Investigation of the Deprotonative Generation and Borylation of Diamine-Ligated α-Lithiated Carbamates and Benzoates by in situ IR spectroscopy

Diamine-mediated α-deprotonation of <i>O</i>-alkyl carbamates or benzoates with alkyllithium reagents, trapping of the carbanion with organoboron compounds, and 1,2-metalate rearrangement of the resulting boronate complex are the primary steps by which organoboron compounds can be stereoselectively homologated. Although the final step can be easily monitored by ¹¹B NMR spectroscopy, the first two steps, which are typically carried out at cryogenic temperatures, are less well understood owing to the requirement for specialized analytical techniques. Investigation of these steps by in situ IR spectroscopy has provided invaluable data for optimizing the homologation reactions of organoboron compounds. Although the deprotonation of benzoates in noncoordinating solvents is faster than that in ethereal solvents, the deprotonation of carbamates shows the opposite trend, a difference that has its origin in the propensity of carbamates to form inactive parasitic complexes with the diamine-ligated alkyllithium reagent. Borylation of bulky diamine-ligated lithiated species in toluene is extremely slow, owing to the requirement for initial complexation of the oxygen atoms of the diol ligand on boron with the lithium ion prior to boron–lithium exchange. However, ethereal solvent, or very small amounts of THF, facilitate precomplexation through initial displacement of the bulky diamines coordinated to the lithium ion. Comparison of the carbonyl stretching frequencies of boronates derived from pinacol boronic esters with those derived from trialkylboranes suggests that the displaced lithium ion is residing on the pinacol oxygen atoms and the benzoate/carbamate carbonyl group, respectively, explaining, at least in part, the faster 1,2-metalate rearrangements of boronates derived from the trialkylboranes

Control in advanced biofuels synthesis via alcohol upgrading: catalyst selectivity to n ‐butanol, sec ‐butanol or isobutanol

Ruthenium complexes with tetradentate PNNP donor ligands demonstrate a marked change in selectivity compared to analogous bis bidentate PN complexes in Guerbet catalysis, producing mixtures of n‐butanol (17 %), sec‐butanol (14 %) and ethyl acetate (66 %) rather than the usual 90 %+ selectivity to n‐butanol. Tridentate PNP ruthenium complexes such as [Ru(H)(Cl)(CO)(Ph2PCH2CH2NHCH2CH2PPh2)] also produce sec‐butanol and, in optimized conditions (120 °C, 10 mol% NaOEt base), achieve 71 % selectivity to this butanol isomer. The same tri‐ and tetradentate complexes are efficient catalysts for the conversion of methanol/ethanol mixtures to isobutanol (up to 97 % selectivity). In this way, judicious choice of ligand within this general catalyst family allows selectivity to three butanol isomers of interest as fuel molecules