Geminal halides reduction

Castro CE, WC Kray (1966) Carbenoid intermediates from polyhalomethanes and chromium (II). The homogeneous reduction of geminal halides by chromous sulfate. J Am Chem Soc 88 4447-4458. 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

A special use of sodium arsenite (applied in aqueous alkaline solutions) is partial reduction of trigeminal halides to geminal halides [220] and reduction of aromatic nitro compounds to azoxy compounds [221]. 

Trialkylaluminums, 21 By geminal alkylation of carbonyl groups Dichlorodimethyltitanium, 216 From reduction of alkyl halides, alkyl sulfonates and similar compounds From acetates or other esters Triphenylsilane, 334 From alcohols... 

The mechanism of these reductions is a bimolecular nucleophilic substitution for the reaction of LAH with most primary and secondary halides [BK5, PCI]. A single-electron transfer (SET) has been proposed in the reduction of sterically hindered primary iodides [AD3, AGl, AWl], although some doubts have been cast [PCI] on this mechanism with bromocyclopropanes [HW2] and aromatic or vinyl halides [Cl], especially in the presence of CeClj [G02]. In this case, some rearrangements may be observed. SET does take place in the reduction of geminal dihalides by LAH [AIM] as well as in the reduction of bromocyclopropanes in the strict absence of molecular oxygen [PNl]. In the presence of oxygen, the C—Br bond of 2,2-diphenyl-l-bromocyclopropanecarboxylic acid is left unchanged [PN1 ]. 

Reductions of C-X Bonds. Organotin hydrides generated in situ via PMHS reduction of Sn-0 or Sn-X precursors easily reduce aromatic and aliphatic halides. These reactions can proceed either thermally or photochemically and have been used successfully to reduce geminal dihalides stepwise (eq 22). Since organotin halides are the hy-products of these reactions, the PMHS/fluoride/R3Sn-X combination allows reactions to be carried out with catalytic amounts of tin. ... 

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