Bis-o-quinodimethanes

Bis-o-quinodimethanes have also been used to functionalize [60]-fullerene by Diels Alder reaction. An example is the preparation of main-chain polymers with incorporated [60]-fullerene units [48] illustrated in Scheme 2.20. Cycloaddition of bis-diene 50 generated in situ from bis-sulfone 49 with [60]-fullerene leads to an oligomer mixture 51. Another type of functionalization is based on the... 

Copolymers of [60] fullerene and in situ generated bis-o-quinodimethanes were prepared by Gtigel and colleagues114. In order to get soluble polymers, it proved necessary to introduce flexible groups on the bis-o-quinodimethanes. A maximum of 10 [60]fullerene units were incorporated into oligomers when [60]fullerene was reacted with a 7 3 mixture of 157 and 158 (i.e. with o-quinodimethanes 159 and 160). Monosulfone 158 was added to induce the formation of triple cycloadducts of [60]fullerene. This prevented polymerization of the oligomer (quadruple cycloadditions to [60]fullerene are hard to accomplish) and enhanced its solubility. 

Other tethers have been employed in the search for regioselectivity in multiple additions to fullerenes. Some of these methodologies include the use of bis(o-quinodimethanes) connected by a, oo-dioxamethylene tethers [40], tethered nucleophilic vinylcarbenes [41] or azides [42],... 

Nishimura and co-workers used bis(o-quinodimethanes) connected by a.n+dioxa-methylene tethers for the regioselective bis-functionalization by double Diels-Alder addition (Scheme 7-14) [93]. The tethered bis(o-quinodimethanes) were intermediately formed by heating the bis(bromomethyl) derivatives 72a-d together with C, in toluene in the presence of KI and [18]crown-6 [90]. With an 0-(CH2)2-0 tether (72a), the cis-2 and cw-3 bis-adducts 73a and ( )-74a were isolated in 10% and 8% yield, respec-... 

Scheme 7-14 Regioselective bis-functionalizations by double Diels-Alder addition of tethered bis(o-quinodimethanes) to Cgo [93, 94].

Thieno-o-quinodimethanes 46 and 48, generated in situ by iodide-induced 1,4-elimination from the respective 2,3-bis(chloromethyl)thiophene 45 and 2,3-bis(bromomethyl)benzo[b]thiophene 47 precursors, undergo Diels Alder... 

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... 

These cycloadditions with o-quinodimethanes provide a broad variety of useful fullerene functionalizations, since o-quinodimethanes can be prepared using several routes and the resulting cycloadducts are thermally stable [42], There exist several alternatives to the iodide-induced bromine 1,4-elimination of 1,2-bis (bromomethyl)-benzenes [44-47]. o-Quinodimethanes have been prepared by thermolysis of 3-isochromanone (42) [43], benzocyclobutenes (43) [48-50], isobenzothiophene 2,2-dioxides (44) [42] and sultines [51,52] or by photolysis of o-alkylphenones such as 45 [53-55] and could be added to Cjq in good yields (Scheme 4.7). Indene, thermally rearranged to isoindene, also adds to Cjq in similar fashion to quinodimethanes [56]. 

In another series of related experiments Errede and coworkers prepared o-quinodimethane itself by the flash pyrolysis of o-methylbenzyltrimethylam-monium hydroxide 23 [72, 73], The conditions of this experiment were such that the o-quinodimethane was quenched soon after it was formed by cooling to - 78 °C. The product trapped out under these conditions was an approximately 25 75 mixture of 1,2,5,6-dibenzocyclooctadiene 20 and the spiro o-quinodimethane dimer 24 (Fig. 14). Dimer 24 can readily be seen to be the result of the Diels-Alder reaction of one o-quinodimethane bis-exo-methylene diene unit across one of the exo-methylene groups of another o-quinodimethane. The spirodimer... 

Very interesting Diels Alder comonomers for BMI are the bis(benzocyclobutenes). Under appropriate thermal conditions, the strained four-membered ring of benzocyclobutene undergoes electrocylic ring opening to generate, in situ, o-quinodimethane, which, in the presence of BMI, reacts via a Diels-Alder reaction (83). The chemical structure of a bis(benzocyclobutene-imide) is provided in Fig. 28. The synthesis and properties of BCB and BMI/BCB blend systems is described in detail in chapter I of this book. 

In a recent re-examination of the thermolysis of benzocyclobutenes for the in situ generation of o-quinodimethanes, the resultant IMDA diastereoselectivity was highly dependent on the nature of the hydroxyl protective group.90 The intramolecular 4 + 2-cycloaddition of o-quinodimethanes (83), derived from ene-bis(sulfinylallenes) (82), with electron-deficient and electron-rich alkenes produced the corresponding polycyclic aromatic compounds (84) (Scheme 22).91 The enantioselective Diels-Alder... 

In the case of the hexaadduct where the symmetry is increased to 7), or D3 compared to C2v for the corresponding monoadduct, the emission band is blue-shifted to 660-690 nm depending on the solvent used [67,111,112], The fluorescence spectra of o-quinodimethane bis-adducts also strongly depend on the addition pattern. The spectra for three different regioisomers (cis-2, cis-3, e) are all remarkably different from one another, as in the case of the absorption spectra. 

Triplet absorption spectra of o-quinodimethane bis-adducts exhibit some differences to the corresponding monoadduct. The absorption band positions are only less affected, whereas the lifetime strongly differs between the regioisomers. Also, the initial absorbance differs noticeably, which may be associated with the quantum yield of intersystem crossing from Si to Tx [110],... 

When A.A-dimethylisoindolinium bromide is treated with phenyl-lithium, it gives V-methylisoindole via the ylid (54).2 59,60 An attempt to prepare benzo[c]thiophene via the analogous ylid (55) failed. Thus, when l,3-dihydrobenzo[c]thiophene methylsulfonium iodide was treated with phenyllithium, it gave a mixture of methyl phenyl sulfide, spiro[5.5]-l-methylthio-2,3-benzo-6-methylthio-methyleneundeca-7,9-diene (56), and 3,4-bis(methylthio)-l,2 5,6-dibenzo-l,5-cyclooctadiene (57).59,60 The formation of methyl phenyl sulfide may be explained by the formation and ring cleavage of compound 58, and compounds 56 and 57 arise by Diels-Alder dimerization of the o-quinodimethane (59) formed by ring cleavage of the ylid (55). 

An interesting templated bis-functionalization was reported by Shinkai and co-workers who introduced two boronic acid groups regio- and diastereo-selectively into C6o by using saccharides or saccharide derivatives as imprinting templates.147-149 Thus, when D-threitol was used as the template, C2-symmetric cis-3 isomers fC-49 and fA-49,150 were obtained in a nearly. 72 28 ratio after removal of the saccharide templates (Scheme 1.6).149 Competitive complexation studies indicated that D-threitol-imprinted boronic acid (fC)-49 and L-threitol-imprinted (A)-49 preferentially rebind their original templates with up to 48% d.e.148 Exploration of other regio- and diastere-oselective double [4 + 2] cycloadditions between C6o and o-quinodimethanes... 

A relatively new route to benzocyclobutenes, without the involvement of o-quinodimethanes, is the cobalt-catalyzed co-oligomerization of bis(trimethylsilyl)acetylene with 1,5-hexadiynes, (580) + (581) - (582) (Scheme 133). Despite its elegance, and the potential to modify the silyl groups, this method suffers from limitations in providing benzocyclobutenes with well-defined substitution patterns. In this... 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

Olefinic pyrazines like 108 were shown to react with QH in the superacid TfOH to give anti-Markovnikov addition products like 109 <050L2505>. The orientation observed is presumed to be due to the multiply charged heterocycle adjacent to the olefin. Pyrazine o-quinodimethanes underwent Diels-Alder condensation with meso-tetraarylporphyrins to give new jt-extended porphyrins <05TL2189>. A one-pot formation of polycyclic - and -lactones like 111 was developed using the reaction of pyridine and pyrazine (110) with bis(trimethylsilyl)ketene acetals. Many of them were characterized by X-ray <05EJO3724>. 

Figure 3.60. Sketch of TP excited excimer laser photolyses of 1,2 bis-[(phenylthio)-methylbenzene] (147) in acetonitrile resulting in generation of o-quinodimethane, which...

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