Migration step

The presence of water in the carbonylation mixture makes it possible to halt the reaction after the second group migration (step 2). Oxidation of the intermediate boraglycol yields the corresponding ketone, and alkaline hydrolysis affords the secondary alcohol, R HOH. A blocking group of low... 

For a discussion of the transition state involved in the migration step, see Wistuba, E. Ruchardt, C. Tetrahedron Lett., 1981, 22, 3389. 

The success of both of these methods depends upon the thexyl group being noncompetitive with the other groups in the migration steps. 

A closely related reaction employs a-diazo esters or a-diazo ketones.25 With these compounds, molecular nitrogen acts as the leaving group in the migration step. The best results are achieved using dialkylchloroboranes or monoalkyldichloroboranes. 

Above 323 K, the surface hydride catalyzes the hydrogenolysis of neopentane, isobutane, and propane, whereas ethane does not undergo any significant hydrogenolysis. The first step of the reaction is the activation of the C—H bond, whereas the next step is the activation of the C—C bond of the alkyl groups via (l-methyl migration steps. 

The methyl-rhodium complex (2) was detected, at low concentration in neat CH3I solution, by a combination of FTIR and NMR spectroscopy.6 The rate of the reaction (2) —> (3) was measured between 5°C and 35°C. An Arrhenius plot yielded activation parameters of A7/= 63 k.lmol 1 and AS= — 59 JmoC1 K 1 for the methyl migration step. 

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

In conclusion to this section, the two step reaction requires the presence of a metal that can be easily oxidized (d8 over d6). However the 1,3 migration step does not appear to be easy in an intramolecular way. An intermolecular 1,3 shift seems feasible but is probably highly sensitive to steric effects. The pathways for the isomerization reactions with d6 and d8 metal complexes are significantly different. They have in common the preference for the hydrogen to move as a proton, whether the shift starts from Ca or from the metal center. 

The possibility exists of an migration of an H atom. Because the migration step... 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

An efficient process for one-carbon homologation to aldehydes is based on cyclic boronate esters.17 These can be prepared by hydroboration of an alkene with dibromobor-ane, followed by conversion of the dibromoborane to the cyclic ester. The homologation step is carried out by addition of methoxy(phenylthio)methyllithium to the boronate ester. The migration step is induced by mercuric ion. Use of enantioenriched boranes and boronates leads to products containing the groups of retained configuration.18... 

There is kinetic evidence that the migration step in these base-catalyzed rearrangements is concerted with ionization. Thus, in cyclopentane derivatives, the rate of reaction depends on the nature of the trans substituent R, which implies that the migration is part of the rate-determining step.26 27 28 29... 

The side reaction of hydrogenolysis of the methyl-ruthenium intermediate to methane also may become predominant when the carbonyl insertion-methyl migration step of the process (Scheme 1) proceeds at a low rate. To reduce this drawback some Lewis acid promoters (i.e. metal alkali cations, classical Lewis acids such as AII3, SbCl etc.)... 

McEwan and co-worker" examined the acid-catalyzed decomposition of unsymmetrical benzhydryl azides 18 and some related species (Fig. 13.11). The aryl migration step showed very little discrimination between aryl rings with electron-donating and those with electron-withdrawing substituents. This low selectivity was judged to be more consistent with formation of a discrete nitrenium ion intermediate (19). These workers reasoned that a concerted migration would exhibit higher selectivity toward donor-substituted arenes, because in that mechanism the electrons from the arene would participate in the reaction. 

In spite of its formal similarity to the above mentioned annulation processes, the reaction shown in 4.37. includes a unique migration step. Oxidative insertion of the palladium into the phenyl-iodine bond is followed by the migration of the palladium onto the more electron rich indole ring. The 2-indolylpalladium complex than carbopalladates the pendant alkync moiety and the process ends by the formal activation of a C-H bond of the phenyl substituent and subsequent reductive elimination, furnishing the pentacyclic product.48 The same strategy has been utilised in the preparation of the indoloindolone framework from /V-bcnzoyl-3-(o-iodophcnyl)-indolc in an oxidative addition - palladium migration - C-H activation sequence.49... 

The formation of [Fen(CN)5H20]3- as a product sets the basis for the catalytic processing of nitrite reduction by hydrazine under appropriate pH conditions. As shown in Fig. 13, nitrite binds to the aqua ion and rapidly converts to NO+. After the attack by N2H4, the adduct reorganization, associated with proton migration steps, favor... 

The rate of the transfer step from a non-EFS chromophore to the closest ring in a nearby EFS is the same as that of a migration step between two non-EFS rings separated by the same distance. 

The intramolecularity of the migration step in the Schmidt rearrangements has been convincingly demonstrated by showing the retention of chirality of the migrating group.166... 

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