Synthesis with heteroatoms Flashcards
(28 cards)
Why can silicon form a pentavalent species
pentavalent silicon species forms during nucleophilic substitution as silicon is a big enough atom to do so.
Why cant silicon do nucleophilic substitution vis Sn1 reaction
because that would mean a silicon+ ion formed which is too slow to happen
What elements does silicon form strong bonds with
oxygen and fluorine
Why is a Si-F bond stronger than C-F bond
large electronegativity difference, more ionic nature so bond is strengthened
Why is silicon a good protecting group
using silicon as a substitute for a proton forms a stronger bond so overall a more stable molecule
How is cyanohydrin formed
Nucleophilic addition of CN- to electrophilic aldehyde ion (oxonium)
Draw mechanism of cyanohydrin formation
check notes for mech
How are enolates formed
Ketone/Aldehyde and LDA at low temperatures as LDA is a strong sterically hindered base that takes away the alpha hydrogen from carbonyl forming lithium enolate
How can the lithium enolate be stabilised
lithium atom replaced with SiMe3 group as stronger bond between Si-O so more stable compound
What are the two different conditions for the trapping of kinetic/thermodynamic enolates?
using silyl ethers as enolate traps by:
LDA + THF (K) or
Me3SiCl + Et3N + THF (T)
both at -78 degrees
Draw mechanism of enolate formation with SiMe3Cl, LDA and THF
show both kinetic and thermodynamic product
how are the enolates recovered?
2 enolate products shown
one formed with LDA
recovered by electrophilic addition, breaking double bond in ring
What other functional groups can silicones protect?
can protect carbons in alkyne bonds and can also mask alcohols
all by formation of silyl ethers
How is silyl ether cleavage done?
by using TBAF, has F-ions which forms strong Si-f bond so silicon group displaces from O to F
used to reform alcohols
What are the effects of the C-Si bond on reactivity
C-Si is a long bond so can stabilise Beta carbocations through hyperconjugation
therefore makes Beta alkenes more nucleophilic
How does Silicon direct Electrophilic aromatic substitution?
silicon directs ipso electrophilic aromatic addition meaning the electrophile will replace into the same position the silicon group was
What is the Peterson olefination?
used for synthesis of alkenes, used with SiMe3.
Basic conditions form Z alkenes, acidic conditions form E alkenes.
How are phosphonium salts formed
formed from phosphines are highly nucleophilic,
can displace Br from benzene ring via SN2
What is a ylide?
compounds with anionic site adjacent to heteroatom with positive charge
e.g phosphonium ylides found in wittig reaction
What is the wittig reaction used for?
converts aldehydes/ ketones into alkenes
intermediate of phosphonium ylide
carbon anion is nucleophilic
2+2 cyclo-addition occurs
How do we get stereoselectivity within the Wittig reaction? what determined this?
unstable phosphonium ylides form Z isomer, stabilised phosphonium ylides form E isomer
a stabilised Ylide has an electron withdrawing group on the nucleophilic carbon, unstabilised as electron donating group at C
What is the TS within the Wittig reaction?
there is a cross shaped intermediate, which exlpains the selectivity due to steric clash or dipole interactions, dependant of the R group on the ylide
What is the Arbuzov-Michaelis reaction
similar to Wittig, but no ylide formed, phosphonium enolate formed instead
Explain some properties of sulphur and its bonding
C-S bond is not polarised, Sulphur is a great nucleophile, can also access many O.S
whats the difference between phosphonium ylide and sulfonium ylide?
P=O bond is strong enough to fully remove O forming alkenes, S=O isnt strong enough to fully remove O so epoxide formed
