Aromatic alkenes styrene derivatives

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

Oligomerization of aliphatic alkenes such as ethylene and propene, and aromatic alkenes, such as styrene, indene and their derivatives, catalyzed by a variety of Lewis acids, such as AlCl.i, BFj, AlBrj, and Brpnsted acids, such as H3PO4, is a well-studied subject. These reactions are important, particularly in the industrial arena. 

The hydroboration of non-aromatic alkenes is regio- and diastereoselective. Only the reaction of styrene derivatives such as 24 results in mixtures of regio-isomers (25 and 26) Eq. (12). 

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. 

Asymmetric hydroformylation, although intensively investigated for 20 years, is still of limited (but growing) value in asymmetric synlhesis of chiral aldehydes and their derivatives. Some substrate types clearly promise better results than others. Thus, reactive symmetrical aliphatic alkenes such as norborncnc, aromatic alkenes (especially styrenes) and some hctcrofunctional-ized alkenes have given the best results up to now with around 90 % ee. In many cases, however, highest asymmetric inductions are still only achieved with loss of productivity in terms of yields, regioseleclivity, conversion rates and reaction times. 

C-H o-bond activation of hydrocarbons by transition metal complexes is of considerable importance in modern organometallic chemistry and catalytic chemistry by transition-metal complexes [1], because a functional group can be introduced into alkanes and aromatic compounds through C-H o-bond activation. For instance, Fujiwara and Moritani previously reported synthesis of styrene derivatives from benzene and alkene via C-H o-bond activation of benzene by palladium(ll) acetate [2]. Recently, Periana and his collaborators succeeded to activate the C-H o-bond of methane by the platinum(ll) complex in sulfuric acid to synthesize methanol [3], Both are good examples of the reaction including the C-H o-bond activation. 

Similar to mercuration, Pd(OAc)2 undergoes facile palladation of aromatic com-povmds. The palladation product 162 is an unstable intermediate. It can be isolated only when stabilized by chelation. The palladation products of aromatics as reactive intermediates undergo three reactions. The reaction with alkenes to afford styrene derivatives 164 is the first one. Pd(II)-promoted alkenylation of aromatic compovmds, discovered by Fujiwara, is a stoichiometric Heck reaction. The second one is homocoupling to form biaryls. The acetoxylation of aromatic rings is the third reaction. These latter two reactions are treated in Chapter 2.7. 

The Pd(II)-mediated reaction of benzene with alkenes affords styrene derivatives 164. The reaction can be vmderstood by palladation, insertion of olefin to give 163, and y3-H elimination [67,68]. In addition to benzene and naphthalene derivatives, electron-rich heteroaromatic compounds such as ferrocene, furan and thiophene react with alkenes to give vinyl heterocycles. The effect of substituents in this reaction is similar to that observed in the electrophilic aromatic substitution [69]. 

Flammable liquids may undergo a chemical reaction called polymerization, in which a large number of simple molecules, called monomers, combine to form long-chained molecule called a polymer. This process is used under controlled conditions to create plastics (see Figure 5.17). Alkene hydrocarbon compounds and hydrocarbon derivatives, such as aldehydes, alkyl halides, and esters, and the aromatic hydrocarbon styrene may undergo polymerization. There are other monomers that are flammable and can polymerize, but their primary hazard is poison. Monomers can be flammable liquids, flammable gases, and poisons. 

Insertion reactions of monoenes into Pd-R bonds also lead to j -allylic palladium complexes provided that either the monoene or the R group contain an unsaturated functionality. Thus, the insertion of an alkene into a Pd—vinyl bond leads to 77 -allylic palladium compounds. The insertion of styrene and other vinyl arenes into a Pd—R bond leads to 77 -benzylic palladium complexes. The aryl ring in these derivatives gets involved in bonding, and the 77 -benzylic coordination can be looked at as a particular case of 77 -allyl. Nonetheless, the adoption of cr-coordination is more facile in -benzylic derivatives than in j -allyls due to the extra stabilization provided by the aromaticity of the aryl ring in the n-form. 77 -Benzylic palladium complexes have been prepared by insertion of styrene into Pd-Pd-Me, and Pd-SiRs bonds, and styrene derivatives into Pd-Me bonds (Equation (33)). Insertion of vinylarenes into a usually non-detected Pd-H bond leads to the synthesis of other -benzylic palladium derivatives. 

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

The application of such complexes in the cyclopropanation of alkenes was investigated, mostly in benchmark reactions between styrene and diazo esters or aromatic diazomethane derivatives (Scheme 9.19) to form the cis- and trans-cyclo-propene derivatives 23 and 24. 

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

The classical Vilsmeier-Haack reaction - involves electrophilic substitution of a suitable carbon nucleophile with a chloromethyleneiminium salt, for example salt (1). Suitable carbon nucleophiles are generally electron-rich aromatic compounds such as V,N-dimethylaniline (2), alkene derivatives such as styrene (3) or activated methyl or methylene compounds such as 2,4,6-trinitrotoluene (4 Scheme I). These compounds (2-4) react with salt (1) giving, after loss of hydrogen chloride, the corresponding im-inium salts (5-7). Hydrolysis of iminium salt (5) affords aldehyde derivative (8) and this transformation (Ar—H - Ar—CHO) is the well-known Vilsmeier-Haack formylation reaction. Hydrolysis of iminium... 

A variety of catalysts with differing aromatic substituents and ethylenediamine cores have been prepared and tested, but few perform better than Jacobsen s catalyst. These catalysts are not general for all alkene substrates and the best results have been achieved using cis-alkenes, trisubstituted alkenes and some tetrasubstituted derivatives. Especially high ees have been obtained with conjugated alkenes, in particular chromenes and P-substituted styrenes while trans-olefins and terminal olefins are poor substrates for this process. 

The arene substrates are not limited to simple benzene derivatives. A variety of het-eroarenes can also participate in alkene arylations to generate the desired coupling products. Stoichiometric oxidative coupling of aromatic heterocycles such as furan, thiophene, selenophene, A-methylpyrrole, benzofiiran and benzothiophene with a variety of alkenes, including acrylonitrile, styrene and methyl acrylate, have been extensively studied by Fu-jiwara and coworkers [8]. Furan, thiophene, selenophene and A-methylpyrrole are easily alkenylated with alkenes to give 2-alkenylated and 2,5-dialkenylated heterocycles in relatively low yields (3 6%) [8a], while the reactions of benzofuran and benzothiophene with alkenes produced a mixture of 2- and 3-alkenylated products [8b]. 

Several difunctional compounds involve a C=C unit and the carbonyl unit of a carboxylic acid (or an acid chloride), ester, or amide. Typical conjugated carbonyl compounds are 2-propenoic acid (12 acrylic acid), but-2E-enoic acid (13 also known as crotonic acid), and 2-methylpropenoic acid (14 also known as methacrylic acid). There are cyclic derivatives of the carboxylic acids in which the carboxyl unit is attached to the ring. A C=C unit or a C=0 unit is considered to be conjugated if it is connected to an aromatic ring. Ethenylbenzne (styrene, 15) is a conjugated alkene and both benzaldehyde (16) and acetophenone (17) are conjugated. 

---