Other Carbon Rearrangements

Other Carbon Rearrangements.—The ring opening of [PtCl(l- H-cyclopropyl)-(PMe2Ph)2] on treatment with AgNOs-KPFe gave the -allyl complex [Pt(7 -CHgCDCHajCPMcaPhjalPFe quite stereospecifically without any H-transfer. 

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Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

One of the benefits of catalytic cracking is that the primary and secondary ions tend to rearrange to form a tertiary ion (a carbon with three other carbon bonds attached). As will be discussed later, the increased stability of tertiary ions accounts for the high degree of branching associated with cat cracking. 

Double bonds having oxygen and halogen substituents are susceptible to epoxi-dation, and the reactive epoxides that are generated serve as intermediates in some useful synthetic transformations in which the substituent migrates to the other carbon of the original double bond. Vinyl chlorides furnish haloepoxides that can rearrange to a-haloketones. 

Different rearrangements were observed in other cases. Thus, Maas22 reported that when photolyzed in benzene the polysilyldiazoketone 180 gave the isomeric ketene 181, the product of a Wolff rearrangement (a 1,2 carbon-to-carbon rearrangement) of the initially formed carbene 182 (Eq. 57). The isomeric bis-silylketene 183 was not observed, but the siloxa-tene 184 was also a product of the reaction. 

It was further reported that olefins such as unbranched hexenes (24, 30) undergo only double bond shift without skeletal rearrangement over alumina. On the other hand, rearrangement of the carbon skeleton has been observed in the interconversion of cyclohexene to methylcyclopentenes (14, 15). 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

In diamond each carbon atom is bonded covalently to four other carbon atoms arranged tetrahedrally and since each bond is common to two carbon atoms, there are twice as many bonds as atoms. Substituting this value for S in equation 11.3 and rearranging, we obtain... 

An iron tetracarbonyl complex (295) ° and a platinum bis(triphenylphosphine) complex of thiete 1,1-dioxide have been prepared. Platinum complexes of 3-phenyl- and 3-(p-bromophenyl) thiete 1,1-dioxide also have been prepared. No complex was obtained with the 3-t-butyl derivative. The pale-yellow, crystalline iron complex decomposes in refluxing hexane in the presence of excess sulfone to Fe2S2(CO)9, indicating a drastic structural rearrangement. Other carbon-containing fragments were not observed. The bis(triphenylarsine)platinum complex of 3-02-bromophenyl) thiete sulfone is decomposed photochemically to the thiete sulfone. The same result is achieved on treatment of the complex with tetra-cyanoethylene. ... 

Nucleophilic displacement of the chlorine atom of 3-chloro-1,2-benzisothiazole has proved to be a popular procedure. Boeshagen and Geiger34 have continued their earlier work on nitrogen nucleophiles, and now include carbon, oxygen, and sulfur nucleophiles.35 In some cases, rearrangements occur, as in the formation of 3-amino-2-acylbenzo[6]thiophenes (20) from reaction of 21 with methyl ketones. Similar results are obtained from the reaction of other carbon nucleophiles, and it has been suggested that attack may be either at the 3-carbon or the sulfur atom.36 The reaction of 3-chloro-1,2-benzisothiazole (8) with the anion of ethyl cyanoacetate, for example,... 

Allyl chloride reacts without rearrangement. Vinyl chloride does not react. The various polyhaloalkanes behave differently from each other carbon tetrachloride is almost as reactive as alkyl halides, chloroform requires heating under reflux, methylene dichloride reacts only on prolonged heating at 100° diphosphonylation is not observed even with 1,4-dichloroalkanes.190,191... 

Since the Ireland-Claisen rearrangement typically begins with deprotonation of an allyhc ester, the scope of the reaction is potentially Hmited by the presence of other acidic protons in the molecule. Several examples of selective deprotonation of esters in the presence of other carbon acids have been reported. 

For a given pH in Hoagland s solution, the carbonate ion concentration is constant, and right-hand portions of the two equations are equal to each other. Thus, rearranging yields... 

Coordination of the n electrons of the double bond with palladium gives a Tr-complex, which rearranges by migration of the substituent R to the less substituted carbon of the double bond while palladium bonds to the other carbon. Dissociation of the complex gives the alkene. Other steps (not shown) restore the original form of the catalyst. 

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