“…The distribution of protons between all of the nucleophiles (bases) present in the system is governed by their relative basicities. Using values of p K B known from the literature, the concentration of protonated EO can be expressed as a fraction of the overall concentration of the secondary oxonium ions. − Then, the final equation, allowing calculation of k 6 / k 2 ratio may be derived (Scheme ). 8 …”
Cationic copolymerization of tetrahydrofuran (THF) with ethylene oxide (EO) in the presence
of diols proceeds with simultaneous participation of secondary and tertiary oxonium ions. This process
comprises therefore some features of the activated
monomer (AM) and the active
chain
end (ACE)
mechanisms. The mechanism of copolymer formation was evaluated, and the kinetics of competing
reactions involving both secondary and tertiary oxonium ions with nucleophiles present in the system
was studied. The apparent rate constants were derived, allowing the estimation of the influence of
copolymerization conditions on the composition and microstructure of copolymers.
“…The distribution of protons between all of the nucleophiles (bases) present in the system is governed by their relative basicities. Using values of p K B known from the literature, the concentration of protonated EO can be expressed as a fraction of the overall concentration of the secondary oxonium ions. − Then, the final equation, allowing calculation of k 6 / k 2 ratio may be derived (Scheme ). 8 …”
Cationic copolymerization of tetrahydrofuran (THF) with ethylene oxide (EO) in the presence
of diols proceeds with simultaneous participation of secondary and tertiary oxonium ions. This process
comprises therefore some features of the activated
monomer (AM) and the active
chain
end (ACE)
mechanisms. The mechanism of copolymer formation was evaluated, and the kinetics of competing
reactions involving both secondary and tertiary oxonium ions with nucleophiles present in the system
was studied. The apparent rate constants were derived, allowing the estimation of the influence of
copolymerization conditions on the composition and microstructure of copolymers.
“…Since the basicity/nucleophilicity of the alcohols is not very different, the decrease of reactivity in the observed direction is probably due to steric crowding in forming the C−O bond between the cation and the oxygen of the alcohol in the reaction in which the mixed acetal 9 is formed. Among the ethers tetrahydrofuran (THF) and 1,4-dioxane, the latter is in all cases much less reactive, which reflects its considerably lower nucleophilicity, caused by the −I effect of the second oxygen in the ring.…”
The acyltrimethylsilanes
4-RC6H4C(O)SiMe3 (R =
H, Me, MeO) and β-naphthylC(O)SiMe3, upon
photolysis
in acetonitrile with 20 ns pulses of 248 nm light from an KrF* excimer
laser, give rise to the corresponding
α-siloxycarbenes ArC:OSiMe3, whose
absorption spectra (λmax between 270 and 310 nm),
lifetimes (between 130
and 260 ns), and reactivities with proton donors (ROH, mainly alcohols)
are reported. With highly acidic ROH,
such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP), the reaction is
of simple second order, rate constants being
in the 109 M-1 s-1 range and
virtually independent of the nature of the aromatic moiety of the
carbene. Isotopic
substitution of H in ROH by D has no effect on the rate constant for
reaction with carbene. For less acidic alcohols
such as, e.g., methanol, the reactivity of the carbenes increases with
increasing [ROH]. This behavior is interpreted
in terms of reversible adduct formation between carbene and alcohol
followed by reaction with further alcohol
molecule(s) to give product. On the basis of experiments in
the acidic and only weakly nucleophilic solvents 2,2,2-trifluorethanol (TFE) and HFIP, protonation of the carbenes leads to the
corresponding carbenium ions, whose
absorption spectra (λmax between 305 and 355 nm),
lifetimes (100 ns−5 μs in TFE), and reactivities with
nucleophiles
(halides, alcohols, and ethers) are reported. In the solvent HFIP,
the reactivities of the carbenium ions with the
alcohols and ethers increase with their concentration, in a way
analogous to that observed in the reaction of the
carbenes with the alcohols. This is explained as resulting from
reversible formation of a cation−nucleophile complex
followed by reaction of the complex with a second nucleophile molecule
which acts as a base. In solvents more
basic than HFIP, it is presumably the solvent which serves this
function.
“…Additionally, the sulfur atom of 13 resides in a six-membered ring, which is thought to be more nucleophilic than the corresponding five-membered ring compounds. An analogous explanation has been suggested to account for differences in complex formation of tetrahydrofuran and tetrahydropyran with dimethylzinc. , The present combination of oxathiane with a sulfur atom as the nucleophile, a C 3 side chain and a bromide as the leaving group had not been previously investigated.…”
[reaction: see text] A new general method for the construction of medium ring ethers has been developed. This involves the ring expansion of halo-O,S-acetals followed by a Ramburg-Bäcklund ring contraction reaction with concomitant extrusion of the sulfur atom. This methodology has been utilized for the synthesis of cis- and trans-lauthisan.
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