Stannosilicate zeolites with nanosheet morphology and
MFI topology
(Sn-MFI-ns) are successfully synthesized with an amphiphilic organic
structure-directing agent. Sn source, Si/Sn ratio, crystallization
time and temperature impact framework Sn incorporation and nanosheet
morphology. Optimal synthesis conditions generate Sn-MFI-ns with high
crystallinity and isolated framework Sn sites. Nanosheets are ∼2
nm thick in the (010) direction, resulting in large intersheet mesopore
volumes and external surface areas exceeding 430 m2/g.
Unlike bulk Sn-MFI, Sn-MFI-ns is highly active in the Baeyer–Villiger
oxidation of bulky cyclic ketones using hydrogen peroxide (H2O2). Although Sn-MFI-ns and Sn-MCM-41 have comparable
activity and oxidant efficiency, the nanosheets exhibit drastically
higher thermal and hydrothermal stability.
The
Brønsted basicities pK
aH (i.e.,
pK
a of the conjugate acids) of 32 pyrrolidines
and imidazolidinones, commonly used in organocatalytic reactions,
have been determined photometrically in acetonitrile solution using CH acids as indicators. Most investigated
pyrrolidines have basicities in the range 16 < pK
aH < 20, while imidazolidinones are significantly less
basic (10 < pK
aH < 12). 2-(Trifluoromethyl)pyrrolidine
(A14, pK
aH 12.6) and the
2-imidazoliummethyl-substituted pyrrolidine A21 (pK
aH 11.1) are outside the typical range for pyrrolidines
with basicities comparable to those of imidazolidinones. Kinetics
of the reactions of these 32 organocatalysts with benzhydrylium ions
(Ar2CH+) and structurally related quinone methides,
common reference electrophiles for quantifying nucleophilic reactivities,
have been measured photometrically. Most reactions followed second-order
kinetics, first order in amine and first order in electrophile. More
complex kinetics were observed for the reactions of imidazolidinones
and several pyrrolidines carrying bulky 2-substituents, due to reversibility
of the initial attack of the amines at the electrophiles followed
by rate-determining deprotonation of the intermediate ammonium ions.
In the presence of 2,4,6-collidine or 2,6-di-tert-butyl-4-methyl-pyridine, the deprotonation of the initial adducts
became faster, which allowed the rate of the attack of the amines
at the electrophiles to be determined. The resulting second-order
rate constants k
2 followed the correlation
log k
2(20 °C) = s
N(N + E), where electrophiles
are characterized by one parameter (E) and nucleophiles
are characterized by the two solvent-dependent parameters N and s
N. In this way, the organocatalysts A1–A32 were integrated in our comprehensive
nucleophilicity scale, which compares n-, π-, and σ-nucleophiles.
The nucleophilic reactivities of the title compounds correlate only
poorly with their Brønsted basicities.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.