Changes in Tricarbastannatrane Transannular N–Sn Bonding upon Complexation Reveal Lewis Base Donicities

Extending Chirality in Group XIV Metallatranes

The syntheses of the racemic amino alcohol rac-N(CH₂CMe₂OH)(CMe₂CH₂OH)(CH₂CHMeOH) (L^22'1*H₃, 2) and its representative N(CH₂CMe₂OH)(CMe₂CH₂OH)(CH₂C(R)HMeOH) (L^22'1RH₃, 3) with the stereogenic carbon center being R-configured are reported. Also reported are the stannatranes L^22'1*SnOt-Bu (4) L^22'1RSnOt-Bu (6) and germatranes L^22'1*GeOEt (5) and L^22'1RGeOEt (7) as well as the trinuclear tin oxocluster [(μ₃-O)(μ₃-O-t-Bu){SnL^22'1R}₃] (8). NMR and IR spectroscopy, electrospray ionization mass spectrometry (ESI MS), and single crystal X-ray diffraction analysis characterize these compounds. Computational studies accompany the experimental work and help understand the diastereoselectivity observed in the course of the metallatrane syntheses.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe₂CH₂C₆H₄)(mesityl)Sb]⁺ (A) and [(NMe₂C₆H₄)(mesityl)Sb]⁺ (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.

Synthesis, structure and properties of trivalent and pentavalent tricarbabismatranes

The first trivalent and pentavalent tricarbabismatranes were synthesized by the reaction of N(CH₂{2-LiC₆H₄})₃ with BiCl₃ and subsequent reaction with XeF₂, respectively. The trivalent bismatrane was easily oxidized by air, while the pentavalent bismatrane difluoride was relatively stable to air. A similar pentavalent bismatrance dichloride was prone to C-Cl bond reductive elimination even at room temperature.

Germatranes and carbagermatranes: (hetero)aryl and alkyl coupling partners in Pd-catalyzed cross-coupling reactions

In the past few decades, palladium-catalyzed cross-coupling reactions have taken root in the construction of a complex synthetic community. The development of organometallics has been an important objective in this field. Our group has focused on exploiting new germanium-based reagents and the corresponding catalytic processes. In the past three years, we have established new methods for the synthesis of structure-modified (hetero)aryl germatranes and alkyl carbagermatranes. Particularly for alkyl carbagermatranes, the stability to be compatible with various derivatization reactions and the high activity for transmetallation (e.g. base/additive-free for primary alkyl carbagermatranes) distinguish them from many reported nucleophiles. In this article, we would introduce (1) the development process of organogermanium reagents in palladium-catalyzed cross-couplings; (2) the history of germatrane-type systems and the breakthrough we have made in the field; (3) the outlook for (carba)germatranes and alkyl-GeMe₃.

Formation of monomeric Sn(ii) and Sn(iv) perfluoropinacolate complexes and their characterization by 119Sn Mössbauer and 119Sn NMR spectroscopies

The synthesis and characterization of a series of Sn(ii) and Sn(iv) complexes supported by the highly electron-withdrawing dianionic perfluoropinacolate (pin^F) ligand are reported herein. Three analogs of [Sn^IV(pin^F)₃]^2- with NEt₃H⁺ (1), K⁺ (2), and {K(18C6)}⁺ (3) counter cations and two analogs of [Sn^II(pin^F)₂]^2- with K⁺ (4) and {K(15C5)₂}⁺ (5) counter cations were prepared and characterized by standard analytical methods, single-crystal X-ray diffraction, and ¹¹⁹Sn Mössbauer and NMR spectroscopies. The six-coordinate Sn^IV(pin^F) complexes display ¹¹⁹Sn NMR resonances and ¹¹⁹Sn Mössbauer spectra similar to SnO₂ (cassiterite). In contrast, the four-coordinate Sn^II(pin^F) complexes, featuring a stereochemically-active lone pair, possess low ¹¹⁹Sn NMR chemical shifts and relatively high quadrupolar splitting. Furthermore, the Sn(ii) complexes are unreactive towards both Lewis bases (pyridine, NEt₃) and acids (BX₃, Et₃NH⁺). Calculations confirm that the Sn(ii) lone pair is localized within the 5s orbital and reveal that the Sn 5p_x LUMO is energetically inaccessible, which effectively abates reactivity.

Mononuclear Pseudostannatranes Possessing Unsymmetrical [4.4.3.01,5]Tridecane Cage: Experimental and Theoretical Aspects of Reverse Kocheshkov Reaction in Phenyl Pseudostannatrane

The synthetic protocols, structural aspects, and spectroscopic aspects of mononuclear pseudostannatranes possessing a [4.4.3.0^1,5]tridecane cage have been reported. A tripodal ligand N(CH₂CH₂OH){CH₂(2-t-Bu-4-Me-C₆H₂OH)}₂ (H₃L) having unsymmetrical arms was reacted with n-butyltrichlorostannane, phenyltrichlorostannane, and tin tetrachloride under different solvent systems to obtain pseudostannatranes (1-3). The reaction of n-butyltrichlorostannane and the ligand in CH₃OH/Na/THF yielded an aqua complex of pseudostannatrane [LSnBu(H₂O)] (1_a), which was crystallized as its acetone solvate (i.e 1_a·Me₂CO). However, the same reactants yielded methanol complex [LSnBu(CH₃OH)] (1_b) when the reaction was carried out in the NaOCH₃/C₂H₅OH system. Similarly, the reaction of phenyltrichlorostannane and the ligand under these solvent systems yielded pseudostannatranes, i.e., an aqua complex [LSnPh(H₂O)] (2_a) and a methanol complex [LSnPh(CH₃OH)] (2_b) (where 2_a was crystallized as 2_a·Me₂CO). The reaction of tin tetrachloride and the ligand in the Et₃N/THF system resulted in the formation of pseudostannatrane [LHSnCl₂] (3). A similar product was isolated as its triethylamine solvate (3·NEt₃) due to the disproportion reaction when PhSnCl₃ was reacted with the ligand in the Et₃N/C₆H₅CH₃ system, which demonstrates the first report on the reverse Kocheshkov reaction in pseudostannatranes. The experimental findings on the formation of 3·NEt₃ due to the reverse Kocheshkov reaction have been corroborated with ¹¹⁹Sn NMR spectroscopy and density functional calculations that provide insightful information about the underlying details of the reaction route.

Understanding Nontraditional Differential Mobility Behavior: A Case Study of the Tricarbastannatrane Cation, N(CH2CH2CH2)3Sn. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2020;31:796-802. [PMID: 32129991 DOI: 10.1021/jasms.9b00042]

The effect of strong ion-solvent interactions on the differential mobility behavior of the tricarbastannatrane cation, N(CH₂CH₂CH₂)₃Sn⁺, has been investigated. Exotic "type D" dispersion behavior, which is intermediate to the more common types C and A behavior, is observed for gaseous N₂ environments that are seeded with acetone and acetonitrile vapor. Quantum chemical calculations and first-principles modeling show that under low-field conditions [N(CH₂CH₂CH₂)₃Sn + solvent]⁺ complexes containing a single solvent molecule survive the entire separation waveform duty cycle and interact weakly with the chemically modified environment. However, at high separation voltages, the ion-solvent bond dissociates and dynamic clustering ensues.

Zn-mediated decarboxylative carbagermatranation of aliphatic N-hydroxyphthalimide esters: evidence for an alkylzinc intermediate

Alkyl nucleophiles synthesized by decarboxylation of the corresponding N-hydroxyphthalimide esters (NHP esters) would inherit the complex structure of natural carboxylic acids and result in useful cross-coupling fragments. Herein, we report the synthesis of alkyl carbagermatranes via Zn-mediated decarboxylation of NHP esters without Ni catalysis or photocatalysis. Mechanistic studies indicate that an alkyl zinc intermediate was involved; however, the generation of alkyl zinc will be inhibited in the presence of Ni. Hence, this study provides valuable resolution to the perplexing problem about whether organozinc was involved in recently emerging catalytic systems of NHP ester-Zn. Meanwhile, alkyl zinc reagents from NHP esters are compatible with aryl/alkyl bromides and iodides; therefore the scope of carbagermatranation in this work precedes that of in situ-generated organozinc from alkyl halides.

Tricyclic tin(iv) cages: synthetic aspects and intriguing features of stannatranes and pseudostannatranes

This perspective summarizes various synthetic routes of stannatranes and pseudostannatranes along with their interesting features, such as appearance of satellites in their NMR spectra, cluster formation upon hydrolysis and exchange reactions.

Preparation of Enantioenriched Alkylcarbastannatranes via Nucleophilic Inversion of Alkyl Mesylates for Use in Stereospecific Cross-Coupling Reactions

We report the preparation of enantioenriched secondary alkylcarbastannatranes via a stereoinvertive S_N2 reaction of enantioenriched alkyl mesylates and carbastannatranyl anion equivalents. Using this process, enantioenriched secondary alcohols may be converted into highly enantioenriched alkylcarbastannatranes, which are useful in stereospecific cross-coupling reactions.

Alkyl Carbagermatranes Enable Practical Palladium-Catalyzed sp2-sp3 Cross-Coupling

Pd-catalyzed cross-coupling reactions have achieved tremendous accomplishments in the past decades. However, C(sp³)-hybridized nucleophiles generally remain as challenging coupling partners due to their sluggish transmetalation compared to the C(sp²)-hybridized counterparts. While a single-electron-transfer-based strategy using C(sp³)-hybridized nucleophiles had made significant progress recently, fewer breakthroughs have been made concerning the traditional two-electron mechanism involving C(sp³)-hybridized nucleophiles. In this report, we present a series of unique alkyl carbagermatranes that were proven to be highly reactive in cross-coupling reactions with our newly developed electron-deficient phosphine ligands. Generally, secondary alkyl carbagermatranes show slightly lower, yet comparable activity to its Sn analogue. Meanwhile, primary alkyl carbagermatranes exhibit high activity, and they were also proved stable enough to be compatible with various reactions. Chiral secondary benzyl carbagermatrane gave the coupling product under base/additive-free conditions with its configuration fully inversed, suggesting that transmetalation was carried out in an "S_E2(open) Inv" pathway, which is consistent with Hiyama's previous observation. Notably, the cross-coupling of primary alkyl carbagermatranes could be performed under base/additive-free conditions with excellent functional group tolerance and therefore may have potentially important applications such as stapled peptide synthesis.

Stereospecific Electrophilic Fluorination of Alkylcarbastannatrane Reagents

We report the use of isolable primary and secondary alkylcarbastannatrane nucleophiles in site-specific fluorination reactions. These reactions occur without the need for transition metal catalysis or in situ activation of the nucleophile. In the absence of the carbastannatrane backbone, alkyltin nucleophiles exhibit no activity towards fluorination. When enantioenriched alkylcarbastannatranes are employed, fluorination occurs predominately via a stereoinvertive mechanism to generate highly enantioenriched alkyl fluoride compounds. These conditions can also be extended to stereospecific chlorination, bromination, and iodination reactions.

B(C6F5)3-Catalyzed transfer 1,4-hydrostannylation of α,β-unsaturated carbonyls using iPr-tricarbastannatrane

Tris(pentafluorophenyl)borane, B(C₆F₅)₃, has been found to be an effective catalyst to access the hydridoborate anion, [N(CH₂CH₂CH₂)₃Sn][HB(C₆F₅)₃], via hydride abstraction from the hypercoordinated tin reagent, iPr-tricarbastannatrane. This process has been applied to the B(C₆F₅)₃-catalyzed transfer 1,4-hydrostannylation of electron-deficient olefins, namely benzylidene barbituric acids. Insights into the mechanism have been obtained via a series of ¹H, ²H, ¹¹B, ¹³C, and ¹¹⁹Sn NMR spectroscopy, mass spectrometry, and labeling experiments.