Active Esters as Pseudostoppers for Slippage Synthesis of [2]Pseudorotaxane Building Blocks: A Straightforward Route to Multi‐Interlocked Molecular Machines

Abstract

The efficient synthesis and very easy isolation of dibenzo[24]crown-8-based [2]pseudorotaxane building blocks that contain an active ester motif at the extremity of the encircled molecular axle and an ammonium moiety as a template for the dibenzo[24]crown-8 is reported. The active ester acts both as a semistopper for the [2]pseudorotaxane species and as an extensible extremity. Among the various investigated active ester moieties, those that allow for the slippage process are given particular focus because this strategy produces fewer side products. Extension of the selected N-hydroxysuccinimide ester based pseudorotaxane building block by using either a mono- or a diamino compound, both containing a triazolium moiety, is also described. These provide a pH-dependent two-station [2]rotaxane molecular machine and a palindromic [3]rotaxane molecular machine, respectively. Molecular machinery on both interlocked compounds through variation of pH was studied and characterized by means of NMR spectroscopy.

How Secondary and Tertiary Amide Moieties are Molecular Stations for Dibenzo-24-crown-8 in [2]Rotaxane Molecular Shuttles?

“…The [2]rotaxane molecular shuttles were obtained from the isolable active ester-containing [2]rotaxane building-block 1 ( Scheme 1 ) [ 24 ]. A very efficient slippage process allowed the formation of the N -hydroxysuccinimide ester-containing [2]rotaxane 1 in very good yield.…”

“…Several secondary stations for the DB24C8 have been reported in the literature to date. As a non-comprehensive list of them, we can cite the following positively charged stations: bipyridinium [ 4 , 5 , 6 , 7 , 8 , 9 , 10 ], 1,2-bis-(pyridinium)ethane [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ], triazolium [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ], pyridinium [ 31 , 32 , 33 ], pyridinium amide [ 34 , 35 , 36 , 37 ], imidazolium [ 38 , 39 , 40 , 41 ] moieties, oxidized ferrocene [ 42 ], etc. Some neutral molecular stations are also important to be mentioned, such as urea [ 43 ], thiourea [ 44 ], carbamate [ 45 , 46 , 47 , 48 , 49 , 50 ], isoxazole [ 51 ], and benzylic ester [ 52 ], that usually interact either through π-π stacking with the aromatic rings of the crown ether, or through hy...…”

Challenges and Opportunities in the Post‐Synthetic Modification of Interlocked Molecules

“…As eries of other [2]rotaxane building blocks containing nitrophenyl esters was described by Coutrot and co-workers in 2016 (Scheme 7). [26] Tw od ifferent synthetic pathways were envisaged for the rotaxane formation, depending on the substitution of the para-nitrophenyl aromatic ring. Thes mall size of the paranitrophenyl ester allowed instantaneous threading of the molecular axle 53 a through DB24C8.…”

Association of liquid-assisted grinding with aging accelerates the inherently slow slipping-on of a dibenzo-24-crown-8 over the N-hydroxysuccinimide ester of an ammonium-containing thread

“…Using the slippage strategy, 4 we recently reported the preparation of an insulated and storable, albeit activated, N-hydroxysuccinimide (NHS) estercontaining [2]rotaxane building block (Scheme 1 and entry 1 of Table 1). 5 This compound is a valuable activated building block for post-interlocking elongation of the encircled axle using bulky amino compounds. 6 As the mechanism of such aminolysis reactions preserves the mechanical bond, it allowed the efficient and straightforward preparation of more sophisticated interlocked compounds such as [2] and [3]rotaxane molecular shuttles.…”

“…6 As the mechanism of such aminolysis reactions preserves the mechanical bond, it allowed the efficient and straightforward preparation of more sophisticated interlocked compounds such as [2] and [3]rotaxane molecular shuttles. 5,6 Improving the access to the NHS estercontaining [2]rotaxane building block 2 is therefore of real interest. This is particularly justied since in acetonitrile solution, the slipping-on of the DB24C8 (3 equiv.)…”