Regioselectivity experimental conditions

Since the Fries rearrangement is a equilibrium reaction, the reverse reaction may be used preparatively under appropriate experimental conditions. An instructive example, which shows how the regioselectivity depends on the reaction temperature, is the rearrangement of m-cresyl acetate 8. At high temperatures the ortho-product 9 is formed, while below 100°C the para-derivative 10 is formed ... 

Carbonyl compounds, such as aldehydes [103, 179], (thio)ketones [31, 94, 180-183], carboxylic acids, and esters [183, 184] with 1 are reduced to alcohols after hydrolysis [5], except in stericaUy hindered cases (see Section 8.5) [185, 186]. Under the same experimental conditions the regioselective reduction of the oxirane ring with 1 gives also the corresponding alcohol [183, 187]. 

Full exploitation of the synthetic potential of enolates requires control over the regioselectivity of their formation. Although it may not be possible to direct deprotonation so as to form one enolate to the exclusion of the other, experimental conditions can often be chosen to favor one of the regioisomers. The composition of an enolate mixture can be governed by kinetic or thermodynamic factors. The enolate ratio is governed... 

The palladium-catalyzed hydrovinylation does not seem to be very useful due to the regioselectivity in favor of the linear compound 143 and the formation of a substantial amount of isomerized product 142 (Scheme 39).148 149 Phosphines of basic nature can improve the selective formation of the desired branched product 141 by limiting the isomerization under the experimental conditions.150... 

A similar process with benzalacetophenone results in different products, the chemoselectivity being controlled by the experimental conditions, namely the solvent and the supporting electrolyte used [261]. Either an acyclic disulfide, bis(2-benzoyl-l-phenylvinyl) disulfide (in Et0H-DMF/Mg(Cl04)2), or two isomeric thiophenes (in DMF/NaCl04) can thus be obtained (Scheme 63). Upon the substitution of a vinylic proton at C(2) with Br, the reaction becomes more regioselective and the only product now formed is an a,a -dibenzoyl thiophene derivative. 

It should be noted here that a regioselective control may also be exerted by just controlling the experimental conditions. Thus, working under strictly kinetic conditions (low temperature, absence of oxygen and slow addition of the ketone to an excess of a solution of an aprotic base) the less substituted enolate of carvomenthone can also be selectively generated and may be then submitted to different kind of reactions. However, reversible reactions like the Michael addition would equilibrate the reaction mixture to the thermodynamically more stable enolate. 

The regioselective reduction of the C=C bond of a, -unsaturated carbonyl compounds, a very important organic reaction, is achieved by means of hydrogen telluride and phenyl tellurol, under appropriate experimental conditions. 

In conclusion, the regioselectivity of the deprotonation a/p competition) is related not only to the structure of the oxirane but also to the experimental conditions (nature of the base, solvent, temperature etc.). Thus, prediction must take into account all of these parameters and evaluate their relative importance. 

Different protecting groups such as acetonides, /-butyl dimethyl silyl derivatives proved to be stable under these experimental conditions, but the benzylidene group is completely cleaved.. The rate of photodeoxygenation is greater at the secondary positions [82], as indicated in Scheme 43, with the regioselective photolysis of dipivaloate to give a mixture of monodeoxy and dideoxy derivatives (81) in low yields. 

Another highlight of the period under review is the report of the total synthesis of the tricyclic alkaloid fawcettimine (22) (Scheme 2).9 Lewis-acid-catalysed Diels-Alder addition of butadiene to 2-allyl-5-methylcyclohex-2-enone provided the ds-octalone (23), which was modified as indicated to give the dialdehyde (24). After considerable experimentation, conditions were defined which led to regioselective ring-closure in the desired direction. The unsaturated aldehyde was treated directly with the Wadsworth-Emmons reagent to provide... 

Similar results tQ those reported for 107 and 110 were found for benzotriazoles 113 and 116, which were efficiently, but not regioselectively, allylated. Many experimental conditions were tested (95JHC1325). However, mixtures of compounds 114 and 115 in one case and of 117-119 in the other were always formed (Scheme 24). Allylbenzotriazoles 114 and 115 do not equilibrate under Pd(0) catalysis. 

A clean insertion reaction of the 2,3-dihydrophosphole 4 into the zirconium-carbon bond of the transient benzyne-zirconocene 1 occurs when 4 is heated with Cp2ZrPh2 at 80°C for 6 h, leading to the tricyclic compound 5 (Scheme 1). The regioselective formation of 5 can be regarded as the result of a strong interaction between the phosphorus lone pair and zirconium. It can be noted that under the same experimental conditions, the corresponding oxide 4 or the 3,4-di-hydrophosphole 4 do not react with 1. 

The use of a metal carbene complex in benzannulations has become one of the most valuable synthetic applications of these organometallic reagents . Because of its applicability to a broad spectrum of substituents, its regioselectivity and its mild experimental conditions, benzannulation has been employed as a key step in the synthesis of a series of natural compounds . 

At ambient temperature, Zn(BH4)2 in EtjO, in the presence of trimethylsilyl chloride, transforms acetals and ketals into ethers. Under these conditions, esters remain intact, but the double bonds are reduced [KUl] (Figure 2.23). According to the experimental conditions, the regioselectivity of the cleavage can vary [JSl], as shown by the reduction of sugar derivative 2.44 (Figure 2.23). 

The regioselectivity of the hydrogen abstraction reaction between the photoinitiator and an amine remains an outstanding problem that is the subject of debates and discussions. A recent paper [264] based on ESR and laser flash photolysis clearly shows the presence of the aminoalkyl radical and the significant absence of any aminyl radical in primary and secondary amines such an aminyl radical is, however, readily observed in the same experimental conditions when using tetramethyl piperidine. 

A similar Rh complex, [HRh(CO)(PPh3)3], immobilized onto AC and CNTs (ends-opened) by an incipient wetness technique was tested as a catalyst for propene hydroformylation [70]. Activity assay of the catalysts showed that the CNT-supported Rh complex displayed not only high activity for propene conversion but also excellent regioselectivity to the butyraldehyde product. Under the experimental conditions used, the molar ratio of normal to branched aldehydes reached 12 to 13 at a TOP of 0.12 s corresponding to a propene conversion of 32%. To understand the excellent catalytic behavior of the CNT-supported catalyst, an ends-unopened CNT was also used as support and its catalytic properties tested In this particular case, propene conversion and the nii ratio reached only 17.2% (corresponding to a TOP of 0.06 s ) and 6.0, respectively. On the basis of this result, the authors concluded that the high propene conversion and excellent regioselectivity demonstrated by ends-opened CNT-supported catalyst was due mainly to the confinement effect induced by the presence of a Rh-phosphine complex in the inner surface of the tubular nanochannels of CNT [70]. 

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