Styrene-type alkenes

Moeller has carried out an extensive series of studies of the electrochemical oxidation of electron-rich w-alkenes. One olefinic component is an enol ether, which is converted into an electrophilic center upon oxidation this center then attacks the other site intramolecu-larly. The anodic oxidation of the bis-enol ethers 21 in methanol25 exemplifies the course of such reactions (Scheme 4). The products are w-acetals (22), formed in 50-70% yield in many cases. The cyclization can be used to produce quaternary25 and angularly fused26 bicyclic and tricyclic structures (equation 11). In its original form, this work involved oxidation of a mono-enol ether bearing a nearby styrene-type double bond27. 

Experimental observations of the aziridination of styrene-type alkenes, catalyzed by CuPF6 in the presence of chiral diimine ligands (such as (lR,2R,A i4A i4)-A A -bis(2,6-dichlorobenzylidene)cyclohexane-l,2-diamine 425), have been taken as evidence of the intermediacy of a discrete, monomeric Cu(lll)-nitrene complex, (diimine)Cu=NTs 423. Variation of the steric properties of the aryl group in the oxidant TsN=IAr (Ar = Ph, 2-/-Bu, 5,6-Me3C6H) has no effect on the enantioselectivities in forming the aziridination products 424 (Scheme 108) <1995JA5889>. 

The results of asymmetric hydroformylation are described here for individual substrate types such as normal open chain and cyclic alkenes, styrene and other alkenylaromatics, dienes, functionalized alkenes and alkynes. Earlier extensive compilations of results concerning this subject are found in several monographs and reviews31 45,54,104,105, l21,177,178,180,, 81. 

Intramolecular Mizoroki-Heck reactions with styrene-type alkenes constitute the second frequently met case. Finding suitable rationales in theses cases seems to be more intricate. There is some evidence for a mechanism involving a radical intermediate [11]. Most explanations, however, cite a facile epimerization at the benzylic position and, thus, furnishing the required cw-stereochemistry or a base-assisted reductive elimination of the palladium species (Scheme 6.3) [11]. In some cases, suitable substrates or conditions can lead to the a r/-elimination products via an Elcb-type mechanism [23,24]. 

Levesque " has observed that dithioacids (221) add to alkenes under AIBN initiation to give 1,3-dithiolanes (223) in high yield when the alkene is of the styrene type. This must involve intrlamolecular addition of the radical 222 to the sulfur atom (Scheme 94). Analogous results are obtained in the free radical addition of thiols to compounds 224 [X = Ikyl, OMe, NMc2, or SMe Z = CHj, (CH2)2, O, or S] where the radical 225 adds intramolecularly to the sulfur atom to give 226. 

Notably, [Rh2(esp)2] (esp = a,a,a, a -tetramethyl-l,3-benzenedipropanaote) was also found to exhibit catalytic activity in the cyclopropanation reaction of styrene-type alkenes with dimethyl diazomalonate at room temperature with very low catalyst loading (0.1 mol%) (eq 42). The corresponding cyclopropanes were formed in good yields. In this reaction, alkene is employed as the limiting reagent, which is in contrast to many related reactions that an excess (>5 equiv) of alkenes was employed. 

The formation of enones (66) by irradiation of the arylcarbothioamides (65) in the presence of styrene derivatives proceeds by initial formation of the thietane (67) this conversion provides a potentially valuable route for the arylation of simple alkenes as shown in (Scheme 9). Several types of photoproducts are obtained by the irradiation of (65) in the presence of alkenes. The type of product obtained depends on the alkenes used and the reaction conditions. 

Heck type vinylation of 4-bromo-l-(4-methylphenylsulfonyl)-indole proceeds in good yield with such alkenes as methyl acrylate, styrene and N-vinylphthalimide using Pd(OAc)2 (5 mol%) and tri-o-tolylphosphine as the... 

Chemoselective alkenylation in the C-3 position of N-substituted 3,5-dichloropyrazin-2(lH)-ones has been described by Van der Eycken et al. [27]. When a mixture of N-substituted 3,5-dichloropyrazin-2(lH)-one, ethyl acrylate, and NEts in DME, using Pd(OAc)2/DTPB [2-(di-f-butylphosphanyl)bi-phenyl] as a precatalyst, was irradiated for 15 min at 150 °C, the desired /1-fimctionabzed ethyl acrylates could be obtained in moderate yields (Scheme 81). When styrene was used as an alkene, a mixture of E and Z products was isolated. The type of catalyst used proved to be important to avoid competitive Diels-Alder reaction of ethyl acrylate with the hetero-diene system of 3,5-dichloro-l-benzylpyrazin-2(lH)-one. 

Meerwein Arylation Reactions. Aryl diazonium ions can also be used to form certain types of carbon-carbon bonds. The copper-catalyzed reaction of diazonium ions with conjugated alkenes results in arylation of the alkene, known as the Meerwein arylation reaction.114 The reaction sequence is initiated by reduction of the diazonium ion by Cu(I). The aryl radical adds to the alkene to give a new (3-aryl radical. The final step is a ligand transfer that takes place in the copper coordination sphere. An alternative course is oxidation-deprotonation, which gives a styrene derivative. 

In aqueous DMF, the reaction can be applied to the formation of C-C bonds in a solid-phase synthesis with resin-bound iodobenzoates (Eq. 6.33).80 The reaction proceeds smoothly and leads to moderate to high yield of product under mild conditions. The optimal conditions involve the use of 9 1 mixture of DMF-water. Parsons investigated the viability of the aqueous Heck reactions under superheated conditions.81 A series of aromatic halides were coupled with styrenes under these conditions. The reaction proceeded to approximately the same degree at 400°C as at 260°C. Some 1,2-substituted alkanes can be used as alkene equivalents for the high-temperature Heck-type reaction in water.82... 

Brunner, Leitner and others have reported the enantioselective transfer hydrogenation of alpha-, beta-unsaturated alkenes of the acrylate type [50]. The catalysts are usually rhodium phosphine-based and the reductant is formic acid or salts. The rates of reduction of alkenes using rhodium and iridium diamine complexes is modest [87]. An example of this reaction is shown in Figure 35.8. Williams has shown the transfer hydrogenation of alkenes such as indene and styrene using IPA [88]. 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

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