M-Xylylene

A rare example of a crystallisable aromatic polyamide used as a plastics material is poly-m-xylylene adipamide. 

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

Wenthold and co-workers have used this approach to generate the negative ion of 5-dehydro-m-xylylene (Eq. 5.19). ° Using bracketing experiments to determine the gas-phase acidity at the 5-position of m-xylylene, and the kinetic method to determine the triradical electron affinity, the BDE at the 5-position of m-xylylene was measured (Eq. 5.20). 

Convenient routes to several new sterically crowded chelating diphosphines have been described.48-50 Thus, e.g., m-xylylene dibromide, on treatment with di-t-butylphosphine, affords a bisphosphonium salt, which on treatment with a weak base affords the diphosphine (58).48... 

The use of the upper rim of calix 4 arcncs, blocked in the cone conformation, as convenient platforms for the introduction of metal ion binding sites is well documented [26e,291. Table 5.9 shows the catalytic performances of regioisomeric vicinal 21-Ba2 and distal 22-Ba2, and of the m-xylylene derivative 17-Ba2 in the basic ethanolysis of esters 14 and 23-25 [30]. 

The formula (58) for di-m-xylylene was established by Baker et al. (1951) and confirmed by Brown s X-ray study. The linkage of the benzene rings by the (CH2)2 groups at the meta positions produces a ten-membered ring with the benzene rings located step-wise to permit adequate clearance at the central carbon atoms (Fig. 7). The molecule... 

Fig. 7. The conformation of di-m-xylylene. Carbon atom 3 lies 0-14 A, and atom 7, 0-04 A, out of the plane of the remaining four atoms in the benzene ring. The separation 3 --3 is 2-69 A.

To apply the rule, take, for example, the MSAD of o-xylylene, 1, p-xylylene, 2 and m-xylylene, 3, in Scheme 8.2. Both 1 and 2 have S. = 0, while 3 has Sz = 1. It follows that o- and p-xylylenes will have singlet ground states, while m-xylylene will possess a triplet ground state. The prediction is correct, but not particularly surprising, since 1 and 2 can each be described by a perfectly paired Kekule structure, while 3, which cannot, is a diradical species that can be reasoned to have a triplet spin based on Hund s rule. 

Even tris- and tetrakis-adducts of C6o were accessible in a regioselective way by sequential tether-directed macrocyclization steps. A clipping reaction starting from mono-adduct 40 with two tether-linked malonate residues thus yielded tris-adduct ( )-41 (Scheme 1.5).125 Under the assumption that both m-xylylene tethers enforce the expected cis-2 addition, the C2-symmetry of... 

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