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Carbon Bond Formation Involving Carbonium Ions

SECTION 9.6 CARBON-CARBON BOND FORMATION INVOLVING CARBONIUM IONS [Pg.461]

Carbon-Carbon Bond Formation Involving Carbonium Ions [Pg.461]

The formation of carbon-carbon bonds in aromatic systems often takes place by an electrophilic attack on the ring by a carbonium ion or a species with carbonium ion character. The large family of reactions related to Friedel-Crafts reaction are of this type. In aliphatic chemistry carbon electrophiles are more likely to be encountered as carbonyl groups or as such compounds as halides and tosylates, which are subject to nucleophilic displacement. Many examples of these types of reactions have been discussed, particularly in Chapters 1, 2, and 6. There are also some valuable synthetic procedures in which carbon-carbon bond formation results from electrophilic attack by a carbonium ion on an alkene. It is this group of reactions that we will now consider. [Pg.461]

Consideration of the energetics of the attack of a carbonium ion on an alkene reveals that such reactions are not likely to be efficient unless the intermediate which is formed, which is itself some type of carbonium ion, is more stable than the attacking reagent. In fact, most of the reactions which have attained synthetic importance involve alkenes which are capable of specific subsequent stabilization of the intermediate carbonium ion. [Pg.462]

One group of alkenes which react efficiently with electrophilic carbon species are the allylsilanes. The carbon-silicon bond is broken as the reaction proceeds, thus providing a low-energy product.Among the electrophilic carbon species [Pg.462]


It has been suggested that the carbon atom in the carbonium ion which is usually represented as containing only six electrons in its outer shell may actually contain six of its own electrons and two electrons which are donated by chlorine atom from aluminum chloride or, presumably, a fluorine from boron fluoride (N. V. Sidgwick, private communication to Hunter and Yohe, 17). Combination with a second molecule of olefin would then involve the breaking of the carbon-chlorine coordinate bond and the formation of a carbon-carbon bond. [Pg.68]

A significant variation in the stereochemistry takes place when the double bond is conjugated with a group that can stabilize a carbonium ion intermediate. Most of the examples that have been studied involve an aryl substituent. Examples of alkenes that give primarily syn addition are cis- and 1-phenylpropene/ cis- and rran5-j8-t-butylstyrene, l-phenyl-4-r-butylcyclohexene/ and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbonium ion center in these molecules, concerted attack by halide ion is not required for carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than rotation takes place, the result will be syn addition, since the proton and halide ion are... [Pg.327]

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. [Pg.176]

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... [Pg.92]

Formation of Carbon-Nitrogen Bonds A ring closure of this type will most often involve either the attack of an electrolytically formed nucleophile (hydroxylamine, amine, or hydrazine) on an electrophilic center (existing or potential carbonyl, cyano, nitro, or nitroso group) or a reaction between a nucleophile and an electrolytically generated electrophilic center (e.g., nitroso group or carbonium ion). [Pg.237]

Conjunct Polymers. Conjunct polymers (frequently called acid-soluble oils in HF alkylation, red oils in sulfuric acid alkylation) are an exceedingly complex mixture of highly unsaturated, cyclic hydrocarbons. These polymers are by-products of tertiary butyl carbonium ions, and their formation undoubtedly Involves a complexity of reactions. Miron and Lee (1963) found the bulk of an HF conjunct polymer to be mode up of molecules containing 2-4 rings with an average ring size of 5-6 carbon atoms. They estimated the number of double bonds per molecule of polymer at about 2.5 to 3. Thus, these polymers are hydrogen-deficient. [Pg.36]

But our description of the molecule is not yet quite complete. Carbon has left a p orbital, with its two lobes lying above and below the plane of the a bonds (F ig. 5.transition states leading to their formation. [Pg.162]

We can now replace Markovnikov s rule by a more general rule electrophilic addition to a carbon-carbon double bond involves the intermediate formation of the more stable carbonium ion. [Pg.195]

One of the subjects of our investigations involved the interaction of allenes with the P=E derivatives. This work provided some very interesting, unexpected results that may well be of use in synthetic organophosphorus chemistry. Thus, in attempting to study the (2+2)-cycloadditions of the phosphinimine and the (methylene)phosphine systems with the allenic C=C bond, we established that the reactions take a different and more interesting course — that of an ene" reaction (eq 1). In the first step of the reaction, the electrophilic phosphorus center apparently attacks the nucleophilic central carbon of the allene system. Then, instead of undergoing nucleophilic attack on the incipient carbonium ion, the anionic center (E) abstracts a proton from the terminal C-H bond, leading to the formation of a new double bond in the phosphorus-substituted 1,3-butadiene derivatives (3). [Pg.77]

The carbonium ions are known to be the important intermediates in the reactions involving formation and breaking of the carbon-carbon bonds. In the case of a solid-acid catalyst, the Bronsted acid sites are considered to be the active sites for initiation of carbonium ion formation, which in turn lead to various reactions such as polymerization, alkylation and aromatization[5]. However, a comparision of the data in Fig.2 and Table 1 shows that the conversion of ethylene over ZSM-5 and HZSM-5 sanq)les is either unaffected or shows an increase while the concentration of the hydroxyl groups reduced to 25-40% with the increase in pretreatment tenq)erature fi om 300 to 700°C. Similarly, while the concentration of the... [Pg.726]

The formation of a carbon-carbon bond via intermediates centered about an electron-deficient carbon atom is generally classed as an SnI reaction, but the formal description involving a free carbonium ion as an intermediate is an over-... [Pg.288]

The formation of terpenes and other isoprenoid lipids involves the condensation of dimethylallyl pyrophosphate with isopentenyl pyrophosphate (29). In this case (Scheme 11), the leaving group is the pyrophosphate (PP) of an allylic pyrophosphate, such as dimethylallyl pyrophosphate, generating an allylic carbonium ion derivative (and inorganic pyrophosphate, which forms as an ion pair with the carbonium ion). The electron-rich agent that adds to the electron-deficient center to form the carbon-carbon bond is the rr-electron density of the isopentenyl pyrophosphate. The stereochemical course of the biosynthesis of complex natural products has been determined and is consistent with such a mechanism (30). [Pg.289]

Reaction of e J<9-5-hydroxymethyl-2-norbornene with dichlorocarbene generated under phase transfer conditions leads to 3-chloro-5-oxatricyclo-[5.2.1.0 ]-dec-2-ene as the major product (see Eq. 2.21). Formation of this product probably involves initial addition of dichlorocarbene to the carbon-carbon double bond to yield a 1,1-dichlorocyclopropane which ionizes and ring-opens to form a chloro-substituted allylic carbonium ion. This cation is then trapped by the intramolecular nucleophilic alcohol [40]. [Pg.30]

The results for butene isomerization are controversial. Kemball et reported that the isomerization of 1-butene occurred readily at temperatures ca. 300 K, with concomitant formation of significant amounts of butadiene. The initial cis/lrans product ratio was 1.2 — 1.5. They postulated the reaction mechanism involving a butadiene surface species formed by the simultaneous loss of two hydrogen atoms from adjacent carbon atoms on the adsorbed Tbutene molecule. They observed different characteristics on the rti -2-butene isomerization. Exclusive cis-trans isomerization was observed with no detectable double-bond migration or butadiene formation. An intramolecular mechanism involving a secondary carbonium ion as an intermediate was assumed for the isomerization of m-2-butene. [Pg.103]


See other pages where Carbon Bond Formation Involving Carbonium Ions is mentioned: [Pg.130]    [Pg.309]    [Pg.113]    [Pg.130]    [Pg.91]    [Pg.551]    [Pg.227]    [Pg.393]    [Pg.52]    [Pg.279]    [Pg.636]    [Pg.1046]    [Pg.39]    [Pg.211]    [Pg.117]    [Pg.119]    [Pg.474]    [Pg.324]    [Pg.132]    [Pg.474]    [Pg.117]    [Pg.119]    [Pg.24]    [Pg.132]    [Pg.82]    [Pg.636]    [Pg.148]    [Pg.81]   


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Carbonates involving

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Carbonium ion formation

Formate ion

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