ester + Base → α-bromoester enolates (e.g. BrCH2CO2Et)
α-bromoester enolates + BR3 → R-CH2CO2Et
What is the mechansim for the reaction between boranes and α-bromoester enolates?
What is the Matteson homologation?
Grignard reagent puts R- onto boronate ester
R group then migrates to displace Br
What can you treat an aldehyde with to get a stereospecific alcohol with an allyl added?
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
What is the Crotyl transfer?
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)
Why is the crotyl transfer important?
OTher common allylic organometallics have poor regio and/or stereo control
As they dont follow the Zimmerman-Traxler TS
How can boron be used in diels-alder reactions?
Diene reacts with boronate with very ewg bonded to it
How can a diene be formed from sulfolene?
3-Sulfolene + Heat → Diene + SO2
