Understanding the reaction mechanisms, particularly the chiral induction mode, is critical for the development of new asymmetric catalytic reactions. Rhodium(III)-catalyzed C–H activation en route to atroposelective [4 + 2] annulative coupling with α-diazo β-ketoesters has been realized, affording axially chiral phenanthrenes in good to excellent enantioselectivity. A combination of experimental and computational studies revealed a nontraditional stereodivergent–convergent chiral induction mode. The reaction proceeded with a rhodafluorene intermediate, followed by competitive, constructive, and stereodivergent migratory insertions of the two Rh–C(aryl) bonds into the carbene species to give β-ketoester intermediates. Then, the other Rh–C(aryl) bond migratorily inserts into the ketone carbonyl group. Following this stereodetermining carbonyl insertion, an ester-chelated rhodium(III) alkoxide species bearing two poorly controlled chiral centers and a well-controlled C(sp2)–C(sp3) chiral axis is generated. The final product is delivered via stereoconvergent elimination of a rhodium(III) species with retention of the well-controlled axial chirality and with loss of the central chirality. 

A Stereodivergent–Convergent Chiral Induction Mode in Atroposelective Access to Biaryls via Rhodium-Catalyzed C–H Bond Activation

E1cB and E2 eliminations have been described as competing mechanisms that can even share a common pathway when the E1cB/E2 borderline mechanism operates. A suitable case study evincing such a mechanistic dichotomy corresponds to the elimination reaction of β-phenylmercaptoethyl phenolate, since its mechanism has been thought to be an E2 elimination. Nonetheless, according to the computational assessment of the substituents on the leaving group, we demonstrate that the reaction proceeds via a borderline E1cB mechanism. Stabilization of the carbanion was provided not only by substituent effects tuning the nucleofugality of the leaving group, but also by a base, since distortion/interaction–activation strain and Natural Bond Order (NBO) analyses suggest a stabilizing interaction between the base and Cβ of the E1cB intermediate. In order to gain insights into these results in a more general context, we have rationalized them with a qualitative picture of the E1cB/E2 mechanistic dichotomy using simple relationships between diabatic parabolas modeling the potential wells of reactants, intermediates, and products. In this Diabatic Model of Intermediate Stabilization (DMIS), the borderline E1cB mechanism for the elimination reaction of β-phenylmercaptoethyl phenolate was discussed in terms of bonding and dynamic stepwise processes. The conceptual model presented herein should be useful for the analysis of any reaction comprising competing one- and two-step mechanisms.

/pdf/theoretical-insights-into-the-e1cb-e2-mechanistic-dichotomy-1o1f8u5k4a.pdf

Theoretical insights into the E1cB/E2 mechanistic dichotomy of elimination reactions

Elimination reactions can be very useful in organic synthesis for producing alkenes, alkynes, and allenes on a preexisting scaffold. Synthetic issues associated with eliminations include the suppression of substitution and rearrangement as well as the control of regiochemistry and stereochemistry. Factors determining selectivity and suitability in synthesis will be discussed and methods involving stereoselective and stereospecific transformations to di-, tri-, and tetrasubstituted alkenes will be disclosed. Elimination reactions that form alkynes and both enantioenriched and racemic allenes are also featured. Recent examples of the use of elimination reactions in the synthesis of complex natural products will be showcased.

6.18 Eliminations to Form Alkenes, Allenes, and Alkynes and Related Reactions

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6.18 Eliminations to Form Alkenes, Allenes, and Alkynes and Related Reactions