Alkyls migration

From a mechanistic perspective, the reaction is similar to those described above. Silver-catalyzed cyclization of the ketone to the allene gave a cyclic oxonium intermediate. A [1,2]- or [1,5]-alkyl shift modified the sigma skeleton leading to an alkylsilver intermediate, which on elimination gave a trisubstituted furan. 

Although acyloxy, phosphatyloxy, and tosyl allenones were obtained from the corresponding propargyl alcohol derivatives via silver catalysis through an overall process that resembled [3,3]-sigmatropic rearrangement (see Sections 3.41 and 3.43), the mechanism was not fully proved and is still in question. 

These results clearly demonstrated that, depending on substrates, apparent [3,3]-sigmatropic shifts actually resulting from 1,2 shifts could occur. Further studies are thus clearly needed to better understand the mechanism of apparent sigmatropic rearrangements catalyzed by silver and other metals. 

---

CO is a representative species for Q, -insertion its insertion into C—Pd bonds affords acylpalladium complexes such as 15. Mechanistically, the CO insertion is 1.2-alkyl migration to coordinated CO. This is an important step in carbonyiation. SO , isonitriies, and carbenes are other species which undergo a.a-insertion. 

Rearrangement of an enamine to a Sehiff s base through N- to C-alkyl migration was reported 729). These authors also found that enamines, rather than aminals, were formed from butyraldehyde and seeondary amines (730). Chloramines and aeetylenes reacted to give chloroenamine intermediates, which hydrolyzed on work-up of the reactions (731). 

This alkyl migration is believed to proceed via ion-pair formation. These and many other simple 0-alkyIated cyclic hydroxamic acids are thermally stablebelow 180°. 

In the case of an appropriate substrate structure, the carbenium ion species can undergo a 1,2-alkyl shift, thus generating a different carbenium ion—e.g. 4. The driving force for such an alkyl migration is the formation of a more stable carbenium ion, which in turn may undergo further rearrangement or react to a final product by one of the pathways mentioned above—e.g. by loss of a proton to yield an alkene 3 ... 

Another aspect of stereochemistry of the CO insertion which has received attention concerns the actual process of formation of the acyl moiety from the coordinated CO and R. Three possible pathways may be envisaged. First, the alkyl moves from the metal onto an adjacent CO. This is known as the alkyl migration mechanism. Second, a coordinated CO moves to insert into the M—R bond—a CO insertion mechanism. Third, both CO and R move in a cooperative manner. These three pathways are represented schematically in Eq. (46). 

XIV) and L [Eq. (50)] may be best rationalized on the basis of the intermediacy of square-pyramidal (XVII), which arises from (XIV) via a concerted approach of Et and CO(l), accompanied by the opening of the Cl(l)—Ir—L bond angle. It is entirely possible that this mechanism of cooperative migration may be as general, if not more so, than that of alkyl migration. 

Since Bruce s pioneering work in the area of ruthenium vinylidene chemistry (1), it has been well known that isomerization of a terminal alkyne to a vinylidene on a metal center is not only favorable but also effects a reversal in the reactivity of the carbon atoms. However, hydration catalysis was not possible, because alkyl migration from a proposed acyl intermediate led to an... 

Photolysis of the methylidyne cluster HRu3(CO)] (/1, 71"COCH3) (A) (14) in cyclohexane solution leads to an unprecedented oxygen-to-carbon alkyl migration to form the bridging acyl complex HRu3(CO)10( i-> 2-C(O)CH3) (B) ... 

The alkyl migration takes place with retention of configuration of the alkyl group which leads to the formation of a trialkyl borate, an ester, B(OR)3. 

Alkyl migration has not been found in protonated furan derivatives. Neither 2,5-dimethylfuran nor 2,3,5-trimethylfuran undergo any change other than protonation in fluorosulfonic acid containing antimony pentafluoride. On the other hand, migrations will occur during cyclization until the product corresponds to a protonated furan (Scheme 29).19... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

The presence of A1H3 would assist the alkyl migration and the intermediate suggested is analogous to those Shriver has isolated. The reaction of CH3C(0)FeCp(C0)2 and CH3FeCp(C0)2 with LAH under 13C0 resulted in no incorporation of 13C into the Cn and Cn+1 products. No incorporation would be expected for Scheme 2. 

The benzene ring containing cyclobutanones (208), (210) and (212) were also oxidized to their y-lactones (209), (211) and (213) respectively71 These examples demonstrate convincingly the order of alkyl migration. 

The most widely used route to l-benzazepin-2-ones involves the Beckmann or Schmidt reaction of the easily accessible 1-tetralones. Many biologically active compounds described in this review have been prepared on the basis of these reactions they have been fully reviewed [2], In the Beckmann reaction of 1-tetralone oximes, polyphosphoric acid is used as a catalyst-solvent in most instances. Aryl migration generally takes precedence over alkyl migration under these reaction conditions, and various 1-tetralone oximes substituted on the aromatic and/or aliphatic rings can be converted to the appropriate 2,3,4,5-tetrahydro-l//-l-benzazepin-2-ones (51) [5, 20-23, 36, 59, 65, 80, 107-112]. Both courses of the rearrangement occur in some instances, yielding l-benzazepin-2-ones (51) and the isomeric 2-benzazepine-l-ones, probably due to electronic effects of the substituents [90, 113, 114]. 

Although both boronates and alanates react with allylic bromides, aldehydes and C02 to afford allenic products in satisfactory yield, the alanates are more efficient in additions to ketones (Table 9.14). Boronate reagents do not require a B-C alkyl migration for their preparation. Thus the starting acetylene possesses the structural elements of the product. Additionally, the issue of dummy ligands is irrelevant. 

The cleavage of C5-C4 and formation of C5-C3 suggests that we have a 1,2-alkyl migration of C5 from C4 to a cationic C3. Then the electrons in the C2-C3 bond can move to form a new n bond between C3 and C4, leaving a stabilized acylium ion at C2. After addition of H2O to the acylium ion, an acid-catalyzed electrophilic addition of the resultant carboxylic acid to the alkene occurs to give the final product. 

---