Functional Group Reduction

The reduction of azobenzene 1 with catalyst 2 was reported (J. Am. Chem. Soc. 2012, 134, 11330. DOI: 10.1021/ja302963p) by Alexander T. Radosevich at Penn. State University, representing a unique example of a non-transition metal-based two-electron redox catalysis platform. Wolfgang Kroutil at the University of Graz found (Angew. Chem. Int. Ed. 2012, 51, 6713. DOI: 10.1002/anie.201202375) that diketone 4 was converted to piperidinium 5 with very high stereoselectivity using a transaminase followed by reduction over Pd/C.

Dennis P. Curran at the University of Pittsburgh reported (Org. Lett. 2012, 14, 4540. DOI: 10.1021/ol302010f) that NHC-borane 7 is a convenient reducing agent for aldehydes and ketones, showing selectivity for the former as in the mono-reduction of 6 to 8. A catalytic reduction of esters to ethers with Fe₃(CO)₁₂ and TMDS, as in the conversion of 9 to 10, was developed (Chem. Commun. 2012, 48, 10742. DOI: 10.1039/C2CC32142) by Matthias Beller at the Leibniz-Institute for Catalysis. Meanwhile, iridium catalysis was used (Angew. Chem. Int. Ed. 2012, 51, 9422. DOI: 10.1002/anie.201205154) by Maurice Brookhart at the University of North Carolina at Chapel Hill for the reduction of esters to aldehydes with diethylsilane (e.g. 11 to 12). As an impressive example of selective reduction, Ohyun Kwon at UCLA reported (Org. Lett. 2012, 14, 4634. DOI: 10.1021/ol302077n) the conversion of ester 13 to aldehyde 14, leaving the malonate moiety intact.

The cobalt complex 16 was found (Angew. Chem. Int. Ed. 2012, 51, 12102. DOI: 10.1002/anie.201206051) by Susan K. Hanson at Los Alamos National Laboratory to be an effective catalyst for C=O, C=N, and C=C bond hydrogenation, including the conversion of alkene 15 to 17. The use of frustrated Lewis pair catalysis for the low-temperature hydrogenation of alkenes such as 18 was developed (Angew. Chem. Int. Ed. 2012, 51, 10164. DOI: 10.1002/anie.201204007) by Stefan Grimme at the University of Bonn and Jan Paradies of the Karlsruhe Institute of Technology. Guanidinium nitrate was found (Chem. Commun. 2012, 48, 6583. DOI: 10.1039/C2CC32611F) by Kandikere Ramaiah Prabhu at the Indian Institute of Science to catalyze the hydrazine-based reduction of alkenes such as 20. The hydrogenation of thiophenes is difficult for a number of reasons, but now Frank Glorius at the University of Münster has developed (J. Am. Chem. Soc. 2012, 134, 15241. DOI: 10.1021/ja306622y) an effective system for the highly enantioselective catalytic hydrogenation of thiophenes and benzothiophenes, including 22.

Prof. Beller has discovered (J. Am. Chem. Soc. 2012, 134, 18325. DOI: 10.1021/ja3069165) that certain Brønsted acids such as 25 selectively catalyze the reduction of phosphine oxides, even in the presence of ketone functionality (cf. 24 to 26). The selective hydrogenolysis of aryl ethers is of high importance for the conversion of biomass lignins, but typical conditions for this process tend to lead to over reduction of the aromatic rings. Now, John F. Hartwig at Berkeley has found (J. Am. Chem. Soc. 2012, 134, 20226. DOI: 10.1021/ja3085912) that well defined nickel complexes such as Ni(CH₂TMS)₂(TMEDA) selectively catalyze the hydrogenolysis of diaryl ethers such as 27 without arene hydrogenation.

Finally, Seth B. Herzon at Yale developed (J. Am. Chem. Soc. 2012, 134, 17376. DOI: 10.1021/ja307145e) multicatalytic systems for the reductive hydration of alkynes such as 30 to produce either the linear (28) or branched (31) alcohols with high selectivity. The process involved the catalytic conversion of the alkyne to the corresponding aldehyde using ruthenium catalysis or to the corresponding methyl ketone under gold catalysis, followed by in situ hydrogenation with the ruthenium dimeric ruthenium catalyst 29.