Hopf cyclization

Scheme 6.40 Formation of 162 by electrocyclization of (Z)-hexa-l, 3-dien-5-yne (Hopf cyclization), interception of 162 by styrene and rearrangement of 162 to benzene.

Scheme 6.44 Hopf cyclizations of cyclic hexa-l,3-dien-5-ynes to give eventually benzocycloalkenes.

Scheme 6.45 Hopf cyclization of aromatic hydrocarbons having a (Z)-4-phenylbut-l-en-3-ynyl side-chain.

O Connor and co-workers have shown that the Hopf cyclization can be carried out efficiently under very mild conditions (23 °C) when dienyne 45 is treated with ruthenium complex 24 in THF-Jg or CDCls. ... 

Aza-Hopf cyclization of azaenediynes. The classic Hopf cyclization of hexa-l,3-diene-5-yne, under thermal conditions [422], requires high temperatures (>274°C) and proceeds via diradical 3.961 followed by the H-shift to give the product—benzene (Scheme 3.144). Replacement of the terminal ethylene carbon atom by nitrogen can sharply reduce the barrier to cyclization. Thus, the 9- and 10-membered azaenediynes. 

Mandal, S. and Basak, A. (2009) Aza Hopf cyclization synthesis and reactivity of cyclic azadieneynes. Tetrahedron Letters, 50(26), 3641-3644. 

In fact, the formation of benzene from hexa-l,3-diene-5-yne is possible by at least three different mechanistic pathways (i) at temperatures below 500 - 600°C preferably by an electrocyclization via the isobenzene followed by a 1,3-hydrogen shift, (ii) at temperatures above 500 - 600 °C preferably by the discussed carbene mechanism, and (iii) in the presence of hydrogen radicals by an intramolecular radical cyclization sequence. For further discussion including labelling studies see Nuchter U, Zimmermann G, Francke V, Hopf H (1997) Liebigs Ann/Recueil 1505 and the references cited herein. 

In addition, [2.2]paracyclophane was transformed into reactive bis-dienes. By reaction of the furan cycloadducts 49 with tetraphenylcyclopentadienone and thermal retro-Diels-Alder reaction [2.2](4,7)isobenzofuranophane 51 was obtained [38]. The highly reactive molecule was trapped in situ with para-ben-zoquinone. The more stable tetraphenylisobenzofuranophane 57 was synthesized via the classical procedure to [2.2]paracyclophanes of Hopf [39] the reaction of 1,2,4,5-hexatetraene 53 with dibenzoylacetylene 54 gave 4,5,12,13-tetra-benzoyl[2.2]paracyclophane (56), that was reduced and cyclized to the target molecule 57 [38]. 

Indeed, Hopf observed that bisallene gave 3,4-dimethylenecyclobutene rapidly at 200°C. That the reaction is a 3,3-shift followed by a cyclization also follows from deuterium labeling studies which indicate conversion of l,6-dideuterio-l,5-hexadiene to l,2-dideuterio-3,3-dimethylenecyclobutene. ... 

Under thermolysis conditions, cyclization of l,3-hexadiene-5-ynes occurs to give substituted arenes in a similar fashion to the Bergman cyclization. This variation on the Bergman cyclization is known as the Hopf... 

Eshdat, L., Berger, H., Hopf, H. and Rabinovitz, M. (2002) Anionic cyclization of a cross-conjugated enediyne. Journal of the American Chemical Society, 124(15), 3822-3823. 

Lu, Y.-X., Zou, J.-W., Wang, H.-Q. and Yu, Q.-S. (2005) Myers-Saito and Schmittel cyclizations of enyne-(hetero)-l,2,3-trienes a DFT study on the structure-reactivity relationship.of Molecular Structure THEOCHEM., 732(1-3), 233-238. Hopf, H. and Kruger, A. (2001) Synthesis of cyclo-1, 3-dien-5-ynes. Chemistry—A European Journal, 7(20), 4378-4386. 

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