1.3- dithiane alkanal

Spirocyclization with 2-silyt- 1,3-dithianes. Treatment of the trimcthylsilyl-alkanal 1 with fluoride ion liberates an anion that undergoes cyclization to the aldehyde group to yield 2. Various Lewis acids do not promote this cyclization. Similar desilylation of 3 is followed by an intramolecular Michael addition to give 4 in 64% yield. 

Activation of two Si—Si bonds in bis(disilanyl)alkanes with palladium(O) bis(tert-alkyl isocyanide) induced the formation of the cyclic bis(silyl)palladium(II) bis(terf-alkyl isocyanide) complexes (100) and disilanes described schematically in Scheme 42. These complexes were found to react with phenylacetylene, affording different amounts of five-membered cyclic products and acyclic products which are derived from the insertion of the alkyne into the general intermediate complex 101 (Scheme 42, equation 54). The bis(silanyl)dithiane palladium complex (102) was isolated and characterized in the solid state the two silicon atoms, the two isocyano carbons and the palladium atom are nearly in a plane with a short cross-ring Si—Si distance of 2.613(2) A, suggesting the possibility of covalently bonded two Si—Si atoms in the four-membered ring. Similar reaction with cyclic disilanes afforded oligomers, and cyclic 20-membered compounds have been prepared in the presence of nitriles248,249. 

Under ordinary conditions, carbonyl groups of aldehydes and ketones will not be reduced by catalytic hydrogenation, but if Raney nickel (Ni-ff2) is used as the catalyst, then a reduction takes place giving an alcohol product. If the ketone or aldehyde is first converted to a dithiane, then Raney nickel reduction will give an alkane product (see Clemmenson and Wolff-Kishner reductions below as alternatives for this same transformation). 

Here is a clever way to alkylate carbonyl compounds at the carbonyl carbon atom. Thioacetals, also called didiianes, can be made from carbonyl compounds and the sulfur counterparts of diols (Rg. 19.114). Dithianes are more acidic than typical alkanes and can be deprotonated with alkyllithium reagents (Rg. 19.115). 

WORKED PROBLEM 19.40 Suggest a synthetic route to alkanes starting from 1,3-dithiane. 

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