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Bruce M Novak - One of the best experts on this subject based on the ideXlab platform.
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living ring opening metathesis polymerizations of 3 4 disubstituted Cyclobutenes and synthesis of polybutadienes with protic functionalities
Macromolecules, 1996Co-Authors: Michael G Perrott, Bruce M NovakAbstract:The compounds bis(phenylmethyl) cis-Cyclobutene-3,4-dicarboxylate (1) and cis-3,4-bis(2-oxa-3-phenylpropyl)Cyclobutene (2) were polymerized by ring-opening metathesis polymerizations using Mo(CHC(CH3)2R1)(NC6H3-2,6-diisopropyl)(OR2)2 (R1 = Ph, R2 = C(CH3)3, I; R1 = CH3, R2 = C(CF3)2CH3, II) as the initiator. For I as initiator, no chain termination or chain transfer processes were observed during the lifetime of the polymerization, showing that the polymerizations of 1 and 2 by I are living. Block copolymers containing 1 and 2 and diethyl cis-Cyclobutene-3,4-dicarboxylate (5) were synthesized. The double bonds of the poly-2 and poly-3 ranged from 70% trans to 92% cis depending on the initiator and monomer used. Poly-1 and poly-2 were treated with iodotrimethylsilane and converted to the acid-functionalized and alcohol-functionalized poly-4 and poly-6, respectively. Block copolymers were treated with iodotrimethylsilane and converted to materials with blocks bearing protic functionalities and blocks bearin...
K. N. Houk - One of the best experts on this subject based on the ideXlab platform.
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altering the allowed forbidden gap in Cyclobutene electrocyclic reactions experimental and theoretical evaluations of the effect of planarity constraints
Journal of the American Chemical Society, 2003Co-Authors: Patrick Lee, Shogo Sakai, Peter Hörstermann, Wolfgang Roth, Adam E Kallel, K. N. HoukAbstract:The allowed conrotatory Cyclobutene ring-opening has a distinctly nonplanar carbon skeleton. Classic experiments by Brauman and Archie, and by Freedman et al., placed the allowed/forbidden gap at greater than 15 kcal/mol. Wolfgang Roth proposed that a system forced to planarity might have a smaller preference for the conrotatory mode than unconstrained systems. Such systems have now been studied theoretically and experimentally, and results that confirm Roth's postulate are presented here. The experiments were performed in Bochum, and the calculations were carried out in Osaka and Los Angeles. As the Cyclobutene ring-opening transition structure approaches planarity, the energy gap between allowed conrotatory and the forbidden disrotatory pathways decreases. For the ring-opening of a Cyclobutene fused to norbornene, the energy gap between the forbidden and the allowed transition state is only 4.1 kcal/mol by CASSCF and 8.0 kcal/mol by CAS-MP2 as compared to 13.4 and 19.2 kcal/mol, respectively, for the parent Cyclobutene. Experimental studies of 3,4-dimethylCyclobutenes fused to various ring systems are reported, and a trend is found toward a reduced allowed/forbidden gap as the planarity of the Cyclobutene is enforced.
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Substituent Effects on Rates and Stereoselectivities of Conrotatory Electrocyclic Reactions of Cyclobutenes. A Theoretical Study.
The Journal of Organic Chemistry, 1996Co-Authors: Satomi Niwayama, E. A. Kallel, David C. Spellmeyer, Chimin Sheu, K. N. HoukAbstract:Substituent effects on the geometries and conrotatory electrocyclic ring openings of Cyclobutenes were studied. This work extends the original investigations to many more substituents and provides a comprehensive theory of substituent effects on geometries and reaction rates. The effects of substitution at the 1 position are minimal; donor substituents raise the activation energy slightly, and powerful acceptor substituents slightly lower the activation energy. Substituents on C3 cause small distortions of the Cyclobutene geometry, in the same direction as the favored stereochemistry of reaction. Donors prefer outward rotation, while strong acceptors prefer inward rotation. The activation energy changes and Cyclobutene geometrical perturbations were found to correlate with Taft σR0 parameters. Amino, hydroxy, fluoro, chloro, methyl, cyano, formyl, and vinyl substituents were studied in the 1 position. Boryl, dimethylboryl, nitroso, formyl, nitro, carboxyl (neutral, protonated, and deprotonated), cyano, tr...
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The torquoselectivity of electrocyclic reactions of 3-donor-3-acceptor-substituted Cyclobutenes
Tetrahedron Letters, 1995Co-Authors: Satomi Niwayama, Ying Wang, K. N. HoukAbstract:Abstract Theory predicts that electron donors have larger outward rotation preferences than acceptors in thermal ring openings of Cyclobutenes. Experimental verifications are presented for several 3,3-disubstituted Cyclobutenes.
Michael G Perrott - One of the best experts on this subject based on the ideXlab platform.
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living ring opening metathesis polymerizations of 3 4 disubstituted Cyclobutenes and synthesis of polybutadienes with protic functionalities
Macromolecules, 1996Co-Authors: Michael G Perrott, Bruce M NovakAbstract:The compounds bis(phenylmethyl) cis-Cyclobutene-3,4-dicarboxylate (1) and cis-3,4-bis(2-oxa-3-phenylpropyl)Cyclobutene (2) were polymerized by ring-opening metathesis polymerizations using Mo(CHC(CH3)2R1)(NC6H3-2,6-diisopropyl)(OR2)2 (R1 = Ph, R2 = C(CH3)3, I; R1 = CH3, R2 = C(CF3)2CH3, II) as the initiator. For I as initiator, no chain termination or chain transfer processes were observed during the lifetime of the polymerization, showing that the polymerizations of 1 and 2 by I are living. Block copolymers containing 1 and 2 and diethyl cis-Cyclobutene-3,4-dicarboxylate (5) were synthesized. The double bonds of the poly-2 and poly-3 ranged from 70% trans to 92% cis depending on the initiator and monomer used. Poly-1 and poly-2 were treated with iodotrimethylsilane and converted to the acid-functionalized and alcohol-functionalized poly-4 and poly-6, respectively. Block copolymers were treated with iodotrimethylsilane and converted to materials with blocks bearing protic functionalities and blocks bearin...
Yannick Landais - One of the best experts on this subject based on the ideXlab platform.
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p anisaldehyde photosensitized sulfonylcyanation of chiral Cyclobutenes enantioselective access to cyclic and acyclic systems bearing all carbon quaternary stereocenters
Organic Letters, 2020Co-Authors: Vincent Pirenne, Iman Traboulsi, Lisa Rouviere, Jonathan Lusseau, Stephane Massip, Frédéric Robert, Dario M. Bassani, Yannick LandaisAbstract:The photosensitized p-anisaldehyde-mediated addition of sulfonylcyanides onto the π-system of Cyclobutenes is shown to afford highly functionalized cyclobutanes in high yields and diastereocontrol. The homochiral Cyclobutene precursors are accessible on multigram scale in two steps through an asymmetric [2 + 2] cycloaddition/vinyl thioether reduction sequence. The enantiopure cyclobutylnitriles can be elaborated further through SmI2-mediated ring opening or converted into new enantiopure Cyclobutenes through base-mediated sulfone elimination.
W. J. Hart - One of the best experts on this subject based on the ideXlab platform.
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Ab Initio calculations on the isomerization of the dimethylene Cyclobutene and 1,2,4,5-hexatetraene radical cations to the benzene structure
Journal of the American Society for Mass Spectrometry, 1997Co-Authors: W. J. HartAbstract:Ab initio calculations at the MRCI//ROHF/6-31G** level suggest that stable C_6H_6 radical cations formed from dimethylene Cyclobutene and 1,2,4,5-hexatetraene either retain the structure of the parent neutral molecule or isomerize to the benzene structure. The previous suggestion from photodissociation experiments that part of the ions from dimethylene Cyclobutene isomerize to the 1,2,4,5-hexatetraene structure is not supported by the present results.
