227 research outputs found
Computational and Experimental Investigations of the Formal Dyotropic Rearrangements of Himbert Arene/Allene Cycloadducts
No full textThe fascinating intramolecular
arene/allene cycloaddition discovered
by Himbert affords dearomatized, polycyclic intermediates with sufficient
strain energy to drive rearrangement processes of the newly formed
ring system. We disclose a detailed examination of a thermally induced
stepwise dyotropic skeletal rearrangement of these cycloadducts, a
reaction also first described by Himbert. We offer computational evidence
for a two-stage mechanism for this formal dyotropic rearrangement
and provide rationalizations for the significant substitution-dependent
rate differences observed in experiments. These investigations led
to the development of a Lewis-acid-catalyzed rearrangement of precursors
that were unreactive under simple thermal instigation. The isolation
of the product of an “interrupted” rearrangement under
Lewis acidic conditions provides further support for the proposed
stepwise mechanism. Computational results also matched experiments
in terms of regiochemical preferences in unsymmetrical rearrangement
precursors and explained why lactam <i>O</i>-, <i>S</i>-, and <i>C</i>-heterologues do not easily undergo this
rearrangement
Complex Polycyclic Scaffolds by Metathesis Rearrangement of Himbert Arene/Allene Cycloadducts
No full textThe intramolecular arene/allene cycloaddition first described 30 years ago by Himbert and Henn permits rapid access to strained polycyclic compounds. Alkene metathesis processes cleanly rearrange appropriately substituted cycloadducts into complex, functional-group-rich polycyclic lactams of potential utility for natural product synthesis and medicinal chemistry
Complex Polycyclic Scaffolds by Metathesis Rearrangement of Himbert Arene/Allene Cycloadducts
No full textThe intramolecular arene/allene cycloaddition first described 30 years ago by Himbert and Henn permits rapid access to strained polycyclic compounds. Alkene metathesis processes cleanly rearrange appropriately substituted cycloadducts into complex, functional-group-rich polycyclic lactams of potential utility for natural product synthesis and medicinal chemistry
Studies on the Himbert Intramolecular Arene/Allene Diels–Alder Cycloaddition. Mechanistic Studies and Expansion of Scope to All-Carbon Tethers
No full textThe
unusual intramolecular arene/allene cycloaddition described
30 years ago by Himbert permits rapid access to strained polycyclic
compounds that offer great potential for the synthesis of complex
scaffolds. To more fully understand the mechanism of this cycloaddition
reaction, and to guide efforts to extend its scope to new substrates,
quantum mechanical computational methods were employed in concert
with laboratory experiments. These studies indicated that the cycloadditions
likely proceed via concerted processes; a stepwise biradical mechanism
was shown to be higher in energy in the cases studied. The original
Himbert cycloaddition chemistry is also extended from heterocyclic
to carbocyclic systems, with computational guidance used to predict
thermodynamically favorable cases. Complex polycyclic scaffolds result
from the combination of the cycloaddition and subsequent ring-rearrangement
metathesis reactions
Studies on the Himbert Intramolecular Arene/Allene Diels–Alder Cycloaddition. Mechanistic Studies and Expansion of Scope to All-Carbon Tethers
No full textThe
unusual intramolecular arene/allene cycloaddition described
30 years ago by Himbert permits rapid access to strained polycyclic
compounds that offer great potential for the synthesis of complex
scaffolds. To more fully understand the mechanism of this cycloaddition
reaction, and to guide efforts to extend its scope to new substrates,
quantum mechanical computational methods were employed in concert
with laboratory experiments. These studies indicated that the cycloadditions
likely proceed via concerted processes; a stepwise biradical mechanism
was shown to be higher in energy in the cases studied. The original
Himbert cycloaddition chemistry is also extended from heterocyclic
to carbocyclic systems, with computational guidance used to predict
thermodynamically favorable cases. Complex polycyclic scaffolds result
from the combination of the cycloaddition and subsequent ring-rearrangement
metathesis reactions
D L McLane
No full text2 x 4 negative, man with suspenders standing under some trees, a fence and more trees are behind himBert Hall Collection Vault F3-D5 Envelope Bert Hall Collection 85.158 D L McLane 195
Studies on the Himbert Intramolecular Arene/Allene Diels–Alder Cycloaddition. Mechanistic Studies and Expansion of Scope to All-Carbon Tethers
No full textThe
unusual intramolecular arene/allene cycloaddition described
30 years ago by Himbert permits rapid access to strained polycyclic
compounds that offer great potential for the synthesis of complex
scaffolds. To more fully understand the mechanism of this cycloaddition
reaction, and to guide efforts to extend its scope to new substrates,
quantum mechanical computational methods were employed in concert
with laboratory experiments. These studies indicated that the cycloadditions
likely proceed via concerted processes; a stepwise biradical mechanism
was shown to be higher in energy in the cases studied. The original
Himbert cycloaddition chemistry is also extended from heterocyclic
to carbocyclic systems, with computational guidance used to predict
thermodynamically favorable cases. Complex polycyclic scaffolds result
from the combination of the cycloaddition and subsequent ring-rearrangement
metathesis reactions
Studies on the Himbert Intramolecular Arene/Allene Diels–Alder Cycloaddition. Mechanistic Studies and Expansion of Scope to All-Carbon Tethers
No full textThe
unusual intramolecular arene/allene cycloaddition described
30 years ago by Himbert permits rapid access to strained polycyclic
compounds that offer great potential for the synthesis of complex
scaffolds. To more fully understand the mechanism of this cycloaddition
reaction, and to guide efforts to extend its scope to new substrates,
quantum mechanical computational methods were employed in concert
with laboratory experiments. These studies indicated that the cycloadditions
likely proceed via concerted processes; a stepwise biradical mechanism
was shown to be higher in energy in the cases studied. The original
Himbert cycloaddition chemistry is also extended from heterocyclic
to carbocyclic systems, with computational guidance used to predict
thermodynamically favorable cases. Complex polycyclic scaffolds result
from the combination of the cycloaddition and subsequent ring-rearrangement
metathesis reactions
Determination of the Boltzmann constant k from the speed of sound in helium gas at the triple point of water
Get PDFThe Boltzmann constant k has been determined from a measurement of the speed of sound in helium gas in a quasi-spherical resonator (volume 0.5 l) maintained at a temperature close to the triple point of water (273.16 K). The acoustic velocity c is deduced from measured acoustic resonance frequencies and the dimensions of the quasi-sphere, the latter being obtained via simultaneous microwave resonance. Values of c are extrapolated to the zero pressure limit of ideal gas behaviour. We find J⋅K−1, a result consistent with previous measurements in our group and elsewhere. The value for k, which has a relative standard uncertainty of 1.02 ppm, lies 0.02 ppm below that of the CODATA 2010 adjustment
Computation and Experiment Reveal That the Ring-Rearrangement Metathesis of Himbert Cycloadducts Can Be Subject to Kinetic or Thermodynamic Control
Get PDFUnusual observations in the ring-rearrangement metathesis of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provides a reasonable explanation for the previously irreconcilable results
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