Migration of bonds

Carbon to carbon a-bond migration processes, which are visibly involved in the transformations of I and III, usually offer savory mechanistic problems whose main difficulty—most people think—resides in the drawing of intermediate structures. These are often complicated by the migration of bonds and atoms, and require the foresight to conceive convoluted spatial transformations. This sort of problem, however, features another major hardship If one makes generous use of one s imagination, a whole series of carbocationic intermediates... 

Examples of the primary stereoelectronic effect have been demonstrated in the literature. Chandrasekhar and Roy showed that rearrangement of 2-oxo-cyclohexyl-peroxyacetic acid 35, derived from acid 34, proceeded via intermediates 36 and 37 to 38 in 62% overall yield.17 Migration of bond a was the only bond which migrated being antiperiplanar to the peroxide no product of migration of bond b was observed. 

The mass spectra of cycloalkenes show quite distinct molecular ion peaks. For many cycloalkenes, migration of bonds gives virtually identical mass spectra. Consequently, it may be impossible to locate the position of the double bond in a cycloalkene, particularly a cyclopentene or a cycloheptene. 

Fig. 7.2 Postulated rearrangement of orthogonene 1 to a earbene. Migration of bond C1-C5 from Cl to C2 through transition state It gives earbene Ic. The reaetion profile, showing activation and reaction energies in kJ moF, was ealeulated by CASMP2/6-31G //CASSCF(4,4)/6-31G (see text)...

The mass spectra of cycloalkenes show quite distinct molecular ion peaks. For many cycloalkenes, migration of bonds gives virtually identical mass spectra. Consequently, it may be impossible to locate the position of the double bond in a cycloalkene, particnlarly a cyclopentene or a cycloheptene. Cyclohexenes do have a characteristic fragmentation pattern that corresponds to a retro Diels-Alder reaction (Fig. 4.4). In the mass spectrum of the monoterpene Umonene (Fig. 4.16), the intense peak at m z = 68 corresponds to the diene fragment arising from the retro Diels-Alder fragmentation. 

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

Step 2 Enzyme catalyzed proton transfer steps cause migration of the double bond converting the mine formed m step 1 to an isomeric mine... 

Conjugate acid (Section 1 13) The species formed from a Brpnsted base after it has accepted a proton Conjugate addition (Sections 1010 and 1812) Addition reaction in which the reagent adds to the termini of the con jugated system with migration of the double bond synony mous with 1 4 addition The most common examples include conjugate addition to 1 3 dienes and to a 3 unsaturated car bonyl compounds... 

Sigmatropic rearrangement (Section 24 13) Migration of a a bond from one end of a conjugated tt electron system to the other The Claisen rearrangement is an example... 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

Medroxyprogesterone acetate (74) is stmcturaHy related to and has been prepared from hydroxyprogesterone (39) (Fig. 10). Formation of the bis-ketal accomplishes the protection of the ketones and the required migration of the double bond. Epoxidation with peracetic acid produces a mixture of epoxides (75), with a predominating. Treatment of the a-epoxide with methyl magnesium bromide results in diaxial opening of the epoxide. Deprotection of the ketones provides (76), which is dehydrated to (77) by treatment with dilute sodium hydroxide in pyridine. Upon treatment with gaseous hydrochloric... 

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

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