C—H Bond Insertion by Rh Carbenoids

Rh-catalyzed asymmetric C—H bond functionalization via a carbene insertion reaction was extensively documented in the early days, especially the intramolecular reactions. Thanks to enormous efforts from the groups of Davies and Doyle, asymmetric intramolecular C—H bond insertion by Rh carbenoids has become a reliable methodology and has been employed frequently in the total synthesis of complex natural products.  

One important advantage of the intermolecular carbene insertion reactions is that simple starting materials can be employed and accordingly there is no need for the construction of complex substrates in advance. However, the intermolecular process requires a delicate balance between electronic and steric effects for metal carbenoids. On the other hand, there are several obstacles to be overcome, including chemo-, regio-, and enantioselectivity. Fortunately, great efforts have been devoted in the past decade and a series of carbene precursors and chiral Rh catalysts have been developed, so satisfactory yields and ee can be obtained in some catalytic systems. Generally, suitable carbene precursors, such as donor/acceptor diazo compounds, could reduce the chance of side product formation due to carbene dimerization. 

On the other hand, the dirhodium bridge caged within a lantern structure is thought to be essential to the success of dirhodium complexes in which two rhodium atoms are surrounded by four ligands in a nominal symmetry. Both computational studies and characterization of dirhodium car-benoid intermediates suggested that the intermediate adopts a Rh—Rh=C framework. In another word, two rhodium atoms are bound to one carbene center, and the bonding scenario obeys the three-center orbital paradigm. As such, metal carbenoids derived from chiral Rh complexes and donor/ acceptor diazo compounds are routinely utilized. 

Ar CgHs Rh2(S-BSP)4 Ar = p- BuCeHt Rli2(S-TBSP)4 Ar = p-(Ci2H25)C6H4 Rti2(S-D0SP)4 

Ar = CgHs Rh2(/ -TPCP)4 Ar = p-BrC6H4 Rh2(R-BTPCP)4 Ar = p-PhC6H4 Rh2(R-BPCP)4 

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Other enantioselective reactions. Several asymmetric reactions worth mentioning are the Cu-cataly/.ed allylic oxidation in the presence of 105, 106, or 107- - with t-butyl perbenzoate, oxidation of sulfides (/-BuOOH-Ti ) in the presence of a 4,4 -dimer of B-aromatic l-hydroxyestrane,-" the reductive amination by chiral t-butylsulfinamidc,- the glyoxylate ene reaction promoted by Yb(OTf), and ent-l ) C-arylation ol phenols with aryllcad reagents under the influence of brucine,- and the C—H bond insertion by Rh-carbenoids."-"... 

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