Although sulfide deprotonations are possible, the protons adjacent to two sulfide sulfur atoms are rather more acidic and alkylation of thioacetals is straightforward

In general, thioacetals can be made in a similar way to ‘normal’ (oxygen-based) acetals—by treatment of an aldehyde or a ketone with a thiol and an acid catalyst—although a Lewis acid such as BF₃ is usually needed rather than a protic acid. The most easily made, most stable toward hydrolysis, and most reactive towards alkylation are cyclic thioacetals derived from 1,3-propanedithiol, known as dithianes.

Dithianes are extremely important compounds in organic synthesis because going from carbonyl compound to thioacetal inverts the polarity at the functionalized carbon atom. Aldehydes, as you are well aware, are electrophiles at the C=O carbon atom, but dithioacetals, through deprotonation to an anion, are nucleophilic at this same atom.

Dithianes in synthesis

nucleophilic carbon atom An example: chemists wanted to make this compound (a ‘metacyclophane’) because they wanted to study the independent rotation of the two benzene rings, which is hindered in such a small ring. An ideal way would be to join electrophilic benzylic bromides to nucleophilic carbonyl groups, if that were possible.

The dibromide and dialdehyde were both available—what they really wanted was a nucleophilic equivalent of the dialdehyde to react with the dibromide. So they made the dithiane. Sulfur is less basic than oxygen, so the protonated species is lower in concentration at a given pH, and the sulfur 3p lone pairs are less able to form a stable π bond to carbon than are the oxygen 2p lone pairs.

After the dithianes have been alkylated, they can be hydrolysed to give back the carbonyl groups. Alternatively, hydrogenation using Raney nickel replaces the thioacetal with a CH₂ group and gives the unsubstituted cyclophane.

Dithianes are rather more stable than acetals, and special reagents have to be used to assist their hydrolysis and reveal the hidden carbonyl group. Sulfur is less basic than oxygen, so the protonated species is lower in concentration at a given pH, and the sulfur 3p lone pairs are less able to form a stable π bond to carbon than are the oxygen 2p lone pairs.

●Sulfur compounds are less basic than oxygen compounds and C=S compounds are less stable than C=O compounds.

The most obvious solution to this problem is to provide a better electrophile than the pro ton for sulfur. Mercury, Hg(II), is one solution. Another is oxidation of one sulfur to the sulfoxide. Protonation can now occur on the more basic oxygen atom of the sulfoxide and the concentration of the vital intermediate is increased.

A third solution is methylation, since sulfur is a better nucleophile than oxygen for saturated carbon. The sulfonium salt can decompose in the same way to give the free aldehyde. There are many more methods for hydrolysing dithioacetals and their multiplicity should make you suspicious that none is very good.

Hydrogenation of C–S bonds in both sulfides and thioacetals is often achieved with Raney nickel, the finely divided form of nickel made by dissolving away the aluminium from a powdered nickel–aluminium alloy using alkali. It can be used either as a catalyst for hydrogenation with gaseous hydrogen or as a reagent since it often contains sufficient adsorbed hydrogen (from the reaction of aluminium with alkali) to effect reductions alone. Thio acetalization followed by Raney nickel reduction is a useful way of replacing a C=O group with CH₂.

●Dithianes are ‘acyl anion equivalents’

 A sequence in which a carbonyl group has been masked as a sulfur derivative, alkylated with an electrophile, and then revealed again is a nucleophilic acylation. These nucleophilic equivalents of carbonyl compounds are known as acyl anion equivalents. In the retrosynthetic terms of Chapter 28 they are d1 reagents corresponding to the acyl anion synthon.

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مواضيع ذات صلة

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation

The Peterson elimination

Nucleophilic substitution at silicon

Silicon has an affinity for electronegative atoms

Silicon and carbon compared

Sulfoxonium ylids

Sulfonium ylids

Sulfonium salts as electrophiles