Abstract
Despite significant advances in the last three decades towards high yielding syntheses of rotaxanes, the preparation of systems constructed from more than two components remains a challenge. Herein we build upon our previous report of an active template copper-catalyzed azide-alkyne cycloaddition (CuAAC) rotaxane synthesis with a diyne in which, following the formation of the first mechanical bond, the steric bulk of the macrocycle tempers the reactivity of the second alkyne unit. We have now extended this approach to the use of 1,3,5-triethynylbenzene in order to successively prepare [2]-, [3]- and [4]rotaxanes without the need for protecting group chemistry. Whilst the first two iterations proceeded in good yield, the steric shielding that affords this selectivity also significantly reduces the efficacy of the active template (AT)-CuAAC reaction of the third alkyne towards the preparation of [4]rotaxanes, resulting in severely diminished yields.
Highlights
Whilst the use of non-covalent interactions to direct the self-assembly of oligo[n]pseudorotaxanes prior to formation of a permanent mechanical bond is the most operationally simple method towards forming multi-component interlocked species [46,47,48,49,50,51], this generally offers little in the way of control over structural or sequence specificity
We have developed a small macrocycle [62] modification of Leigh’s active template Cu-mediated azide-alkyne cycloaddition (AT-CuAAC) methodology [63] that enables the preparation of rotaxanes in very high yield, and removes the need for extraordinarily large stoppering units [64]
We have shown that the steric encumbrance of a macrocycle can be used in lieu of protecting group chemistry to successively prepare [2], [3]- and [4]rotaxanes from a simple triethynylbenzene unit
Summary
The first synthesis of a rotaxane was reported in 1967 [1], the development of convenient and high yielding methods for preparing mechanically interlocked molecules (MIMs) remains an active area of research due to their potential applications [2,3,4,5,6] in fields as diverse as sensing [7,8,9], catalysis [10,11,12], medicine [13,14,15,16,17,18] and as artificial molecular machines [19,20,21]. Following Sauvage’s ground-breaking use of metal ions as templating species for the synthesis of MIMs [22,23], a variety of interactions have been utilized to arrange components prior to formation of the final covalent bond that establishes the mechanical bond [24,25]. These have included coordination bonds with a range of main group and transition row metal ions [26,27,28], anion-templation [29,30,31,32], π-stacking interactions [33], H-bonding [34,35] and radical-radical interactions [36], amongst others. Iterative approaches [55,56,57,58,59,60,61] tend to offer the best opportunity for rapid and economical formation of precision designed oligo[n]rotaxanes
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