Hydrogenation insertion reactions

With nonracemic chiral diazoacetates the insertion process occurs with evident match/mismatch characteristics. This has been demonstrated in reactions of optically pure 2-methylcyclohexyl diazoacetates (Eq. 9) [85] and in carbon-hydrogen insertion reactions of steroidal diazoacetates (Eq. 10) [86], as well as with the synthesis of pyrrolizidines 36 and 37 [84]. The mechanistic preference for formation of a /J-lactone in Eq. 10 over insertion into the 4-position is not clear,but there are other examples of /J-lactone formation [87]. In these and related examples, selectivities in match/mismatch examples are high, and future investigations are anticipated to show even greater applicability. 

One of the most dramatic recent developments in metal carbene chemistry catalyzed by dirhodium(II) has been demonstration of the feasibility and usefulness of intermolecular carbon-hydrogen insertion reactions [38, 91]. These were made possible by recognition of the unusual reactivity and selectivity of aryl- and vinyldiazoacetates [12] and the high level of electronic control that is possible in their reactions. Some of the products that have been formed in these reactions, and their selectivities with catalysis by Rh2(S-DOSP)4, are reported in Scheme 10. 

Stang etal. (94JA93) have developed another alkynyliodonium salt mediated approach for the synthesis of y-lactams including bicyclic systems containing the pyrrole moiety. This method is based on the formation of 2-cyclopentenones 114 via intramolecular 1,5-carbon-hydrogen insertion reactions of [/3-(p-toluenesulfonyl)alkylidene]carbenes 113 derived from Michael addition of sodium p-toluenesulfinate to /3-ketoethynyl(phenyl) iodonium triflates 112 (Scheme 32). Replacing 112 by j8-amidoethynyl (phenyl)iodonium triflates 115-119 provides various y-lactams as outlined in Eqs. (26)-(30). 

Chiral Dirhodium(ll) Carboxamidates for Asymmetric Cyclopropanation and Carbon-Hydrogen Insertion Reactions... 

Intramolecular carbon-hydrogen insertion reactions have well known to be elTectively promoted by dirhodium(ll) catalysts [19-23]. Insertion into the y-position to form five-membered ring compounds is virtually exclusive, and in competitive experiments the expected reactivity for electrophilic carbene insertion (3°>2° 1°) is observed [49], as is heteroatom activation [50]. A recent theoretical treatment [51] confirmed the mechanistic proposal (Scheme 15.4) that C-C and C-H bond formation with the carbene carbon proceeds in a concerted fashion as the ligated metal dissociates [52]. Chemoselectivity is dependent on the catalyst ligands [53]. 

Alkylidenecarbenes undergo a carbon-hydrogen-insertion reaction that leads to a mixture of two isomeric products, 2-substituted furo[3,2- ]pyridines, 109, and 2-substituted furo[2,3-. 

Intramolecular Carbon-Hydrogen Insertion. The advantages of rhodium(II) catalysts for carbenoid transformations are nowhere more evident than with carbon-hydrogen insertion reactions. Exceptional regio- and diastereocontrol has been observed for Rh2(OAc)4 catalyzed transformations of a broad selection of diazoketones, a-diazo-p-ketoesters, a-diazo-P-keto-sulfones and -phosphonates which yield cyclopentanone derivatives in moderate to good yields (57-54). In contrast, poor yields and low regioselectivities characterize the corresponding copper catalyzed reactions. Applications of dirhodium(II) catalysts for C-H insertion reactions have even been extended to the synthesis of y-lactones (55), 3(2//)-furanones (56,57), P-laetones (58), and P-lactams (59,60). 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

Mnch of the reaction chemistry of alnminnm trialkyls can be classified by the following reaction types, some of which are illnstrated in Scheme 1 oxidation, alkane elimination in the presence of protic reagents, Al-C bond cleavage via halogenation and hydrogenation, insertion reactions, j6-hydrogen elimination (eqnation 5), and coordination of Lewis bases. 

Chiral Ligands. Bidentate chelation of dirhodium(II) compounds by chiral oxazolidinones creates asymmetric sites on the metal, leading to induction in cyclopropanations and carbon-hydrogen insertion reactions. The oxazolidinones are less effective in this capacity than are the pyrrolidines. ... 

Metal Carbene TVansformations. The effectiveness of Rh2(55 -MEPY)4 and its 5R-form, Rh2 5R-MEPY)4, is exceptional for highly enantioselective intramolecular cyclopropanation and carbon-hydrogen insertion reactions. Intermolecular cyclopropanation occurs with lower enantiomeric excesses than with alternative chiral copper salicylaldimine or C2-symmetric semicorrin or bis-oxazoline copper catalysts, but intermolecular cyclopropenation exhibits higher enantio-control with Rh2(MEPY)4 catalysts. The methyl carboxylate attachment of Rh2(55-MEPY)4 is far more effective than steri-cally similar benzyl or isopropyl attachments for enantioselective metal carbene transformations. The significant enhancement in enantiocontrol is believed to be due to carboxylate carbonyl stabilization of the intermediate metal carbene and/or to dipolar influences on substrate approach to the carbene center. 

Enantioselective Intramolecular Carbon-Hydrogen Insertion Reactions. The suitability of Rh2(55-MEPY)4 and Rh2(5R-MEPY)4 for enantioselective intramolecular C-H insertion reactions is evident in results with 2-alkoxyethyl diazoacetates (eq 4). Both lactone enantiomers are available from a single diazo ester. Other examples have also been reported, especially those with highly branched diazo substrate structures. ... 

Quantum dynamics of atom-hydrogen insertion reactions Lendvay G. 

Carbon Dioxide Reduction with an Electric Field Assisted Hydrogen Insertion Reaction... 

In an effort to overcome the limitations of electrochemical and photoelectrochemical cathodic reduction of carbon dioxide, and to draw upon insights of hydrogen insertion reactions studied in homogeneous... 

Figure 1. (a) Cell arrangement for metal hydride hydrogen insertion reaction. Left side, acid reduction and hydrogen atom incorporation in palladium (Pd) foil membrane. Right side, electrostatic field for enhancement of carbon dioxide/bicarbonate adsorption on foil membrane, (b.) Blow-up of palladium/hydride foil showing hydrogen insertion into carbon dioxide. 

Rhodium(II)-MEPY and rhodium(II)-MACIM (methyl 1-acetylimidazolidin-2-one-4-carboxylate) complexes are efficient chiral catalysts for intramolecular carbon-hydrogen insertion reactions of diazoacetates (224) and metal carbene transformations (225). Dirhodium(II) carboxylates of similar structure (eg, piperidinonate complexes of the Rh2(ligand)4 type) have been found efficient catalysts for asymmetric cyclopropanation of olefins (226). 

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