Alkenes attack

As the Br2 molecule gets closer to the alkene, this temporary effect becomes more pronounced. Now we can understand why Br2 functions as an electrophile in this reaction there is a temporary 5+ on the bromine atom that is closer to the pi bond of the alkene. When the electron-rich alkene attacks the electron-poor bromine, we get the following hrst step of our mechanism ... 

This methodology was later successfully applied to cyclic alkenes, dienes and trienes [59]. The tentative mechanism proposed by Li et al. involves the activation of the —H bond by gold(I) species (from in situ reduction of gold(III)). Then, the reaction is followed by the alkene attack on the alkylgold hydride intermediate. 

Notes Useful for the c/s-dihydroxylation of alkenes. Attacks alkenes from the less-hindered face. 

The catalytic behaviour of palladium is believed to involve coordination of the metal to the alkene. Attack by the carboxylate ion then effects cyclization and isomerization completes the process. 

A similar cyclisation of an alkene derived from geranyl acetate 24 by dihydroxylation and formation of the epoxide 26 leads to a substituted cyclohexane 28. The Lewis acid ZrCU is used to open the epoxide and the alkene attacks intramolecularly 27 to give eventually the ryn-compound 28 with both substituents equatorial. The alignment of the alkene and the epoxide in a chair conformation 27a is responsible for the diastereoselectivity Note the regioselectivity the less substituted end of the alkene attacks the more substituted end of the epoxide 27. These are just two examples of the very many ordinary ionic reactions that can be used to make six-membered rings. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

The alkene s HOMO interacts with the isocyanate s LUMO, and the most electrophilic atom is the carbonyl carbon so this is where the terminal carbon atom of the alkene attacks. The chlorosulfonyl group can be removed simply by hydrolysis under mild conditions via the sulfonic acid. 

The first formed product now cyclizes to form the second six-membered ring. This recreates a carbocation at the tertiary centre like the one that set off the fragmentation as the more nucleophilic end of the isolated alkene attacks the end of the conjugate electrophile. This is a thermodynamically controlled reaction with the new stereogenic centre choosing an equatorial substituent. 

The reactive conformation of the alkene is probably the Houkf conformation (Chapter 34) with the hydrogen atom on the stereogenic centre eclipsing the alkene. Attack occurs syn to the OH group and anti to the larger butyl group. 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

The work of Davis was, however, unable to distinguish which oxygen was attacked on the titanium peroxo complex when the alkene co-ordinates. Therefore, Neurock and Manzer conducted a theoretical study of the mechanism of alkene epoxidation using TS-1 with aqueous hydrogen peroxide.32 The workers concluded that their calculations to predict both the structure and relative bands in the IR spectra for TS-1 were in good agreement with experimental data. The calculations indicated that the oxygen closest to the titanium centre was the active site for alkene attack. The result was the direct formation of... 

II is easy to draw a mechanism for the second cyclization as the nucleophilic end of the exomethylene alkene attacks the allylic pyrophosphate to form a new six-membered ring. 

As you will expect, the alkene attacks the peroxy-acid from the centre of the HOMO, its n orbital. First, here is the orbital involved. 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

The meta photocycloaddition of ( )-l,2-dichloroethene with benzonitrile, toluonitriles, benzene, phenol, cresols, fluorobenzene, chlorobenzene, trifluoromethylbenzene and 3-trifluoro-methylbenzonitrile shows not only the influence of the aromatic substituents on the direction of the alkene attack, but also a control of the cyclopropane closure by the vinyl chlorine to give exclusively the exo-(>, ent/o-7-dichloro adduct isomer 5 which was, in the most favorable cases, isolated in gram quantities. ... 

Aldehydes may be formed as intermediates in the oxidation of alkanes and alkenes, initiated by a hydroxyl radical, and by direct attack of ozone upon alkenes. Attack by OH is the dominant sink as shown in reaction (23)... 

Fused ring C-glycosides. Ionization of glycosides in the presence of appropriate alkenes leads to C-C bond formation. When C-2 is protected by an acid- cleav-able group, formation of a tetrahydrofuran ring ensues. Since the alkene attacks from the axial direction, the C-2 oxygen function must be equatorial in order to form the new ring. 

The alkene attacks die protonated epoxide, producing its opening mid die formation of a tertiary cm bocation. 

The alkene attacks the IBr, resulting in the bromide displacement and formation of a iodonium ion inside a three-membered ring. 

Roush and Works reported a novel diastereofacially selective Ireland-Claisen rearrangement of Fe-complexed trienic allylic esters (Scheme 4.108) [101]. Although the rearrangement proceeded with excellent facial selectivity, with the allylic alkene attacking anti to the Fe-complex in the conformer shown, only modest syn/ anti selectivity was obtained. 

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