1,3-Dithianes

Question 1

Define umpolung

Answer 1

Reverse of natural polarity

Question 2

What are 1,3 dithianes equivalents of?

Answer 2

Acyl anions

Question 3

How are 1,3 dithiane anions stabilised?

Answer 3

E- density in large S atoms readily polarised inducing stabilising dipoles (most important).

Oxygen in Period 2 so no low-lying empty orbitals c.f. S period 3 with empty 3d AOs allowing p pi to d pi electron donation.

sp3 HAO of lithio-anion can overlap with empty C-S sigma* orbital when anion is equatorially orientated.

Question 4

How are S,S acetals formed from carbonyl compounds?

Answer 4

Use propane 1,3-dithiol and Lewis/protic acid catalyst

Question 5

Why are S,S acetals harder to hydrolyse than their O congeners?

Answer 5

Reduced basicity of S c.f. O. Higher barrier to formation of thiocarbenium cationic intermediate

Question 6

How are S,S acetals hydrolysed to umask carbonyl group?

Answer 6

Deprotection by metal coordination - using heavy metal salts. Formation of strong S-Hg bonds renders process essentially irreversible.

Alkylative deprotection- LG ability of S enhanced by alkylation.

Oxidative deprotection - oxidation of S with a range of OAs to facilitate hydrolysis.