Induction diene complex

In contrast, molybdenum and iron diene complexes undergo the same type of reaction with chiral lithium imide enolates, with moderate to good induction at the p-position (eq 16). ... 

Palladium(II) acetate and palladium(II) chloride (often applied as the soluble dibenzonitrile complex) are especially suited for cyclopropanation of strained double bonds as well as styrene and its ring-substituted derivatives.152,154,155 The good coordinating abilities of these palladium ) compounds, however, somewhat complicate the catalytic action and may even limit it. Thus, the presence of phosphane ligands in palladium(II) halides causes a significant induction period for decomposition of the diazocarbonyl compound,156 and the formation of stable palladium diene complexes may even prevent the cyclopropanation reaction.155,157 Furthermore, alkenes such as 4-dimethylaminostyrene and 4-vinylpyridine cannot be cyclo-propanated since their basic center deactivates the catalyst.155... 

Table IV and Fig. 3 provide a comparison of the effectiveness of some of the activators. These comparisons are based on the amounts of C6 dienes produced within a given period of time after the addition of the organic chloride to a totally inactive Rh1 complex. As can be seen from Fig. 3, the very best activator triggers the catalytic reaction with almost unnoticeable induction period, while a substantial induction period is apparent with the least efficient activator.

In the literature it has been generally assumed that hydrogenation of the spectator dienes with cationic Rh(I)-complexes [13] proceeds rapidly before the hydrogenation of the prochiral alkene. These induction periods, which were found in many hydrogenation reactions, however, prove without doubt the slower hydrogenation of the dienes. 

Further proof for the fact that these induction periods are caused by slow hydrogenation of the diene ligand was obtained by NMR-spectroscopic measurements under hydrogenation conditions [10f, 15]. The registration of 31P- and 1H-spectra allows the simultaneous monitoring of changes in the bisphosphine complexes and substrate conversion. The results of the hydrogenation of methyl-... 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Grubbs synthesized molybdenum catalyst 74 and reported the first example of asymmetric induction by kinetic resolution of diene using RCM with complex 74." After 90% conversion of the reaction of diene with 74, an enantiomeric excess (ee) of the recovered staring material 78 showed 84% and an absolute configuration is determined to be 5 ... 

While some molybdenum complexes such as Mo03(dien) were found to be inactive,236 the rates of molybdenum-catalyzed epoxidation of alkenes were found to be independent of the structure of the complex used, after an induction period representing the time for exchange of anionic ligands by the alkyl hydroperoxide. cis-Dioxomolybdenum(VI) diolates such as (78) were isolated... 

In spite of the high asymmetric induction observed in the Mo-catalyzed ARCM of 1,6-dienes, when complexes 4a and 4b are used in reactions involving 1,7-dienes, inferior asymmetric induction is obtained. For example, as illustrated... 

The chiral boron complex prepared in situ from chiral binaphthol and B(OPh)3 is utilized for the asymmetric aza-Diels-Alder reaction of Danishefsky s diene and imines [67] (Eq. 8A.43). Although the asymmetric reaction of prochiral imine affords products with up to 90% ee, the double asymmetric induction with chiral imine by using oc-benzylamine as a chiral auxiliary has achieved almost complete diastereoselectivity for both aliphatic and aromatic aldimines. This method has been successfully applied to the efficient asymmetric synthesis of anabasine and coniine of piperidine alkaloides. 

In spite of the high asymmetric induction observed in the Mo-catalyzed ARCM of 1,6-dienes, when 4a and 4b are used in reactions involving 1,7-dienes, inferior asymmetric induction is obtained. For example, as illustrated in Scheme 2, dienes 12 and 13 are not resolved with useful selectivity (krei < 5) when 4a is employed as the catalyst. To address this shortcoming, we took advantage of the modular character of the Mo complexes and prepared a range of chiral complexes as potentially effective catalysts. Accordingly, as depicted in Scheme 2, we... 

The discovery that chiral Lewis acids can catalyze the asymmetric Diels-Alder reaction is a major milestone for the scale up and practice of this reaction on an industrial scale. The use of such a catalyst obviates the need for a chiral auxiliary on the diene or dienophile. The vast majority of chiral auxiliaries that have been used in the Diels-Alder reaction are either not commercially available or are expensive. In addition, the chemical steps needed to attach and remove the chiral auxiliary increase the cost and complexity of the synthesis. Chiral catalysts may also be recovered or recycled, further decreasing cost.47 Research in this area is very active, and catalysts based on a number of metals (Table 26.1) have shown encouraging asymmetric induction.21 Our understanding of the role these catalysts play in the asymmetric induction of Diels-Alder reactions is increasing, and more general reagents should appear. 27-48 54... 

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