Electrophilic attack on alkenes

Certain metal cations are capable of electrophilic attack on alkenes. Addition is completed when a nucleophile adds to the alkene-cation complex. The nucleophile may be the solvent or a ligand from the metal ion s coordination sphere. 

Although iodine(III) reagents attack double bonds, the rearrangement of the amide group is, at least in some cases, more rapid than electrophilic attack on alkenes. Thus 3-cyclohexene-l-carboxamide rearranges smoothly to the corresponding amine as long as only one equivalent of [1,1-bis(trifluoroacetoxy)iodojbenzene is used. 

Five-membered rings also allow us to explore electrophilic attack on alkenes. A simple 4-substi-tuted cyclopentene has two different faces—one on the same side as the substituent and one on the opposite side. Epoxidation with a peroxy-acid occurs preferentially on the less hindered face. 

Electrophilic attack on alkenes by bromine often goes through three-membered ring cyclic bromoni-um ions and we can sometimes tell that this is so by studying the stereochemistry. Here are two reactions of styrenes that look very similar—a reaction with bromine and one with PhSCl. With no further information, we might be tempted to assume that they both go by the same mechanism. However, the Hammett p values for the two reactions are rather different. 

Problems of orientation of attack and reactivities of fluorinated alkenes arise in a way that is analogous but entirely complementary to the classical problems of electrophilic attack on alkenes. For example, typical of the results that we must be able to account for is the reaction of methoxide in methanol which occurs specifically at the Cp2= site in perfluoropropene (Figure 7.15). Also, there is a very wide range of reactivity with perfluoroalkenes for example, reactions of tetrafluoroethene usually require base catalysis, whereas perfluoroisobutene reacts with neutral methanol. 

Reaction control experiments show that the syn-isomer is the kinetic product of the addition. The results achieved are in accordance with Houk s52 analysis of electrophilic attack on alkenes bearing an adjacent stereogenic center. It predicts that the kinetic product is the syn-isomer and the anti-isomer 3 A should be kinetically favored in the addition to silyl ether 1 (R1 = TBDMS R2 = CHj),... 

Electrophilic attack on alkenes is normally stereospecifically syn (as in hydroboration) or anti (as in bromination). In this section we consider stereoselective aspects of these reactions. In the simplest cases, the two faces of the alkene are diastereotopic because of some stereogenic centre elsewhere in the molecule. Reaction will then occur on the less hindered face opposite the substituent already present. In favourable cases, where the substituent is large and close to the alkene, the selectivity may be high. 

This synthesis involves four electrophilic attacks on alkenes - two hydroborations, one alkylation by a mesylate, and one ozonolysis. This is possible - even though the starting material had only two alkenes - because one of the alkenes is recreated three times. You will have noticed that each time it reappears, it moves round the structure so that electrophilic attack followed by double bond (re-)creation enables the formation of several new bonds as well as the introduction of new functionality. 

Now that we have introduced all three kinds of selectivity in the context of electrophilic attack on alkenes, we shall look at some ways to control selectivity by special devices. In the examples which follow it is more convenient to treat the three selectivities together as they are often interdependent. 

We shall end this chapter with three ways to make carbon-carbon bonds by electrophilic attack on alkenes. Though useful, they are not as important as the functionalisation reactions we have discussed so far, but they are each special in their own way. The first two, the Prins reaction and hydroboration revisited, use one-carbon electrophiles. 

In chapter 21 we emphasise that the most reliable method of controlling one chiral centre by another in open chain compounds is by Felkin-Anh orbital control of nucleophilic attack on a carbonyl group next to a chiral centre. We shall start this discussion with that method and follow with the next most reliable - Houk control of electrophilic attack on alkenes also next to a chiral centre. These are of course both 1,2-control and we shall deal with that before discussing 1,3-, 1,4- and remote control. 

Houk control concerns electrophilic attack on alkenes, enolates and the like. The alkylation of enolate 56 would be an example if it were not held in a ring by chelation. It can in fact be difficult to tell whether chelation is involved or not with many enolates and the outcome of the reaction may tell which. Chamberlin s asymmetric preparation of both pyrrolidine 2,3-dicarboxylic acids 141 and 142 from natural aspartic acid illustrates this perfectly. The key to the stereochemical control is the very large protecting group 9-phenylfluorenyl-143 introduced by Rapoport.22... 

Visual images and additional information available at springer.com/cary-sundberg Certain metal cations promote addition by electrophilic attack on alkenes. Addition is completed when a nucleophile adds to the alkene-cation complex. The nucleophile may be the solvent or a ligand from the metal ion s coordination sphere. The same process occurs for other transition metal cations, especially for Pd +, but the products often go on to react further. Synthetically important reactions involving Pd are discussed in Section 8.2 of Part B. The mercuration products are stable, and this allows study of the addition reaction itself. We also consider reactions of the Ag+ ion, which give complexes, but usually do not proceed to adducts. 

You met electrophilic attack on alkenes in Chapter 19 and we shall just briefly revisit its regioselectivity. Unsymmetrical alkenes add HBr to give the more stable of the two possible cations. If R is alkyl or aryl, this means the more substituted cation. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine is formally a substitution process, it is mechanistically closely related to electrophilic additions to alkenes. The enol or enolate derived from the carbonyl compound is the reactive species, and the initial attack is similar to the electrophilic attack on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition. The acid- and base-catalyzed halogenations of ketones, which were discussed briefly in Part A, Chapter 7, are the best-studied examples of the reaction. The most common preparative procedures for such... 

The same ions can also be produced by electrophilic attack on alkenes by species that should generate positive halogen ... 

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