5-methylene-1,4-cyclohexadiene

When heated, benzocyclobutene reacts as a bis-methylene cyclohexadiene and polyaddition with maleimide has been shown [66] (Fig. 16). 

This observation also supports the suggestion that the exo-methylene cyclohexadiene is involved in the rearrangement of the benzyltrimethyl-ammonium ion. 

CaHaCt, Dimoiea 1.3-DimethyI-l-di<>blar< Di l-4-methylen-cyclohexadien-(2,5)... 

As the argument has been presented above, this rationalization does not appear to be impossible. However, Figure 3 shows a tabulation of all of the model compounds synthesized to date in an attempt to model the behavior of AH. Although the second and third compounds in the list are reported to initiate polymerization, MCH does not. Furthermore, as shown in Figure 3, a compound that would appear to give an even more hindered ene reaction transition state than AH also does not initiate the polymerization of styrene (private communication from D. Aue). This is hard to reconcile with a rationalization of the lack of MAH reaction by methylene-cyclohexadiene based on the hindrance of its ene reaction. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

THBA from butadiene 2,5- (7( -methylene from 1,3-cyclopentadiene 4-methyl-THBA from 2-methylbutadiene 2,5- (7( -ethylene from 1,3-cyclohexadiene. 

Ai- 4-(bis[4-(phenylamino)phenyl]methylene)-2,5-cyclohexadien-l-ykdene -3-methyl-ben2eneaminesulfate [57877-94-8] (20) have been claimed as positive CCAs (65). The absorption spectra of the triaryknethane dyes can be extended into the near-infrared region. The use of triaryknethane dyes as infrared absorbers for optical information recording media (66) and as infrared color formers in carbonless copy paper has been claimed. 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

The next intermediate, 5-diazo-6-methylene-l,3-cyclohexadiene (6.75) was postulated by Trondlin et al. (1978) because unsolvated ( naked ) acetate ions in benzene are strong proton acceptors. Experimental evidence for its role in this mechanism was given by these authors in two ways. 

Clear evidence in favor of 6.75 being an intermediate came, however, from stereochemistry. If the indazole cyclization takes place at a chiral carbon atom in the exposition of the alkyl group in 6.78, the stereochemistry of the 3-i/-indazole 6.79 can indicate whether the 5-diazo-6-methylene-l,3-cyclohexadiene 6.75 is an intermediate or whether, on the other hand, deprotonation and cyclization are synchronous. In the first case a racemic indazole 6.79 is expected. In the case of a synchronous reaction, however, a stereospecific product, probably with retention of the chirality at Ca, should be observed. 

Briliant Blue FCF (E 133, FD C Blue No. 1, Cl Food Blue 2) is a triarylmethane dye disodium 3-[N-ethyl-N-[4-[[4-[N-ethyl-N-(3-sulfonatobenzyl)-amino] phenyl] (2-sulfonatophenyl)methylene]-2,5-cyclohexadiene-l-ylidene]ammoniome-thyl] benzenesulfonate (or disodium (4-(N-ethyl-3-sulfonato-benzylamino) phenyl)-a-(4-N-ethyl-3-sulfonatobenzylamino)cyclohexa-2,5-dienylidene)-toluene-2-sul-... 

Green S (E 142, Cl Food Green 4, Brilliant Green BS) is a triarylmethane dye, with the chemical name sodinm N-[4-[[4-(dimethylamino)phenyl](2-hydroxy-3,6-disnlfo-l-naphthalenyl)-methylene]-2,5-cyclohexadien-l-ylidene]-N-methylmetha-naminium. The calcium and potassinm salts are also permitted. Green S is a dark bine or green powder or grannies, solnble in water, slightly solnble in ethanol, with a maximum absorption in water at 632 nm = 1720). It is not permitted as a... 

Da Silva, G. Chen, C.-C. Bozzelli, J. W. Quantum chemical study of the thermal decomposition of o-quinone methide (6-methylene-2,4-cyclohexadien-l-one). J. Phys. Chem. A 2007, 111, 7987-7994. 

Rabin, O. Vigalok, A. Milstein, D. A novel approach towards intermolecular stabilization of para-quinone methides. First complexation of the elusive, simplest quinone methide, 4-methylene-2,5-cyclohexadien-l-one. Chem. Eur. J. 2000, 6, 454-462. 

For the parent 6-methylene-1,4-cyclohexadiene a planar structure was found149 the d -dimethyl derivative, however, gave a dihedral angle of 8°152, which should diminish slightly the cross-conjugation. 

Intermolecular addition of activated methylenes to unsaturated systems has been investigated with silver,36 silver/ gold, and palladium catalysts. Thus, C-H addition of 2,4-pentandione to 1,3-cyclohexadiene occurs in THF at 0°C with 5mol% of palladium(ll) catalyst without base. Josiphos ligand 20 is used as a chirality source to induce... 

Coumassie R250 N-[4-[[4-[(4-ethoxyphenyl)amino]phenyl][4-[ethyl[(3-sulphophenyl) methyl] amino]phenyl]methylene] -2,5-cyclohexadien-1 -y lidene] -N-ethyl-3-sulphobenzenemethan-aminium monosodium salt... 

Crystal Violet, Gentian Violet N-[4-[Bis[4-dimethylamino)-phenyl]methylene]-2,5-cyclohexadien-1 -y lidine] -N-methyl-methan-aminium chloride... 

Cycloalkenone-2-carboxylates tautomerize to conjugated dienols in the presence of either acids or bases. Iron(III) catalysts have also been found to promote enone-dienol equilibration, and, at room temperature, dimerization64. Thus, treating 87 with 1 mol% iron(III) chloride hexahydrate in methylene chloride at room temperature affords 88 in 81% yield (equation 46). The cyclohexadiene-cyclohexanone is in a rapid equilibrium with its triendiol tautomer, 89 (equation 47). 

The usefulness of 1,3-cyclohexadiene complexes is enhanced by their conversion to stable cationic complexes. The if -cationic complex 102 is prepared as a stable salt by the hydride abstraction from the neutral complex 66 via 101, and its highly regio- and stereoselective reaction with nucleophiles is used for synthetic purposes. Complex 102 reacts with nucleophiles such as amines, active methylenes, alkyl copper or alkoxides at C(l) or C(5) from the uncomplexed exo side. In other words, the nucleophilic attack occurs regioselectively at a dienyl terminus, and stereoselectively anti to Fe(CO)3 to give 103. Hydride abstraction from 103 affords 104, which reacts with a nucleophile to form 105. Decomplexation of 105 produces the 5,6-disubstituted-l,3-cyclohexadiene 106. 

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