Abstract:The combination of experimental data and results of DFT calculations indicates that the catalytic activity of chalconium and halonium salts serving as sigma-hole donating organocatalysts cannot be clearly estimated via...
“…The reversible amine-aldehyde coupling giving a Schiff base (Figure 4) was carried out in the presence of any of Cat1 OTf - Cat4 OTf (10 mol %) or AgOTf (10 mol %), or the mixture of any of Cat1 OTf -Cat4 OTf and AgOTf (1 : 1, total 20 mol % of the electrophiles). The kinetic data, obtained by us using the same kinetic equations as in our previous work on Schiff condensation, [21] indicate that this reaction is catalyzed either by the silver(I) triflate or the iodonium triflates (Table 1, entries 1, 2, 3, 5, 7, 9). A rough approximation of the expected reaction rate constants for separate catalysis of the reaction by the mixture of silver(I) and iodine(III) centers [k f (AgOTf…”
Section: Electrophilic Activation Of the Carbonyl Group In Schiff Con...mentioning
Kinetic data based on the 1H NMR monitoring and computational studies indicate that in a solution pyrazole‐containing iodonium triflates and silver(I) triflate bind each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine–isocyanide coupling, or the nucleophilic attack on a triple carbon–carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole‐containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.
“…The reversible amine-aldehyde coupling giving a Schiff base (Figure 4) was carried out in the presence of any of Cat1 OTf - Cat4 OTf (10 mol %) or AgOTf (10 mol %), or the mixture of any of Cat1 OTf -Cat4 OTf and AgOTf (1 : 1, total 20 mol % of the electrophiles). The kinetic data, obtained by us using the same kinetic equations as in our previous work on Schiff condensation, [21] indicate that this reaction is catalyzed either by the silver(I) triflate or the iodonium triflates (Table 1, entries 1, 2, 3, 5, 7, 9). A rough approximation of the expected reaction rate constants for separate catalysis of the reaction by the mixture of silver(I) and iodine(III) centers [k f (AgOTf…”
Section: Electrophilic Activation Of the Carbonyl Group In Schiff Con...mentioning
Kinetic data based on the 1H NMR monitoring and computational studies indicate that in a solution pyrazole‐containing iodonium triflates and silver(I) triflate bind each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine–isocyanide coupling, or the nucleophilic attack on a triple carbon–carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole‐containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.
“…The nucleophilic species accumulating during the reaction progress can occupy the catalyst's σ-holes and, thus, effectively compete with the starting species for the coordination to the catalyst. 35 This can explain the high initial reaction rate, leading to approximately 5% conversion of the starting imine A within the first 4 min of the reaction (Figure 3) followed by a notable decrease of the reaction rate. This suggestion is indirectly confirmed by the presence of such a conversion leap in the case of σ-holedonating 1(BH 3 CN)−6(BH 3 CN) reagents and its absence in the case of utilization of Na(BH 3 CN) and n Bu 4 N(BH 3 CN), as well as by the retention of this leap in the case of reaction carried out in the presence of 2,6-di-tert-butylpyridine (10 mol %), which would neutralize traces of a Brønsted acid if they were present in 1(BH 3 CN)−6(BH 3 CN) as an impurity and catalyze the reaction at the beginning of its progress (see Figure S2 for the sulfonium cyanoborohydride).…”
Section: ■ Results and Discussionmentioning
confidence: 99%
“…For XB donors, it has been demonstrated that iodoazoliums 11−15 and iodopyridiniums 16 featuring an exocy-clic iodine(I) center effectively catalyze an extensive series of organic transformations, whereas hypervalent iodine(III) species (iodonium salts) possess even a greater catalytic activity than the iodine(I) derivatives. 17−23 For ChB donors, recent publications unambiguously indicate that telluronium salts (R 3 Ch + X − , Ch = Te IV ) exhibit a significantly higher Lewis acidity compared to the tellurium(II) derivatives and lighter chalcogen(IV)-derived species, 24−32 whereas selenonium salts (Ch = Se IV ) have a higher 33,34 or comparable 35 activity than their sulfonium analogues (Ch = S IV ), which, in turn, exhibit a sufficient catalytic activity toward a series of model organic transformations. 34,36 Although some of the σ-hole carriers 36−43 �similarly with well-studied HB donors 44 �were tested in the reaction of hydrogenation of the imine moiety by the Hantzsch esters (Scheme 1), 45 no examples were known for their applications involving simple inorganic reducing agents, such as hydrides, which might be explained by possible redox side reactions between the cationic σ-hole carrier and the reducing agent.…”
Section: ■ Introductionmentioning
confidence: 99%
“…Although, in early works, some catalytic effect was observed for noncharged XB and ChB donors, , the cationic σ-hole donors exhibit a significantly higher catalytic activity (Figure ). For XB donors, it has been demonstrated that iodoazoliums − and iodopyridiniums featuring an exocyclic iodine(I) center effectively catalyze an extensive series of organic transformations, whereas hypervalent iodine(III) species (iodonium salts) possess even a greater catalytic activity than the iodine(I) derivatives. − For ChB donors, recent publications unambiguously indicate that telluronium salts (R 3 Ch + X – , Ch = Te IV ) exhibit a significantly higher Lewis acidity compared to the tellurium(II) derivatives and lighter chalcogen(IV)-derived species, − whereas selenonium salts (Ch = Se IV ) have a higher , or comparable catalytic activity than their sulfonium analogues (Ch = S IV ), which, in turn, exhibit a sufficient catalytic activity toward a series of model organic transformations. , …”
Sulfonium,
selenonium, telluronium, and iodonium cyanoborohydrides
have been synthesized, isolated, and fully characterized by various
methods, including single-crystal X-ray diffraction (XRD) analysis.
The quantum theory of atoms in molecules’ analysis based on
the XRD data indicated that the hydride···σ-hole
short contacts observed in the crystal structures of each compound
have a purely noncovalent nature. The telluronium and iodonium cyanoborohydrides
provide a significantly higher rate of the model reaction of imine
hydrogenation compared with sodium and tetrabutylammonium cyanoborohydrides.
Based on the NMR and high-resolution electrospray ionization mass
spectrometry data indicating that the reaction progress is accompanied
by the cation reduction, a mechanism involving intermediate formation
of elusive onium hydrides has been proposed as an alternative to conventional
electrophilic activation of the imine moiety by its ligation to the
cation’s σ-hole.
“…Moreover, the pre-association processes involving the catalyst was not considered in the calculations despite its important role in catalysis, as recently demonstrated by the same authors. [62] Nevertheless, this study emphasized the role of side-interactions in chalconiums and pnictoniums which cooperate with the σ-hole interaction for the stabilization of the reactions transition states (Scheme 13).…”
: Chalcogen bonding (ChB) is the non‐covalent interaction occurring between chalcogen atoms as Lewis acid sites and atoms or groups of atoms able to behave as Lewis bases through their lone pair or p electrons. Analogously to its sister halogen bonding, the high directionality of this interaction was implemented for the precise structural organization in the solid state and in solution. Regarding catalysis, ChB is now accepted as a new mode of activation as demonstrated by the increased number of examples in the last five years. In the family of ChB catalysts, those based on tellurium rapidly appeared to overcome their lighter sulfur and selenium counterparts. In this review, we highlight the Lewis acid properties of tellurium‐based derivatives in solution and summarize the start‐of‐the‐art of their applications in catalysis.
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