4-methoxystyrene

Sawamoto and Higadiimura have continued their investigation of the stop-flow spectroscopic behaviour of the polymerisation of p-methoxystyrene and added triflic acid to the series of catalysts previously tried. Again, they observed the typical absorption around 380 nm, attributed to the protonated monomer and calculated an initiaticm rate constant at 30 °C in ethylene chloride (assuming a bimolecular interaction orders not determined) kj = 5 x 10 s . This value is expectedly much 

Sawamoto and Higashimura also determined values of the propagation rate constant by measuring at the same time the amount of monomer consumed and the integrated amount of active species. This was done in media of decreasing dielectric con- 

It is interesting that pseudocationic polymerisaticm can occur even with the strongest acid known and erne of the most basic mcmcmers available, if the sdvent has a relatively low polarity. The presence of propagating ester molecules is therefore much more widespread than originally thew t, and must be cemsidered a totally general phenomenon in the cationic polymerisatiem initiated by Br sted acids. 

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When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

In a related photoreaction, hexafluorobutadiene forms the [2+4] adduct with p (Al.lV-dimethylamino)styrene in 11% yield, whereas no such adduct could be detected for the same reaction with p-methoxystyrene [124] (equation 104) The analogous thermal reaction at 120 °C leads only to [2+2] adducts... 

Fenoldopam (76) is an antihypertensive renal vasodilator apparently operating through the dopamine system. It is conceptually similar to trepipam. Fenoldopam is superior to dopamine itself because of its oral activity and selectivity for dopamine D-1 receptors (D-2 receptors are as.sociated with emesis). It is synthesized by reduction of 3,4-dimethoxyphenylacetonitrile (70) to dimethoxyphenethylamine (71). Attack of diis last on 4-methoxystyrene oxide (72) leads to the product of attack on the epoxide on the less hindered side (73). Ring closure with strong acid leads to substituted benzazepine 74. O-Dealkylation is accomplished with boron tribromide and the catechol moiety is oxidized to the ortho-quinone 75. Treatment with 9NHC1 results in conjugate (1,6) chloride addition and the formation of fenoldopam (76) [20,21]. 

Fig. 10. Influence of the initiator concentration on the rate of polymerization of o-methoxystyrene. (J. Geerts, M. Van Beylen, G. Smets, Ref. 69))...

The latter relations are relevant for polymerization of ortho-methoxystyrene initiated in toluene by BuLi, and investigated by Smets, Van Beylen and Geerts 69). Calculations based on the subsequently obtained results show that the fraction of the active non-associated lithium poly-ortho-methoxystyrene vary in this system from 15% at 18 10 3 M concentration of lithium to 60% as its concentration decreases to 0.5 10 3 M. The experimental data published in their paper are listed in Table 3, and presented graphically in Fig. 10. 

B. Reactions.—This year has seen the publication of a number of papers on the reactions of olefins and acetylenes with phosphorus pentachloride, to produce new phosphorus-carbon bonds. An investigation into the structural requirements of trisubstituted olefins (40) undergoing the above reaction has shown that both steric and electronic factors are important, e.g. an adduct forms with (40 X = CH3) but no reaction occurs for (40 X = Ph). Further examples of the reactions of unsaturated ethers include the formation and decomposition of adducts from a-methoxystyrene... 

Butadiene p-Methoxystyrene Vinyl acetate Vinyl chloride Diethyl maleate Isopropenyl acetate Vinyl chloride... 

The initial steps in the degradation of 2 -methyl-4-methoxystyrene (tran -anethole) by a strain of Arthrobacter aurescens are comparable to those already noted, except that the epoxide is hydrolyzed to a diol to produce 4-methoxybenzoate (Shimoni et al. 2002). 

Better yields result from more nucleophilic styrene dienophiles. For example, method F proves successful with the benzaldehyde 5 and a-methoxystyrene to afford the benzopyran 52 in a 55% yield (Fig. 4.28).27 The preferred diastereomer reflects an endo orientation with the more reactive moiety, which in this case is the vinyl ether portion of the dienophile. However, the diastereoselectivity for this and other 1,1-substituted alkenes is less than that for the corresponding mono-substituted systems. 

A total synthesis of O-methylarnottianamide (223) was performed by Falck et al. (177) (Scheme 34). The regio- and stereospecific cycloaddition of the 2,4-dinitrophenyl (DNP) salt of 6,7-methylenedioxyisoquinoline (218) with a-methoxystyrene 219 resulted in 220. Compound 220 was hydrolyzed, then aromatized, and the resultant aldehyde was oxidized to carboxylic acid 221. Curtius rearrangement of the appropriate azide yielded urethane 222, which... 

This procedure has been used in the preparation of other nitrostyrenes in the following yields o-nitrostyrene (40%),2 / -nitrostyrene (41%),2 and 3-nitro-4-hydroxystyrene (60%).2 A better procedure for more volatile styrenes involves simultaneous decarboxylation and codistillation with quinoline from the reaction flask. This method has been used to prepare the following styrenes o-chlorostyrene (50%),3 4 m-chlorostyrene (65%),4 -chlorostyrene (51%),4 m-bromostyrene (47%),4 o-methoxystyrene (40%),4 -methoxystyrene (76%),4 m-cyano-styrene (51%),3 and j -formylstyrene (52%).9... 

For some of the reactions depicted in Scheme 14, copper(II) hexafluoroacetyl-acetonate [Cu(hfacac)2] proved to be a better catalyst than other chelates neither cyclopropanes nor allylic insertion products were found, and the yield of dihydrofuran was not affected by temperature in the range 70-132 °C 130). However, in phenylogous vinyl ethers such as 4-methoxystyrene or trans-anethole, cyclopropanes (58a, b) rather than dihydrofurans resulted 131>. 

From the copper-catalyzed reaction of methyl 2-diazo-3-oxobutyrate 57 a with Z-3-methoxystyrene, dihydrofuran 59 (formed with retention of olefin configuration) and butadienol 60 result130). Such an acyclic by-product also occurs when benzofuran is the cycloaddition partner. In that case, however, regioisomers 61 and 62, arising from the connection of the former diazo carbon with either the 2- or 3-position of the heterocycle, are obtained similarly, two isomeric dihydrofurans 63 and 64 are formed under Cu(hfacac)2 catalysis130). 

Easily available copper(II) tartrate has also been used for an enantioselective cyclopropanation. From 3-methoxystyrene and 4-bromo-l-diazo-2-butanone, the cyclopropanes cis/trans-204 were obtained the mainly formed frans-isomer displayed an enantiomeric excess of 46% i99>. This reaction constituted the opening step of asymmetric total syntheses of equilenin and estrone. 

Similarly, hydroboration of p-methoxystyrene with HBcat using zwitterionic [(P-P)Rh(//6-catBcat)] ((4), P-P = dppe, dippe) or [(dippe)Rh(r/3-2-IVIc-a 11 y 1)] (dippe= l,2-bis(diisopropylphosphine)ethane) gave 99% of the internal hydroboration product. Complications arose in studying reactions of hindered styrenes due to the appearance of not only the hydroboration products (A and B), but also the dehydrogenative borylation products C and D (Scheme 5).32 The ratio of products was found to be highly catalyst dependent (Table 2).33... 

The QUINAP ligands are highly active for reaction of -substituted vinylarenes, thus, p-methoxystyrene gives the corresponding branched alcohol in 95% yield (57% ee) within 15 min at room temperature. Similarly, a combination of hydroboration/amination using (13) has allowed primary amines to be formed in a one-pot reaction of vinylarenes upon reaction of the B H addition product with MeMgCl/H2N0S03H (Scheme 10).5°... 

A value of kjkp = 17 000 has been determined for partitioning of the acetophenone oxocarbenium ion [12+] in water.15,16 It is not possible to estimate an equilibrium constant for the addition of water to [12+], because of the instability of the hemiketal product of this reaction. However, kinetic and thermodynamic parameters have been determined for the reaction of [12+] with methanol to form protonated acetophenone dimethyl ketal [12]-OMeH+ and for loss of a proton to form a-methoxystyrene [13] in water (Scheme 10).15,16 Substitution of these rate and equilibrium constants into equation (3) gives values of AMeoH = 6.5 kcal mol-1 and Ap = 13.8 kcal mol-1 for the intrinsic... 

Finally, as shown in Table 13, p for an aromatic ring is also strongly dependent on the other substituents at the double bond it varies from —1.6 to — 5.5 on going from a-methoxystyrenes to stilbenes. This variation, which is related to the well-known non-additivity of multiple substituent effects, and contrasts with what is observed for alkene bromination, is discussed in the next paragraph, devoted to substituent interaction and selectivity relationships in bromination. 

Fig. 12 Selectivity-reactivity relationships in arylolefin bromination. Line A correlates the data for styrenes, stilbenes and their a-methyl derivatives, and line B those of 1,1-diarylethylenes and a-methoxystyrenes (Tables 15 and 17).

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

Arya et al. used solid phase synthesis to prepare immobilised dendritic catalysts with the rhodium centre in a shielded environment to mimic nature s approach of protecting active sites in a macromolecular environment (e.g. catalytic sites inside enzymes) [51], Two generations PS immobilised rhodium-complexed dendrimers, 6 and the more shielded 7, were synthesised.The PS resin immobilised rhodium-complexed dendrimers were used in the hydroformylation of styrene, p-methoxystyrene, vinyl acetate and vinyl benzoate using a total pressure of 70 bar 1 1 CO/H2 at 45 °C in CH2C12. 

Using styrene as substrate both 6 and 7 could be recycled four times, apparently without loss in activity or selectivity (>99% conversion after 20 hours b l ratio i 18 1). When p-methoxystyrene was used as a substrate the first three cycles did not suffer... 

Fig. 9. Exdplex formation between excited 9-cyanophenanthrene and p. methoxystyrene. Er is the ground-state repulsion energy hv is the emission maximum of the exdplex and Hq the stabilization energy of the exdplex...

In comparison with the platinum catalysts, rhodium catalysts are much more reactive to effect addition of bis(catecholato)diboron even to non-strained internal alkenes under mild reaction conditions (Equation (5)).53-55 This higher reactivity prompted trials on the asymmetric diboration of alkenes. Diastereoselective addition of optically active diboron derived from (li ,2i )-diphenylethanediol for />-methoxystyrene gives 60% de (Equation (6)).50 Furthermore, enantioselective diboration of alkenes with bis(catecolato)diboron has been achieved by using Rh(nbd)(acac)/(A)-QUINAP catalyst (Equation (7)).55,56 The reaction of internal (A)-alkenes with / //-butylethylene derivatives gives high enantioselectivities (up to 98% ee), whereas lower ee s are obtained in the reaction of internal (Z)-alkenes, styrene, and a-methylstyrene. 

Matsubara, C. and Kojima, M. (2001). Photo-oxygenation and photodimerization of 4-methoxystyrene in NaY. The role of co-adsorbed water and ankene-oxygen charge-transfer complex. Res. Chem. Int. 27, 975-989... 

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