Cyclization unimolecular reaction

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The competitive kinetics of Scheme 3.1 can also be applied to calibrate the unimolecular radical reactions provided that kn is a known rate constant. In particular the reaction of primary alkyl radicals with (Mc3Si)3SiH has been used to obtain kinetic data for some important unimolecular reactions such as the p-elimination of octanethiyl radical from 12 (Reaction 3.5) [12], the ring expansion of radical 13 (Reaction 3.6) [8] and the S-endo-trig cyclization of radical 14 (Reaction 3.7) [13]. The relative Arrhenius expressions shown below for the... 

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

At this time, no absolute rate constants have been determined for a reaction of an aminium cation radical. However, for synthetic utility, one needs to consider the relative rate constants for competing reactions. Competition between two unimolecular reactions depends only upon the relative rate constants for the processes. For competition between a unimolecular and a bimolecular reaction whose rate constants are comparable, product distributions can easily be controlled by the concentration of the second species in the ratio of rate laws. The ratio of reaction products from cyclization (unimolecular) versus hydrogen atom trapping before cyclization (bimolecular) can be expressed by the equation %(42 + 65)/%41 = Ar/(A H[Y - H]) (Scheme 20). Competition between two bimolecular reactions is dependent on the relative rate constants for each process and the effective, or mean, concentration of each reagent. The ratio of the products from H-atom transfer trapping of the cyclized radical versus self-trapping by the PTOC precursor can be expressed by the equation %42/%65 = (kH /kT) ([Y - H]/[PTOC]). 

Unimolecular reactions of organic radical cations are fragmentation, rearrangement and cyclization, as illustrated by the following generic examples. The specific details of each of these transformations are included in Section 7.4.6. (Note Ar and R represent aryl and alkyl groups, respectively.)... 

High Dilution Although the preparation of five- and six-mem-bered rings from appropriate acyclic precursors usually proceeds without difficulty, the synthesis of larger rings requires more elaborate techniques since bimolecular condensations tend to become more favorable than simple cyclization. To favor the unimolecular reaction at the expense of the bimolecular condensation, high dilution techniques are used, and the apparatus (Fig. 1-14) used for the Dieckmann cyclization of diethyl tetradecan-1,14-dicarboxylate is excellent for this purpose.f... 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

The Arrhenius expression for the reaction of the o(allyloxy)phenyl radical (9) with (Me3Si)3SiH relative to this unimolecular rearrangement [Eq. (4)] has been measured, v/z., og(kclku) (M) = 2.6 - 1.6/0.36 When the competition study was performed, however, reliable absolute rate constants for the cyclization of radical 9 to radical 10 were not available, although a... 

The overall reactivities of these radicals in their ummolecular 5-hexenyl cyclization processes reflects those same factors which affect the reactivity of partially-fluorinated radicals in their bimolecular addition reactions with alkenes, such as styrene. Table 17 indicates this clearly, and it also reflects the general leveling effect which would be expected for the more facile unimolecular cyclization processes which have log A s about 1-2 units larger than those for the bimolecular additions. 

Our earlier studies of the bimolecular alkene addition reactivity of a,a-difluoro alkyl radicals indicated that they exhibited little philicity , reacting with styrene and pentafluorostyrene (IPs 8.43 and 9.20, respectively) at virtually the same rate [70]. Their significantly greater reactivity in bimolecular additions, hydrogen-abstraction reactions and unimolecular cyclizations can therefore be largely attributed to their pyramidal nature, with some possible thermodynamic con-... 

Another common scenario in competition kinetics utilizes unimolecular radical reactions as a clock against which other reactions can be timed. Among the most commonly used free radical clocks are the cyclization of 1 -hexenyl and other radicals with double or triple bonds in the chain,33 ring opening,34 and p-elimination from alkoxyl radicals.35... 

Although some interesting synthetic processes of cyclic polymers have been addressed, the cyclization principle can be classified into two main methods as indicated in Fig. 2. One is the utilization of the ring-chain equilibrium that occurs in many polycondensation and ring opening polymerization. Another is the end-to-end cyclization method that can be used for synthesizing cyclic polymers from a/o-difunclional linear precursors. The ring-closure reaction by the end-to-end cyclization is further divided into intermolecular reaction and intramolecular reaction, i.e., bimolecular process and unimolecular process, respectively. 

The proposed mechanism for this reaction involves the initial coordination of styrene oxide to Tbt(Tip)Sn=Se followed by nucleophilic attack of a second Tbt(Tip)Sn=Se molecule on the less substituted carbon with the formation of cis- and trans-195. Alternatively, the unimolecular cyclization of the initial styrene oxide-Tbt(Tip)Sn=Se complex leads to the formation of compound 196 as a minor product (Scheme 10). 

A significantly more important tool to decipher the reaction mechanism is probe reactions (Fig. 8). Most commonly used are cyclopropylcarbinyl radical ring opening reactions and radical 5-exo cyclizations to intercept coupling reactions with metal centers. Cyclopropylmethyl bromide 20 is reduced by a metal complex and generates cyclopropylcarbinyl radical 20A. Unimolecular ring opening to... 

The Heck reaction may also proceed in an intramolecular fashion if the aryl halide is covalently bound to the allyl substrate. In this case the sequential allylic substitution-Heck-cyclization with Pd acetate as a catalyst takes place very smoothly as an unimolecular process and furnishes the polycon-densed pyrrolidones 91 in excellent yields (Scheme 29). 

Both direct and sensitized irradiations of cyclohexene 78Z to give trans-anti-transcis-trans-, and cis-anti-cis- 2 + 2]-cyclodimers 80-82 in different ratios [59]. This photodimerization is believed to proceed through the initial formation of the highly strained ( )-isomer 78E, which is followed by the thermal concerted and/or stepwise cyclodimerization with 78Z, although no direct evidence for the intervention of 78E has been obtained and the cyclization mechanism(s) involved are not very clear. In this photocyclodimerization, the enantiodifferentiation occurs not in the cyclodimerization but in the initial photoisomerization step hence we classify this formally bimolecular reaction as a unimolecular enantiodifferenti-ating photosensitization. 

---