Two C₂-symmetric chelating P₂-bisphosphazene superbases connected via a binaphthyl backbone – synthesis, structural features and preparation of a cationic alkyl aluminum complex

Abstract

Two P₂-phosphazenyl groups were linked via a C₂-symmetric binaphthyl backbone resulting in two novel chiral superbases with dimethylamino and pyrrolidino substituents. We investigated their basic properties and coordination chemistry towards a cationic alkyl aluminum fragment. The outstanding basicity of the chiral tetrasphosphazenes presented herein leads to interesting perspectives for application in asymmetric Brønsted base catalysis.

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Chiral amines play a very important role in asymmetric catalysis – especially with regard to extensive scientific activities in the field of organocatalysis during the past decade.¹ However, due to their low basicity the role of chiral amines in enantioselective reactions in which they act as Brønsted-bases is strongly limited to comparably acidic substrates. Thus, chiral representatives of all classes of organic superbases such as amidines,² guanidines,³ azaphosphatranes,⁴ proton sponges⁵ or phosphazenes⁶ have been prepared and investigated with respect to their applicability to asymmetric synthesis.⁷ This renaissance of well-established concepts of superbasicity becomes manifest in a cluster on superbases published in Synlett quite recently.⁸ So far, the class of chiral guanidines is the most advanced with respect to its structural variety and utilization in enantioselective reactions. The area has already been the subject of review articles by Ishikawa and Tan.⁹ Chiral guanidines were applied to asymmetric Michael addition, silylation, nucleophilic epoxidation and TMS cyanation reactions.^3a,b Taking into consideration that the pK_BH⁺ values of organic superbases range from 25 to about 45 on the acetonitrile scale, it must be stated that chiral guanidines presented to date only populate the very low end of the basicity scale (pentamethylguanidine: pK_BH⁺ (MeCN) = 25.00¹⁰). Extension of the synthetic scope of chiral superbases requires the design of compounds with higher pK_BH⁺ values by making use of the high intrinsic basicity of phosphazenes with multiple PN units or the combination of several superbasic features in one molecule. In this context Terada et al. recently published helically chiral spirocyclic P₃ phosphazenes and a series of highly basic chiral guanidinophosphazenes that were used in enantioselective amination reactions of ketones.¹¹ Raab¹² and Himmel et al.¹³ linked two guanidinyl moieties via a C₂-symmetric binaphthyl backbone allowing the formation of an energetically favorable [N–H⋯N] hydrogen bond after protonation. Since we have thoroughly studied the basicity-enhancing effect of proton chelation,¹⁴ we were interested to combine this proton pincer ligand concept with a chiral binaphthyl skeleton to achieve a unique interaction of two amino-substituted Schwesinger-P₂ bases in a chiral environment. A related bis(monophosphazene) with two PPh₃ units was prepared by Reetz et al. in 1998, but the electron-withdrawing triphenylphosphonio substituents were not able to grant a high basicity or even superbasicity with a pK_BH⁺ value (MeCN) > 25.00.¹⁵

We chose a Kirsanov condensation to achieve the linkage between two P₂ fragments and the aromatic diamine. The required bromophosphonium bromide precursors 1 and 2 were prepared by the bromination of the literature known P(III) compounds that were accessible via three-step procedures from tris(dimethylamino) phosphane and tris(pyrrolidino)phosphane.^14b,17,18 The reaction of (S)-(–)-1,1′-binaphthyl-2,2′-diamine with the bromophosphonium bromides in the presence of the auxiliary base triethylamine and subsequent deprotonation with metal bis(trimethylsilyl)amides gave the two chiral tetraphosphazenes 3 and 4. They were characterized via¹H, ¹³C and ³¹P spectroscopy, ESI mass spectrometry, elemental analysis and in the case of the dimethylamino-substituted species also X-ray crystallographic analysis (Fig. 1). The two independent molecules found in the asymmetric unit reveal a large distance of 3.98(1) and 4.10(1) Å between the two basicity centers. Unlike in proton sponges, their N-centered lone pairs avoid each other by suitable torsion of the two naphthalene moieties around the axial carbon–carbon bond. Hence, no steric strain in the aromatic backbones or repulsion of the lone pairs of the nitrogen atoms destabilizing the free base form of tetraphosphazene is observed.

Fig. 1 Molecular structure of 3 (ellipsoids with 30% probability). Hydrogen atoms are omitted for clarity. Selected bond lengths/Å and angles/°: d(N1⋯N21) 3.98(1), d(N1–P1) 1.567(6), d(P1–N4) 1.593(7), d(N4–P2) 1.536(7), d(N21–P3) 1.561(6), d(P3–N24) 1.627(7), d(N24–P4) 1.545(7), d(N41⋯N61) 4.10(1), d(N41–P5) 1.557(6), d(P5–N44) 1.609(6), d(N44–P6) 1.549(6), d(N61–P7) 1.542(7), d(P7–N64) 1.596(6), d(N64–P8) 1.539(7).¹⁶

Protonation of the two tetraphosphazenes was studied with p-toluenesulfonic acid. It is accompanied by a low-field shift of the signals in the ³¹P NMR spectrum and an increase in the ²J(P,P) coupling constants. In the case of the pyrrolidino-substituted species, the acidic proton exhibits a triplet with a ²J(P,H) coupling constant of 7.4 Hz indicating interaction with two phosphorous atoms and thus the formation of an intramolecular [N–H⋯N] hydrogen bond.

The pK_BH⁺ values were determined via NMR titration experiments versus bisphosphazene proton sponges.²⁰ As expected, 4 exhibits a higher pK_BH⁺ value of 30.8 on the acetonitrile scale than its dimethylamino-substituted counterpart 3 which shows a pK_BH⁺ value of 29.3. These results also suggest the energetically favorable chelation of the acidic proton since the basicity of 4 exceeds the pK_BH⁺ value of the non-chelating naphthalene-substituted P₂ Schwesinger base (pyr)P₂-1-Naph (pK_BH⁺ (MeCN) (calc.) = 26.0)^14b by nearly five orders of magnitude. For Terada's chiral guanidinophosphazenes pK_BH⁺ values (THF) close to 26.8 were estimated.¹¹

The coordination behavior of proton sponges²¹ or other chelating superbases towards electrophiles other than protons has attracted considerable scientific interest.^13,22 However, whereas metal complexes of macrocyclic phosphazenes have been reported,²³ the coordination chemistry of classical open chain Schwesinger superbases is widely unexplored.²⁴ We were interested to demonstrate the use of the title compounds not only as a base but also as a chiral donor ligand towards Lewis acids of catalytic interest. The reaction of 4 with two equivalents of trimethylaluminum resulted in the asymmetric dissociation and formation of [4-AlMe₂]⁺[AlMe₄]⁻ (Scheme 1) which could be investigated concerning its structural features (Fig. 2). An angle of 69.3(9)° is observed between the two naphthalene planes. The cationic fragment [AlMe₂]⁺ is chelated by the two basic nitrogen atoms showing an N–Al–N angle of 98.3(2)° and a non-bonding N⋯N distance of 2.931(6) Å.

Scheme 1 Preparation of the chiral tetraphosphazenes 3 and 4via a Kirsanov reaction, protonation with para-toluenesulfonic acid and reaction of 4 with trimethylaluminum.

Fig. 2 Molecular structure of [4-AlMe₂]⁺[AlMe₄]⁻ (ellipsoids with 30% probability). The disorder in the [AlMe₄]⁻ anion is not displayed. Hydrogen atoms are omitted for clarity. Selected bond lengths/Å and angles/°: d(N1–Al1) 1.938(5), d(N1–P1) 1.633(5), d(P1–N4) 1.582(5), d(N4–P2) 1.558(4), d(N8–Al1) 1.938(5), d(N8–P3), d(P3–N11) 1.576(5), d(N11–P4) 1.548(5), N1–Al1–N8 98.3(2).¹⁹

In the state of the art the concept of Schwesinger polyphosphazene superbases has not been extended to chelating chiral representatives, favorably with a rigid backbone. Furthermore, Schwesinger polyphosphazene bases have not been used as superstrong chelating neutral donor ligands in coordination chemistry and catalysis. In this respect the chiral tetraphosphazenes communicated herein add valuable perspectives to the highly topical field of chiral superbases with expected applications as enantioselective organocatalysts with an extraordinarily high Brønsted-basicity and as chiral superdonor ligands for cationic metal complexes of catalytic interest.

We thank Fabian Schröder and Lars Finger for their valuable support with crystal structure refinement. Financial support by the Fonds der Chemischen Industrie (doctoral scholarship for J.K.) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures and analytical data. CCDC 977138–9771340. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc00773e

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