Catalytic reductions of secondary amides and esters DIBAL reductions in organic chemistry have limitations associated with the by- products generated and, in some cases, modulation of reactivities. Consequently, two papers by Cheng and Brookhart on catalytic reductions of amides and esters by diethylsilane are likely to have a
O
N H
significant impact, particularly in process chemistry.
In their paper on amides (J. Am.
Chem. Soc., 2012, 134, 11304) the researchers describe how relatively simple iridium complexes can be used to give amines at room temperature (Reaction 1). This reaction can be stopped at the imine stage by limiting the amount of Et2
the catalytically active species because a material with tBu in place of Et was crystallised from similar starting materials. Formation of aldehydes using DIBAL can be notoriously difficult for some esters, due to over-reduction, hence I suspect the iridium-mediated processes will be even more useful than the amide
H OBn O
0.05 Ru3(CO)12 0.15
neat, 135 ° NC12
H25 OH N C
reductions (Angew. Chem. Int. Ed., 2012, 51, 9422). These reactions from Brookhart have
quite broad chemoselectivities – note the alkyne is not hydrosilylated or hydrogenated in Reaction 2 – except that nitro groups are not tolerated. Another paper from Brookhart’s
group (J. Am. Chem. Soc., 2012, 134,
11404) describes reduction of CO2 down to methane. A pincer complex
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mechanism, the process proceeds via Cu-mediated generation of a ruthenium substituted enolate, and condensation of this with the aldehyde component (A).
Enantioselective hydroformylation of terminal alkenes
Clarke and co-workers have communicated (Angew. Chem. Int.
Improved procedures for catalysed hydroesterification Hydroesterification of alkenes tends to be much harder to achieve than hydroacylation reactions, but a recent paper by Manabe et al brings us closer to adding formate esters to alkenes under controlled conditions. Their innovation (Org. Lett., 2012, 14, 4722) is to use imidazoles in the reaction. Under these conditions there is no need to add CO, or make use formates with coordinating functional groups. Manabe’s group succeeded in hydroesterifying some internal alkenes, and intramolecular ring closures. The temperatures used for these transformations (Reaction 3) are still high, but this is definitely a step in the right direction.
Aldehyde reshuffle The unlikely looking Reaction 4 is one from Chao-Jun Li’s lab (J. Am. Chem. Soc., 2012, 134, 16468). The confusing aspect of this process is that the aldehyde that goes into the reaction is not the same CHO in the product. According to Li’s postulate for the