Rearrangement skeletal

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

Little essential differences can be found between the intermediate structures suggested by these hypotheses. The dispute concerns only the extent to which it can be regarded as a cyclopropane species. We can agree with 

Anderson that at this level the mechanistic details are a matter of opinion (7). There is, however, a difference as far as the number of surface atoms participating in the reaction is concerned. Mechanism A requires more than one Mechanism C, however, requires only one metal atom. Van Schaik et al. 89) reported skeletal isomerization according to Mechanism A over platinum-rich platinum-gold alloys, whereas over gold-rich catalysts, isolated platinum atoms could promote Mechanism C only. Garin and Gault (82) assumed the formation of a C4 cyclic intermediate with the insertion of a platinum atom as the fourth member of the ring. This concept of Mechanism B would also involve one metal atom. 

Pines and Csicsery (90, 90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over non-acidic chromia-alumina. A similar, 1,3-methyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (90b). Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 

This pathway is restricted to four metals platinum, palladium, iridium, and rhodium (Section II,A,3). In addition to cyclization, it should also involve the opening of the C5 cycle (see Section III,C,1). 

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Acetoxylchlorination of norbornene (138) proceeds with skeletal rearrangement in the presence of an excess of CuCl2 to give e.Yo-2-chloro-iyn-7-acetoxy-norbornane (139). This is a good synthetic method for ivn-7-norbornenol[163]. 

The PdCli-catalyzed instantaneous rearrangement of A -carbethoxy-S-azabi-cyclo[5.1.0]oct-3-ene (60) takes place at room temperature to give A -car-bethoxy-8-azabicyclo[3.2.1]oct-2-ene (61)[50], The azepine 62 undergoes a smooth skeletal rearrangement to give 63, and the diazepine 64 is converted into the open-chain product[51]. Beckmann fission of the oxime 65 of ketones and aldehydes to give the nitrile 66 is induced by a Pd(0) complex and oxygen [52,53]. 

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]. 

Sesquiterpenes are formed by the head-to-tad arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

A book (B-71MS) and a review by Nishiwaki (74H(2)473) contain much information about the behaviour of pyrazoles under electron impact. The Nishiwaki review covers mainly the hydrogen scramblings and the skeletal rearrangements which occur. One of the first conclusions reached was that pyrazoles, due to their aromatic character, are extremely stable under electron impact (67ZOR1540). In the dissociative ionization of pyrazole itself, the molecular ion contributes about 45% to the total ion current thus, the molecular ion is the most intense ion in the spectrum. 

Reforming is the conversion primarily of naphthenes and alkanes to aromatics, but other reactions also occur under commercial conditions. Platinum or platinum/rhenium are the hydrogenation/ dehydrogenation component of the catalyst and alumina is the acid component responsible for skeletal rearrangements. 

Similar preference in replacement by fluorine of tertiary versus secondary and secondary versus primary hydrogens is observed in the fluorination of alkanes with chlorine trifluoride in 1,2-difluorotetrachloroethane at room temperature (Table 3). Skeletal rearrangements accompany the fluorination [31]... 

Skeletal rearrangements in volving a ring size change are observed in reactions of DAST with some cyclic alcohols [131, 134, 755] (equations 69 and 70). 

Trimethylacetaldehyde reacts with sulfur tetrafluoride with a skeletal rearrangement 2,3-difluoro-2-methylbutane is formed in high yield as the only fluo-roalkane along with bis(l-fluoro-2,2-dimethylpropyl) ether [169] (equation 85)... 

Hexafluoroacetone azine accepts nucleophiles (ROH, RSH, R NH) in positions 1 and 2 to yield hydrazones [27] Phosphites give open-chain products via a skeletal rearrangement [22] Radical addition reactions are also reported [22] Treatment of tnfluoropyruvates with tosylhydrazine and phosphorus oxychlo-ride-pyndme yields tnfluoromethyl-substituted diazo compounds [24] (equation 3)... 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Mercury(II) trifluoroacetate is a good electrophile that is highly reactive toward carbon-carbon double bonds [52, 53, 54] When reacting with olefins in nucleophilic solvents, it usually gives exclusively mercurated solvoadducts, but never products of skeletal rearrangement Solvomercuration-demercuratton of alkenes with mercury(II) trifluoroacetate is a remarkably effective procedure for the preparation of esters and alcohols with Markovnikov s regiochemistry [52, 5J] (equation 24)... 

Triflic acid is strong enough to protonate polycyclic saturated hydrocarbons [77, 78, 79], and even -butane [80, 81], and to initiate skeletal rearrangements Acidic treatment of homoadamantane [77] (equation 32), 2-homoprotoadamantane [78] (equation 33), or as 2,3-trimethylenebicyclo[3 3 Ojoctane [79] (equation 34) causes their rearrangement to isomenc hydrocarbons... 

Other methods resulted in skeletal rearrangement. This study also showed that the rate of acid-catalyzed MOM cleavage increases in the following... 

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

Reduction with sodium iiaphthalide followed by treatment with Mel gave 127 and 128 in an excellent combined yield. Exposure of 73 to AgBp4-EtsSiH resulted in a novel skeletal rearrangement leading to compound 129 (90JA3029). 

Skeletal rearrangements of carbenium ion species 2, that involve nucleophilic 1,2-migrations of alkyl groups, are called Wagner-Meerwein rearrangements... 

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

Pedersen and coworkers10 studied the El mass spectra of several alkyl 2-hydroxyphenyl sulfoxides (10) and found that, contrary to methyl phenyl sulfoxide2,11 and the corresponding sulfones10, they do not show any abundant skeletal rearrangement ions (see Section III). This is obviously due to an ortho effect as shown in structure 10. 

Both benzothieno[3,2-b]pyridine 5-oxide (31) and thieno[3,2-b 4,5-b ]dipyridine 5-oxide (32) exhibit competitive loss of oxygen either as an atom or as carbon monoxide after initial skeletal rearrangement, e.g. to sulfenate esters (equation 10)18b. These results together with some data for Y-oxides indicate that the presence of an intense [M — 16] + peak is not diagnostic for the latter only. 

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