Carbonyl compounds electrophilic assistance

Leaving precise mechanistic arguments aside, it should be stressed that an a,/9-unsatu-rated carbonyl compound can behave as a latent nucleophile with the assistance of a hydrosilane and a low-valence rhodium complex. With this simplification, isocyanates 18 and aldimines 20 were used as electrophiles for a similar protocol. Both were successfully incorporated with the aid of [Rh(COD)(P(OPh)3)2]OTf to afford 19 and 21, respectively (Scheme 6.5) in a reaction that was similar to the aldol-type coupling [11]. 

Aldehydes and ketones have been alkynylated using indium(III) and Hunig s base (PrjNEt) as catalysts.213 IR and NMR evidence support a dual-activation role for indium it is a Lewis acid for the hard electrophile (carbonyl compound), and has sufficient n -coordination ability for a soft nucleophile such as a terminal alkyne. For the latter substrate, the amine then assists proton abstraction. 

The reactivity of a carbonyl compound is increased by coordination to M+, by a process known as electrophilic assistance. The metal cation withdraws electron density from the... 

Although many nucleophiles dehalogenate a-halocarbonyl compounds, the most generally used have been halides, especially iodide ion, thiols and other nucleophilic sulfur species, and phosphines. All of these are effective by themselves, but all benefit by addition of an electrophile to the medium to assist cleavage by coordination to the carbonyl (equation 11). 

Although the mechanism of gas phase neutral hydrolysis of this compound is known, much uncertainty on details of the reaction in the condensed phase remains. There are some mechanisms by which this reaction can proceed in an aqueous medium. In this work we study a concerted mechanism in which there is a nucleophilic attack of the hydroxyl OH ion from a water molecule on the ester s carbonyl carbon, while the hydrogen H+ ion attacks the methoxy oxygen, electrophilically, via a transition state TS before forming the methanol and formic acid products. This concerted mechanism can be assisted by some water molecules. 

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