Double bond formation intramolecular cyclization

The intermolecnlar process involves cross-linking between polymer chains. The intramolecular processes involve B-N double bond formation and cyclization reactions within polymer chains. As... 

An alternative cyclization of the radical-cation intermediate has been observed in the irradiation of 40a that affords, in addition to the cyclopropane 42a, the dihydroquinoline 50. The formation of 50 is in agreement with a mechanism involving the generation of the radical-cation centered in the CC double bond 51. Intramolecular cyclization in 51, by electrophilic attack on the phenyl ring attached to the nitrogen atom, generates the intermediate 52, which by aromatization and hydrogen abstraction yields 50 (Scheme 12). 

Another approach is based on the palladium-catalyzed intramolecular carbocyclization of the allylic acetate moiety with the alkene moiety (Scheme 96). After the formation of a 7t-allylpalladium complex, with the first double bond the intramolecular carbometallation of the second double bond occurs to form a new C-C bond. The fate of the resulting alkylpalladium complex 393 depends on the possiblity of /3-elimination. If /3-elimination is possible, it generates a metallated hydride and furnishes the cycloadduct 394. This cyclization could be viewed as a pallada-ene reaction, in which palladium replaces the hydrogen atom of the allylic moiety.231... 

The mechanism of formation of the initial benzene molecule in combustion of hydrocarbons depends on both the fuel and the reaction conditions. Two classes of ring-forming reactions have been proposed. The first class involves addition of acetylene to vinyl-type radicals (i.e., short-chain radicals containing a double-bond), with subsequent cyclization to a five or six membered ring, depicted in Fig. 14.8. In these reactions the adduct does not require intramolecular rearrangements prior to cyclization [318]. However, under most conditions these reactions are thought to be of secondary importance. 

The intramolecular C—C bond formation (or cyclization) mediated by (TMS)3SiH has been the subject of numerous publications in the last few years. For example, radical cyclization of bromides 73 was found to be totally regioselective following 5-exo mode ring closure and afforded tetrahydrofuran derivatives (equation 37)89. Furthermore, the chiral auxiliary together with the configuration of the double bond allow for complete diastereocontrol. 

In this context we have recently reported that A-2-bromoallyl-iV-(3-functionalized) allylamines 315 undergo, after formation of the corresponding vinyllithiums, intramolecular carbolithiation processes giving rise to functionalized methylenepyrrolidines 316 in good yields. We have shown that a moderately activating group at the terminal position of the double bond favours the cyclization reaction and we have presented the first... 

The bicyclic a-hydroxy acid 10a was obtained from the ring contraction of the 7,8-bis(trimethyl-siloxy)bicyclo[4.2.0]oct-3,7-diene (product of an acyloin cyclization) either by selective bro-mination at — 40 °C and subsequent hydrolysis or by simple autoxidation under an air stream. On further bromination at 20 °C and subsequent solvolysis in aqueous sodium hydroxide the hydroxycyclopropanecarboxylic acid 10a was transformed into 4-bromo-6-oxatri-cyclo[3.2.1.0 ]octane-7-carboxylic acid (11) by intramolecular cyclization, resulting from the addition of the c <7o-hydroxy group to the cyclohexene double bond. Formation of the product resulting from the addition of the carboxy group was not observed. ... 

Radical cyclization. Amido radicals are generated from A -methyldithiocarbonyl-hydrazides by heating with dilauroyl peroxide. Setting up the functional group in juxtaposition to a double bond invites intramolecular addition that even further implications in ring formation are envisaged. As a key step in a synthesis of the amaryllidaceae alkaloid fortucine the value of such a process is demonstrated. ... 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene imdergo Pd-catalyzed addition reactions. Inter-molecular reaction is initiated by jr-complexation of the two double bonds, followed by cyclization to form the palladacyclopentane 7. This is called oxidative cyclization. The oxidative cyclization of 1,6-diene affords the palladacyclopentane 8, which undergoes further transformations. Similarly, the oxidative cyclization of a,co-enyne affords the palladacyclopentene 9. Formation of these five-membered rings occurs stepwise and can be understood in terms of the formation of either palladacyclopropene or palladacyclopropane. Then the inter- and intramolecular... 

As a natural consequence of the crosslinking reaction process, the density of the primary polymer differs depending on the time of this primary polymer formation. That is, in the case of the copolymerization of vinyl and divinyl monomers, the generally formed inhomogeneous crosslink formation can be regarded as a natural consequence of the mechanism of crosslink formation. This is true except for die special reaction conditions by favorable timing of the incorporation of divinyl monomer in the polymer chain (formation of pendant double bonds) and consumption of pendant double bonds (formation of crosslinks). These special reaction conditions are used by Flory as simplified conditions when the Flory-Stockmayer theory is applied to the copolymerization of vinyl and divinyl monomers. Flory s simplified conditions include die following three assumptions (1) the reactivities of the monomer and die double bonds in the polymer are all equal (2) any double bond reacts independently and (3) there will be no intramolecular reactions (cyclization) within the finite size molecules (sols). 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

Many of the crown ether syntheses with which we are concerned in this book are one form or another of the Williamson ether synthesis. Although the simplest example of such a reaction would involve an co-haloethylene glycol oligomer which undergoes intramolecular cyclization, it is more common for two new bonds to be formed in crown syntheses. An early example of the formation of a crown by a double-Williamson can be found in Dale s synthesis of 18-crown-6. The rather obvious chemical steps are shown in Eq. (2.1). 

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 radical addition/cyclization process has been described for the formation of benzindolizidine systems from l-(2-iodoethyl)indoles and methyl acrylate <00TL10181>. In this process, sun-lamp irradiation of a solution of the l-(2-iodoethyl)ethylindoles 149 in refluxing benzene containing hexamethylditin and methyl acrylate effects intermolecular radical addition to the activated double bond leading to the stabilized radical 150. Intramolecular cyclization to the C-2 position of the indole nucleus then affords the benzindolzidine derivatives 151 after rearomatization of the tricyclic radical. 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

Intramolecular arylation of alkenes. The intramolecular palladium-catalyzed condensation of aryl halides with an alkene group (Heck arylation5) can actually proceed more readily than the original bimolecular version. These intramolecular cyclizations typically proceed in acetonitrile at room temperature, particularly when catalyzed by Ag2C03, which also inhibits isomerization of the double bond of the product. They can be used to obtain spiro, bridged, and fused systems. Even tetrasubstituted alkenes can participate, with formation of quaternary centers.6... 

Intramolecular alkoxycarbonylation of alkynols is parallel to what has been described for alkenols except that functionalization of the triplebond produces a double bond. No lactone formation is observed in the Pd(II)-catalyzed oxidative cyclization-carbonylation of alkynes. Instead [(methoxycarbonyl)methylene]tetrahydrofurans are selectively formed [134, 135]. Moreover, starting from an enynol, furan-2-acetic ester is obtained resulting from a final aromatization step [136]. 

Brummond [28] was the first to illustrate that cross-conjugated trienes could be obtained via an allenic Alder-ene reaction catalyzed by [Rh(CO)2Cl]2 (Eq. 14). Selective formation of the cross-conjugated triene was enabled by a selective cycloisomerization reaction occurring with the distal double bond of the aUene. Typically directing groups on the allene, differential substitution of the aUene termini, or intramolecularization are required for constitutional group selectivity. However, rhodium(f), unlike other transition metals examined, facihtated selective cyclization with the distal double bond of the allene in nearly aU the cases examined. 

It is the opinion of the present authors that isomerization of a tertiary alkyl radical to a primary radical as in the formation of II from I is improbable. The formation of IV is similarly unlikely. The cycliza-tion of V by intramolecular alkylation seems quite plausible however, equation 9 does not explain either the formation of V or its subsequent cyclization. The following mechanism has the advantages that, like the generally accepted free radical-initiated mechanisms, it postulates a chain reaction and that the intramolecular alkylation step is directly analogous to that proposed for thermal alkylation, namely addition of an alkyl radical to the double bond of the alkene (Frey and Hepp, 12). The method of formation of the chain initiator, R —, again is not critical since R —, merely starts the first cycle of the chain reaction it may be formed by decomposition of the isobutylene. 

Formation of 1,3-dioxanes was also effected by intramolecular cyclization of suitable precursors possessing a double bond either two or three carbons away from the oxygen functional group. For example, /3-trimethylsilylethoxymethyl (SEMI-protected allylic alcohols reacted with bromonium dicollidine hexafluorophosphate (BrDCH) to 5-bromo-l,3-dioxanes in acceptable yields and with a high preference for the 4,6-tfr-addition product (Equation 83) <2000JCX32797>. 

The formation of substituted quinolines 35 (58-90% yield) by intramolecular cyclization of 4-arylamino-l-azabutadienes 2 has been carried out at 100°C using one equivalent of aluminium chloride the reaction involves the diene moiety and one carbon—carbon double bond of the aromatic ring (87S82). The participation of the phenyl group attached to the a-... 

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