“…Addition of ·CH 2 OH to one of the double bond of unpolymerized DADMAC is not a viable first-order termination in CH 3 OH. As shown earlier, addition of alcohol radicals to the monomer yields an intermediate that undergoes cyclization, but the resulting cyclic radical abstracts a H atom from the solvent, regenerating another alcohol radical. Steps 5−8 were found to be important reactions during the chain reduction of H 2 O 2 in methanol, and are useful to explain the atypical independence of PE(−AuCl 4 - ) on light intensity presented in the inset of Figure .…”
AuCl4
- ions are transformed into Au crystallites via two consecutive photoreactions inside cross-linked polymers
of diallyldimethylammonium chloride swollen with methanol. The photoreactions are markedly influenced
by the gel matrix, which facilitates pathways not observed in homogeneous solutions and controls the direction
of propagation. Initially, the reactions are repressed by air, but at longer times they are aided by products
from the O2-reduction. The first process is an efficient monophotonic chain reduction of AuCl4
- with chain
lengths in excess of 80 and an atypical pseudofirst-order termination. Kinetic data from fully swollen gels is
understood in terms of a mechanism involving solution reactions of ·CH2OH radical chain carriers. Light-absorbing products of the first photoreaction, believed to be gold clusters, initiate the second process that
yields metal particles. Several common kinetic features are displayed by both photoreactions. However,
generation of Au crystallites takes place via an unusual biphotonic chain reaction, with reaction rates that are
correlated to the number of particle formation and decay cycles.
“…Addition of ·CH 2 OH to one of the double bond of unpolymerized DADMAC is not a viable first-order termination in CH 3 OH. As shown earlier, addition of alcohol radicals to the monomer yields an intermediate that undergoes cyclization, but the resulting cyclic radical abstracts a H atom from the solvent, regenerating another alcohol radical. Steps 5−8 were found to be important reactions during the chain reduction of H 2 O 2 in methanol, and are useful to explain the atypical independence of PE(−AuCl 4 - ) on light intensity presented in the inset of Figure .…”
AuCl4
- ions are transformed into Au crystallites via two consecutive photoreactions inside cross-linked polymers
of diallyldimethylammonium chloride swollen with methanol. The photoreactions are markedly influenced
by the gel matrix, which facilitates pathways not observed in homogeneous solutions and controls the direction
of propagation. Initially, the reactions are repressed by air, but at longer times they are aided by products
from the O2-reduction. The first process is an efficient monophotonic chain reduction of AuCl4
- with chain
lengths in excess of 80 and an atypical pseudofirst-order termination. Kinetic data from fully swollen gels is
understood in terms of a mechanism involving solution reactions of ·CH2OH radical chain carriers. Light-absorbing products of the first photoreaction, believed to be gold clusters, initiate the second process that
yields metal particles. Several common kinetic features are displayed by both photoreactions. However,
generation of Au crystallites takes place via an unusual biphotonic chain reaction, with reaction rates that are
correlated to the number of particle formation and decay cycles.
“…In these compounds, free-radical addition to the diene system followed by cyclization of the resultant 5-hexenyl radical has the potential of generating a variety of carbocyclic nucleotide analogues (eq 1). Although there have been numerous studies of cyclizations of 5-hexenyl radicals derived from 1,6-heptadiene and from other sources, we know of none where a nucleic acid base is substituted in the 4 position of the 1,6-heptadiene. …”
A series of 1,6-heptadienes, substituted in the 4 position with nucleic acid bases 1-6, have been synthesized via Mitsunobu condensations. Guanine, adenine, thymine, and uracil derivatives can be prepared directly by coupling the protected base with 1,6-heptadien-4-ol (7). However, coupling protected cytosine and 7 gives an O-alkylated product. Thus, the cytosine derivative must be prepared from the uracil-substituted heptadienes via the triazole. The free-radical addition of CCl(4) and BrCCl(3) to these adducts was investigated. In all cases, both 1:1 and 1:2 adducts were obtained. The 1:1 adduct was identified as the cyclized product of the initially formed 5-hexen-1-yl radical. The cyclization takes place in a stereospecific manner, with only one of the four possible diastereomers resulting. NMR studies indicate that all substituents are cis in this product. In the case of the addition of CCl(4) to the uracil-substituted heptadiene, this conclusion was confirmed by an X-ray structure determination of the isolated cyclized product. The free-radical-initiated cyclocopolymerizations of 1-6 with SO(2) gave 1:1 copolymers with cis-linked five-membered rings. Two-dimensional NMR studies on poly(2-SO(2)) showed predominately the cis-syn isomer while poly(6-SO(2)) has an approximately equal amount of cis-syn and cis-anti isomers.
“…Hyperconjugative mixing of the half-filled p orbital and the C-H σ and σ* orbitals produce a semioccupied delocalized orbital which is of matching symmetry to the alkene π* orbital. Therefore in the transition state, the cis-isomer exhibits a second interaction between the carbon-centered radical and remaining olefin, offsetting the nonbonded repulsion between the vicinal substituents of the newly formed pyrrolidine [32][33][34]. Simple substitution at the alkene terminus does not affect the cyclization, and the process continues to follow a 5-exo-trig pathway.…”
Section: Introductionmentioning
confidence: 99%
“…Therefore in the transition state, the cis-isomer exhibits a second interaction between the carbon-centered radical and remaining olefin, offsetting the nonbonded repulsion between the vicinal substituents of the newly formed pyrrolidine. [32][33][34] Simple substitution at the alkene terminus does not affect the cyclization, and the process continues to follow a 5-exo-trig pathway. Padwa et al also reported the addition of an acetylthiyl radical onto another homogenously unsaturated substrate; the corresponding N,N-dipropargylsulfonamide 11 (Scheme 3B).…”
Sulfur centered radicals are widely employed in chemical synthesis, in particular for alkene and alkyne hydrothiolation towards thioether bioconjugates. The steadfast radical chain process that enables efficient hydrothiolation has been explored in the context of cascade reactions to furnish complex molecular architectures. The use of thiyl radicals offers a much cheaper and less toxic alternative to the archetypal organotin-based radical methods. This review outlines the development of thiyl radicals as reactive intermediates for initiating carbocyclization cascades. Key developments in cascade cyclization methodology are presented and applications for natural product synthesis are discussed. The review provides a chronological account of the field, beginning in the early seventies up to very recent examples; a span of almost 50 years.
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