Dimerization and Self-Sorting of Tetraurea Calix[4]arenes

Abstract

Calix[4]arenes, substituted at their wide rim by four urea functions, form hydrogen-bonded dimeric capsules. Covalent connection of adjacent urea functions within a calixarene molecule creates compounds with one to four loops. They show selective dimerization, since an overlap of loops is not possible within a dimer, and the urea substituent of the partner must be able to penetrate the loop. These selectivities can be developed into a sorting scheme for eleven tetraureas, where only six of potentially 35 dimeric combinations are realized. Together with the similar dimerization of triureas derived from triphenylmethane and the formation of tetramers from triurea monoacetamide calix[4]arenes, it is possible to design building blocks that form dendritic assemblies uniform in size and structure.

1 Introduction

1.1 Self-Assembly: General Points

1.2 Dimerization of Tetraureas Derived from Calix[4]arenes

1.3 Properties

1.3.1 Stability

1.3.2 Guest Exchange

1.3.3 Symmetry

2 Synthesis of Tetraurea Calix[4]arenes

2.1 General Pathways and Compounds with Identical Urea Groups

2.2 Selective Syntheses

2.2.1 Protective Groups

2.2.2 Stepwise Syntheses

2.3 Tetraurea Derivatives with Loops

2.3.1 Conventional Synthesis

2.3.2 Olefin Metathesis and Template Syntheses

3 Selective Dimerization

3.1 Early Examples

3.2 The Use of Loops and Bulky Groups

3.2.1 Urea Groups Connected by Loops

3.2.2 Urea Groups with Bulky Substituents

3.2.3 Bulky Substituents in Combination with Loops

3.3 Proofs for Selectivity

4 Self-Sorting

4.1 General Rules

4.2 A Sorting Scheme for Programmed Self-Assembly

4.3 Experimental Proofs

4.3.1 NMR Spectroscopy

4.3.2 Mass Spectrometry

5 Self-Assembly to Dendrimers

5.1 General Idea

5.2 Additional Assemblies

5.2.1 Dimers of Triurea Triphenylmethanes

5.2.2 Tetramers of Triurea Monoacetamides

5.3 Examples of Dendritic Assemblies

6 Outlook

Key words

calixarenes - metathesis - dimerizations - self-assembly - dendrimers

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10

Analogous dimers are formed by methyl ethers, although they are not fixed in the cone conformation (see ref. 46).

24

By A, B, and C, different types of phenolic units are indicated.

34

It should be noted that the 1,2-substitution is also statistically favored by a factor of two over the 1,3-substitution.

46

In the case of tetramethyl ethers, which usually prefer the partial cone conformation, the calix[4]arenes are also prearranged into the cone conformation.

53

Strictly spoken, this is true for the loops studied so far in which the meta positions of the adjacent phenylureas are connected by an α,ω-oxyalkane chain with up to 20 CH₂ groups. For other loops, the situation may be qualitatively different.

55

Bolte, M.; Vysotsky, M. O.; Böhmer, V. unpublished results

62

Regioisomeric dimers and enantiomers are not distinguished.

63

For the bis-loop derivative I, a similar compound with a bulky residue is possible, and for the mono-loop derivative G, two additional compounds with slim residues are possible. However, this would not enlarge the sorting possibilities.

64

From n objects, the first can form n different dimers, including its homodimer. The second one can form n - 1 dimers additionally, the dimer with the first object already being counted. The third one can form n - 2, and so on, and for the last one only its homodimer remains as a possibility. Thus, the number of dimers is given by: S = n + (n - 1) +
(n - 2) + ... + 2 + 1. When the first and the last term are added, as well as the second and the penultimate, and so on, this can be rearranged to: S = [n + 1] + [(n - 1) + 2] + [(n - 2) + 3] + ... = 0.5˙n˙(n + 1).

65

It is not even necessary that all eleven tetraureas are present in the same quantity; only the pairs K/A, J/B, I/C, H/D, and G/E must be present in stoichiometric amounts.

66

For compounds where Y = C₅H₁₁, the slim residue is tolyl and the bulky residue is 3,5-bis(4-tert-butylphenyl)-4-propoxyphenyl. The loops were formed by -O(CH₂)₁₀O- chains connecting adjacent phenyl groups

67

Rudzevich Y., Rudzevich V., Klautsch F., Schalley C. A., Böhmer V. Angew. Chem. Int. Ed. 2009, 48, 3867; Angew. Chem. 2009, 121, 3925.

71

In principle, the synthetic sequence allows also the introduction of two different alkyl ether residues Y if the respective ethers of the benzaldehyde 30 or phenol 31 are used.

74

In polar solvents, the NMR spectra reveal the expected (time-averaged) C _S symmetry.

75

From two regioisomeric dimers, only the distal arrangement is formed.