Norbomadiene rearrangement

Experimentally, this reaction sequence was observed with hydrogen 70) or a substituted octenyl group721 in position 7 of the norbomadiene. In the calculations a methyl group was used as marker, to show the course of the rearrangement. 

The DMS method has not been employed yet for the generation of 117 and 123, since the dibromocarbene adducts of norbomadiene and norbornene rearrange under the usual conditions for the preparation [89]. However, they could be synthesized at -60 °C by taking advantage of tetrabromomethane and methyllithium as a source of the carbene [90] and could prove stable enough to serve as precursors of 117 and 123. On the other hand, the adducts of bromofluorocarbene to norborna-diene and norbornene having the fluorine atom in a cis-orientation should be isol-able at room temperature and hence be usable as stable precursors of 117 and 123. These variations ofthe DMS method were published on the occasion ofthe preparation of cycloadducts of l-oxa-2,3-cyclohexadiene (351) (Section 6.3.6) [35, 91], 1,2,4-cyclohexatriene (162) and 3d2-lJ-f-naphthalene (221) (Section 6.3.4) [35, 92],... 

Olefinic compounds such as a-pinene (309) and /Dpi none (312) undergo hydrogen abstraction followed by rearrangement and amino-oxyl insertion (310, 313) and addition reactions (311, 314, 315), as shown in reactions 43 and 44627. Other olefinic compounds such as norbomadiene, cyclo-octene and cyclo-octa-1,5-diene gave analogous results628. 

Wagner-Meerwein rearrangements occur also when arylsulfenyl chlorides or the mixtures R2NCI + SO3 (R = piperidino, morpholino) add to norbomadiene 393196,197. An addition of 2-nitro- (NBSC) or 2,4-dinitrobenzenesulfenyl chloride to the polyenes 395-397 in AcOH and under doping conditions (AcOH, with LiClO/f) is accompanied by Wagner-Meerwein rearrangements (equations 145-147)198. Thus, new types of struc-... 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

There is an important class of rearrangements of strained cyclic a-bonded systems to give less strained ir-bonded qrstems which occur under the influence of transition metal catalysts although the uncatalysed proce is Woodward-Hoffman forbidden and slow. Examples are the conversion of cubanes XXII and bis-homocubanes XXIll to syn-tricyclooctadienes XXIV and related species XXV and of quadricyclene (XXVI) to norbomadiene (XXVII) [Ag, however, converted cubane and related species to the previously unrecognised species cuneane (XXVIII) and its relatives as do some electrophiles with incompletely filled d-subshells ... 

In a study of the addition of nitrosyl chloride or nitrosyl bromide to norbor-nene and norbomadiene, it was observed that (a) there was no structural rearrangement during the reaction, (b) a cis addition had taken place, (c) nucleophilic solvents such as ethanol or acetic acid were not incorporated in the products. These facts seem to speak against an ionic addition mechanism, while a free radical initiated by NO radicals was considered unlikely since nitric oxide is inactive toward norbomadiene. Therefore a four-center mechanism has been suggested [70], However, when a relatively simple, unstrained olefin such as A9-octalin was subjected to the reaction, only blue, crystalline, monomeric 9-nitroso-10-chlorodecalin was produced. This product had a trans configuration. Thus it is evident that the structure of the olefin has a significant bearing on the steric course of the addition [71]. 

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

The process is induced photochemically and involves the single-electron transfer oxidation of cubane then completed with a backward electron transfer to the transient radical cations. A Li+ salt with a weakly coordinating anion is able to induce pericyclic transformations, including the rearrangement of cubane to cuneane, quadricyclane to norbomadiene, and basketene to Nenitzescu s hydrocarbon 392... 

Cyanogen azide reacts with olefins at 0-35° to afford alkyl-idene cyanamides and/or N-cyanoaziridines arising from decomposition of an intermediate unstable triazoline. 54 With norbomadiene the isolated product is N-cyano-3-azatricyclo-[3.2.1.0s-4-M<,]oct-6-ene (58) which is unstable and rearranges into N-cyano-2-azabicycIo[3.2.1]octa-3,6-diene (59).167... 

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbomadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

This was followed by a long series of studies in which the rearrangement of other strained organic ring systems was catalyzed by transition metal salts or complexes. A typical example is the conversion of quadricyclene to norbomadiene, catalyzed by Rh(I) or Pd(II) (equation 4), for which Halpem provided the accepted mechanism. An extensive review of this early work is available. 

The intermediate halonium ion may undergo well-established carbocationic rearrangements, for instance in the halofluorination of norbomadiene [276] (Figure 3.63). 

In a similar way, the 1,3-diradicals postulated in the di-ir-methane rearrangement of several benzonor-bomadienes and related species have been produced via denitrogenation of the appropriate azoalkanes (27)-(30). The products obtained in the thermal and photochemical denitrogenation of (27), for example, are the norbomadiene and the tricyclic di- ir-methane product (occasionally other products not derived from denitrogenation have been observed). The latter hydrocarbon, which derives from direct collapse of the 1,3-diradical, is formed almost exclusively (equation 37). 

Remote double bonds often participate in epoxide openings, sometimes with extensive skeletal changes. ejco-Norbomadiene oxide (15 equation 6) provides an interesting example. Meinwald et alP found that this very acid-sensitive material rearranged simply on standing to give the endo-aldehyde (17), presumably via the intermediate nortricyclanol ion (16). 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

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