Polycyclics, reductive isomerization

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

By proper choice of reaction conditions (metal, solvent, the order of addition of reagent and reactant), fused polycyclic aromatics can be converted to different partially reduced derivatives with high selectivity. When the red complex of naphthalene, for example, formed in the Na-NH3 solution, is quenched with aqueous ammonium chloride solution, 1,4-dihydronaphthalene is formed209 [Eq. (11.58)]. Reaction in the presence of an alcohol yields 1,4,5,8-tetrahydronaphthalene210 [Eq. (11.59)]. The Benkeser reduction affords further saturation to the isomeric octalins211 [Eq. (11.60)] or even to decalin212 [Eq. (11.61)] ... 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

If the correct stereoelectronic relationship for ring opening is difficult to achieve under the reaction conditions applied, then the cyclopropane moiety will be retained even when a radical is formed a to the ring. This is the case when the cyclopropane ring is incorporated in a strained, polycyclic compound. Thus, l,5,6,7-tetrachloro-8,8-dimethoxy-3,3-dimethyl-en /o-tricyclo[3.2.1.0 ]oct-6-ene (5) reacted with sodium in the presence of er/-butyl alcohol to give 8,8-dimethoxy-3,3-dimethyl-enafo-tricyclo[3.2.1.0 ]oct-6-ene (6) in 75% yield.Similarly, l,l-dimethyl-4,5,6,6-tetrachlorospiro[2.3]hex-4-ene underwent electrolytic reduction in moist dimethylformamide to give a 1 1 isomeric mixture of l,l-dimethyl-4,5,6-trichloro-spiro[2.3]hex-4-ene. ... 

The stereochemical implications of the reductive opening of the cyclopropane ring in polycycles has been studied. The three-membered ring in the isomeric 3a,5-cyclo-5a- (24) and 3, 5-cyclo-5)S-cholestan-6-one (25) was opened differently. In the latter case, the cyclopropylmethylene group became a methyl group. The reduction of 4,5-methano-3-oxosteroids was shown to be stereospecific with respect to the orientation of the methyl group formed. ... 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Sodium borohydride in excess of triflic acid at low temperature provides" a highly efficient reductive superacid system, which was found effective for the reductive superacid isomerization of unsaturated polycyclic precursers to cage hydrocarbons in high yields (equation 25). The key to the success of the ionic hydrogenation system appears to be due to the in situ formation of boron tris(triflate), a highly superacidic Lewis acid. 

The cationic cyclization of polyisoprene with acid catalysts is well documented. The same reaction in polybutadienes requires much more severe conditions, higher temperatures and more acidic catalysts, and until recently has received much less attention. A cyclized polymer with a reduction of 35—40% of the initial unsaturation, can be prepared by treating cis-l,4-polybutadiene with an alkyl aluminium chloride-organic halide catalyst in xylene solution at >100 C."- Such polymers, containing polycyclic sequences apparently at random within the chains, have better skid resistance and tensile properties than the parent polymer. Cyclization has been reported to accompany other reactions in polydienes, for example the radiation-induced addition of carbon tetrachloride to 1,2-polybutadiene, and the direct addition of a o j unsaturated carboxylic acids (acrylic and cinnamic) to polydienes and polypentenamers. It is reported that the thermal isomerization of cis-transoidal poly(phenylacetylene) is accompanied by cyclization, and additionally chain scission and aromatization at temperatures >120°C. ... 

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