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Electrophiles tt bonds

Nucleophilic Addition to and Substitution at Electrophilic tt Bonds 127 Table 3.1. Interconversions of ketones and their equivalents... [Pg.127]

As conjugated systems with alternating TT-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized TT-bond. If the nucleophilic and electrophilic reactions are charge-controHed, reactants can attack regiospeciftcaHy. [Pg.494]

We can consider the hydroboration step as though it involved borane (BH3). It simplifies our mechanistic analysis and is at variance with reality only in matters of detail. Borane is electrophilic it has a vacant 2p orbital and can accept a pair- of electrons into that orbital. The source of this electron pair is the tt bond of an alkene. It is believed, as shown in Figure 6.10 for the example of the hydroboration of 1-methylcyclopentene, that the first step produces an unstable intenrrediate called a tt complex. In this tt complex boron and the two carbon atoms of the double bond are joined by a three-center two-electron bond, by which we mean that three atoms share two electrons. Three-center two-electron bonds are frequently encountered in boron chemistry. The tt complex is fonrred by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... [Pg.252]

The reaction begins with an attack on the electrophile, HBr, by the electrons of the nucleophilic tt bond. Two electrons from the 7t bond form a new u bond between the entering hydrogen and an alkene carbon, as shown by the curved arrow at the top of Figure 6.7. The carbocation intermediate that results is itself an electrophile, which can accept an electron pair from nucleophilic Br ion to form a C Brbond and yield a neutral addition product. [Pg.188]

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... [Pg.204]

The methyl groups on the pyridine ring result in a major difference in the reactivity of lutidines. In 3,5-lutidine the methyl groups act as electron donors tending to increase the stability of the tt-bonds, and activating the ring for electrophilic attack at the a-positions. The MOs in 3,5-lutidine show the it-levels pushed to lower energy... [Pg.97]

If the double bonds of a polyene are not conjugated with other 7r-systems in the molecule, addition to one of the tt-bonds will proceed in a similar fashion to addition to a simple alkene. Usually addition to one of the bonds is preferred, because it is either more highly substituted and, as a result, has enhanced electron density, or because it has fewer substituents and is less hindered and more accessible to the electrophile. Alternatively, one of the double bonds of a polyene may be activated by the presence of a heteroatom at the allylic position. [Pg.694]

Under proper conditions a carbocation (R" ), formed by adding an electrophile such as H or BF, to an alkene, may add to the C==C bond of another alkene molecule to give a new dimeric R here, R" acts as an electrophile and the tt bond of 0=C acts as a nucleophilic site. R " may then lose an to give an alkene dimer. [Pg.103]

The carbonyl group (2a) has an easily polarisahle TT-bond with an electrophilic carbon atom at one end easily attacked by nucleophiles ... [Pg.70]

On the basis of spectroscopic and thermodynamic data, it has been concluded that TT-bonding is significant in difluorocarbene [86] and to a degree which accounts for the fact that it is surprisingly stable and relatively unreactive compared with CH2 and other carbenes [86]. That the balance between the two effects is clearly dominated by tt-bonding is illustrated by the relative reactivities of various carbenes with different alkenes it is concluded that electrophilicity decreases in the series CH2 > CBr2 > CCI2 > CFCl > CF2 and CH2 > CHF > CF2. Indeed, CF2 is considered to be amphiphilic [87]. [Pg.156]

Electronic effects in bispolyfluoroalkylcarbenes (6.61A) are clearly defined, in that the aheady electron-deficient carbon is made even more electrophilic by the strong electron withdrawal by polyfluoroalkyl groups (Figure 6.61). Polyfluoroalkylfluorocarbenes (6.61B) are, however, an intermediate situation with the possibility of compensating TT-bonding, as described earlier. [Pg.158]

The Si-Si bond is quite different from the C-C bond and actually resembles more the C = C bond in its chemistry and properties. For example, disilanes readily undergo electrophilic cleavage by the same reagents that add to olefins by cleavage of the tt bond. [Pg.12]

Classification of carbene complexes as Fischer or Schrock perhaps focuses loo much on their differences and too little on their similarities. Both contain a metal-carbon bond of order greater than one. Whether the carbene carbon tends to seek or provide electrons will depend on the extent of tt bonding involving the metal and the carbon substituents. Some carbene complexes lie between the Fischer/Schrock extremes, behaving in some reactions as nucleophiles and in others as electrophiles. ... [Pg.342]

Addition of electrophiles is a reaction typical of aliphatic tt bonds (see Example 4.3). Such additions involve two major steps (1) addition of the electrophile to the nucleophilic tt bond to give an intermediate carbocation, and (2) reaction of the carbocation with a nucleophile. Typical electrophiles are bromine, chlorine, a proton supplied by HCl, HBr, HI, H2SO4, or H3PO4, Lewis acids, and carbocations. The nucleophile in step 2 is often the anion associated with the electrophile, e.g., bromide, chloride, iodide, etc., or a nucleophilic solvent like water or acetic acid. [Pg.208]

Thus, if the olefin is strongly bonded to Pd (II) before the transition state, Pd(II) attack cannot be a simple electrophilic attack on the olefin, such as is the proton attack in olefin hydration. It is rather a rearrangement of a TT-bonded to a o-bonded adduct. There is no reason to expect that the transition state for this rearrangement would have much polar character. The small effect of olefin substitution on rate indicates that this transition state is, in fact, nonpolar. [Pg.135]


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See also in sourсe #XX -- [ Pg.32 , Pg.53 , Pg.54 , Pg.55 , Pg.56 ]




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