Electron-rich alkenes, brominations, bromine

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 ... 

The electron-rich alkene double bond repels the electrons in the bromine molecule to create a partial positive charge on the bromine atom near the double bond. An intermediate bromonium ion is formed, which reacts to give the /ram-dibromide derived from anti- addition (i.e. the two Br groups add to the alkene from the opposite sides). 

The reaction is faster with electron-rich alkenes. The rate constants for addition of bromine to a series of alkenes were found to increase in the order ethene < propene < 2-butene isobutene < 2-methyl-2-butene. In other words, each methyl group that replaces a hydrogen atom on ethene increases reactivity. The addition of bromine to substituted ethenes in methanol with added sodium bromide can be correlated to the equation... 

So, we have an electron-rich alkene, which can attack the nearby, electron-poor bromine atom. This gives the reaction that we saw in the first semester of organic chemistry ... 

Because vinyl azides are electron-rich alkenes, treatment of these substrates with electrophiles was studied in detail. In most cases, the attack of the a-nitrogen atom or the j3-carbon of the olefinic part at the electrophile led to final products without an intact azido group such as amides or ketones. Only a few exceptions were reported, for instance, the reaction of the terminal enazides 40 or 54 with bromine in methanol yielding mainly the -azido ethers 227 (Scheme 5.28). Already in 1910, Forster and Newman investigated the conversion of the parent compound 1 with bromine in aqueous solution.Due to the explosion-like course of the reaction and the high sensitivity of the addition product to hydrolysis generating bromoacetaldehyde, however, the desired substance 228 could not be characterized. Quite recently, is has been shown that the transformation 1 228... 

The metal catalyst is not absolutely required for the aziridination reaction, and other positive nitrogen sources may also be used. After some years of optimization of the reactions of alkenes with positive nitrogen sources in the presence of bromine equivalents, Sharpless et al. reported the utility of chloramine-T in alkene aziridinations [24]. Electron-rich or electron-neutral alkenes react with the anhydrous chloramines and phenyltrimethylammonium tribromide in acetonitrile at ambient temperature, with allylic alcohols being particularly good substrates for the reaction (Schemes 4.18 and 4.19). 

Selective bromine-mediated addition of BOC-protected-guanidine 81 to dihydropyridine 56 occurs across the electron-rich 5,6-alkene to give, after acid deprotection, r-2-amino-l,3a,5,7a-dihydroimidazo[4,5-b]pyridine 82 (Scheme 23). Aminal bond cleavage under basic conditions affords substituted 2-aminoimidazole 83 <2004OL3933>. Replacement of guanidine 81 with urea or thiourea leads, similarly, to 2-aminooxazoles or 2-ami-nothiazoles, respectively however, the yields are considerably lower than that of 82 due to the sensitivity of the ureas to bromine oxidation <2005JOC8208>. 

When the bromine molecule approaches end-on to the alkene double bond and an electrophilic centre is included (Following fig.). Since the alkene double bond is electron rich, it repels the electrons in the bromine molecule and this results in a polarisation of the Br-Br bond in such a way that the nearer bromine becomes electron deficient (electrophilic). In this way, when an electrophilic centre has been generated, the mechanism is the same as before. 

With the exception of hydrogenation, the addition reactions of alkenes presented in this text occur by an electrophilic addition mechanism. The electrophile (H+ or X+) attacks the electron-rich pi-bond of the double bond. The pi electrons are used to form a single bond between the carbon and attacking species the other carbon becomes a carbocation. The carbocation is then neutralized by halide ion or water the addition is complete. In bromination reactions, the bromine adds in a trans fashion. 

The bromine molecule becomes polarised as it approaches the alkene. The bromine atom nearest the double bond becomes electrophilic (as the electrons in the Br-Br bond are repelled away from the electron-rich double bond)... 

Notice that the bromine reacts only with the most electron-rich trisubstituted alkene and not with the disubstituted alkene or the alkyne. 

Bromine is certainly not a Br0nsted-Lowry acid, but can diatomic bromine be categorized as a Lewis acid Bromine and indeed all of the diatomic halogens are polarizable. This means that Br-Br covalent bond will become polarized with a 6+ Br and a 6- Br when brought into proximity of an electron-rich species such as the Ji-bond of an alkene (see 37). In the proximity of a 6+ bromine, the electron-rich 7i-bond of the alkene will donate electrons to this electrophilic bromine atom. This is formally analogous to the C=C unit reacting as a Lewis base with respect to the bromine. A cautionary note is that the final... 

The bromine molecule (Br2) is normally symmetrical. However, as it approaches the nucleophilic and electron-rich tt bond of the alkene, it becomes polarized by induction and can then function as the electrophile in an addition reaction. The result is the generation of a cyclic bromonium ion ... 

The following observations have been made (a) UVA sible (UVA IS) spectroscopy (following e.g., disappearance of the color of bromine [Br2]) suggests that a n-complex is formed (b) in what is apparently the rate-determining step, the electron-rich 7i-cloud attacks the bromine molecule (Br2) forming a bromonium ion (the onium ion) and bromide anion (Br, or its equivalent. The bromine atom in the bromonium ion apparently bonds equivalently to both carbon atoms of what was the symmetrical alkene, forming a bridged ion (c) bromide anion (Br ,or its... 

The reaction is an electrophilic addition, like most of the reactions of alkenes. The n hond of ethene is an electron-rich area (8-). Repulsion of the electrons of the Br-Br hond hy the Jt electron cloud induces a temporary dipole in an approaching bromine molecule (Figure 20.26a). 

Bromination of Olefins. Another example of TMS-Br as a source of bromine is its use in the bromination of alkenes (eq 42). This transformation is achieved via the use of tetradecyltrimethyl-ammonium permanganate [(Ci4H29(CH3)3N)(JCMn04)]. This reagent performs a trans relative addition of the bromine atoms and is selective for electron-rich olefins over electron-deficient ones. The yields for these reactions are generally high (60-91%). 

How does bromine attack the electron-rich double bond even though it does not appear to contain an electrophilic center The answer lies in the polarizability of the Br-Br bond, which is prone to heterolytic cleavage upon reaction with a nucleophile. The ir-electron cloud of the alkene is nucleophilic and attacks one end of the bromine molecule, with simultaneous displacement of the second bromine atom as bromide ion in an SN2-like process. 

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