In spite of relatively few examples, two‐center catalysis is proving to be a clever and effective means of performing diverse reactions with both high regio‐ and enantioselectivity. A simple scheme allows classification of the efficient M1–M2–binol complexes 1 as well as the enzymes urease and carbon monoxide dehydrogenase. M1 = La, Al,…; M2 = Li, Na, K,….

Palladium-catalyzed allylic substitution reactions are well established in organic synthesis."] We and others showed that an enantioselective reaction can be realized with ligands containing electronically different donor centers;gz1 of these, chiral phosphinoaryldihydrooxazole (phosphinooxazoline) ligands were found to be particuIarIy useful.[31 With the prototype P 1 of this ligand class it was possible to obtain substitution products with enantiomeric purity of 2 98 % ; 1 ? from acyclic substrates. The postulated catalytic cycle for the examined system is shown in Scheme 1. The cycle contains the essential intermediates 2-5, of which only the easily preparable allylic complexes 4 could be characterized so far. In the case of an unsymmetrical ligand such as 1, an exo-(4x) and an endo-n-complex ( 4 4 are fundamentally possible; these are in equilibrium through n-o-n rearrar~gement!~] As a consequence, there are in principle four reaction pathways, of which two lead to the preferred product (S)-6. Attack of the nucleophile can occur at C-3 o f 4 x or at C-1 of 4 n . In spite of contrary assumptions (earlyr4,5b1 and late N PPhz = ''Y [*I Priv.-Doz. Dr M. Reggelin 2 R h N U r L 4x (exo) R A N " 5a 5b Scheme 1. Catalytic cycle of the Pd-catalyzed allylic substitution; R = Ph, X = OAc, NU = CH(COOCHI),.transition state[5a1), mechanistic interpretations up to now were all based on the postulate that the first reaction pathway i s the faster one. We were now able to detect a primarily formed v2olefin -Pdo complex in the catalytic cycle and unambiguously determine its conformation. Therefore, it is now possible to make a well-founded statement about a late transition state.Initially the stoichiometric reaction between the n-ally1 complexes 4I61 (10: 1 mixture of 4 x and 4n) and sodium dimethylmalonate (NaDMM) as a nucleophile was examined. A solution of the complexes in [DJTHF (0.5 mL, 8 8 m~) , prepared at room temperature, was cooled to -78 "C, and a precooled solution of NaDMM (20.3 mg, 0.132 mmol) in [DJTHF (0.5 mL) added. The sample was allowed to warm to room temperature inside the NMR probehead, and the progress of the reaction was monitored by 3 1 P NMR spectroscopy (Figure 1). A 4x (21 .O) 4n (25.2) I

Despite a separation of 7 Å the angles between C–H and H–H vectors (e.g., C3–H3/H25–H26) in 1 can be determined by NMR spectroscopy by the measurement of cross‐correlated double‐ and zero‐quantum relaxation rates in solution. As shown by comparison with a crystal structure of 1, the intervector projection angles can be measured with an error of at most ±3°.

No abstract

Described herein is a practical method for the generation of o-azaxylylenes and their application to the synthesis of a variety of hydroquinoline derivatives. The formation of oazaxylylenes has previously been reported by photochemical fragmentation, [1,2] fluoride induced elimination of o-trime-Figure 2. The molecular structure of 4. Selected bond lengths [] and angles [8]: Hf1ÀC1