[reaction: see text] Co(2)(CO)(6)-complexed alkynyl pinacolborane derivatives are readily transformed with functional group tolerance into fused arylboronates via the [2 + 2 + 2]cycloaddition to alpha,omega-diynes.
The four N‐(ω‐oxo‐ω‐phenylalkyl)‐substituted imidazolidinones 5–8 were prepared from N‐acetylimidazolidinone (4). Upon irradiation, these substrates underwent Norrish–Yang cyclization to the racemic products rac‐9–rac‐12 (51–75 %). The reactions of the N‐2‐oxoethylimidazolidinones 5 and 6 were conducted in tBuOH, and yielded 1:1 mixtures of exo/endo diastereoisomers rac‐9 a/rac‐9 b and rac‐10 a/rac‐10 b, accompanied by Norrish type II cleavage products. The reactions of the N‐3‐oxopropylimidazolidinones 7 and 8 were performed in toluene. The exo diastereoisomers rac‐11 a and rac‐12 a were the major diastereoisomers (d.r. ≅4:1). In the presence of the chiral compounds 1–3, the photocyclization of substrate 8 proceeded with significant enantiomeric excess (5–60 % ee). The more sophisticated complexing agents 3 and ent‐3 provided better enantiofacial differentiation (up to 60 % ee) than the lactams 1 and 2 (up to 26 % ee). Low temperatures and an excess of the complexing agent helped to increase the enantioselectivity. The absolute configuration of the major exo product 12 a obtained from compound 8 in the presence of complexing agent 3 was unambiguously established by single‐crystal X‐ray crystallography of its chiral N‐methoxyphenylacetyl derivative 15 a. In a similar fashion, the absolute configurations of the endo products 12 b and ent‐12 b were established. The N‐2‐oxoethylimidazolidinone 5, which crystallized in a chiral space group, was irradiated in the solid state. At low levels of conversion, the product 9 a/ent‐9 a was formed with high enantiomeric excess (78 % ee). The enantioselectivity deteriorated at higher levels of conversion.
The renaissance in radical chemistry during the last years and decades can be explained in large part by the improved prediction and control of the important parameters chemo-, regio-, and stereoselectivity.[1] Recent investigations show that radical reactions can be carried out enantioselectively without an auxiliary being attached covalently to the substrate.[2] Two different strategies have been reported. On the one hand, it is possible to differentiate the enantiotopic faces of a prochiral radical with chiral reagents (reagent control). For this purpose chiral hydrogen-atom donors have been used most often. [3] On the other hand, face differentiation is possible by a Lewis acid, which forms a chelate complex with the substrate, and which is in turn coordinated to chiral ligands.[4] Alternative chiral templates that are based upon noncovalent interactions and are similarly effective have, to the best of our knowledge, not yet been established. In a recent study we were able to show that high enantioselectivity (up to 84 % ee) in radical reactions can be achieved with the help of a hydrogenbonding chiral template. Our preliminary results are presented in this communication.We investigated the enantioselectivity of the reductive radical cyclization of 3-(w-iodoalkylidene)piperidin-2-ones (1). These compounds can be synthesized by the aldol condensation of N-tert-butyloxycarbonyl(Boc)piperidin-2-one with w-tert-butyldimethylsilyl(TBDMS)oxyaldehydes followed by conversion of the protected hydroxy group into an iodo group (1. tetrabutylammonium fluoride (TBAF), THF; 2. PPh 3 , imidazole, I 2 ). [5] In the presence of an initiator and Bu 3 SnH the alkenyl iodides reacted in a 5-or 6-exo-trig-cyclization [1, 6] (e.g. 2 a! 3 a, Scheme 1). The intermediate radicals 3 exhibit a prostereogenic center in a-position to the carbonyl function which is transformed by an intermolecular reaction with Bu 3 SnH to a stereogenic saturated carbon atom. In the case of the alkenyl iodide 1 a both enantiomeric cyclization products 4 a and ent-4 a are formed during the reaction.The reaction proceeded smoothly for the three substrates investigated. The appropriate cyclization products were
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