Regioselectivity, in reactions

Regioselectivity in reactions of heterocyclic anions of allyl-type with electrophiles 99CRV665. 

V. REGIOSELECTIVITY IN REACTIONS OF POLYENYL RADICALS A. Trapping with Closed-shell Molecules... 

Scheme 3.26. Regioselectivity in reactions between silylcuprate reagents and allylic acetates and carbamates [103].

Scheme 3.27. Regioselectivity in reactions of silyllithium and silyl cuprate reagents with allylic carbamates and epoxides...

A -Silylmethyl-amidines and -thioamides (42) (X=NR or S) undergo alkylation at X with, for example methyl triflate, and then fluorodesilylation to give the azomethine ylides 43 (identical with 38 for the thioamides) (25,26). Cycloaddition followed by elimination of an amine or thiol, respectively, again leads to formal nitrile ylide adducts. These species again showed the opposite regioselectivity in reaction with aldehydes to that of true nitrile ylides. The thioamides were generally thought to be better for use in synthesis than the amidines and this route leads to better yields and less substituent dependence than the water-induced desilylation discussed above. 

Diels-Alder reactions (6, 65-66). 2-Melhoxy-5-methylbenzoquinone shows no regioselectivity in reactions with pipcrylcnc or isoprene. These cycloadditions are catalyzed by both BF3 and SnCl, but they favor different adducts in each case. The difference is believed to arise from different types of complexes with the quinone.1... 

The regioselectivity in reactions of fluoroalkenes to maximize the number of fluorines in the /f-position illustrates that whether hyperconjugative interactions are important or not, fluorine in the /i-position can have a profound effect on the outcome of reactions.75,81,82... 

Reductive lithiation of allyl phenyl sulfides.1 This reagent is particularly useful for preparation of allyllithium reagents at temperatures at which the anions are stable. Moreover, regioselectivity in reactions can be achieved by conversion to allyltitanium(IV) complexes by metal exchange with Ti(0-/-Pr)4. Thus the un-symmetrical anion formed from the allyl sulfide 1 with LDMAN reacts with cro-tonaldehyde to give a mixture of 1,2- and 1,4-adducts. The 1,2-adduct 2 can be obtained in high yield as two diastereomers (9 1) by use of the allyltitanium complex (equation I). 

Kinetic Aspects of Ar-SE Reactions Reactivity and Regioselectivity in Reactions of Electrophiles with Substituted Benzenes... 

Similarly, although the yields are lower, the Co catalysts also show generally reversed regioselectivity in reactions with carbodiinudes as compared with isocyanates. °- In fact, the 4,6-disubstituted product is formed with complete regioselectivity in reactions of di(p-tolyl)carbodiimide with terminal alkynes. 

Scheme 5.1.18 Regioselectivity in reactions of the monostannylene acetal of methyl (i-lactoside ...

The regioselectivity in reactions of Bredt-olefins has been studied. Typical electrophilic and nucleophilic reactions, the latter supported by only two examples, show high regioselectivity and syn-stereoselectivity. With the exception of diphenylketene, the [2 + 2] and [2 + 3]cycloadditions of Bredt-olefins have negligible regioselectivity. 

Experience has shown, however, that simple resonance considerations are not satisfactory for predicting and understanding regioselectivity in reactions with un-symmetric dipolarophiles. 

Review. These products have become of special interest since they are now available in optically pure form by asymmetric dihydroxylation of alkenes. In addition, the regioselectivity in reactions with nucleophiles can differ from that of epoxides and the yields can be superior. The reference covers literature from 1863 through 1991 (104 references). 

Double activation of the hydrogens of the 17-methylene function by the adjacent keto function and oxazole nucleus induced a regioselectivity in reactions which could in principle have involved either of the methylene functions a- to the... 

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