Efficient End-Capping Synthesis of Neutral Donor-Acceptor [2]Rotaxanes Under Additive-Free and Mild Conditions

Efficient end‐capping synthesis of neutral donor–acceptor (D–A) [2]rotaxanes without loading any catalysts or activating agents was achieved by utilizing high reactivity of a pentacoordinated hydrosilane toward salicylic acid derivatives. As components of [2]rotaxanes, an electron‐deficient naphthalenediimide‐containing axle with a salicylic acid terminus and several electron‐rich bis(naphthocrown) ether macrocycles were employed. End‐capping reactions with the pentacoordinated hydrosilane underwent smoothly even at low temperature to afford the corresponding [2]rotaxanes in good yields. A [2]rotaxane containing bis‐1,5‐(dinaphtho)‐38‐crown‐10 ether as a wheel molecule was synthesized and isolated in 84 % yield by the end‐capping at −10 °C, presenting the highest yield ever reported for the end‐capping synthesis of a neutral D–A [2]rotaxane. It was found that the yields of the [2]rotaxanes in the end‐capping reactions were almost parallel to the formation ratios of the corresponding pseudo[2]rotaxanes estimated by utilizing model systems. These results indicate that the end‐capping reaction using the pentacoordinated hydrosilane proceeded without perturbing the threading process, and most of the pseudo[2]rotaxanes underwent efficient end‐capping reaction even at low temperature. Towards a facile synthetic protocol: A pentacoordinated hydrosilane works as an end‐capping agent for the synthesis of neutral donor–acceptor [2]rotaxanes (see scheme). This protocol is performable at low temperature without loading of any catalysts and activating agents and applicable to the synthesis of rotaxanes based on weak intercomponent interactions.