Zigzag Phenylenes

The solid-state characterization of the bifunctional radical [4,4 -(5-cyanobenzene)-l,3-bis(l,2,3,5-diselenadiazolyl)] 28 has been studied <1993CM820>. The crystal structure consists of stacks of diradicals running parallel to radical dimerization up and down the stack generates a zigzag arrangement, as seen in the related 1,3-phenylene structures. Along the stacking axis the mean intradimer Se-Se contact is 3.23 A, while the mean interdimer Se—Se distance is 3.91 A. 

The angle between the plane of the p-disubstituted phenylene group and the alkylene chain is similar to that of PET, but the sawtooth formed is deeper than PET due to the greater length of the methylene chain. It is nowhere near the deep zigzag of PTT as shown in a-c plane models. 

Figure 11.2 Examples of thiophene/phenylene co-oligomers (TPCOs) and their various molecular shapes (pseudo-straight, zigzag, bent, etc)...

Figure 4.2. Simple phenylene topologies (a) linear [4]-, (b) angular [4]-, (c) zigzag [4]-, (d) branched [4]-, (e) (mixed) bent [4]-, and (f) circular [6 phenylene.

Figure 4.3. Phenylenes and topologically related PAHs (a) linear [/VJphenylenes and polyacenes (b) angular/zigzag [/VJphenylenes and polyphenanthrenes/helicenes (c) circular [6]phenylene sheet and graphite (d) archimedene (C120) and fullerene C o).

A variant of the double cycloisomerization route to angular [5]phenylene (Scheme 4.6) was used to prepare zigzag [5]phenylene 44 (Scheme 4.11) [72]. Starting once more with 39, double alkynylation with 23 assembled tetrayne 42, which was further substituted with TMSA. Removal of all the silyl protecting groups provided 43, a regioisomer of 24b (Scheme 4.6). Compound 43 was then cydized to 44 in 2% yield [72]. 

Scheme 4.10. Two syntheses of zigzag [4]phenylene 38 (a) intramolecular approach,...

To summarize this section, to date 19 phenylenes have yielded to synthesis. They can be divided broadly into 5 families ( of examples) linear (3), angular/ helical (7), zigzag (2), bent (3) and branched (4). Their topologies have been accessed through 26 different routes, 15 of which involved in the crucial step an all-intramolecular cobalt-catalyzed cyclization, 9 used intermolecular variants and 2 a combination of the two strategies. 

Table 4.2. HOMO-LUMO gaps and 2max values in angular, zigzag and linear phenylenes.

Preparation oT Bis(trimethylsilyl) Zigzag [4]Phenylene A One-pot Intra-and Intermolecular [CpCo(CO)2]-Mediated Cyclization... 

Eickmeier, C., Holmes, D., Junga, H., et al. (1999) A novel phenylene topology total syntheses of zigzag [4] and [5]phenylene. Angewandte Chemie International Edition, 38(6), 800-804. 

---