Ligands cycloaddition

A closely related formation of >/3-cydopropenyl molybdenum complex Cp P(OMe)3)2Mo( 3-C3Ph2Me), possibly involving an interamolecular ligand cycloaddition within a carbyne-acetylene intermediate complex, has been accomplished by treating the acetylenic cation complex [Cp f P(OMe)3 2Mo( 2-PhC=CPh)]BF4 with vinylmagne-sium bromide in THF (equation 253)321. 

Keywords Asymmetric catalysis Azomethine ylides Chiral ligands Cycloaddition ... 

While these methods were so far mostly shown to allow the incorporation of dyes, affinity handles, and convertible groups, they also enrich the arsenal of methods for the introduction of transition-metal ligands. Cycloaddition and cross-coupling reactions were successfully utilized to attach a variety of diene. 

The intramolecular [In + 2 rr] cycloaddition ofmethylenecyclopropane with the alkyne in 117 using isopropyl phosphite as a ligand affords the methylene-cyclopentene 118[55]. 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

Cationic complexes of rran.s-chelating tridentate ligand, (/ ,/ )-4,6-dibenzo-furandiyl-2,2 -bis(4-phenyloxazoline), with transition metal(II) perchlorates as effective catalysts for asymmetric cycloaddition of nitrones 98YGK368. 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

On the other hand, the corresponding tin precursor (63) undergoes smooth cycloaddition with a wide variety of aldehydes to produce the desired methylene-tetrahydrofnran in good yields [32, 33]. Thus prenylaldehyde reacts with (63) to give cleanly the cycloadduct (64), whereas the reaction with the silyl precursor (1) yields only decomposition products (Scheme 2.20) [31]. This smooth cycloaddition is attributed to the improved reactivity of the stannyl ether (65) towards the 7t-allyl ligand. Although the reactions of (63) with aldehydes are quite robust, the use of a tin reagent as precursor for TMM presents drawbacks such as cost, stability, toxicity, and difficult purification of products. 

A series of chiral binaphthyl ligands in combination with AlMe3 has been used for the cycloaddition reaction of enamide aldehydes with Danishefsky s diene for the enantioselective synthesis of a chiral amino dihydroxy molecule [15]. The cycloaddition reaction, which was found to proceed via a Mukaiyama aldol condensation followed by a cyclization, gives the cycloaddition product in up to 60% yield and 78% ee. 

The cycloaddition reaction between ethyl glyoxylate 4a and Danishefsky s diene 2a has been investigated by Ghosh et al. applying catalyst systems derived from Cu(OTf)2 and ligands (S)-Ph-BOX (S)-21a, (S)-t-Bu-BOX (S)-21b, and the confer-... 

This methodology has been used for the synthesis of the C3-C14 segment 24 of the antitumor agent laulimalide 23 (Scheme 4.22) [35]. The constrained chiral BOX ligand 21c in combination with Cu(OTf)2 afforded dihydropyrane 6f by a cycloaddition reaction in good yield and ee this was converted to the C3-C14 segment 24 via a Ferrier-type rearrangement in several steps. 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

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