Hypophosphorus acid, reduction

The best general reagent for the reductive elimination of the diazonium group is hypophosphorus acid. Reduction proceeds readily at 0-5° with an aqueous solution of the reagent. The yields of hydrocarbons are in the range of 60-85%. Hydrochloric acid is recommended for the diazo-tization except in certain cases in which nuclear halogenation occurs as a side reaction when this acid is used. 

Hypophosphorus acid, H3P02 in the presence of AIBN and a base (amine or NaHC03) can reduce organic halides and xanthates in dioxane or aqueous alcohols. Eq. 12.6a to 12.6d shows the reduction of xanthate, thionocarbonate, and iodoarene, and 5-exo-trig cyclization respectively [35-43]. 

Apart from catalytic hydrogenation and dissolving metal methods, very few other techniques exist for the reduction of the pyrrole ring. However, one example is the improved use of hypophosphorus acid that has been described by Scott and coworkers in a reduction of pyrrole-2-carboxylic acid to the prolyl hydroxylase inhibitor (5)-3,4-dehydroproline (after resolution) (equation 10). r,—A i, HI, AcOH, H3PO2... 

Zinc reduction of [TcClg] in HCl (aq) produces the mixed-valence dimer [TcaClg] ". The analogue of Ref[TciClg] , is isolated by workup of products from reduction of pertechnetate with hypophosphorus acid. The [n-Bu4N]2 [TC2CI8] is isolated and converted to [Tc2Br8] salts . 

Reduction of the Diazonium Group to Hydrogen Deamination of Anilines Hypophosphorus acid (H3PO2) reacts with arenediazonium salts, replacing the diazonium group with a hydrogen. In effect, this is a reduction of the arenediazonium ion. 

Finally, it is possible to remove nitrogen completely from the molecule and replace it with a hydrogen (a reduction). This reduction is commonly done by heating a diazonium salt with hypophosphorus acid (H3PO2). 

Finally, in this vein, amino groups attached to aromatic rings can be replaced by hydrogen via diazotization and reduction of the diazotized species with hypophosphorus (H3PO2) acid and copper(I) salts (Scheme 10.22). 

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