Intramolecular cycles/reactions

Serious deviations of the polymer network structure from the ideal one can have several causes. One of them is the crosslinking agent involvement in intramolecular cycle formation. The contribution of this reaction grows with the system dilution as well as when the crosslinker units in the chain are close one to the other, i.e. its fraction in the copolymer increases. All this is in good agreement with the observed trend. 

Analysis of data pertaining to the modulus of PEO gels obtained by the polyaddition reaction [90] shows that even in this simplified case the network structure substantially deviates from the ideal one. For all samples studied, the molecular weight between crosslinks (M p) exceeds the molecular weight of the precursor (MJ. With decreasing precursor concentration the M xp/Mn ratio increases. Thus, at Mn = 5650 a decrease in precursor concentration from 50 to 20% increases the ratio from 2.3 to 12 most probably due to intramolecular cycle formation. 

Some Bis reactions may lead to intramolecular cycles that will not affect the elasticity behavior of the gel, and pendant, unreacted, gronps may form where only the crosslinker reacts at one of its double bonds [315]. Tobita and Hamielec [393] found that primary cyclization, i.e., the formation of loop cycles, may consnme as much as 80% of... 

A tandem palladium-catalysed ort/io-alkylation/intramolecular Heck reaction coupling sequence was used effectively to access in fair yields the tetrahydro 1-benzoxepines 67 from the iodoaryl precursor 66 and the appropriate alkyl bromide. The norbornene plays a relay role in the proposed reaction cycle <06JOC4937>... 

The proposed catalytic mechanism for intramolecular McMurry reaction begins with the reduction of TiCl3 by zinc metal to generate the activated titanium species A-19. Reductive cyclization of the dicarbonyl substrate forms the McMurry coupling product, along with titanium oxide complex B-15. To close the catalytic cycle, the oxide complex B-15 is converted to TiCl3 by Me3SiCl (Scheme 63).8d,8e... 

In the absence of chloride ion, the Cu(I)/Cu(II) catalytic cycle could be initiated by an intramolecular redox reaction of the [Cu2(HA)2]2+ dimer to yield unsymmetrical products ... 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

One way to reduce the intramolecular cycle formation, is to add AB2-mono-mer successively throughout the reaction in a so-called concurrent slow-addi-tion . Several authors have shown that slow addition of monomer leads to a reduction in side reactions and increased molecular weight [5], while others have studied the occurrence of cyclization in hyperbranched systems [6]. 

Stannane based Hnkers (Tab. 3.8) are used for the StiUe reaction (see below, section 3.3.2.1) onsoHd supports [119]. Nicolaouetal. developed polymer-bound aUcenylstan-nanes to obtain the natural product (S)-zearalenone by an intramolecular StiUe reaction and a cyclative cleavage [120]. 

The seven membered core of iboga alkaloids has also been constructed in an intramolecular Heck reaction. Insertion of a pendant olefin into the indolylpalladium complex, formed from iodoindole, followed by / -hydride elimination gave the complex framework of the natural product (5.5.), Although the insertion step could have led to the formation of a six membered ring, the formed palladium complex would have contained a quaternary carbon center in the -position, blocking the closure of the catalytic cycle under the applied conditions.5... 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

Equation (3.23) has no physical sense for x > xgei, because intramolecular reactions in the gel cannot be neglected so, the relationship between the number of moles in the system and the conversion is no longer valid. For an actual system, the departure of the theoretical prediction of Mn and the experimental value in the pregel stage may be ascribed to the formation of intramolecular cycles. This constitutes the usual way of determining the significance of cyclization for a particular system. 

Equation (3.79) was obtained by dividing the total mass of the system by the total number of moles (Af and Bg represent the initial number of moles of each one of the monomers). The factor xAfAf in the denominator represents the moles of type A functionalities that have reacted, which is equal to the number of moles that are lost by reaction provided that no intramolecular cycles are formed. As 1 mole of A reacts with 1 mole of B, it must be verified that... 

In any case, the macromolecules formed at the beginning of polymerization exhibit a large number of intramolecular cycles and are very compact. Cyclization reactions are favored by dilution with a nonreactive solvent. 

The catabolism of certain amino acids (e.g., valine, isoleucine, methionine) and odd-chain fatty acids (17 0) produces propionyl-CoA. Propionyl-CoA enters the TCA (citric acid) cycle following conversion to succinyl-CoA, as shown in Fig. 28-7. Propionyl-CoA is first carboxylated to produce D-methylmalonyl-CoA, which in turn is then racemized to L-methylmalonyl-CoA. In an intramolecular rearrangement reaction catalyzed by L-methylmalonyl-CoA mutase, a vitamin B12-... 

Apart form the aforementioned highly enantioselective hetero-Diels-Alder reactions, that proceed with very low catalyst loadings, the catalytically accessible enolates have also been used for related intramolecular Michael reactions (Philips et al. 2007) and for the desym-metrization of 1,3-diketones yielding cyclopentenes via an intramolecular aldol reaction (Wadamoto et al. 2007). The formation of cyclopentenes, however, presents a special case, so—depending on the stereochemical nature of the enone substrates (s-cis or s-trans) and the stereochemistry of the final products—two different mechanisms are discussed in the literature. Whereas /ran.v-cycl open (cries are proposed to be available upon conjugate addition of a homoenolate to chalcones,... 

Many biologically active peptides are cyclic in nature, and the SPS of this class of peptides, exemplified by 2.7, has also received attention with several different strategies for the final cyclization. The phenomenon of pseudodilution on bead in which each resin site is essentially isolated from its neighbors favors the intramolecular cyclization reaction compared to the intermolecular dimerization, which occurs in solution even at high dilutions. The SPS of cyclic peptides has recently been covered in two excellent reviews (31, 32). The technique of cyclative cleavage via the N- or the C-terminus (see Section 1.2.7) has been used, as has anchoring through amino acid side chains with sequential cyclization and peptide release. 

Figure 7. Catalytic cycles and description of the mechanisms of the intramolecular isomerization reactions of toluene catalyzed by an acidic zeolite.

In contrast to the alkene theory the predominant mode of oxidation of the alkyl radicals is by oxygen addition and the alkylperoxy radical so formed then undergoes homogeneous intramolecular rearrangement (reaction (14)). Decomposition of the rearranged radical (reaction (16)) usually leads to a hydroxyl radical and stable products which include O-heterocycles, carbonyl compounds and alcohols with rearranged carbon skeletons relative to the fuel and alkenes. The chain-cycle is then completed by unselective attack on the fuel by the hydroxyl radical (reaction (12)). 

C (66). If electron transfer from type 1 to type 3 copper couples the two halves of the enzyme cycle, as proposed for laccase, then this intramolecular redox reaction must be extremely rapid to account for the effects of trace dioxygen on the reduction of the type 1 copper. Consequently, despite the fact that an ambiguous assignment of a type 1 to type 3 transfer is not possible in this example, facile intramolecular electron transfer processes probably ensure a rapid distribution of electrons among the type 1 and type 3 copper centers, at least in some of the enzyme molecules. The equilibrium distribution, and quite conceivably the relative rates of approach to this state, should be influenced by the oxidation-reduction potentials, which, as described earlier in this chapter (Figure 5), favor electron occupancy of the type 3 copper pairs at 10.0°C. 

In the same sense that a-alkoxy radicals derived from carbohydrates can be generated and utilized in intramolecular addition reactions, a-amino radicals can be employed by the reduction of A-(a-benzotriazolylalkyl)alkenylamines in syntheses of amino-substituted carbocycles and C-substituted pyrrolidines [20]. In the carbo-cycle synthesis, cu-unsaturated aldehydes are condensed with benzotriazole and secondary amines to provide (a-benzotriazolylalkyl)alkenylamines (Scheme 1). These intermediates rapidly ionize in solution to the corresponding iminium ions... 

Indolizine (37) has been transformed into cycl(2.3.3)azine (38) by an intramolecular condensation reaction (Equation (1)) <81JCS(P1)3150>. 

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