Anethole, dimerization

Although the chemistry of anethol dimers has been studied in detail because of their natural occurrence the effects of reaction conditions in acid-catalyzed dimerization... 

The case of anethole, An-CH=CH-CH3, is interesting because of the relation between the cation-radical cyclodimerization and reduction processes. Although the rapid cation-radical dimerization of trani -anethole is strongly predominant, the cyclobutadimer formed is also ionized. Therefore, reductive cleavage of the dimer proceeds efficiently (80%). 

In the presence of catalytic amounts of tris(4-bromophenyl)ammoniumyl hexachloroantimonate in methylene chloride at 0°C, trani -anethole is smoothly converted in less than 5 min to the cyclobutane dimer (Bauld et al. 1996 Scheme 7.16). 

Of conrse, the cyclic cation-radical formed should be less stable than the alkene cation-radical (which contains a double bond that is favorable for the spin-charge scattering). However, the cation-radical product and corresponding nentral species are generated in a concerted process. The process involves simultaneous covalent bond formation and one-electron reduction of the cyclic product (Karki et al. 1997). Similar to other branched-chain processes, the cation-radical dimerization is characterized by an activation enthalpy that is not too high. These magnitudes are below 20 kJ mol for the pair of cyclohexadiene and trani-anethole (p-MeOCgH4CH=CHCHMe, Z-form Lorenz and Bauld 1987). It is clear that the cation-radical variant of cyclodimerization differs in its admirable kinetic relief. For cyclohexadiene and tran -anethole, catalytic factors are 10 and 10, respectively (Bauld et al. 1987). 

Lewis investigated the photodimerization of cis- and /ran.v-anethole[l-(p-methoxyphenyl)propane, c-A or f-A] in the singlet state. Whereas the reaction of singlet t-A with another ground state t-A or c-A occurs with retention of stereochemistry, the dimerization of c-A occurs with retention at one double bond and inversion at the other. This is proposed to proceed via sequential formation of a singlet exdplex with optimized n-orbital overlap and a singlet 1,4-diradical intermediate [106] ... 

Reactions that include the dimerization of two radical cations are, for example, those of anethole [313], pyrroles [314], oligopyrroles [315], thiophenes [316], oligo-thiophenes [317], diphenylamine [318], triphenylamine [319], WWdimethylaniline [320], and diphenylpolyene [321]. 

Scheme 11. Syn/anti diastereoselectivity in the dimerization of tran -anethole.

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

Figure 25. SECM working curve for the RR dimerization mechanism together with experimental points obtained for the oxidation of fra/j -anethole. The ratio Ig/It at the figure corresponds to —. Jh in the text and K is defined in Eq. (63). (From Ref. 199).

An example of the application is the oxidation of tran -anethole [trans- - A-methoxyphenyl)propene], the radical cation of which by LSV was found to undergo rapid RR dimerization (through C-2). The rate constant measured by SECM is as high as 4 X 10 M s [199]. The fit of the experimental data to the working curve is shown in Fig. 25. The SECM technique has also been used to determine the rate constant for the dimerization of the radical anions of acrylonitrile, the prototype example of electrohydrodimerization (Chapter 21). The value of k was found to be 6 x 10 M s [201]. 

Figure 1.15 Synthesis of 20 by means of oxidative dimerization of anethole (19). Modified by permission of Shokabo Publishing Co., Ltd...

ESI droplets can be considered as microvessels for the study of fast reactions in solution in short time intervals. In one representative study, different arrangements of reactant sprayers were used to detect intermediate in electron-transfer-catalyzed dimerization of trani -anethole [100], Most recently, Lee et al. [101] took advantage of the fusion of high-speed liquid droplets to record the kinetics of liquid-phase chemical reactions in the microsecond timescale (Figure 4.5). The reaction was quenched when the species entered a heated transfer capillary. This approach resembles, to some extent, the concept of fused... 

Marquez, C.A., Wang, H., Fabbretti, F, Metzger, J.O. (2008) Electron-transfer-catalyzed Dimerization of fratis-Anethole Detection of the Distonic Tetramethylene Radical Cation Intermediate by Extractive Electrospray Ionization Mass Spectrometry. J. Am. Chem. Soc. 130 17208-17209. 

Metzger and coworkers have studied reactive intermediates of chemical reactions in solution by using a microreactor coupled to an ESI mass spectrometer. The highly stereo- and regioselective dimerization of trows-anethole to give the head-to-head trans, anti, trans-cydobutane initiated by aminium salt proceeds by a radical cation chain mechanism (Scheme 4.4) and this method was further used to study the transient radical cations intermediates in electron transfer-initiated D-A reactions [12-14]. 

Scheme 4.4 Radical chain reaction of trans-anethole mediated by tris(p-bromophenyl)aminium hexachloroantimonate to give the dimer product 1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane via two reaction intermediates.

Furthermore, Fiirmeier and Metzger used the same procedure to study the electron transfer-initiated Diels-Alder reaction of isoprene and anethole, as well as the Diels-Alder dimerization of 1,3-cyclohexadiene. They unambiguously detected and characterized the respective transient radical cations of the dienophiles and of the Diels-Alder addition products. 

Styrene derivatives with a bulky substituent at the a or P position to the vinyl group cannot be made into high polymers because of the steric hindrance of the substituent. Consequently, these monomers give dimers exclusively at high temperatures irrespective of the kind of initiator few studies, however, deal with the dependence of the structure of these dimers on reaction conditions and on the nature of counteranions. This section briefly discusses cationic dimerization of a-methylstyrene ( MS) and anethol (P-methyl-p-methoxystyrene) as a- and P-substituted styrenes, respectively. 

From the unsaponifiable matter of Spheciospongia vesparia, Bergmann and McAleer (22) isolated a crystalline fraction which was shown to be metanethole (225), the dimerization product of anethole. From the lack of optical activity the authors suspected that this sponge product was an artifact formed during the saponification procedure. 

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