Skeletal rearrangements, catalytic

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Hydrosilation of alkylcyclohexenes illustrates the ability of active catalytic complexes to isomerize olefins and the tendency chlorosilanes have to form primary alkyl adducts even if this requires a skeletal rearrange-... 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

In 1974, the same workers 134) reported an unusual skeletal rearrangement of miyaconitine. Treatment of miyaconitine (140) with 1 N methanolic potassium hydroxide for 1 hr yielded miyaconine (145). The latter on acetylation with acetyl chloride afforded diacetylmiyaconine (146). Reaction of miyaconine with 1 N methanolic potassium hydroxide under reflux for 2 hr yielded apomiyaconine (147). Acetylation of the latter with acetic anhydride in the presence of a catalytic amount of />-toluensulfonic acid on a steam... 

Matsuda and Sugishita found that cyclooctatetraene monoepoxide (233 equation 99) gave only skeletally rearranged products, e.g. (234), when treated with Grignard reagents. In an effort to isolate the presumed intermediate cycloheptatrienecarbaldehyde, (233) was subjected to a catalytic amount of MgBra, but this resulted in the formation of phenylacetaldehyde. 

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

Rearrangements of strained skeletal rearrangements of highly strained orbital symmetry restrictions.4 Thus treatment of homo-cubane(l) in dilute CDC13 or acetone-4 solution with catalytic amounts of silver fluoroborate results in quantitative conversion into (2), pentacyclo[4.3.0.02-4. (F-MF- nonane, within 1 day at 25°. In the absence of silver ion, (1) is stable to 2400.5... 

In isomerization reactions only platinum, palladium and iridium are active metals for the skeletal rearrangement of alkanes. Hence, in the first part of this chapter we shall mainly focus on the catalytic behavior of these three metals. 

Of Morphinandienols. The natural base laurifonine has been synthesized by a route that may parallel the biogenetic pathway for the formation of this and related alkaloids. O-Methylflavinantine was reduced with sodium borohydride to supply a separable mixture of epimeric dienols. Skeletal rearrangement of the mixture to the neospirene 13 was effected by boron trifluoride-etherate, and subsequent catalytic reduction produced laurifonine in excellent yield ... 

Other Catalytic Systems Causing Skeletal Rearrangement of Epimeric Aldoses... 

Taking into account the catalytic results during olefin oxidation, it is observed that methacrolein is again mainly obtained over MoV-based ° or VPO catalysts, " while MA is also observed with the latter catalyst. In this way, it has been suggested that a skeletal rearrangement of a carbocation or radical intermediate could occur due to the presence of strong Br0nsted acid sites on the VPO catalyst surface, which could explain the formation of maleic anhydride. 

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