1,2‐Chirality Transfer in the Synthesis of Cyclopropanes

Abstract

In the last few years organic photochemistry has made remarkable progress in the area of asymmetric synthesis.[1] A range of new strategies has been developed that drastically enhance the diastereo-and enantioselectivity of certain photochemical processes. Here we report on an entirely novel concept concerning the synthesis of enantiomerically enriched cyclopropanes based on a photochemically induced intramolecular 1,2-chirality transfer.During chirality transfer a prochiral moiety incorporates the chiral information of a chiral moiety, which simultaneously loses this information. A distinguishing feature in the intramolecular transfer of central chirality is the distance between the source and the target of the chiral information. While relatively many examples of 1,3-chirality transfer exist, [2] which are mainly based on the transfer of chirality along an allylic system, there are only a few examples of 1,2-chirality transfer. [3] Some time ago we developed the concept of spin-center shift, [4, 5] which provides a completely new access to cyclopropanes. In contrast to the classical Norrish-Yang reaction, [6] phenyl ketones 1 with a suitable leaving group X in a position are used. At the stage of 1,4-diradicals 2 this results in a shift of the radical center from the carbonyl C atom to the adjacent C atom as a result of HX elimination. The resulting 1,3-diradicals 3 cyclize with often complete diastereoselectivity and good to very good yields to give cyclopropanes 4 (Scheme 1). [4] By extensive quantum chemical calculations we could elucidate details of the mechanism and found that the leaving group X already has a significant impact on the first reaction step, the photochemical H-shift (1!2, Scheme 1). [4] Owing to hyperconjugation between the s* orbital of the CÀX bond and the p system of the excited carbonyl group, X prefers a pseudoaxial arrangement with respect to the approximately chairlike six-membered transition state.[4b] If we consider enantiomerically pure ketones of type 5 with two identical substituents in b position, we can formulate two different transition states for the g-H abstraction (TS-5 a and TS-5 b), which differ in the arrangement of the leaving group X (axial or equatorial, Scheme 2). Based on the quantum chemical calculations, [4b] TS-5 a with X in the axial position should be favored over TS-5 b. In TS-5 a the g-methylene group is attacked, whereas in TS-5 b the attack takes place at the g'-methylene group, which means that the carbonyl oxygen atom must distinguish between the two diastereotopic methylene groups. During the photochemically induced H-transfer and the following elimination of HX, the enantiomeric diradicals 6 a and 6 b are formed, and the chiral information of the a-C atom has been transferred to the adjacent prochiral b-C atom (1,2-chirality transfer). Consequently, after the cyclization the enantiomeric cyclopropyl ketones (R)-7 and (S)-7 are obtained, whereby the formation of (R)-7 should be favored over (S)-7. Thus, during such a process desymmetrization relat...

Carbon–Carbon Bond Forming Reactions via Photogenerated Intermediates

“…Thus, γ-hydrogen abstraction was followed by the elimination of MsOH and cyclopropylketones 13a , b were formed upon cyclization of the resulting 1,3-biradicals ( 12a , b , Scheme ). , The reaction required N -methylimidazole in order to buffer the acidity liberated and gave exclusively the trans -adducts depending on the substituent R.…”

Formation of a Three‐Membered Ring

“…Wessig and colleagues have shown that ketones with a variety of leaving groups, such as OMs (OSO 2 CH 3 ), OTs (OSO 2 C 6 H 4 CH 3 -p), OPO(OEt) 2 [72a]. In addition, the photochemistry of enantiomer-rich samples of the ketones 107 showed a 1,2-chirality transfer (Scheme 4.50) [73]. Although the ketones 107 give the cyclopropyl ketones 108 at room temperature in 28-52% ee yields, the rise in temperature enhances the stereoselectivity.…”