Expanding the Boundaries of Water‐Tolerant Frustrated Lewis Pair Hydrogenation: Enhanced Back Strain in the Lewis Acid Enables the Reductive Amination of Carbonyls

Abstract

The development of a boron/nitrogen-centered frustrated Lewis pair (FLP) with remarkably high water tolerance is presented. As systematic steric tuning of the boron-based Lewis acid (LA) component revealed, the enhanced back-strain makes water binding increasingly reversible in the presence of relatively strong base. This advance allows the limits of FLP's hydrogenation to be expanded, as demonstrated by the FLP reductive amination of carbonyls. This metal-free catalytic variant displays a notably broad chemoselectivity and generality.

Solid Molecular Frustrated Lewis Pairs in a Polyamine Organic Framework for the Catalytic Metal‐free Hydrogenation of Alkenes

“…To lower the moisture sensitivity of the system in future developments, a feasible method might be to substitute the BCF with compounds that exhibit a lower Lewis acidity and, hence, a higher moisture tolerance. In literature, there are already some examples for moisture‐tolerant FLPs, in which the LA exhibits, for example, partial chlorine instead of fluorine substitution …”

Direct Reductive Amination of Carbonyl Compounds Catalyzed by a Moisture Tolerant Tin(IV) Lewis Acid

“…Very recently Soós et al . reported the first example of FLP‐catalysed RA (Scheme ) employing a triarylborane as LA ( I in Scheme ), which is impressive given the factors outlined above. The authors noted, however, that “electronic tuning [in BAr 3 species] has reached its limit” for refining moisture tolerance, and their success was based upon very careful and specific design of the triarylborane used, which focused on steric modification.…”

“…While the reaction times using 1 are mostly shorter than using Soós’ catalyst I for identical coupling partners ( vide supra ; Scheme ), these side reactions were not observed by the latter, which is highly likely a result of the lower operating temperature. Since our attempts to reduce the reaction temperature with 1 detrimentally affected the rate of turnover, we considered substrates which were more problematic for I , namely those exhibiting a larger steric profile …”

Recent Advances in Water-Tolerance in Frustrated Lewis Pair Chemistry

“…, the absence of a pnictogen-F bond and the steric congestion created by the organic ligands boosted the stability of this Sb-based Lewis acid, which could be stored in air for several days without decomposition. However [27] + still shows significant oxophilicity as demonstrated by the Lewis acid initiated THF polymerization. Also notable is its ability to abstract fluoride from [(C 6 F 5 ) 3 B-F]indicating a higher fluorophilicity than 1.…”