Diethers

Some studies relative to the influence of Lewis bases,124-127 1,3-diethers in particular,125-127 and the MgCl2 support have also been recently reported. The dependence of the industrially relevant isotactic indexes on the chemical structure of the 1,3-diether donor has been rationalized in the assumption that donor coordination competes with Ti catalytic species formation and xylene-insoluble (highly isotactic) and xylene-soluble (poorly isotacUc) fractions are mainly obtained by polymerization on (100) and (110) cuts, respectively.127... 

The role of Lewis bases in MgCl2-supported Ziegler-Natta catalysts for x-olefin polymerisation is still a subject of continuous interest. Recently [67], the use of 1,3-diethers [of the general formula R1OCH2C(R2)(R3)CH20R4], e.g. 2,2-diisobutyl-l,3-dimethoxypropane, as the internal Lewis base during... 

A different principle was employed to obtain inherently chiral calix[4]arenes. This approach involves desymmetrizing C2v or Cs symmetrical calix[4]arenes or calix[4]arene analogues (see Section 4.5) by etherification. 1,3-Diethers 78a of AABB-type calix[4]arenes are (as usually) easily obtained in a stereochemically fixed syn-arrangement.146 Since the symmetry plane of the calix[4]arene skeleton does not coincide with the two symmetry planes of the diether-pattem, the whole molecule is C, symmetrical, as revealed by the NMR spectrum. The chirality has been additionally demonstrated by further splitting of the signals in the presence of Pirkle s reagent.147 For the same reasons 1,2-diether derivatives 78b of ABAB-type calix[4]arenes are chiral, although less easily accessible.148... 

Figure 25. Calix(4]arene 1,3-diethers with chiral ether residues possible conformations and their symmetry.

It should be mentioned that a tetraether in the 1,3-altemate conformation (54) or 1,2-altemate conformation (Cj) or an anti-1,3-diether (C,) are not chiral. 

While all known examples of 1,3-diethers with an anti-orientation were obtained via protection/deprotection strategies , the acylation with excess benzoyl chloride using NaH as base was reported to give the anti-isomer in boiling toluene, while the iyn-isomer is formed in THF at 0... 

The dependence on the reaction conditions was shown also for the iyw-l,3-diallyl ether of which in CH3CN gives 5yn-5yn-diether/ester with PhCOCl/NaH, while reaction with PhCOCl in the presence of pyridine led to the awfi-yyw-diether/ester °. The formation of an anti-syn 1,3-diether/ester derivative was also achieved by barium(II) ion assisted monodeacylation of a l,3-crown-5 diacetate in the partial cone conformation . ... 

Various 5yn-5yn-triethers, among them inherently chiral derivatives, were prepared starting with mono-, 1,2- or 1,3-diethers . Allyl, benzyl or benzoyl groups have been used as protective groups in triether synthesis. 

Among various other 1,3-0-bridged derivatives calix[4]spherands 36f should be mentioned, which form kinetically very stable complexes with alkali cations (among which Rb+ is especially interesting for diagnostic purposes) . 1,3-Bridged compounds have also been derived from 1,3-diethers bearing acid or amino functions in the ether residues. 

Selective ipio-substitutions (reaction e)) are less frequently reported. The ipio-nitration of 1,3-diethers of 2a gave not only the desired product 48a, but by ipio-attack at the methylene bridges also the 6-nitrocyclohexa-2,4-dienone derivative The exhaustive... 

Monobenzoylation of 1,3-diethers under similar conditions leads also to the anti-syn isomers see Reference 122a. 

For the preparation of glycerol ditrityl ether (m.p. 170-171° presumably the 1,3-diether), glycerol (1 g) is heated with triphenylmethyl chloride (7 g) in pyridine (20 ml) for 30 min on the water-bath. Working up is as described above. 

Recently, Basell (currently, LyondelBasell) reported the use of catalysts classified as fifth-generation Ziegler-Natta systems, through the use of Spheripol technology [34]. Very high productivities, around 100 kg polypropylene/g catalyst have been reported in this process, using 1,3-diether and succinate compounds as internal and external donors, respectively. 

DIBP = Diisobutyl phthalate TMPIP = 2,2,6,6-tetramethylpiperidine DE = 1,3-diether. 

Fig. 10 Morphology of MgCl2 grown from ethanol solution over silicon wafer in the presence of (a) 1,3-diether and (b) phthalate (reproduced from [43]). MgCl2 formed in the presence of 1,3-diether exhibits only 120° comers, indicating the exposure of only one type of lateral cut, i.e., (1 1 0). On the other hand, 90° comers for phthalate indicate coexposure of the (1 1 0) and (10 4) lateral cuts...

Although the above paragraphs focused on the roles of donors in the formatitHi of solid catalysts and the active sites thereon, the interactiOTi between donors and alkylaluminum also plays a crucial role in catalysis. Lewis basic donors not only form a complex with alkylaluminum, but also react with highly reactive Al-R bonds internal donors except 1,3-diether desorb from MgQ2 surfaces through... 

Calix[4]arenes consisting of two different p-substituted phenolic units are readily prepared by fragment condensation ( 2+2 for AABB, 3+1 for ABAB). In the cone conformation their molecules possess Cs and C2V symmetry. 1,3-Diethers... 

With calix[4]arene-like macrocycles of type 25 having two different bridges X and Y [33-35] all mono- and triether derivatives are asymmetric. Their 1,3-diethers have C2 symmetry, while the two possible 1,2-diethers have a symmetry plane. The first examples of such compounds were mentioned by Nishimura [35]. 

Due to the equivalence of the phenolic units, it is now possible to construct in an unambiguous way all kinds of 0-alkylation products (from mono- to tetraethers), as with achiral calix[4]arenes. Of course, the symmetry is reduced in partially 0-alkylated derivatives. Mono- and triether derivatives are asymmetric (Ci) while 1,3-diether derivatives with the usual jyn-arrangement of the O-alkyl groups have C2 symmetry. An anri-1,3-diether (the formation of which is not observed under usual reaction conditions) would have an inversion center in its 1,2-altemate conformation, hence being achiral. 

Not only tertiary ethers can be generated in good yield from methoxy or benzyloxy transfer, but also other ethers including naphthalen-2-ylmethoxy proceed efficiently. As shown in Scheme 37.33, homoallylic ether 171 underwent stereoselective ether transfer to provide orthogonally protect 5y/j-1,3-diether 172. As a complement to benzyl transfer, naphtyhnethyl ether is easily removed by oxidation (Scheme 37.34). 

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