A-Methoxystyrene

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... 

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... 

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. 

Knowles reported the hydrogenation of a-phenylacrylic acid and itaconic acid with 15% and 3% optical purity, respectively, by using [RhCl3(P )3] [P = (R)-(-)-methyl-n-propylphenylphosphine] as homogeneous catalyst [38]. Horner found that a-ethylstyrene and a-methoxystyrene can be hydrogenated to (S)-(+)-2-phe-nylbutane (7-8% optical purity) and (R)-(+)-l-methoxy-l-phenylethane (3-4% optical purity), respectively, by using the complex formed in situ from [Rh(l,5-hexadiene)Cl]2 and (S)-(-)-methyl- -propylphenylphosphine as catalyst [39]. 

Wilkinson s (I) discovery that the soluble rhodium(I) phosphine complex, [Rh(PPh3)3Cl], was capable of homogeneous catalytic hydrogenation of olefins immediately set off efforts at modifying the system for asymmetric synthesis. This was made possible by the parallel development of synthetic methods for obtaining chiral tertiary phosphines by Horner (2) and Mislow (3,4, 5). Almost simultaneously, Knowles (6) and Horner (7) published their results on the reduction of atropic acid (6), itaconic acid (6), a-ethylstyrene (7) and a-methoxystyrene (7). Both used chiral methylphenyl-n-propyl-phosphine coordinated to rhodium(I) as the catalyst. The optical yields were modest, ranging from 3 to 15%. 

Apparently, these results implied an inverse relationship between reactivity and selectivity, with the reactivity of the carbocation measured by the inverse of the rate constant for solvolysis. This indeed was not unexpected in the context of a general perception that highly reactive reagents, especially reactive intermediates such as carbocations, carbanions, or carbenes are unselective in their reactions.257 259 Such a relationship is consistent with a natural inference from the Hammond postulate258 and Bell-Evans-Polanyi relationship,260 and is illustrated experimentally by the dependence of the Bronsted exponent for base catalysis of the enolization of ketones upon the reactivity of the ketone,261,262 and other examples21,263 including Richard s careful study of the hydration of a-methoxystyrenes.229... 

The evidence for perfect synchronization between bond cleavage, bond formation and positive charge delocalization was obtained for the proton transfer from hydronium ion to substituted a-methoxystyrenes ArC(OMe)=CH264. 

Nevertheless, to arrive at the sought-after cyclopropane derivatives, we treated the same reactants with each other thermally in benzene in the absence of carbon monoxide. In this reaction as well we did not get the desired compounds, but obtained instead, again surprisingly, the corresponding substituted a-methoxystyrenes (83) (Scheme 4). 

Hofmann degradation, styrene 468 was formed. Epoxidation of 468 with m-chloroperbenzoic acid from the less hindered side and lithium aluminum hydride reduction gave ( )-epicorynoline (469). Moreover, slow addition of the a-methoxystyrene 471 to isoquinolinium salt 470 gave cycloadduct 472 in 90% yield. The adduct was hydrolyzed by acid and the resultant aldehyde oxidized to naphthoic acid by Jones oxidation. Modified Curtius rearrangement of 473 with added benzyl alcohol afforded benzyl urethane 474, which was reduced by lithium aluminum hydride and formylated with chloral to give 0-methylarnottiamide (475) (Scheme 60). 

Vinyl ethers react in the same manner, the reaction being somewhat slower. a-Methoxystyrene and -nitrophenyl azide yielded (93%) the triazoline 237 °°. 

Dihydrofuran derivatives 319 are formed as major products in Rh2(OAc)4-catalyzed reactions of a-diazoacetophenone with 2-methoxy-propene or a-methoxystyrene (84MI1). Copper chelate or rhodium(II) acetate-catalyzed 1,3-dipolar cycloaddition of metal carbenoids, generated from ethyl formyldiazoacetate (90JOC4975), ethyl diazopyruvate... 

In these 1968 papers, the substrates to be hydrogenated were a-ethylstyrene, a-methoxystyrene, a-phenylacrylic acid, itaconic acid, etc. The hydrogenation of dehydroamino acid derivatives entered the literature with the papers of Kagan and co-workers [59, 60] and Knowles et al. [61]. Actually, the hydrogenation of (Z)-c -acetamidocinnamic acid to give A-acetylphenylalanine (eq. (3)) became the most frequently studied test system for the evaluation of new catalysts. 

On the other hand, the Cr-carbene complex reacts with ethylvinyl ether or isobutylvinyl ether, under milder conditions, to give the metathesis product, a-methoxystyreneS ... 

The same Cr-carbene complex reacts with l-vinyl-2-pyrrolidone to give the metathesis product a-methoxystyrene and with 1-phenyl-1-piperidinoethylene to give both the cyclopropane derivative and a new carbene (the inorganic metathesis product) ... 

The effect of wide and polymodal MW distribution of the synthesised polymen Polymers synthesised in various ionic and ionic-coordination polymerisation reactions demonstrate a broader MWD and sometimes clearly indicated bimodal or trimodal distribution. Broad MWD may indicate the presence of AC with different reactivity. Ultraviolet-spectroscopy and adiabatic calorimetry methods have been used to study a-methoxystyrene cationic polymerisation kinetics in dichloromethane, with a Ph3C SnCl catalyst [78]. MWD curves of forming polymers have been found to be polymodal, while three of the four fractions revealed themselves at the very beginning of the process. Such a distribution has been concluded to be caused by chain growth on three types of AC. 

Figure 22-2. Free radical copolymerization of Ma (methyl methacrylate or methyl acrylate) with Mb (a>methoxystyrene) at 60 C. Since the copolymerization is carried out above the ceiling temperature of a-methoxy styrene, no di-, tri-, etc. sequences of this monomeric unit can be formed because of the rapid depolymerization. The copolymerization parameter rs is then equal to zero, and the resit is a simple alternating copolymerization. (After data by H. Liissi.)...

Figure 5. Correlation of rate of hydration of terminal alkenes with

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