Stereospecific migration

Toluenesulfonic acid Cyclic ethers from diols with 1,2-arylthio group migration Stereospecific ring closure... 

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. 

The reaction is stereospecific the alkyl group migrates with retention of configuration O O... 

Diazo compounds react with alkenes to afford A -pyrazolines, which in turn izomerize to A -pyrazolines if there is a hydrogen atom a to the N=N bond (Scheme 54). In those cases where two possible ways of isomerization exist, the more acidic hydrogen migrates preferentially. The alkene configuration is conserved on the A -pyrazoline (stereospecificity) but the regioselectivity depends on the substituents of both the alkene and the diazo compound. 

The mechanism of the reaction is unknown. The stereospecificity observed with (E)- and (Z)-l-methyl-2-phenylethylene points to a one-step reaction. The very low Hammett constant, -0.43, determined with phenylethylenes substituted in the benzene ring, excludes polar intermediates. Yields of only a few percent are obtained in the reaction of aliphatic alkenes with (52). In the reaction of cyclohexene with (52), further amination of the aziridine to aminoaziridine (99) is observed. Instead of diphenylazirine, diphenylacetonitrile (100) is formed from diphenylacetylene by NH uptake from (52) and phenyl migration. 

Karrer thought that the dihydro-1,3-oxazine skeleton can possibly be formed in proteins during their transformation to polypeptides. Fodor - and Fiescr explained the stereospecific migration of acyl groups from N to 0 and vice versa in tropine alkaloids by an intermediate formation of a 1,3-oxazine ring. 

The B - C migration can also be induced by other types of electrophiles. Trimethylsilyl chloride or trimethylsilyl triflate induces a stereospecific migration to form (3-trimethylsilyl alkenylboranes having cis silicon and boron substituents.34 It has been suggested that this stereospecificity arises from a silicon-bridged intermediate. 

Entry 8 involves a migration initiated by epoxide ring opening. This reaction involves migration of a vinyl substituent. Entry 9 is a stereospecific migration of the aryl group. The DiBAlH both promotes the rearrangement and reduces the product aldehyde. 

Shimizu and co-workers reported that thermal decomposition of A4-thiabenzenes ylides afforded both thienofuran and thiophene derivatives in addition to the expected alkyl-rearranged products. A plausible mechanism was proposed with a [3.1.0] bicyclic sulfonium salt 9 as the key reactive intermediate <2001J(P1)2269>. Warren and co-workers, in their study of stereospecific phenysulfanyl migrations, found that [l,4]-sulfanyl participation could compete with the usual [l,2]-sulfanyl participation <1999SL1211>. Rearrangement of alcohol 18 with TsCl in pyridine gave an inseparable mixture of isomeric chlorides, 19 and 20, in a ratio of 52 48, as shown in Equation (3). 

Cationic Fp (olefin) complexes [Fp = f/5-C5H5Fe(CO)2] undergo regio-specific addition of heteroatomic nucleophiles.32 Subsequent ligand transfer (carbonyl insertion) occurs with retention of configuration at the migrating center (R—Fe—CO -> RCOFe).33 A combination of these processes has provided a novel stereospecific azetidinone synthesis which can also be applied to condensed systems.34... 

Deuterium substitution for the migrating 4/ proton demonstrates that the enzyme transfers it by a stereospecific intramolecular path... 

There are two aspects to the stereospecificity of the [1,2]-Wittig rearrangement the steric course (inversion vs. retention) at the migrating carbon atom and at the Li-bearing carbanion center (equation 6). 

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... 

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