Lewis acids tacticity

Smith exploited a Lewis-acid-mediated asymmetric Diels-Alder reaction34 between 1,3-butadiene and the Oppolzer acryloyl camphorsultam 78 to set the remote C(29) stereocentre in phenylsulfone 57 (Scheme 17.17). This tactic procured acid 8 in 93% enantiomeric excess (ee), after base-promoted hydrolysis of the chiral auxiliary. Utilising a procedure published by Martin and co-workers,35 Smith iodolactonised acid 8, and... 

A tactic for the synthesis of C3 imines has been developed by the Chatani group that involves Lewis acid-promoted insertion of isocyanides into indole C-H <07OL3351>. In a... 

A wide range of trialkylaluminum with different bulkiness and Lewis acidity was investigated They influenced differently the PMMA tacticity. EtsAl and i-BusAl led to syndiotactic PMMA, whereas atactic PMMA was formed in the presence of f-BusAl. Ballard and coworkers successfully polymerized MMA in toluene at 0 °C in the presence of a mixture of f-BuLi and (2,6-di-f-butyl-4-methylphenoxy)diisobutylaluminum, that was prepared by reaction of triisobutylaluminum with 2,6-di-f-butyl-4-methylphenol. In order to shed light on the origin of the polymerization control, a mechanistic study was undertaken by NMR techniques. In the f-BuLi/EtsAl system, the actual polymerization initiator is a f-butyl carbanion. Actually, f-BuLi forms an ate complex with EtsAl. Later on, Muller and coworkers analyzed the unimeric model in the presence of EtsAl and MMA, and they confirmed the formation of a bimetallic ate complex It was accordingly proposed that the propagating species is an ate complex, which exhibits a decreased nucleophilicity compared to the uncomplexed species. 

The chain tacticity of PMMA synthesized by GTP catalyzed by nucleophiles at different temperatures was analyzed by Webster and coworkers The syndiotactic content increases from 50% at 60 °C up to 80% at —90°C in THF, using tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2 ] as catalyst . In contrast to the anionic polymerization of MMA, the stereoselectivity of GTP is less sensitive to solvent. It must be noted that PMMA is less syndiotactic when the GTP is catalyzed by nucleophiles rather than by Lewis acids . GTP was extended to the living polymerization of many acrylates and methacrylates, such as nBuMA, glycidyl-MA, 2-ethylhexyl-MA, Me3SiOCH2CH2-MA, sorbyl-MA, allyl-MA, lauryl-MA), acrylates (EA, BuA), acrylonitrile, methacrylonitrile and Al,A-dimethylacrylamide . 

In general, radical reactions are not selective. The polymerizations allow only a limited degree of control. For instance, tacticity of the polymerization may be controlled by the addition of Lewis acids.6 The reactivity ratios of monomers and rates of polymerization can also be controlled by the addition of Lewis acids.7-9... 

One important application of Lewis acid to asymmetric radical reactions is in the control of tacticity in free radical polymerizations. Recently, Porter [38] showed that Sc(OTf)3 modulates the polymerization of oxazolidinone acrylamides to produce highly isotactic copolymers (Scheme 12). The same study described homopolymerizations in which the m/r dyad ratio was dependent on the reaction temperature. 

The copolymerizations of 2-methyl-1-propene (isobutylene) and acrylamides 13a f were studied in Lewis acid-promoted copolymerizations (Fig. 8). Although 13a-f can be homopolymerized in the presence of Lewis acids, poor conversions are obtained except with 13a. Presumably, complexation renders the radical and monomer too electron-deficient to react efficiently. This effect, however, should enhance the reactivity of the complexed radical toward more electron-rich alkenes and has been observed to increase the alternating character of copolymers of isobutylene and methyl acrylate [9], Isobutylene also is an ideal choice for a comonomer as it does not homopolymerize by radical pathways, and the analysis of the copolymer s tacticity is not complicated by additional stereocenters as would be the case with monosubstituted vinyl comonomers. 

Further discussion on the effects of the reaction media and Lewis acids on tacticity appears in Section 7.2. Attempts to control tacticity by template polymerization and by enzyme mediated polymerization are described in Section... 

Lewis acids are known to form complexes both with monomers and with propagating species. Their addition to a polymerization medium, even in catalytic amounts, can bring about dramatic changes in rate constants in homopolymerization (Section 8.3.4.1) and reactivity ratios in copolymerizalion (Section 8.3.4.2). Early work in this area has been reviewed by Bamford and Barton and Borsig. fhere is significant current interest in using Lewis Acids in establishing tacticity control in homopolymerization (see 8.3.4.1). 

Although it is clear that added Lewis acids affect the rate of polymerization and the molecular weight of homopolymers formed in their presence/ the effect on polymer structure is small. There are reports that Lewis acids affect the tacticity. Otsu and Yaniada found a slightly greater proportion of isotactic (ww) triads in PMMA fbnned by bulk polymerization of a 1 1 complex of MMA with zinc chloride than is observed for a similar polymerization of MMA alone. However, for polymerizations carried out in solution or in the presence of lesser amounts of zinc chloride, no effect was observed. For MMA polymerization in solution at 60 °C, a small though significant effect on tacticity... 

Each monomer addition step interconverts the two organometallic components. The poly(methyl methacrylate) (PMMA) obtained is predominantly syndio-tactic, although isotactic PMMA has been obtained by using chiral indenyl zirconocenes in combination with non-zirconocene Lewis acids. No reports of attempted ethylene or a-olefin copolymerizations have been described. 

Simultaneous control of stereosequence and molecular weight distribution has long been one of the holy grails in the field of radical polymerization. Nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and RAFT all offer control over molecular weight distribution. However, polymers produced by these methods show similar tacticity to those obtained by the conventional process. Recently there have been reports of tacticity control of homopolymers " (which enables the synthesis of stereoblock copolymers ) and control of the alternating tendency for copolymerizations in ATRP or RAFT polymerization through the use of Lewis acids as additives. 

Ge and Sn halides are Lewis acids. SnCl4, which is a strong Lewis acid, can abstract a halogen anion from an alkyl halide to give the alkyl carbocation and is widely used for living cationic polymeriza-tions. ) On the other hand, Ge and Sn iodides (used in this work) are relatively weak Lewis acids. The tacticities of the produced polymers and the complete inhibition of the polymerization in the presence of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) confirmed that the polymerizations in this work proceeded in a radical mechanism. 

Polymerization of vinyl monomers under mild conditions using Lewis acid catalysts to give a stereoregulated, or tactic, polymer. 

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