Regioselectivity replacement

This procedure also allows to invert the regioselectivity. Replacement of the usual phtha-lazine ligand by an AQN-based one leads to an AA in which the formation of the reversed regioisomer 39 is preferred [77], Apart from the obvious conclusion that the substrate orientation within the binding pockets of the different ligands is altered, the mechanistic reason for this reversal remains unknown. From a synthetic point of view this observation is important because it allows the conversion of cinnamate esters into the corresponding phenyl serine derivatives. 

Although not as extensively employed as vinyl esters, vinyl ethers find occasional use as intermediates in the conversion of ketones into alkenes. Once again, vinyl ethers can be generated regioselectively by kinetic or thermodynamic enolate generation and, therefore, offer another route by which ketones can be regioselectively replaced by alkenic double bonds. 

Similar reactions can occur using tetraalkylsilanes including benzyl- and allyl-silanes and, for example, photo-induced electron transfer in the presence of aromatic nitriles can afford alkylated products. These reactions can lead to the regioselective replacement of a cyano substituent with an alkyl group as shown in Scheme 6. Mechanistic studies support the unimolecular fragmentation of the intermediate radical cation [Bu4Si],+ followed by radical-radical combination. ... 

Bromination of Allylic Alcohols. The reaction of an allylic alcohol with PBrs in ether at 0°C leads to both stereoselective and regioselective replacement of the hydroxy group by bromine. Exarhples of this transformation are given in eqs 4 and 5. The latter example shows that the reaction can be run in the presence of significant unsaturation. There are many examples of similar reactions. ... 

In principle, the direct hydride addition or catalytic hydrogenation, which did not give chlorins, was replaced by an electrocyclic intramolecular addition which is much easier with the above system. Complete regioselectivity was also achieved since electrocyclization did not occur with the resonance-stabilized ring C. 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

Bu2SnO, benzene BnBr, DMF, heat, 80% yield. This method has also been used to protect selectively the anomeric hydroxyl in a carbohydrate derivative. The replacement of Bu2SnO with Bu2Sn(OMe)2 improves the process procedurally. The use of stannylene acetals for the regioselective manipulation of hydroxyl groups has been reviewed. ... 

Sauer and Heldmann [97] recently reported an interesting application of ethynyltributyltin as an electron-rich dienophile in an inverse electron-demand Diels-Alder reaction with the electron-deficient triazine derivative 94. This method is interesting because the reaction is highly regioselective and the trialkylstannyl group is easily replaced by several groups under mild conditions, leading to substituted pyridines 95 (Scheme 2.41). 

Synthesis of 3,5-disubstituted pyridazines by regioselective [4 + 2] cycloadditions with ethynyltributyltin and subsequent replacement of the organotin substituent [160]... 

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

A new structural feature is obtained by replacement of the phosphite donor within BINAPHOS by a phosphoramidite system. Improved enan-tioselectivities were noted, albeit the problem of regioselectivity persists (Scheme 20) [69]. 

Sodium iodide can also be used as a bromide replacement, yielding Pd TON of 1195. In this case, bisphosphine ligands are necessary to maintain high regioselectivity (DPC vs. phenylsalicilate) of carbonylation reaction (96 % vs. 72 % for Pd(acac)2). 

In view of our earlier statement, that the regioselectivity remains unaffected by the ligands of the catalyzing metal, the reversal of regioselectivity for myrcene (21), when Cu(OTf)2 is replaced by CuS04, comes as a surprise. As the two observations... 

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