2- -1.3-xylyl-18-crown

Sr2+ so well that it was selectively extracted from a bulk sample of a barium salt (Helgeson et al., 1973a). Binding constants for metal-cation complexes of 1,3-xylyl-crown ethers [66]—[69] carrying an additional carboxylate binding... 

Effect of remote and intra-annular substituents on log K values for t-BuNHjX complexes of 1,3-xylyl-crown ethers 1256] in CDC13 at 24°C ... 

An unprecedented metallation was observed when 1,3-xylyl crown ethers are reacted with diarylmagnesium compounds. Reaction of l,3-xylene-15-crown-4 with diphenyl-magnesium gives in quantitative yield 2-(phenylmagnesio)-l,3-xylene-15-crown-4 (81), the structure of which was established by an X-ray crystal-structure determination... 

The modified crown ligand (15), which is sterically unsuitable for coordination of all six ether oxygen atoms, forms a complex [Sm(N03)3(H20)(L)] where L is the 2-methoxy-l,3-xylyl crown (15).456 This is an irregular bicapped square antiprism in which three consecutive oxygen atoms of the crown ligand (the third being the OMe) are coordinated. 

Figure 16 Coordination of Ph2Zn with 1,3-xylyl crown ethers.

Markies PR, Nomoto T, Schat G, Akkerman OS, Bickelhaupt F (1991) Unusual Metalation and Halogen-Metal Exchange Reactions between 1,3-Xylyl Crown Ethers and Organomagnesium Reagents. X-ray Structure of 2-[(p-ter/-Butylphenyl)magnesio]-l,3-xylylene-18-crown-5. Organometallics 10 3826-3837... 

The ortho-xylyl unit may be considered as the synthetic complement of the catechol unit so often used in crown synthesis. This is especially so for the 1,2-bromomethyl compounds which may be used as electrophiles. These units have been incorporated in numerous crowns over the years and the syntheses are all quite similar. 

The 1,3-xylyl trick was also used for the incorporation of phenols into crown ethers. Three classes of phenols [46a]-[46c] have been investigated. They differ by their substituents in the 4-position. In Table 22 the pX a values of different macrocyclic phenols [46a]-[46c] are compared. The data obtained for the phenol-containing crowns [46a] and [46b] show very little evidence for a macrocyclic effect. No extra stabilization of the protonated (acidic) form by a macrocycle of appropriate ring size was found. The acidities of the macrocyclic phenols [46a] and [46b] were independent of the ring size and comparable to non-macrocyclic analogues. However, the azo-substituted crowns [46c] showed a difference of 0.8 pACg units which was not expected from the pAfa values of [46a] and [46b]. TTiis different behaviour of [46c] is not yet understood. 

When diphenyhnagnesium is crystallized from a solution containing l,3-xylyl-18-crown-5, an X-ray crystal-structure determination showed the formation of rotaxane 116 (Figure 56) . Only four of the five oxygen atoms of the crown are involved in coordination to magnesium, two with a relatively short bond distance [2.204(3) and 2.222(4) A] and two with a longer bond distance [2.516(4) and 2.520(4) A]. The C(l)-Mg-C(2)... 

Assembling [2] rotaxanes (wheel and axle) can involve three basic processes (Scheme 13). One of these (93CC1269) involves slippage, in which the axle (115) is linked by a 4,4 -bipyridine, and the wheel (116) is a bisparaphenylene-34-crown-10 ether. Heating the two components in acetonitrile at 60°C yields the rotaxane, which can be characterized by FABMS and H and l3C NMR, but extrusion of the wheel occurs at 100°C. In other developments (94NAT(369)133), the pyridine component may be incorporated in the wheel, as in structure (117), where two bipyridinium units are connected by p-xylyl groups, and here the rotaxane acts as molecular switch. At room temperature in acetonitrile the wheel... 

The Re(I) bipyridyl unit has been exploited in another series of ion-pair sensors. Molecules 61-64 incorporate crown ether components to act as cation receptors [43,44]. H NMR titrations revealed 61-64 to be selective for acetate over chloride. In all the receptors (except 62 and 64 with acetate) enhanced binding was observed in the presence of K+ cations. The degree of enhancement is lower in the xylyl-spaced molecules 62 and 64 (40-50%) than in 61 and 63 (80-110%). It is also small compared to that seen in the calix[4]arene receptor 60. 

A different coordination mode has been found with 1,3-xylyl 15-crown-4 and 1,3-xylyl 18-crown-5 ethers. The crystal structure of three adducts with Ph2Zn shows that bonding takes place only with two oxygens of the cryptand see... 

Since the publication of 26, we have determined the crystal structures of three additional diarylmagnesium-polyether complexes. Instead of diphenylmagnesium, its 4,4 -bis(tm-butyl) derivative was used in the com-plexation experiments since this species as a rule gives higher quality crystals. Complexation with 1,3-xylyl-18 crown-5 resulted in the formation of a rotaxane complex (27), too. Bond angles and distances, and even the... 

The crown ether-substituted aryl-Grignard 2-bromomagnesio-l,3-xylyl-15-crown-4 (40) crystallized as a solvent-free complex from its THF solu-... 

The analogous ether cleavage reaction of the crown-5 analog of 40 rendered its synthesis from the aryl bromide and magnesium next to impossible isolation of crystalline 2-bromomagnesio-I,3-xylyl-18-crown-5 (41), formed in 16% yield, from the crude reaction mixture was not feasible. Therefore, pure 41 had to be obtained by treatment of the aryl bromide with /j-butyllithium, followed by the addition of 1 equiv magnesium bromide. The crystal structure of 41 contained two independent residues (41a and 41b) and proved to be very irregular the conformation of its crown... 

A remarkable halogen - metal exchange reaction was observed between diphenylmagnesium and 2-bromo-1,3-xylyl-15-crown-4 in diethyl ether, yielding the crystalline compound 2-phenylmagnesio-1,3-xylyl-15-crown-4 (42) (50). Reaction of diphenylmagnesium with l,3-xylyl-15-crown-4 gave... 

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