O-Quinodimethanes as intermediates

Diene synthesis with unstable intermediates o-Quinodimethanes as intermediates... 

Ito and coworkers developed a mild and efficient procedure for generating o-quinodimethanes" as reactive intermediates in [4 -I- 2] cycloadditions. The key step in the sequence, illustrated here by the synthesis of Estrone methyl ether (146 Scheme 53) involves a fluoride-induced fragmentation of the o-[a-(trimethylsilyl)alkyl]benzyltrimethylammonium iodide (144) to give the o-quinodimethane (145), which underwent stereoselective intramolecular [4 + 2] cycloaddition to give the desired tetracyclic framework in (146). The entire process was conducted at room temperature. The annulation reaction is not limited to intramolecular cycloadditions intermolecular versions of the reaction proceed equally well. 

Diene synthesis with unstable intermediates—o-Quinodimethanes as inter-... 

The mechanism of these reactions is not straightforward, however, as the dienophile can act as a redox mediator in the reduction of the a,a-dibromo-l,2-dialkylbenzene to o-quinodimethane as well as tfapping the reactive intermediate [26]. 

The Parylene family has very attractive properties for use as dielectric materials as was noted above, but their thermal stability at the temperatures used in the fabrication of electronic devices is less than optimum. When considering alternatives as possible precursors for VDP, the isomeric ortho-xylylene (o-quinodimethane) is a likely candidate (Scheme 4). This approach involves the thermolysis ofbenzocyclobutene derivatives to generate a reactive dieneoid intermediate (o-quinodimethane),... 

The reaction conditions (80 °C) used for the addition of 33 and 34 to the o-quinodimethane 32 are incompatible with the presence of 1,2-bromochlorocy-clopropene (27), thus the potential of this approach was for quite a while not further exploited. However o-quinodimethanes may be synthesized under much milder conditions, and trapped as reactive intermediates with 27. Thus base-induced isomerization of cw-oct-4-ene-2,7-diyne (38) at -78 °C leads to bis-allene (39). Upon warming of 39, rearrangement to o-quinodimethane (40) occurs between -20 and -10 °C this adds smoothly to 27 and furnishes the adduct 41. Conversion of... 

An alternative polymerization mechanism and polymer architecture has been proposed by Kirchhoff [1, 2, 3], Tan and Arnold [77], By this mechanism, polybenzocyclobutenes which do not contain reactive sites of unsaturation are proposed to polymerize by the 1,4 addition of the o-quinodimethane intermediates to give a substantially linear poly(o-xylylene) structure. Since the monomers all contain at least two benzocyclobutene units the net result of this reaction will to a first approximation be a ladder type polymer as shown in Fig. 17. The formation of a true ladder polymer however would require that all... 

Formation of ring c ( disconnection b) was readily accomplished by addition of o-quinodimethane intermediates, e.g. (562) or 7,8-dibromo-o-quinodimefliane, to tetrahydronqihthoquinones such as racemic or enantiomerically pure (632) (Scheme 141). 

While the electrocyclic ring opening to o-quinodimethanes is the major reaction pathway in the irradiation of substituted benzocyclobutenes (cf. Example 6.14), the irradiation of unsubstituted benzocyclobutene yields 1,2-dihydropentalene (119) and 1,5-dihydropentalene (120) as major products. The mechanism shown with prebenzvalene (118) as primary photochemical intermediate has been proposed to explain the formation of the isomeric dihydropentalenes (Turro et al.. 1988). Supporting calculations that yield the same mechanism for the benzene-to-fulvene transformation have been published (Dreyerand Klessinger, 1995). 

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