1.3- dipolar cycloaddition reactions catalysis

The author has been involved for quite a long time in the study of Lewis acid catalysis of 1,3-dipolar cycloaddition reactions. From his research group, a series of methodologies directed to the Lewis acid-mediated stereochemical and regiochem-ical control of 1,3-dipolar cycloaddition reactions has been reported this includes ... 

Attempts to Catalyze [3 + 2]-Cycloaddition of Nitronates to Olefins In Section 3.2.1.2.2.2, it was noted that [4+ 2]-cycloaddition reactions of nitro-alkenes and alkenes proceed much faster in the presence of LA. At the same time, in the presence of LA, nitronates can rapidly decompose (49) or undergo rearrangements (see Section 3.4.2.5.6 ). Hence, it is not surprising that catalysis of 1,3-dipolar cycloaddition reactions of nitronates with alkenes by LA has attracted little attention until very recent times. An exception is the study by the Japanese... 

The rhodium( 11)-catalyzed formation of 1,3-dipoles has played a major role in facilitating the use of the dipolar cycloaddition reaction in modern organic synthesis. This is apparent from the increasing number of applications of this chemistry for the construction of heterocyclic and natural product ring systems. This chapter initially focuses on those aspects of rhodium(II) catalysis that control dipole formation and reactivity, and concludes with a sampling of the myriad examples that exist in the Hterature today. 

Nitrones are the most widely studied of the 1,3-dipoles in the field of catalyzed enantioselective 1,3-dipolar cycloaddition reactions. Effective catalysis using a variety of chiral Lewis acid catalysts has been reported for the nitrone cycloaddition... 

Padwa and Prein presented an extensive experimental and theoretical study of the 1,3-dipolar cycloaddition reactions of isomunchnones with olefinic dipolaro-philes. The a-diazo carbonyl isomunchnone precursors were synthesized in the usual fashion from amides and diazoethylmalonyl chloride. For example, isomunchnone 457 was readily generated from 456 using rhodium catalysis to form... 

In all instances only starting material was isolated with the and C-NMR confirming the absence of any triazole signals. This clearly illustrates that the dipolar cycloaddition reaction only takes place in the presence of cucurbituril, which has a dual function to catalyze the cycloaddition reaction and in the process become inextricably threaded onto the polymer chain. The sequence of events involved in this catalytic self-threading process is presented in Fig. 1.45. The combination of using monomers with bulky in-chain stopper groups and the catalyst self-threading onto the polymer backbone limits this catalysis to a turnover of exactly 1. 

Scheme 3.5 Domino carbonyl ylide formation-l,3-dipolar cycloaddition reaction catalysed by a combination of rhodium catalysis and chiral nickel catalysis.

Scheme 8.3 Tandem reduction-ring-closure-ring-opening-l,3-dipolar cycloaddition reaction catalysed by whole-ceU catatysis and copper catalysis.

Subsequently, Wang s group presented an unprecedented asymmetric exo-selec-tive 1,3-dipolar cycloaddition reaction of azomethine ylides with a-methylene-y-butyrolactone under the catalysis of a Cu(l)/DTBM-B1PHEP complex, providing an expedient and straightforward access to a series of enantiomerically enriched spiro-[butyrolactone-pyrrolidine] derivatives bearing one to two spiro quaternary stereogenic centers with excellent levels of stereocontrol (Scheme 24) [44]. 

The discovery of Cu(I) catalysis for the Huisgen 1,3-dipolar cycloaddition reaction has led to a tremendous number of synthetic developments [104, 143]. Even if the exact mechanism of the reaction is still debated, the process involves formation of a transient 5-cuprotriazole, which is protodemetalated to close the catalytic cycle. If stoichiometric or an excess of copper is used, one can conceive to trap this transient metalated intermediate by electrophiles (Fig. 7). 

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