Reactions of Olefins Alkenes

The orbital interaction treatment of C=C n bonds and other types of p-p n bonds is given here. Carbonyl compounds are treated separately in Chapter 8, and organo-metallic rc-type bonding is briefly described in Chapter 13. 

Ethylene is the template for olefin reactions, but ethylene itself is rather unreactive, undergoing electrophilic attack by moderately strong Lewis acids. Nucleophilic attack on the n bond even by the strongest Lewis bases has not been reported. The following sequence involves intramolecular addition of a carbanion to an unactivated olefin [111, 112]. The reaction is undoubtedly facilitated by active participation of the lithium cation as a Lewis acid [113]. 

The normal course of reaction of alkenes involves addition of Lewis acids (electrophiles) yielding an intermediate carbocation which is trapped by a weak nucleophile [114]. The most common electrophilic addition reactions are summarized in Ligure 6.1. If the olefin is unsymmetrically substituted, the question of regioselectivity arises. We begin by examining the effects on the olefin n system of three classes of substituents as 

---

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

The reactions of olefins with non-organometallic Tc(VII) compounds behaved similarly. In a recent study, [Tc03C1(AaA)] (86a) (in which AA stands for aromatic diamine derivatives) was shown to react quantitatively with olefins, and produce the corresponding Tc(V) diolato-complex [TcOC1(OaO)(AaA)] (87a). The process could not be run catalytically, as Tc(V) complexes tend to undergo disproportionation rather than reoxidation in the presence of water [97]. These alkene-glycol interconversions could not be performed with the analog Re(VII) compound. Rhenium displays completely contrary behaviour, in that alkenes can... 

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

Attempts to Catalyze [3 + 2]-Cycloaddition of Nitronates to Olefins In Section 3.2.1.2.2.2, it was noted that [4+ 2]-cycloaddition reactions of nitro-alkenes and alkenes proceed much faster in the presence of LA. At the same time, in the presence of LA, nitronates can rapidly decompose (49) or undergo rearrangements (see Section 3.4.2.5.6 ). Hence, it is not surprising that catalysis of 1,3-dipolar cycloaddition reactions of nitronates with alkenes by LA has attracted little attention until very recent times. An exception is the study by the Japanese... 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Most, perhaps all, of the reactions that simple alkenes undergo are also available to allenes. By virtue of their strain and of the small steric requirement of the sp-hybrid-ized carbon atom, the reactions of allenes usually take place more easily than the corresponding reactions of olefins. Because the allenes can also be chiral, they offer opportunities for control of the reaction products that are not available to simple alkenes. Finally, some reaction pathways are unique to allenes. For example, deprotonation of allenes with alkyllithium reagents to form allenyl anions is a facile process that has no counterpart in simple alkenes. These concepts will be illustrated by the discussion of cyclization reactions of allenes that follows. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Cycloaddition of a nitrile oxide to a substituted olefin can lead to two regio-isomers, the 4- and/or 5-substituted 2-isoxazoline. Reactions of monosubstituted alkenes give the 5-substituted isomers 18 with almost complete regioselectivity (10,15,30,109). This result is also supported by ab initio and FMO calculations (114,119). Change of substituents in the dipole has little effect on the regioselectivity of such reactions when monosubstituted alkenes are used (Table 6.4). 

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

Reaction of an alkene with a nitronium ion involves donation of the n -electron pair of the alkene into an orbital of the nitronium ion. Donation of a bonded electron pair necessarily means that the bond from which it comes is broken. Likewise population of an antibonding level by electron donation generally results in breaking of the bond to which the antibonding orbital corresponds. In this case electron donation of the olefinic r-electron pair results in the rupture of file olefinic n bond and acceptance into file N-O n orbital results in breakage... 

Carreira and coworkers recently investigated hydrohydrazination reactions of olefins 201 with azodicarboxylate 202 (Fig. 56) [300, 301]. Similar to the hydration described above, 2 mol% of Mn(dpm)3196 as the catalyst and phenylsilane as the reducing agent proved to be optimal to obtain alkylhydrazines 203 in 45-98% yield. The manganese catalyst is considerably more reactive than cobalt catalysts applied in the same reaction (see Part 2, Sect. 5.7). Even tetrasubstituted alkenes underwent... 

In contrast to Lindlar catalyst we have found that the hydrogenation of an alkyne over ethylenediamine-poisoned "P-2" nickel boride or quinoline-poisoned palladium-on-barium sulfate always gives a minor amount of the saturated hydrocarbon in addition to the olefin. The ratio of saturated hydrocarbon to olefin (about 0.01) also is nearly constant throughout the hydrogenation until the alkyne is consumed, and then it increases. Further reaction of the alkene on the catalyst surface before desorption would explain these results. 

With regard to the preparation of the Zn/Cu couple, several methods have been developed Dibromomethane may be used instead of diiodomethane when a specifically prepared Zn/Cu couple is applied . The use of Zn/Ag couple often gives better results . Ultrasound irradiation of the reaction mixture has been shown to facilitate the reaction . The cyclopropanation of alkenes with diiodomethane and diethylzinc can be carried out in hydrocarbon solvents and is particularly suitable with easily polymerizable olefins such as vinyl ethers . It has been reported that molecular oxygen remarkably promotes the reaction of diiodomethane with diethylzinc and substantially increases the yield of the adducts . A convenient modification which avoids the handling of pyrophoric diethylzinc has been reported . In reaction of olefins which are sensitive to the unavoidably produced zinc iodide (the Lewis acid), the addition of one equivalent of dimethoxyethane (DME) to the solvent has been recommended . Zinc iodide is then precipitated as the 1 1 DME complex as it is formed. Zinc salts, which often complicate the workup of the reaction mixture, can also be removed as precipitates by the addition of pyridine prior to the workup . ... 

The additions of tellurium tetrahalides to olefins in the presence of alcohols proceed equally well when the tetrahalides are generated in the reaction mixture from tellurium dioxide and chlorotrimethylsilane in methanoP or concentrated hydrochloric acid/methanol. trans-2-Methoxycyclohexyl tellurium trichloride was obtained in this manner in quantitative yield . In the reactions of terminal alkenes, the TeCl3 group (from TeO and concentrated hydrochloric acid) added according to the Markovnikov rule producing 2-alkoxy-1 -alkyl tellurium trichlorides. The trichlorides were not isolated but were reduced with disodium disulfite to the ditellurium compounds, which, in turn, were converted to the tellurium trichlorides by treatment with sulfuryl chlorides. For data on these tellurium trichlorides see p. 316. The overall yields range from 15 to 70%. 

The hydroformylation of alkenes is commonly run using soluble metal carbonyl complexes as catalysts but there are some reports of heterogeneously catalyzed reactions of olefins with hydrogen and carbon monoxide. Almost all of these are vapor phase reactions of ethylene or propylene with hydrogen and carbon monoxide catalyzed by rhodium, " 20 ruthenium,nickel, 22,123 cobalt, 23,124 and cobalt-molybdenum 23 catalysts as well as various sulfided metal catalysts. 23,125,126... 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

The reaction of olefins with lead tetraacetate has not been a useful method in organic synthesis, because reactions such as addition of an oxygen functional group to the double bond, substitution of hydrogen at the allylic position, and C-C bond cleavage can occur to give complex mixtures of products. With some specific alkenes, however, reaction with lead tetraacetate can afford synthetically important compounds cleanly. For instance, reaction of the diacid with 6 equiv. lead tetraacetate in acetonitrile gave the dilactone in excellent yield (Scheme 13.36) [59]. 

Although [2+2] photocycloaddition is not limited to reactions of olefins with enone derivatives (styrene and other aromatic alkenes can also undergo this type of photoreaction [11]), the majority of the recent literature has focused on the enone systems, and this will make up the bulk of our discussion. 

---