Abstract
A short approach to chiral diaza-olefines from protected 2,2′-diamino-1,1′-binaphthyl is presented. Cis- and trans-olefines can be selectively obtained by twofold N-allylation followed by RCM or by bridging a 2,2′-diamino-1,1′-binaphthyl precursor with trans-1,4-dibromo-2-butene. Deprotection afforded cis- and trans-dihydro[1,6]diazecines 1 in 58 and 64% overall yield. The reactivity of the but-2-ene-1,4-diyl fragment was investigated yielding corresponding epoxides, diols, and mono- and dibromo products. In several cases rearrangements and participation of the proximate N-Boc group was observed. In no case could allylic substitution be accomplished. From 13 compounds X-ray structure analyses could be obtained.
Highlights
Monoolefine and diolefine ligands are often key players in homogeneous catalysis and have found various applications in asymmetric transformations [1,2]
Cis- and trans-olefines can be selectively obtained by twofold N-allylation followed by ring closing metathesis (RCM) or by bridging a 2,20 -diamino-1,10 -binaphthyl precursor with trans-1,4-dibromo-2-butene
The preferred structures are either rigid, based on bicyclic diene skeletons [3,4,5], semi-rigid, consisting of a mono-ene as part of a cycle which is linked to P [6,7] or S [8,9] functionalities as second coordination site, or flexible with the olefin part being a freely rotating pending side arm attached at a chiral back bone [10,11,12,13,14,15,16,17,18,19,20]
Summary
Monoolefine and diolefine ligands are often key players in homogeneous catalysis and have found various applications in asymmetric transformations [1,2]. The requirement for an efficient chiral ligand in transition metal catalysis is its ability to form only a few, conformative stable diastereomeric intermediates during the catalytic cycle These show highly differing stability and/or traversing transition states with significantly different activation energy on the reaction path to product enantiomers. As the search for proper catalysts is a largely empirical and time consuming process, easy access to ligand libraries to be tested is desired To this end, structural modification should be done at a late stage of the synthesis, preferably as the last step. As a further extension of ligand design, we considered the incorporation of an atropomeric biaryl unit as part of a cycloolefine A or –diolefine moiety B (Figure 2) This would place corresponding olefine complexes in a chiral environment with a variable degree of conformative freedom depending on the size and rigidity of the perimeter. The aim of the present investigation was to synthesize the simplest candidate 1 (R=H) through bridging of 2,20 -diamino-1,10 -binaphthyl, exploring stereochemistry and reactivity [21]
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