5 research outputs found
Synthesis of Enantiopure Tertiary Skipped Diynes via One-Pot Desymmetrizing TMS-Cleavage
Enantiopure tetrasubstituted skipped diynes were readily synthesized from N-protected amino esters upon addition of lithium TMS-acetylide which was found to be desymmetrizing through one-pot selective TMS-cleavage. The deprotection of the TMS group was realized through a one-pot silicon atom attack by the liberated methoxide, which was diastereoselective due to a conformational favorable chelate
Theoretical Study To Explain How Chirality Is Stored and Evolves throughout the Radical Cascade Rearrangement of Enyne-allenes
This
article reports a theoretical study to explain how the intrinsic
property of chirality is retained throughout the radical cascade rearrangement
of an enantiopure chiral enyne-allene (bearing one stereogenic center)
selected as a model for this family of reactions. Calculations at
the MRPT2/6-31GÂ(d)//CASSCFÂ(10,10)/6-31GÂ(d) level of theory were used
to determine the entire reaction pathway which includes singlet state
diradicals and closed-shell species. The cascade process involves
three elementary steps, i.e., by chronological order: MyersâSaito
cycloaromatization (M-S), intramolecular hydrogen atom transfer (HAT),
and recombination of the resulting biradical. The enantiospecificity
of the reaction results from a double transmission of the stereochemical
information, from the original center to an axis and eventually from
this axis to the final center. The first two steps lead to a transient
diradical intermediate which retains the chirality via the conversion
of the original static chirogenic element into a dynamic one, i.e.,
a center into an axis. The only available routes to the final closed-shell
tetracyclic product imply rotations around two Ï bonds (ÏÂ(CâC)
and ÏÂ(CâN), bonds ÎČ and α respectively). The theoretical calculations
confirmed that the formation of the enantiomerically pure product
proceeds via the nonracemizing rotation around the ÏÂ(CâC)
pivot. They ruled out any rotation around the second ÏÂ(CâN)
pivot. The high level of configurational memory in this rearrangement
relies on the steric impediment to the rotation around the CâN
bond in the chiral native conformation of the diradical intermediate
produced from tandem M-S/1,5-HAT
Mechanistic Investigation of Enediyne-Connected Amino Ester Rearrangement. Theoretical Rationale for the Exclusive Preference for 1,6- or 1,5-Hydrogen Atom Transfer Depending on the Substrate. A Potential Route to Chiral Naphthoazepines
Memory of chirality (MOC) and deuterium-labeling studies
were used
to demonstrate that the cascade rearrangement of enediyne-connected
amino esters <b>1a</b> and <b>1b</b> evolved through exclusive
1,5- or 1,6-hydrogen atom transfer, subsequent to 1,3-proton shift
and SaitoâMyers cyclization, depending on the structure of
the starting material. These results were independently confirmed
by DFT theoretical calculations performed on model monoradicals. These
calculations clearly demonstrate that in the alanine series, 1,5-hydrogen
shift is kinetically favored over 1,6-hydrogen shift because of its
greater exergonicity. In the valine series, the bulk of the substituent
at the nitrogen atom has a major influence on the fate of the reaction. <i>N</i>-Tosylation increases the barrier to 1,5-hydrogen shift
to the benefit of 1,6-hydrogen shift. The ready availability of 1,6-hydrogen
atom transfer was explored as a potential route for the enantioselective
synthesis of naphthoazepines
Metal AcetylacetonateâBidentate Ligand Interaction (MABLI) (Photo)activated Polymerization: Toward High Performance Amine-Free, Peroxide-Free Redox Radical (Photo)initiating Systems
Free radical polymerization
(FRP) initiation from metal acetylacetonateâbidentate
ligand interaction (MABLI) under mild conditions (room temperature,
under air) is discussed here for different metal centers (Mn, V, and
Cu). First, without light, in MABLI systems such as VÂ(acac)<sub>3</sub>/2-diphenylÂphosphinobenzoic acid (2dppba), MnÂ(acac)<sub>3</sub>/2dppba, or CuÂ(acac)<sub>2</sub>/2dppba, electron-rich aryl (Ar<sup>âą</sup>) radicals were conveniently and efficiently produced
by the reaction of electron-poor acac<sup>âą</sup> radicals
with the iodonium salt (Iod), leading to an enhancement of pure redox
(no light) polymerizations. Second, it was found that VÂ(III)*/Iod
reaction is generating aryl (Ar<sup>âą</sup>) radicals at a
high enough rate to initiate a photopolymerization process upon mild
irradiation (LED@405 nm). This reaction can be implemented in redox
photoactivated systems (e.g., using three-component VÂ(acac)<sub>3</sub>/2dppba/Iod systems) in order to spectacularly enhance a slow redox
process from 40 °C exothermicity in 800 s to 93 °C in less
than 200 s. Third, an impressive chemical bleaching (without light)
was reported for the MnÂ(acac)<sub>3</sub>/2dppba reaction. Photoactivation
of the MnÂ(acac)<sub>3</sub>/2dppba/Iod system led to outstanding FRP
initiation efficiencies (<20 s for more than 85% Cî»C conversion
of a low-viscosity methacrylate resin). Light enhancement of surface
curing was confirmed for all the redox photoactivated polymerizations
using Raman confocal microscopy. Overall, amine-free peroxide-free
MABLI radical initiating systems were highly improved for safer and
even more efficient redox (photoactivated) polymerizations. This original
combination of redox polymerization and photopolymerization will be
highly worthwhile to combine in one approach the advantages of both
techniques
Polyaromatic Structures as Organo-Photoinitiator Catalysts for Efficient Visible Light Induced Dual Radical/Cationic Photopolymerization and Interpenetrated Polymer Networks Synthesis
Different polyaromatic structures (truxene derivatives
and trisÂ(aza)Âpentacene) are presented as new metal-free organic photocatalysts
(OPC) to promote free radical polymerization FRP and ring-opening
polymerization (ROP) under halogen lamp, household LED bulb, and laser
diode (405 nm). These OPCs exhibit interesting light absorption properties
and lead, through an oxidative catalytic cycle, to the formation of
radicals and ions that can initiate both free radical polymerization
FRP and ring-opening polymerization ROP. Interestingly, excellent
polymerization profiles are obtained even upon visible light exposure.
Using these very soft irradiation conditions, acrylate/epoxide blends
are also easily polymerized under air and lead to the formation of
interpenetrated polymer networks IPN exhibiting no phase separation