Alkene also substitution reactions

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

Many of the reactions of amines are familiar from past chapters. Thus, amines react with alkyl halides in S 2 reactions and with acid chlorides in nucleophilic acyl substitution reactions. Amines also undergo E2 elimination to yield alkenes if they are first qualernized by treatment with iodomethane and then heated with silver oxide, a process called the Hofmann elimination. 

These reactions are found to be promoted by electron-donating substituents in the diene, and by electron-withdrawing substituents in the alkene, the dienophile. Reactions are normally poor with simple, unsubstituted alkenes thus butadiene (63) reacts with ethene only at 200° under pressure, and even then to the extent of but 18 %, compared with 100% yield with maleic anhydride (79) in benzene at 15°. Other common dienophiles include cyclohexadiene-l,4-dione (p-benzoquinone, 83), propenal (acrolein, 84), tetracyanoethene (85), benzyne (86, cf. p. 175), and also suitably substituted alkynes, e.g. diethyl butyne-l,4-dioate ( acetylenedicarboxylic ester , 87) ... 

Since the early work dealing with Zeise s salt, many complexes have been prepared with the formula [PtL(C2H4)X2], where L = quinoline, pyridine, or ammonia and X=C1 , Br , I, or N()2. Similar compounds have been prepared that contain other alkenes than C2H4. Many of the complexes containing dienes, trienes, and tetraenes as ligands also contain carbonyl ligands. In fact, metal carbonyls are frequently starting complexes from which alkene complexes are obtained by substitution reactions. 

From Chapter 7 it is apparent that the trichloromethyl anion is formed under basic conditions from chloroform, as a precursor of the carbene. The anion can also react with Jt-deficient alkenes (see Section 7.3) and participate in nucleophilic substitution reactions, e.g. 1,1-diacyloxy compounds are converted into 1,1,1-trichloroalkan-2-ols [58] (Scheme 6.35). Similarly, benzyl bromides are converted into (2-bromoethynyl)arenes via an initial nucleophilic displacement followed by elimination of hydrogen bromide [59] (Scheme 6.35). 

The inverse-electron-demand Diels-Alder reaction of 3,6-dichloro[l,2,4,5]tetrazine with alkenes and alkynes provides the synthesis of highly functionalized pyridazines. ° Also, the 4 + 2-cycloaddition reactions of the parent [l,2,4,5]tetrazine with donor-substituted alkynes, alkenes, donor-substituted and unsubstituted cycloalkenes, ketene acetals, and aminals have been investigated. ... 

Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether. 

Step (1) is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation (a benzenonium ion) loses a cation (most often to give the substitution product, rather than adding a nucleophile to give the addition product. The benzenonium ion is a specific example of an arenonium ion, formed by electrophilic attack on an arene (Section 11.4). It is also called a sigma complex, because it arises by formation of a o-bond between E and the ring. See Fig. 11-1 for a typical enthalpy-reaction curve for the nitration of an arene. 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

The more basic and less hindered pyridines undergo nucleophilic attack at an ethylene coordinated to platinum(II). Pyridine substitution reactions at platinum also occur, and in the presence of excess ethylene, alkene replacement is observed.72 ... 

The presence of solids such as clays, zeolites, silica or ion-exchange resins may allow catalysis or control of organic reactions. Often, yields are higher and work-up procedures simpler than for the corresponding homogeneous reactions, and product distributions may also be improved. Examples of selective substitution reactions in aromatic and heteroaromatic systems and of selective reactions of alkenes are discussed, and the wider potential for synthesis of fine chemicals is discussed. 

Triarylphosphines, which are often employed with palladium in catalysts, also can transfer aryl groups to the palladium and cause vinyl substitution reactions with alkenes.49,30 Fortunately, this reaction is usually slower than other methods for generating arylpalladium derivatives so that it usually is not a problem, but there are exceptions (equation 16). 

The n/b ratios are also affected by the alkene structure and are sensitive to steric hindrance. For example, dicobaltoctacarbonyl hydroformylates longer chain 1-alkenes more slowly than the shorter alkenes. Methyl substitution on the alkene chain not only slows the reaction as the methyl group is moved closer to the double bond, but the n/b ratio increases (Table 1). A quaternary carbon does not undergo CO insertion and, as a result, the aldehyde is almost never attached at a fully substituted sp1 carbon of an alkene. 

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