Abstract
In contrast to the established dogma that B(C6F5)3 is irreversibly poisoned by excess H2O/amine (or imine) bases, B(C6F5)3 is actually a water-tolerant catalyst for the reductive amination of primary and secondary arylamines with aldehydes and ketones in “wet solvents” at raised temperatures and using only 1.2 equiv of Me2PhSiH as reductant. Arylamines/N-arylimines do not result in the irreversible deprotonation of H2O–B(C6F5)3, allowing sufficient B(C6F5)3 to be evolved at raised temperatures to effect catalytic reductions. Stronger Bronsted basic amines such as tBuNH2 (and derived imines) result in irreversible formation of [HO–B(C6F5)3]− from H2O–B(C6F5)3, precluding the formation of B(C6F5)3 at raised temperatures and thus preventing any imine reduction. A substrate scope exploration using 1 mol % nonpurified B(C6F5)3 and “wet solvents” demonstrates that this is an operationally simple and effective methodology for the production of secondary and tertiary arylamines in high yield, with imine reduction...
Highlights
The past decade has witnessed spectacular advances in metalfree catalytic reductions, using “frustrated Lewis pairs” (FLPs).[1]
Me2PhSiH was used for direct comparison to the previously reported hydrosilylation of N-benzylideneaniline under anhydrous conditions.2d At 20 °C there was significant imine formation (>90%) but no reductive amination and no consumption of the silane, indicating that 1 is effectively poisoned (Table 1, entry 1)
This is attributed to Nbenzylidene-n-butylamine coordinating strongly to 1 (19F resonances for 1-nBuN C(H)Ph persist on prolonged heating in the presence of Me2PhSiH). This observation is consistent with previous reports in which 1 does not catalyze the hydrosilylation of the sterically similar imine benzylidenemethylamine under anhydrous conditions due to its strong coordination to 1.2d. These NMR spectroscopic studies allow the key requirements for successful reductive amination using 1/Me2PhSiH at raised temperatures to be defined as (i) the amine and corresponding imine need to be of sufficiently low Brønsted basicity to reversibly deprotonate 1-OH2 and (ii) the amine and imine need to be of sufficiently low nucleophilicity to reversibly bind to 1
Summary
The past decade has witnessed spectacular advances in metalfree catalytic reductions, using “frustrated Lewis pairs” (FLPs).[1] Numerous advances built on the pioneering work of Piers (using hydrosilanes)[2] and Stephan (using H2)[3] have led to B(C6F5)[3] (1) being established as a versatile reduction catalyst.[1,4] While 1 and its derivatives are effective metal-free catalysts for the reduction of imines,1,2d,4 including enantioselective variants,[5] these procedures require prior synthesis and isolation of the imine and rigorous exclusion of H2O in the subsequent reduction process The latter necessitates anhydrous solvents and inert conditions or the in situ removal of low levels of H2O from “wet” solvents/reagents using sacrificial hydrides or molecular sieves (Scheme 1, right).[6] These requirements have limited the wider uptake of 1 as a catalyst and prevented the use of 1 as a catalyst for reductive amination where the imine is formed (along with 1 equiv of H2O) and reduced in situ. Research Article the reductive amination of a range of arylamines using only 1.2 equiv of silane as reductant
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