The unimolecular nucleophilic substitution (SN1) mechanism figures prominently in every introductory organic chemistry course. In principle, stepwise displacement of a leaving group by a nucleophile via a carbocationic intermediate allows for the construction of highly congested carbon centers. However, the intrinsic instability and high reactivity of the carbocationic intermediates render it very difficult to control product distributions and stereoselectivity in reactions proceeding via SN1 pathways. Here we report asymmetric catalysis of an SN1-type reaction mechanism resulting in the enantioselective construction of quaternary stereocenters from racemic precursors. The new transformation relies on the synergistic action of a chiral hydrogen bond donor (HBD) catalyst with a strong Lewis acid promoter to mediate the formation of tertiary carbocationic intermediates at low temperature and achieve high levels of control over reaction enantioselectivity and product distribution. The work presented here provides a foundation for the enantioconvergent synthesis of other fully-substituted carbon stereocenters.

In an effort to reduce the flammability of polyurethane foam, a thin film of renewable inorganic nanoparticles (i.e., anionic vermiculite [VMT] and cationic boehmite [BMT]) was deposited on polyurethane foam via layer-by-layer (LbL) assembly. One, two, and three bilayers (BL) of BMT-VMT resulted in foam with retained shape after being exposed to a butane flame for 10 s, while uncoated foam was completely consumed. Cone calorimetry confirmed that the coated foam exhibited a 55% reduction in peak heat release rate with only a single bilayer deposited. Moreover, this protective nanocoating reduced total smoke release by 50% relative to untreated foam. This study revealed that 1 BL, adding just 4.5 wt % to PU foam, is an effective and conformal flame retardant coating. These results demonstrate one of the most efficient and renewable nanocoatings prepared using LbL assembly, taking this technology another step closer to commercial viability.