Migration from C to

In the benzylic acid rearrangement of 1,2-diketones, HO adds to one ketone. The tetrahedral intermediate can collapse by expelling HO-, giving back starting material, or it can expel Ph-. The Ph- group leaves because there is an adjacent electrophilic C to which it can migrate to give the product. 

The Favorskii rearrangement differs from the benzylic acid rearrangement in that the a-carbon is a cr-bond electrophile rather than a 7r-bond electrophile. 

The mechanism shown, the semibenzilic mechanism, operates only when the ketone cannot enolize on the side opposite the leaving group, as in aryl ketones. When the ketone can enolize, a different mechanism is operative. This mechanism is discussed in Chapter 4. 

In the Wolff rearrangement, an a-diazoketone is heated to give a ketene. The mechanism of the Wolff rearrangement consists of one step the carbonyl susstituent migrates to the diazo C and expels N2. When the reaction is executed in H20 or an alcohol, the ketene reacts with solvent to give a carboxylic acid or an ester as the ultimate product. (The mechanism of this reaction was discussed in Section 2.3.1.) 

Light can be used to promote the Wolff rearrangement, too, and in this case the reaction is called the photo-Wolff rearrangement. The photo-Wolff rearrangement is probably a nonconcerted reaction. First, N2 leaves to give a free carbene, and then the 1,2-alkyl shift occurs to give the ketene. 

The adjacent electrophilic atom is a 7r-bond electrophile in the benzylic acid rearrangement, but it is a cr-bond electrophile in the Favorskii rearrangement. 

---

Under thermal conditions, the silyl group of conjugated SMA imines undergoes 1,2-migration from C to N with concomitant cyclization when the structure is favorable. 

The silyl group in an SMA derivative is able to migrate from C to N under thermal conditions (see Section VI.B. 4). This is the case with SMA imines, and such rearrangement leads directly to the corresponding azomethine ylid which may be intercepted by a dipolarophile.402... 

Another 1,2-silyl migration from C to C was found during the reaction of an a-silyl cyclopropyllithium 42 with dichloromethyl methyl ether 43 to give the silyl ketone 44 (equation 36)87. 

Anionic 1,2-silyl migrations from C to O were applied to the [3 + 2] annulations of /J-X-substituted a,/J-unsaturated acylsilanes (X=SPh and SiMe3) 153 with lithium enolates 154,... 

Komatsu and coworkers had found that heating of tV-(silylmethyl)imines 186 gave diazomethyne ylide 187 via a 1,2-silyl migration (equation 110). This type of migration from C to N was used in the stereoselective synthesis of c/s -2,3 -d i ary I az i r i d i n c244 and pyrroline derivatives245. 

The 1,3-silyl migrations from C to C occurred with inversion of configuration at silicon and with a rather high activation enthalpy for the 1,3-silyl migration of a-methylallyltrimethylsilane studied by Kwart and Slutsky, AH =47.7 kcalmol-1 and AS = —6.2 calmol-1 K-1297. On the other hand, 1,3-silyl migrations in ketosilanes... 

Among such migrations, the Peterson reactions to give olefins have been shown to proceed via intramolecular 1,3-silyl migrations from C to O (equation 129). 

An anionic 1,4-silyl migration from C to C was observed during the lithiation of 9,10-dihydroanthracene (DHA) derivatives (equation 140)349 - 353. Typically, treatment of DHA 212 (R=SiMe3) with butyllithium followed by hydrolysis gave only ds-9,10-bis(trimethylsilyl)-DHA 213 (R=SiMe3, E=H) stereospecifically. The crossover and deuterium labeling experiments confirmed the intramolecular and irreversible feature of the... 

An anionic 1,4-migration from C to C was also observed during the lithiation of 2-trimethylsilyl-l,3-dithiane derivatives 221 (equation 144)354. 

A 1 1 mixture of (E)- and (Z)-allyloxysilanes 231 was prepared by the reactions of ( )-trimethylsilylvinyl sulfone 229 with aryllithiums followed by the addition of aliphatic aldehydes and cyclohexanone (equation 147). An anionic 1,4-silyl migration from C to O in the lithium alkoxide 230 was proposed to be involved. On the other hand, lithium alkoxide 232 lacking an aryl group was stable and gave no rearranged product even at room temperature (equation 148)360. 

An anionic 1,6-silyl migration from C to O (1,6-Brook rearrangement) was observed during the deprotonation of e-silyl alcohol 324, which gave the corresponding silyl ether 325 (equation 199)466. 

A 1,6-silyl migration from C to O was also observed in the reaction of allylepoxysi-lane 326 with t-BuLi in a THF/HMPA (25 1) mixed solvent at low temperature, which afforded 5-silylpentenal 327 (equation 200). It was proposed that an initial proton abstraction from 326 gave w-silyl alkoxide 328, which underwent 1,2-silyl and then 1,6-silyl migrations to afford dienolate 329. Hydrolysis of 329 provided 327 (equation 201). The intramolecular nature of the transformation was suggested because the facile reaction occurred even in very dilute (0.02 lmol-1) and low-temperature (—78°C) conditions467. 

Table 8.1 Synthesis of aromatic amines by rearrangements involving migration from C to an electron-deficient N atom...

Besides the reactions discussed at 18-13-18-17, a number of other rearrangements are known in which an alkyl group migrates from C to N. Certain bicyclic A-haloamines, for example A-chloro-2-azabicyclo[2.2.2]octane (above), undergo... 

---