Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions.

A series of alkenylboranes were prepared by 1,1-carboboration routes and used as Lewis acid components for the generation of frustrated Lewis pairs (FLPs). The reactions of 1-alkynes with B­(C6F5)3 gave the RCHC­(C6F5)­B­(C6F5)2 systems 4a (R = n-C3H7), 4b (R = n-C4H9), 4c (R = Ph), and 4d (R = t-C4H9), respectively. The alkenylborane/tBu3P FLPs derived from compounds 4a–d reacted rapidly with dihydrogen (2.5 bar) at ambient temperature. The bulky system 4d left the CC double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product [tBu3PH]­[tBuCHC­(C6F5)­BH­(C6F5)2] (10d). In contrast, the less bulky systems 4a/tBu3P and 4b/tBu3P split dihydrogen under these conditions and had their CC double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [tBu3PH]­[RCH2CH­(C6F5)­BH­(C6F5)2] 9a (R = n-C3H7) and 9b (R = n-C4H9), respectively. The 4c/tBu3P FLP gave a mixture of both product types (9c/10c). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R2CC­(C6F5)­B­(C6F5)2 4e (R = C2H5), 4f (R = n-C3H7), 4g (R = Ph), and 4h (R = p-MeC6H4), respectively. The 4e–h/tBu3P FLPs cleaved dihydrogen under mild conditions but retained their CC double bonds to give the respective [tBu3PH]­[R2CC­(C6F5)­BH­(C6F5)2] products (10e–h). Selected examples of these alkenylboranes undergo FLP reactions acting as catalysts for the hydrogenation of imines. Perhaps most remarkably, some of these alkenylboranes retain the CC double bonds under FLP/H2 reaction conditions and heterolytically split dihydrogen in the presence of the Lewis base DABCO and catalyze the hydrogenation of the electron-poor CC double bonds of diaryl-substituted enones.

Acid-catalyzed condensation of furan and pentafluorobenzaldehyde yielded conjugated macrocycles with six and eight furan rings. They represent systems similar to annulenes with 30pi and 40pi electrons. From their structural analyses, it was found that furan rings in both the molecules were inverted in an alternative fashion and displayed nontwisted conformations. [structure: see text].