Alkyl migration Stereochemistry

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). 

Participation means acceleration and retention of stereochemistry and may mean rearrangement Participating groups can have lone pairs or 1C electrons Carbocatlons often rearrange by alkyl migration... 

A change in e stereochemistry of 2,2-dimethylchroman-4-one oximes in the reaction medium accounts for the preferential alkyl migration and formation of... 

The stereochemistry of base-induced alkyl migration from boron to carbon in y-acetylvinylboranes (36) has been studied and migration shown to occur predominantly in an anti fashion with respect to the leaving group, as in (37), to produce a trans-sAlylic alcohol on oxidation (Scheme 13). The availability of optically active propargylic alcohols (see refs. 36, 37 2, 116 4, 142) as precursors for (36) means that the overall sequence accomplishes alkylation and enantioselective [l,3]-hydroxyl transposition enantiomer ratios up to 87 13 were achieved in the alcohol product. 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

The stereochemistry of the products is often controlled through control of the reaction temperature. For instance, use of low temperatures, where the alkyl shift and migration is retarded, favors formation of syndiotactic polypropylene (sPP). Commercial iPP is produced at room temperatures. 

The R—0—B bonds are hydrolysed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of B-to-0 migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least substituted carbon of a double bond results in the alcohol being formed with regiochemistry which is complementary to that observed in the case of direct hydration or oxymercuration, that is, anti-Markownikoff. 138... 

Reactions of acyclic derivatives with carbon electrophiles have also been examined.33,34 An illustrative reaction involving methylation of the unsubstituted complex [MnCr 4-butadiene)(CO)3], (19), is shown in Scheme 16. Again, the reaction is presumed to occur via a methylmanganese species (20) and after methyl migration the unsaturated metal center is stabilized by formation of a Mn—H—C bridge (isomers 21a and 21b). Deprotonation of equilibrating (21a and 21b) yields the [Mn(l-methylbutadiene)(CO>3]-complex (22), which has exclusively trans stereochemistry.34 This sequence represents alkylation of the terminal carbon of butadiene and complements the iron carbonyl chemistry, where terminal acylation has been achieved as described above. Unpublished results indicate that a second methylation of (22) occurs... 

Deconjugation of a,fi-unsaturated esters. It has been known for some time that deprotonation and reprotonation (or alkylation) of a,/J-unsaturated esters is accompanied by migration of the double bond to the /l,y-position (thermodynamically the less stable position). The stereochemistry of the isomerization has now been elucidated by two groups.4,5 A (Z)-2-alkenoate isomerizes almost exclusively to the (E)-3-alkenoate. The (E)-2-alkenoate isomerizes to (Z)- and (E)-3-alkenoates in a ratio of ubout 85 15. Both the (E)- and (Z)-3-alkenoates are unchanged on re-exposure to this sequence. Both papers suggest possible reasons for this stereochemical outcome. 

The alkyl group migrates with the two electrons from its bond to boron and as a result the migration occurs with retention of the stereochemistry of the alkyl group. 

Only one alkyl group is transferred from boron to carbon in the reaction and so generation of the required dialkylborane from thexylborane prevents wastage of more valuable alkyl groups (Equation B4.2). Note that the migrating alkyl group involved in the reaction sequence depicted in Equation B4.2 retains its stereochemistry. 

The next molecule of propene will coordinate onto the complex with the methyl group pointing upwards (6.19.d) migration of the new alkyl anion gives e with the stereochemistry shown. When this process is repeated several times, forming the polymer carbon chain in the plane of the figure, structure f is obtained. From this structure we cannot immediately deduce what the microstructure is, since the polymer chain is not stretched as shown before (Fig. 6.11). By rotating... 

Borodkin et al. (1979a) used a benzyl-substituted acenaphthenium ion to study the stereochemistry at the migrating tetragonal carbon in degenerate 1,2-alkyl shifts under stable ion conditions. The l-p-chlorobenzyl-2,2-dimethyl-l-acenaphthenium ion [335], prepared from the corresponding alcohol in FSO3H at —70°C, rearranged at —20°C (AG = 17.4 kcal mol" at —28°C) to the more stable 2-p-chlorobenzyl-l,2-dimethyl-l-acenaphthenium ion [336]. 

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