4 research outputs found
Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α‑Aryl Esters
The
synthesis and characterization of six new classes of higher-order
superbases, including five that incorporate cyclopropenimine functionality,
has been achieved. We propose a nomenclature that designates these
as the CG<sub>2</sub>, GC<sub>2</sub>, PC<sub>3</sub>, PC<sub>1</sub>, C<sub>3</sub>, and GP<sub>2</sub> classes of superbases. The p<i>K</i><sub>BH+</sub> values were measured to be between 29.0
and 35.6 in acetonitrile. Linear correlations of ten superbase basicities
vs that of their substituents demonstrated the insulating effect of
the cyclopropenimine core. The molecular structures of several of
these materials were obtained by single-crystal X-ray analysis, revealing
interesting aspects of conformational bias and noncovalent organization.
The types of superbasic cores and substituents were each reliably
shown to affect selectivity for deprotonation over alkylation. Higher-order
cyclopropenimine and guanidine superbase stability to hydrolysis was
found to correlate to basicity. Finally, a GC<sub>2</sub> base was
found to catalyze conjugate additions of α-aryl ester pronucleophiles,
representing the first report of a neutral Brønsted base to catalyze
such reactions
Cyclopropenone Catalyzed Substitution of Alcohols with Mesylate Ion
The cyclopropenone catalyzed nucleophilic substitution of alcohols by methanesulfonate ion with inversion of configuration is described. This work provides an alternative to the Mitsunobu reaction that avoids the use of azodicarboxylates and generation of hydrazine and phosphine oxide byproducts. This transformation is shown to be compatible with a range of functionality. A cyclopropenone scavenge strategy is demonstrated to aid purification
Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α‑Aryl Esters
The
synthesis and characterization of six new classes of higher-order
superbases, including five that incorporate cyclopropenimine functionality,
has been achieved. We propose a nomenclature that designates these
as the CG<sub>2</sub>, GC<sub>2</sub>, PC<sub>3</sub>, PC<sub>1</sub>, C<sub>3</sub>, and GP<sub>2</sub> classes of superbases. The p<i>K</i><sub>BH+</sub> values were measured to be between 29.0
and 35.6 in acetonitrile. Linear correlations of ten superbase basicities
vs that of their substituents demonstrated the insulating effect of
the cyclopropenimine core. The molecular structures of several of
these materials were obtained by single-crystal X-ray analysis, revealing
interesting aspects of conformational bias and noncovalent organization.
The types of superbasic cores and substituents were each reliably
shown to affect selectivity for deprotonation over alkylation. Higher-order
cyclopropenimine and guanidine superbase stability to hydrolysis was
found to correlate to basicity. Finally, a GC<sub>2</sub> base was
found to catalyze conjugate additions of α-aryl ester pronucleophiles,
representing the first report of a neutral Brønsted base to catalyze
such reactions
Spin-Center Shift-Enabled Direct Enantioselective α‑Benzylation of Aldehydes with Alcohols
Nature routinely
engages alcohols as leaving groups, as DNA biosynthesis
relies on the removal of water from ribonucleoside diphosphates by
a radical-mediated “spin-center shift” (SCS) mechanism.
Alcohols, however, remain underused as alkylating agents in synthetic
chemistry due to their low reactivity in two-electron pathways. We
report herein an enantioselective α-benzylation of aldehydes
using alcohols as alkylating agents based on the mechanistic principle
of spin-center shift. This strategy harnesses the dual activation
modes of photoredox and organocatalysis, engaging the alcohol by SCS
and capturing the resulting benzylic radical with a catalytically
generated enamine. Mechanistic studies provide evidence for SCS as
a key elementary step, identify the origins of competing reactions,
and enable improvements in chemoselectivity by rational photocatalyst
design