Westwood Flashcards
in a double bond, which side does a boron prefer to add to?
least substituted
name some alternative boron agents
thexylborane, 9-BBN, disiamylborane, dicuclohexylborane, catechol borane, pineocamphylborane
what is the benefit of catechol borane? How?
doesn’t reduce an alkyne to an alkane - stops at alkene due to O lone pairs donating into empty p orbital on B
what is a good activating solvent in an organoboron experiment?
THF
How would you go R-B to R-X-B?
R migration
in an R migration, how would the migrating and leaving group be orientated?
antiperiplanar
what nucleophile is used to go from R-B to B-OH?
peroxide (O-OH)-
what reagents used in a protonolysis?
diglyme
what charge does boron need to have to do a migration?
negative
how would you get evidence of mechanisms of reduction of alkynes/alkenes?
D/H
for alkenylborane reactions, does a halogenation or amination retain or inverse the configuration?
halogenation = inversion amination = retention
how would a NCl3 react with an alkenylborane
via radicals
in a carbonylation of alkenylboranes, what reagent gives an aldehyde?
i) LiAlH(OMe)3
ii) H2O2 in NaOH
in a carbonylation of alkenylboranes, what reagent gives a ketone?
i) H2O
ii) H2O2 in NaOH
in a carbonylation of alkenylboranes, what reagent gives a tertiary alcohol?
i) ethylene glycol
ii) H2O2 in NaOH
in the cyanidation of alkylboranes, what type of compound do you get if you use 2 equiv OTf2?
tertiary alcohol
in the cyanidation of alkylboranes, what type of compound do you get if you use 1 equip OTf2 followed by H2O2 in base?
ketone
what would you use to aminate an alkenylborane?
Cl-NH2 or HSO3O-NH2 (NH2 with good leaving group)
what products would you get if you react BR3 with an a-halocarbonyl? And what intermediate?
3 x a-R-carbonyl
intermediate = enolate
If you have an alkyne (R—H), what reagents would you use to get a cis alkene product?
i) R2BH
ii) I2 in NaOH
If you have an alkyne (R—X), what reagents would you use to get a trans alkene product?
i) R2BH
ii) NaOMe
iii) AcOH
If you have an alkyne (R—H), what reagents would you use to substitute another R group?
i) nBuLi
ii) R3B
iii) I2
what happens when you react disiamylborane with alkynes?
can combine 2 alkynes to forma dialkyne product (with the help of I2)
how do you make a trans dialkene from a dialkyne
i) 2x sia2BH
ii) RCO2H
how would you retroborate? Do you get the same alkene out that you put in?
apply heat to remove a proton - alkene generated depends on which proton is removed in the retroboration.
Which is unstable to rearrangement of allylboranes: B(OR)2 or BR2 (AKA which undergoes the rearrangement?)
BR2
what structure does an allylborane reaction with carbonyls look like?
a chair conformation of a cyclohexane
what is a thiirane?
an epoxide (ring) with an S in it
what is a sulfide?
an ether with an S instead
which bond is stronger OH or SH?
OH (orbital overlap)
What type of addition does PhCH2SH do?
1,4 (as it is very nucleophilic)
what reagents could you use to synthesise thiols?
Ph—X with S=C(NH2)2 + NaHCO3 // Na2S2O3 + H3O+ // KSCsOEt + LiAlH4 //Na2S2 (non Ph R group) + Na/liqNH3 + H3O+
how would you synthesise aromatic thiols?
ClSO3H (chlorosulfonylation) + Zn, H3O+ // S8 with PhLi
what bonds are changing in the oxidation of thiols?
2xSH to SS
what reagents are used to break up a thioacetal ring?
Hg2+, HgCl2 in H2O
How would you substitute H for R on a thioacetal?
i) nBuLi + ii) RX
2 methods of sulphide synthesis?
di-Cl alkane treated with Na2S to substitute 1xCl then nucleophilic attack to remove the Cl.
Attack of nucleophilic carbon atom at electrophilic sulfur
What is an ylide?
a molecule with a permanent positive and negative charge.
is a stabilised ylide hard or soft?
hard
does sulfonium prefer axial or equatorial attack at a C=O on a cyclohexane ring?
axial
When attacking a C=O-C=C, depending on if the ylide is soft or hard, an epoxide and a cyclopropane are formed. Which forms which?
unstabilised, soft, forms epoxide
stabilised, hard, forms cyclopropane
what type of elimination does a sulfoxide do?
syn
what reagents do you need to use for an elimination by a sulfoxide?
i) LDA
ii) (PhS)2
iii) NaIO4 + Heat
what can dimsyl sodium do?
a) form a 1-alkene on long chain alkane
b) substitute with OH in COOH to form a ketone
c) add to a ketone to form an alcohol
what are the three sulfur compounds that are oxidised and reduced to each other - similar to alcohol to aldehyde to carboxylic acid?
sulfure –> sulfoxide –> sulfone
—> oxidation
how does the stern oxidation work?
DMSO+(COCl)2 makes Cl-S-Me2 which is attacked by a terminal alcohol —-> oxidised to an aldehyde
what are the different fates of alkoxysulfonium salts?
elimination, substitution at S, substitution at C, Pumerer reaction
what does the Pummerer reaction do?
acylation of sulfoxide –> elimination to sulfonium ion –> acetation of sulfonium ion –> a-acyloxy-thioether
What are some ways of synthesising sulphones?
oxidation of sulfides and sulfoxides // alkylation of sulfinate salts // rearrangement of sulfinate esters // friedel-crafts alkylation of sulfonyl halides // addition of sulfonyl halides to alkene or alkynes // cycloaddition reactions of SO2
What is Bredt’s rule?
states that: a double bond cannot be placed at the bridgehead of a bridged ring system
What is the Ramberg-backlund reaction
effectively removes an SO2 from the middle of a chain to form an alkene. with KOHH, tBuOH in CCl4, H2O
Which isomer is produced during the Julia reaction? and what is it?
E (trans)
reductive elimination
What are some methods for losing sulfone group?
Ramberg-Backlund // sulfinic acid elimination // reductive desulfonation // reductive elimination
what reagents are used in the Eschenmosser fragmentation?
i) H2O2 in NaOH
ii) TsNHNH2
What does Tschugaev elimination do?
removes two trans alcohols on an 8-membered ring to twist the ring and form a double bond (using P(OR)3 and a sulfur thing
basic: removes an OH to form an alkene using O-C=S bonds.
What is the shapiro reaction?
removes a ketone to replace with a double bond next to it. [arylsulfonylhydrazones]
how would you synthesise a sulfonamide/sulfonate ester?
Sn2 substitution with a Cl for an NH2R/ROH
In which of Si-O vs C-O, Si-F vs C-F and Si-H vs C-H does the silicon version have a weaker bond?
Si-H
How does silicon stabilise the B-cation?
donates e density into empty p orbital on the cation
How does Si stabilise an a-C-M bond?
overlap of the bond with a d orbital on silicon
What is a Brook rearrangement?
inserts O from C-OH to C-O-Si
How do you synthesise a silyl enol ether?
trapping of enolate ethers // conjugate addition/silylation // hydrosilation
what is an exceptional use of silyl enol ether?
formation of a quarternary center on a cyclohexane
What reagent is useful for reactions of silyl enol ether with strong electrophiles
TiCl4
what is siloxybuta-1,3-diene used in?
diels alder
How do you synthesise B-hydroxysilanes?
reaction of a-metallated silanes with carbonyl compounds (Li or Mg good)
or ring-open with epoxysilanes
what Si compound can act as a ‘masked’ carbonyl group?
vinyl silanes (Me3Si-=-R)
Where does a compound substitute if its ipso?
on the same carbon in an aromatic ring
What is hydrosilylation?
reduction of alkyne into an alkene using a Si compound
What compound can be used to stop hydrosilylation reduction from going too far?
Pd-CaO3-PbO
What intermediate does an electrophilic substitution reaction of a vinyl silane go through?
a stabilised B-carbocation
In the Peterson Olefination (step 2) what reagent is used to produce an E alkene product?
NaH (causes a twist to coordinate the O to the Si which leaves)
In the Peterson Olefination (step 2) what reagent is used to produce a Z alkene product?
AcOH (Si kicks off the app OH group)
What reagent could be used to turn an OH into an X
Ph3P, CCl4 // POCl3, DMF
What reagents would be used to form RCO2R (using a P reagent)
i) X2 + P(OEt)3 + pyridine
ii) RCO2H
iii) ROH
reagents in the Mitsonobu reaction
DEAD and PH3P (takes PCO2H + alcohol –> ester)
What ionic nitrogen intermediate compound can POCl3 make?
imine
What phosphorus reagent splits up an ozonide ring?
PR3
Julia Peterson reaction: what are the reagents used and what isomer is generated?
PPh3 is the nucleophile needs a base, an ylide is formed and reacted with a C=O, oxygen and phosphorus form an oxasphosphetane ring, either E/Z isomers are generated.
In the Wittig reaction, using a stabilised ylide, which isomer is formed?
E only
In the Wittig reaction, using an ustabilised ylide, which isomer is formed?
Z only
What does the Wittig Reaction do?
complexes an ylide with a C=O to produce an alkene
What does the HWE reaction do? Which isomer, why?
forms an E alkene from CHO - E alkene is faster for thermodynamic reasons
What is the arbusov reaction?
Goes from (RO)3P + R-Cl to (RO)2RP=O
