Aryl aldehydes, reductive coupling bromides

The use of a relatively soluble base such as CS2CO3 allows good product yield. No products are formed via carbopalladation. Therefore the reaction is considered to occur on a dienolate anion generated from the enal to give an aryl(7r-allyl)palladium intermediate. The regioselectivity seems to be determined in the reductive elimination of the product. Treatment of aliphatic aldehydes with aryl bromides brings about aldol condensation followed by y-aryla-tion to afford 2 1 coupling products (Eq. 27). Note that y-arylation products are also produced in the arylation of a tin-masked dienolate [65,66]. 

Cheng has published a convenient and synthetically useful alternative method to the NHK reaction for the arylation of aromatic aldehydes in a mild and selective way with nickel(ll) bromide/zinc/dppe mediated protocol for the synthesis of diaryl carbinols. Durandetti reported an electrochemical coupling of aryl halides with aldehydes for the synthesis of diaryl carbinols which was catalytic in chromium and nickel salts. Comins utilized the NHK reaction to prepare 5-(l-hydroxyalkyl)-2,3-dihydro-4-pyridones, which were then explored in reductive, oxidative and substitutive reactions. " The first asymmetric catalytic synthesis of 5y -alk-l-ene-3,4-diols was developed the regio-, diastereo- and enantioselective addition of 3-chloropropenyl pivaloate to aldehydes was made possible by exploiting Salen r(II) species in a catalytic version of the NHK reaction. ... 

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