Norbornene mechanism

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

The mechanism of the cycloaddition of phenyl azide to norbornene has been shown to involve a concerted mechanism with a charge imbalance in the transition state (199). In a similar manner the cycloaddition of phenyl azide to enamines apparently proceeds by a concerted mechanism (194, 194a). This is shown by a rather large negative entropy of activation (—36 entropy units for l-(N-morpholino)cyclopentene in benzene solvent at 25°C), indicative of a highly ordered transition state. Varying solvents from those of small dielectric constants to those of large dielectric constants has... 

As the mechanism, a radical and a cationic pathway are conceivable (Eq. 31). The stereochemical results with rac- or mcjo-1,2-diphenyl succinic acid, both yield only trans-stilbene [321], and the formation of a tricyclic lactone 51 in the decarboxylation of norbornene dicarboxylic acid 50 (Eq. 32) [309] support a cation (path b, Eq. 31) rather than a biradical as intermediate (path a). 

The formation of cyclic sulfinic esters (sultines) from vinyl sulfenes is known , and the trapping of the expected intermediate vinyl sulfene in the thermolysis of thiete dioxide (6fc and 194) has been convincingly achieved . Specifically, thermolysis of thiete dioxide 6b in the presence of norbornenes gave cycloadducts of the Diels-Alder type (i.e. 252b), resulting from the trapping of the vinyl sulfene formed. The accumulated evidence thus supports the proposed mechanism for these thermolytic reactions. 

A P NMR study of stoichiometric reactions using the di-primary phosphine H2PCH2CH2CH2PH2 provided more information on the reaction mechanism (Scheme 5-12, Eq. 2). Norbornene was displaced from Pt(diphosphine)(norbornene) by ethyl acrylate. Reaction with the diphosphinopropane was very fast this gave the hydrophosphination product, which, remarkably, did not bind Pt to give Pt(diphos-phine), instead, Pt(diphosphine)(norbornene) was observed [12]. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

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. 

The nitronate anion 414 derived from phenylnorbornene reacts with benzenesulphinate, thiocyanate, nitrite or 4-chlorobenzenethiolate anions in the presence of Fe(III) by the Siwl mechanism to give the norbornenes 415 (R = C SPh, SCN, O2N, or SC6H4CM, respectively). No cyclization occurred450. 

Zhou and Hartwig recently discovered the beneficial effect of added potassium hexamethyldisilazanide (KHMDS) base for the asymmetric addition of aniUnes to norbornenes, thereby widening the synthetic scope of the original CMM system (see Table 6.2) [17]. [IrCl(COE)2]2 and two equivalents of variants of the Segphos and Biphep Ugands first presumably form complexes 8, 11, and 12 in situ (see Chart 1) and then in combination with co-catalytic KHMDS generate the catalytically active species (see Table 6.2 and Section 6.4 for a discussion of the mechanism). 

A variety of substituted seven-membered annulated pyrroles can be synthesized in a one-step process in good yields from readily accessible N-bromoalkyl pyrroles 75 and aryl iodides. The synthesis is based on a palladium-catalyzed/ norbornene-mediated sequential coupling reaction involving an aromatic sp C-H functionalization as the key step. The proposed mechanism suggests that orffzo-alkylation with the formation of intermediate 76 most likely precedes aryl-heteroaryl coupling (Scheme 15 (20060L2043)). 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

The reaction of Co(tc-Cp)(CO)2 and of [Co(7t-Cp)NO]2 with nitric oxide in the presence of norbornene has been reported. In both cases the species shown in Figure 14 may be isolated in high yield.125 The mechanism of these three component syntheses could well be related to that of the NO insertion reactions (Scheme 2) in that here NO insertion might occur into the metal-carbon tt-bond of a cobalt-norbornene intermediate. 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

Addition of osmium tetroxide to norbornene 2 followed by reductive cleavage with sodium sulfite gives the exo,exo diol 3. The same reaction sequence carried out on 7,7-dimethylnorbornene 4 gives endo,endo diol 5. From these results deduce the mechanism of the addition and facial selectivity for these two substrates. 

The first successful catalytic animation of an olefin by transition-metal-catalysed N—H activation was reported for an Ir(I) catalyst and the substrates aniline and norbornene 365498. The reaction involves initial N—FI oxidative addition and olefin insertion 365 - 366, followed by C—FI reductive elimination, yielding the animation product 367. Labelling studies indicated an overall. vyw-addition of N—FI across the exo-face of the norbornene double bond498. In a related study, the animation of non-activated olefins was catalysed by lithium amides and rhodium complexes499. The results suggest different mechanisms, probably with /5-arninoethyl-metal species as intermediates. 

The isomerization of 5-vinyl-2-norbornene to 5-ethylidene-2-norbornene has been performed using a catalytic system consisting of an alkali metal hydride and an amine. The activity of the alkali metal hydride increased with increasing size of the alkali metal KH > NaH > LiH. Among the various amines tested, only aliphatic 1,2-diamines exhibited the activity for the isomerization. Electron paramagnetic resonance (EPR) and UV-visible spectroscopic experiments on the active species suggest that the isomerization of 5-vinyl-2-norbornene proceeds through a radical mechanism.167... 

The tungsten(II) carbonyl complex (CO)4W(/u.-Cl)3W(SnCl3)(CO)3 has been identified as a very effective catalyst for the hydroarylation of norbornene conducted in arene solution at room temperature. Norbornene adducts with benzene, toluene, p-xylene, and mesitylene have been isolated. On the basis of XH NMR monitoring of several catalytic reactions, a possible mechanism, involving coordination of norbornene to the W(II) atom and its activation, has been proposed.123... 

Note the chain end stereocontrol mechanism operating in cyclopentene polymerisation, which is similar to norbornene polymerisation but differs from the case of norbornene in the fact that in cyclopentene polymerisation the chiral centres of the growing polymer chain are located in the a and y position [15]. 

Interesting evidence supporting the mechanism of polymerisation of acetylenes via carbene species is provided by the block and random copolymerisation of acetylenic monomers with cycloolefins. For instance, block copolymers of acetylene and cyclopentene with the WC —AlEtCT catalyst [41] and block copolymers of acetylene and norbornene with the (MeA. Oj2W(=NAr)= CHMe3 catalyst [42] have been obtained moreover, random copolymers of phenylacetylene and norbornene with the WC16 catalyst have also been obtained [149, 150],... 

However, if reaction does involve bromine atoms as intermediates, one would expect to find products (including acetolysis) derived from addition of these radicals to the double bond, especially with olefms, such as norbornene, that possess unreactive allylic positions. Obviously the detailed mechanism of this reaction remains to be resolved. 

Norbornene, which has been used as a mechanistic probe to elucidate the reaction mechanism of acid-catalyzed liquid-phase fluorination with xenon difluoride69-72, reacted in acetonitrile under photochemical conditions and two unrearranged 2,3-difluoronorbor-nanes (33-51%) were the sole difluoro products formed, while up to three monofluoro-substituted norbornanes were also present in the crude reaction mixture, the amounts and product distribution depending on the reaction conditions73 (Scheme 19). 

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