Insert induction bonding

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

Numerous studies have been directed toward expanding the chemistry of the donor/ac-ceptor-substituted carbenoids to reactions that form new carbon-heteroatom bonds. It is well established that traditional carbenoids will react with heteroatoms to form ylide intermediates [5]. Similar reactions are possible in the rhodium-catalyzed reactions of methyl phenyldiazoacetate (Scheme 14.20). Several examples of O-H insertions to form ethers 158 [109, 110] and S-H insertions to form thioethers 159 [111] have been reported, while reactions with aldehydes and imines lead to the stereoselective formation of epoxides 160 [112, 113] and aziridines 161 [113]. The use of chiral catalysts and pantolactone as a chiral auxiliary has been explored in many of these reactions but overall the results have been rather moderate. Presumably after ylide formation, the rhodium complex disengages before product formation, causing degradation of any initial asymmetric induction. 

In addition, the infrared examination of the mechanism of propane and oxygen interaction with the sample (Fig. 6) indicates the different mechanism of interaction of the intermediate propylene as compared to other supported vanadium catalysts such as V-Ti02 (10). In particular, the formation of a 7t-bonded complex stabilized by a nearlying silanol with weak basic character due to the inductive effect of vicinal vanadium is shown. This indicates the relative inertness of the V sites in the silicalite towards 0-insertion or allylic H-abstraction on the adsorbed propylene. It is evident that the reduced reactivity of V sites in these reactions limits the consecutive reactions of intermediate propylene, thus enhancing the selectivity in the formation of this product. 

The rest of the catalyst cycle is identical to that illustrated for the rhodium complex-catalyzed reactions in Scheme 1. It has been proposed that the asymmetric induction occurs during the formation of alkyl-Pt(CO)L2 intermediate through olefin insertion into the Pt-H bond [13]. 

We first established that hydrocarbonylation reactions occur with cis-stereochemistry (29, 16) and that asymmetric induction occurs before or during the formation of the metal alkyl intermediate (5, 6). This means that is either during the 7r-olefin complex formation between catalyst and substrate or during the insertion of the 7r-complexed olefin into the M-H bond. Therefore, the model should focus on the interactions between the substrate double bond and the catalytically active metal atom of the catalyst. 

Although the Chalk-Harrod mechanism has been widely accepted,69 some phenomena (include an induction period for many precatalysts and the formation of vinylsilanes) cannot be explained well by the Chalk-Harrod mechanism. An alternative mechanism to the Chalk-Harrod mechanism involves insertion of the alkene into the M-Si bond instead of insertion of the alkene into the M-H bond (Fig. 5).70... 

In summary, the C-H insertion chemistry of rhodium carbenoids is a very powerful method for transformation of C-H bonds. Highly regioselective and stereoselective reactions are possible and several classes of chiral catalyst are capable of very high asymmetric induction. The chemoselectivity in this chemistry is exceptional, as illustrated by the numerous intermolecular and intramolecular reactions described in this overview. Most notably, this chemistry offers new and practical strategies for enantioselective synthesis of a variety of natural products and pharmaceutical agents. 

For the aziridination of 1,3-dienes, copper catalysis gave better yields of A-tosyl-2-alkenyl aziridines with 1,3-cyclooctadiene, 1,4-addition occurred exclusively (50%) [46]. Good results were also obtained on rhodium catalysed decomposition of PhI=NNs (Ns = p-nitrophenylsulphonyl) with some alkenes the aziridination was stereospecific, whereas with chiral catalysts asymmetric induction (up to 73% ee) was achieved. However, cyclohexene gave predominantly (70%) a product derived from nitrene insertion into an allylic carbon-hydrogen bond [47]. 

The rate of addition to a double bond increases in the sequence shown above, i.e. the cycloalkenecarbenes are less reactive (more stable) from left to right. The position of 2g is not reliable since no insertion with simple olefins has been reported. Again the Ka-values would also be a measure of inductive and resonance effects. The A/I rate with polyenes is modified in comparison with the A/I values for simple olefins. 

Studies on host-guest peptides have shown that incorporation of a proline residue into peptides which have a high tendency to fold into ordered secondary structures disrupts the onset of helical as well as P-sheet conformations and increases solubility and coupling rates.f Based on this observation, serine- and threonine-derived oxazolidine, and cysteine-derived thiazolidine derivatives (pseudoprolines) were proposed as valuable tools for combining protection of their side-chain functions with the simultaneous solubilization of the peptide chain. Due to the induction of kink conformations in the peptide backbone, originating from the preference of these pseudoproline residues to adopt the cis-imide bond configuration, insertion of such derivatives into peptides prevents self-association and P-structure formation. 

Hydroformylation with platinum complexes proceeds as described in Scheme 5 when a Lewis acid, e.g., SnCl2, is added. The Lewis acid removes the chloride from the platinum center to afford a vacant coordination site to which the olefins can coordinate. Asymmetric induction occurs during the formation of alkyl intermediates via olefin insertion into the Pt-H bond [8]. Most importantly, the re-gio- and enantioselectivities are strongly influenced by the reaction temperature in the Pt(II)-catalyzed asymmetric hydroformylations [10, 70, 71, 72, 73]. Re-... 

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