Development of P-Spiro Chiral Aminophosphonium Salts as a New Class of Versatile Organic Molecular Catalyst

Uraguchi, Daisuke; Ooi, Takashi

doi:10.5059/yukigoseikyokaishi.68.1185

Cited by 45 publications

(11 citation statements)

References 87 publications

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“…Much to our surprise, our data clearly showed that the C(a) À H protons of 2 a BPh 4 À do not experience any significant ring current effect in CD 2 Cl 2 solution. Considering the relevance of phosphonium ion/anion interactions in crystal engineering, [21] anion recognition, [22] saltbased solvent systems, [23,24] photochemistry, [19,20,25] structure determination, [10,26] and organic synthesis, [27,28] we decided to carry out a more detailed investigation of the ion pairing in 2 a X À and related phosphonium salts.…”

Section: Introductionmentioning

confidence: 99%

Ion‐Pairing of Phosphonium Salts in Solution: CH⋅⋅⋅Halogen and CH⋅⋅⋅π Hydrogen Bonds

Ammer

Nolte

Karaghiosoff

et al. 2013

Chemistry A European J

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The (1) H NMR chemical shifts of the C(α)H protons of arylmethyl triphenylphosphonium ions in CD2 Cl2 solution strongly depend on the counteranions X(-) . The values for the benzhydryl derivatives Ph2 CHPPh3 (+) X(-) , for example, range from δH =8.25 (X(-) =Cl(-) ) over 6.23 (X(-) =BF4 (-) ) to 5.72 ppm (X(-) =BPh4 (-) ). Similar, albeit weaker, counterion-induced shifts are observed for the ortho-protons of all aryl groups. Concentration-dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl(-) > Br(-) ≫ BF4 (-) > SbF6 (-) . For the less bulky derivatives PhCH2 PPh3 (+) X(-) , we also find CH⋅⋅⋅Ph interactions between C(α)H and a phenyl group of the BPh4 (-) anion, which result in upfield NMR chemical shifts of the C(α)H protons. These interactions could also be observed in crystals of (p-CF3 -C6 H4 )CH2 PPh3 (+) BPh4 (-) . However, the dominant effects causing the counterion-induced shifts in the NMR spectra are the CH⋅⋅⋅X(-) hydrogen bonds between the phosphonium ion and anions, in particular Cl(-) or Br(-) . This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh4 (-) anions. The concentration dependence of the (1) H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD2 Cl2 solution. The cation-anion interactions increase with the acidity of the C(α)H protons and the basicity of the anion. The existence of CH⋅⋅⋅X(-) hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X-ray analyses of the crystals. The IR spectra of the Cl(-) and Br(-) salts in CD2 Cl2 solution show strong red-shifts of the CH stretch bands. The CH stretch bands of the tetrafluoroborate salt PhCH2 PPh3 (+) BF4 (-) in CD2 Cl2 , however, show a blue-shift compared to the corresponding BPh4 (-) salt.

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Section: Introductionmentioning

confidence: 99%

Ion‐Pairing of Phosphonium Salts in Solution: CH⋅⋅⋅Halogen and CH⋅⋅⋅π Hydrogen Bonds

Ammer

Nolte

Karaghiosoff

et al. 2013

Chemistry A European J

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“…37 Mannich reactions have been catalyzed by cyclopropenimine 69 and iminophosphorane bases. 70 Recently, Terada et al designed a superbasic organocatalyst consisting of a phosphazene and a chiral guanidine unit. 71 The cooperative effect of these two units leads to unprecedented enantioselectivity in the direct aza-Mannich reaction of less acidic pronucleophiles, such as αphenylthioacetate.…”

Section: Carbon-centered Superbasesmentioning

confidence: 99%

“…71 The cooperative effect of these two units leads to unprecedented enantioselectivity in the direct aza-Mannich reaction of less acidic pronucleophiles, such as αphenylthioacetate. Uraguchi and Ooi 70 73 for the preparation of chiral derivatizing agents 74 and in Strecker reactions at very low catalyst loading (0.1 mol %). 75 A growing field for superbase applications is also ring-opening polymerization processes.…”

Section: Carbon-centered Superbasesmentioning

confidence: 99%

Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality

et al. 2021

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Conspectus One of the constant challenges of synthetic chemistry is the molecular design and synthesis of nonionic, metal-free superbases as chemically stable neutral organic compounds of moderate molecular weight, intrinsically high thermodynamic basicity, adaptable kinetic basicity, and weak or tunable nucleophilicity at their nitrogen, phosphorus, or carbon basicity centers. Such superbases can catalyze numerous reactions, ranging from C–C bond formation to cycloadditions and polymerization, to name just a few. Additional benefits of organic superbases, as opposed to their inorganic counterparts, are their solubility in organic reaction media, mild reaction conditions, and higher selectivity. Approaching such superbasic compounds remains a continuous challenge. However, recent advances in synthetic methodology and theoretical understanding have resulted in new design principles and synthetic strategies toward superbases. Our computational contributions have demonstrated that the gas-phase basicity region of 350 kcal mol–1 and even beyond is easily reachable by organosuperbases. However, despite record-high basicities, the physical limitations of many of these compounds become quickly evident. The typically large molecular weight of these molecules and their sensitivity to ordinary reaction conditions prevent them from being practical, even though their preparation is often not too difficult. Thus, obviously structural limitations with respect to molecular weight and structural complexity must be imposed on the design of new synthetically useful organic superbases, but strategies for increasing their basicity remain important. The contemporary design of novel organic superbases is illustrated by phosphazenyl phosphanes displaying gas-phase basicities (GB) above 300 kcal mol–1 but having molecular weights well below 1000 g·mol–1. This approach is based on a reconsideration of phosphorus(III) compounds, which goes along with increasing their stability in solution. Another example is the preparation of carbodiphosphoranes incorporating pyrrolidine, tetramethylguanidine, or hexamethylphosphazene as a substituent. With gas-phase proton affinities of up to 300 kcal mol–1, they are among the top nonionic carbon bases on the basicity scale. Remarkably, the high basicity of these compounds is achieved at molecular weights of around 600 g·mol–1. Another approach to achieving high basicity through the cooperative effect of multiple intramolecular hydrogen bonding, which increases the stabilization of conjugate acids, has recently been confirmed. This Account focuses on our efforts to produce superbasic molecules that embody many desirable traits, but other groups’ approaches will also be discussed. We reveal the crucial structural features of superbases and place them on known basicity scales. We discuss the emerging potential and current limits of their application and give a general outlook into the future.

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“…The most prevalent superbase incorporated into organocatalysts is guanidines . In 1999, Corey described the use of a chiral C 2 -symmetric guanidine catalyst for the enantioselective Strecker reaction, and later Tan reported that a t Bu-substituted variant was effective in the enantioselective sulfa-Michael addition of aryl thiols to activated α-phthalimido acrylate esters. , Ishikawa, and more recently Terada, have shown important applications of other chiral guanidine constructs. , Ooi, and subsequently Terada also reported chiral P 1 phosphazene catalyst systems for activation and enantioselective reactions of challenging ketone substrates. , Lambert and Bandar described the application of chiral cyclopropenimine-containing catalysts as the superbase for enantioselective Michael and Mannich reactions of glycine-derived pronucleophiles . These catalyst systems constitute the state-of-the-art in chiral superbase chemistry (Figure C).…”

Section: Introductionmentioning

confidence: 99%

“…27,28 Ooi, and subsequently Terada also reported chiral P 1 phosphazene catalyst systems for activation and enantioselective reactions of challenging ketone substrates. 29,30 Lambert and Bandar described the application of chiral cyclopropenimine-containing catalysts as the superbase for enantioselective Michael and Mannich reactions of glycine-derived pronucleophiles. 31 These catalyst systems constitute the state-of-the-art in chiral superbase chemistry (Figure 1C).…”

Section: ■ Introductionmentioning

confidence: 99%

Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis

Formica

Rozsar

et al. 2020

Acc. Chem. Res.

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Development of P-Spiro Chiral Aminophosphonium Salts as a New Class of Versatile Organic Molecular Catalyst

Cited by 45 publications

References 87 publications

Ion‐Pairing of Phosphonium Salts in Solution: CH⋅⋅⋅Halogen and CH⋅⋅⋅π Hydrogen Bonds

Ion‐Pairing of Phosphonium Salts in Solution: CH⋅⋅⋅Halogen and CH⋅⋅⋅π Hydrogen Bonds

Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality

Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis

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