Carbonyl addition-elimination-hydrogenation reagent

Note that the approach outlined in Scheme 5.10 uses piperonal as the aldehyde component and requires funetionali/alion at die h -posiiion of 14-16 after the key addition step and auxiliary removal. Kelly Rein tried a more direct approach initially, as shown in. Scheme 5.11. but reduction of the aldehyde was the major pathway 68. No addition product was found, hut the isoquinolylox-a/oline could he recovered in MY/i yield. Grignard reagents may reduce carbonyls by cither fi-hydride elimination or electron transfer. Since there are no / -hydrogens in this Grignard. SET is the only possible alternative lor die production of the observed lactone. 

One can also envision that the quinone could trap the hydridopalladium species resulting from the B-hydrogen elimination that releases the oxidized organic product (Equation 16.129). Insertion of quinone into the palladium hydride would form an enolate that would tautomerize to the phenoxide complex. Protonation with the reagent containing an 0-H or N-H bond would generate the free hydroquinone and Pd(II). As noted in Chapter 17 on carbonylation, quinone has been used as an additive with this mechanism in mind to prevent tire Pd(II) hydroesterification catalysts from undergoing reduction to palladium(O). ... 

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