Styrene oxazoline

Polymers containing oxazoline groups are obtained either by grafting the 2-oxazoline onto a suitable existing polymer such as polyethylene or polyphenylene oxide or alternatively by copolymerising a monomer such as styrene or methyl methacrylate with a small quantity (<1%) of a 2-oxazoline. The grafting reaction may be carried out very rapidly (3-5 min) in an extruder at temperatures of about 200°C in the presence of a peroxide such as di-t-butyl peroxide Figure 7.13). 

The aza-bis(oxazoline) 14, bearing sterically hindering groups, led to very good results in terms of activity and selectivity, comparable to those obtained from corresponding aza-semicorins or bis(oxazolines). For the enantioselec-tive cyclopropanation of styrene, the trans isomer was obtained in 92% ee... 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

Table 11 Effect of bis(oxazoline) on the aziridination of styrene with zeolite-supported ...

Scheme 6.5 Bis(oxazolines)bithiophene ligands for Cu-catalysed cyclopropanation of styrene with EDA.

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines, N-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

Other oxazoline containing ligands have been reported for the copper-catalyzed cyclopropanation reaction. These are outlined in Scheme 5 where the reaction of ethyl diazoacetate and styrene is compared. Conditions are more or less standard (halogenated solvent, 0°C to ambient temperature, CuOTf without reduction). For more information on these ligands, the reader is referred to the primary publications (49-54). 

Scheme 5. Various oxazoline ligands and selectivities in the cyclopropanation of styrene.

Optimal selectivities of cinnamate aziridines are provided by the phenyl-substituted bis(oxazoline) (55d). In contrast, /ra .v-p-mcthyl styrene proved to be most selective with ferf-butyl substituted 55c and acetonitrile as solvent, Eq. 56, conditions that afford the cinnamate ester-derived aziridine in only 19% ee. Styrene is a poor substrate for this catalyst and provides the opposite antipode as the major enantiomer from the same ligand, Eq. 57. 

A number of other bis(oxazolines) have been applied as ligands in the copper-catalyzed aziridination reaction. Knight and co-workers (80) examined tartrate-derived ligands. Diastereomeric bis(oxazolines) 110 and 111 were each found to be poorly effective in mediating the asymmetric aziridination of styrene, Eq. 63. 

Interestingly, the Fe2+ ion in the core can be easily removed by base, the complex dissociates and the individual polymer dimers can be analyzed. Block copolymers of 2-ethyl-2-oxazoline with other substituted oxazolines have also been made [121]. Ru2+(4,4 dichloromethyl-2,2 bipyridine)3 has also been used as the multifunctional initiator for the ATRP of styrene at 110°C [122], It is interesting to note that the Cu+ ions necessary for the polymerization reaction are solubilized via complexation with other bipyridine species. 

Scheme 6.6 Heck coupling of iodobenzene and styrene as a model reaction for micellar catalysis using triphenylphosphine-functionalized poly(2-oxazoline)s as macroligands...

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