Pd-Catalyzed Asymmetric Allylic Alkylation of Cyclobutenes: From Double Inversion to Double Retention

The Pd-catalyzed allylic alkylation of 3,4-disubstituted, racemic cyclobutene electrophiles exhibits a highly unusual stereoselectivity that allows for controlling diastereo- and enantioselectivity only by the choice of ligand and independent of the configuration of the substrate. In order to shed light on the origin of stereoinduction, we performed a systematic mechanistic investigation, including preparation of various putative Pd-allyl intermediates, ¹H/³¹P NMR reaction monitoring, ²H-labeling studies, ESI-HRMS and ³¹P NMR analysis of reaction mixtures, and DFT structural computations. The mechanism disclosed exhibits several steps with stereospecificities deviating from the commonly accepted "double inversion rule": oxidative addition was found to follow a stereoconvergent course, giving anti-configured η¹-Pd-cyclobutene species as detectable on-cycle intermediates irrespective of the configuration of starting material, while the subsequent nucleophilic attack features a stereodivergent behavior. In stark contrast to their highly reactive anti-analogues, syn-Pd-cyclobutene complexes that can be formed as side products are rendered entirely unreactive by strong internal Pd-O chelation, preventing the formation of undesired product diastereomers.

Probing Long-Range Anisotropic Interactions: a General and Sign-Sensitive Strategy to Measure 1 H-1 H Residual Dipolar Couplings as a Key Advance for Organic Structure Determination

Residual dipolar couplings (RDCs) are amongst the most powerful NMR parameters for organic structure elucidation. In order to maximize their effectiveness in increasingly complex cases such as flexible compounds, a maximum of RDCs between nuclei sampling a large distribution of orientations is needed, including sign information. For this, the easily accessible one-bond ¹ H-¹³ C RDCs alone often fall short. Long-range ¹ H-¹ H RDCs are both abundant and typically sample highly complementary orientations, but accessing them in a sign-sensitive way has been severely obstructed due to the overflow of ¹ H-¹ H couplings. Here, we present a generally applicable strategy that allows the measurement of a large number of ¹ H-¹ H RDCs, including their signs, which is based on a combination of an improved PSYCHEDELIC method and a new selective constant-time β-COSY experiment. The potential of ¹ H-¹ H RDCs to better determine molecular alignment and to discriminate between enantiomers and diastereomers is demonstrated.