Deconvoluting Nonlinear Catalyst-Substrate Effects in the Intramolecular Dirhodium-Catalyzed C-H Insertion of Donor/Donor Carbenes Using Data Science Tools.

Interactions between catalysts and substrates can be highly complex and dynamic, often complicating the development of models to either predict or understand such processes. A dirhodium(II)-catalyzed C-H insertion of donor/donor carbenes into 2-alkoxybenzophenone substrates to form benzodihydrofurans was selected as a model system to explore nonlinear methods to achieve a mechanistic understanding. We found that the application of traditional methods of multivariate linear regression (MLR) correlating DFT-derived descriptors of catalysts and substrates leads to poorly performing models. This inspired the introduction of nonlinear descriptor relationships into modeling by applying the sure independence screening and sparsifying operator (SISSO) algorithm. Based on SISSO-generated descriptors, a high-performing MLR model was identified that predicts external validation points well. Mechanistic interpretation was aided by the deconstruction of feature relationships using chemical space maps, decision trees, and linear descriptors. Substrates were found to have a strong dependence on steric effects for determining their innate cyclization selectivity preferences. Catalyst reactive site features can then be matched to product features to tune or override the resultant diastereoselectivity within the substrate-dictated ranges. This case study presents a method for understanding complex interactions often encountered in catalysis by using nonlinear modeling methods and linear deconvolution by pattern recognition.

Organocatalytic Asymmetric Synthesis of Si-Stereogenic Siloxanols.

We report the organocatalytic synthesis of Si-stereogenic compounds via desymmetrization of a prochiral silanediol with a chiral imidazole-containing catalyst. This metal-free silylation method affords high yields with enantioselectivity up to 98:2 for various silanediol and silyl chloride substrate combinations (including secondary alkyl, vinyl, and H groups), accessing products with potential for further elaboration. NMR and X-ray studies reveal insight into the H-bonding interactions between the imidazole organocatalyst and the silanediol and the dual activating role of the Lewis basic imidazole to account for the high enantioselectivity.

Hydrobismuthation: Insertion of Unsaturated Hydrocarbons into the Heaviest Main Group Element Bond to Hydrogen.

The bismuth hydride (2,6-Mes2H3C6)2BiH (1, Mes = 2,4,6-trimethylphenyl), which has a Bi-H 1H NMR spectroscopic signal at δ = 19.64 ppm, was reacted with phenylacetylene at 60 °C in toluene to yield [(2,6-Mes2C6H3)2BiC(Ph)=CH2] (2) after 15 min. Compound 2 was characterized by 1H, 13C NMR, and UV-vis spectroscopy, single crystal X-ray crystallography, and calculations employing density functional theory. Compound 2 is the first example of a hydrobismuthation addition product and displays Markovnikov regioselectivity. Computational methods indicated that it forms via a radical mechanism with an associated Gibbs energy of activation of 91 kJ mol-1 and a reaction energy of -90 kJ mol-1.

Overcoming a Radical Polarity Mismatch in Strain-Release Pentafluorosulfanylation of [1.1.0]Bicyclobutanes: An Entryway to Sulfone- and Carbonyl-Containing SF5-Cyclobutanes.

The first assortment of achiral pentafluorosulfanylated cyclobutanes (SF5-CBs) are now synthetically accessible through strain-release functionalization of [1.1.0]bicyclobutanes (BCBs) using SF5Cl. Methods for both chloropentafluorosulfanylation and hydropentafluorosulfanylation of sulfone-based BCBs are detailed herein, as well as proof-of-concept that the logic extends to tetrafluoro(aryl)sulfanylation, tetrafluoro(trifluoromethyl)sulfanylation, and three-component pentafluorosulfanylation reactions. The methods presented enable isolation of both syn and anti isomers of SF5-CBs, but we also demonstrate that this innate selectivity can be overridden in chloropentafluorosulfanylation; that is, an anti-stereoselective variant of SF5Cl addition across sulfone-based BCBs can be achieved by using inexpensive copper salt additives. Considering the SF5 group and CBs have been employed individually as nonclassical bioisosteres, structural aspects of these unique SF5-CB "hybrid isosteres" were then contextualized using SC-XRD. From a mechanistic standpoint, chloropentafluorosulfanylation ostensibly proceeds through a curious polarity mismatch addition of electrophilic SF5 radicals to the electrophilic sites of the BCBs. Upon examining carbonyl-containing BCBs, we also observed rare instances whereby radical addition to the 1-position of a BCB occurs. The nature of the key C(sp3)-SF5 bond formation step - among other mechanistic features of the methods we disclose - was investigated experimentally and with DFT calculations. Lastly, we demonstrate compatibility of SF5-CBs with various downstream functionalizations.

Group 13 ion coordination to pyridyl breaks the reduction potential vs. hydricity scaling relationship for dihydropyridinates.

The relationship Epvs. ΔGH- correlates the applied potential (Ep) needed to drive organohydride formation with the strength of the hydride donor that is formed: in the absence of kinetic effects Epvs. ΔGH- should be linear but it would be more energy efficient if Ep could be shifted anodically using kinetic effects. Biological hydride transfers (HT) performed by cofactors including NADH and lactate racemase do occur at low potentials and functional modeling of those processes could lead to low energy HT reactions in electrosynthesis and to accurate models for cofactor chemistry. Herein we probe the influence of N-alkylation or N-metallation on ΔGH- for dihydropyridinates (DHP-) and on Ep of the DHP- precursors. We synthesized a series of DHP- complexes of the form (pz2HP-)E via hydride transfer from their respective [(pz2P)E]+ forms where E = AlCl2+, GaCl2+ or Me+. Relative ΔGH- for the (pz2HP-)E series all fall within 1 kcal mol-1, and ΔGH- for (pz2HP)CH3 was approximated as 47.5 ± 2.5 kcal mol-1 in MeCN solution. Plots of Epvs. ΔGH- including [(pz2P)E]+ suggest kinetic effects shift Ep anodically by ∼215 mV.

Sn(II)-carbon bond reactivity: radical generation and consumption via reactions of a stannylene with alkynes.

Thermal Sn-C cleavage in the diarylstannylene Sn(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) was used to generate ˙Sn(AriPr4) and ˙AriPr4 radicals for alkyne arylstannylation. The radical pair and RCCR' (R = H, R' = Ph; R = Ph, R' = Ph; R = H, R' = C4H9; R = H, R' = SiMe3) in refluxing benzene generate the aryl vinyl stannylene complexes, AriPr4Sn{C(C6H5)-C(H)(AriPr4)} (1), AriPr4Sn{C(C6H5)-C(H)(C6H5)} (2) and AriPr4Sn{C(C4H9)-C(H)(AriPr4)} (3) respectively. For HCCSiMe3, the known distannene {Sn(CCSiMe3)AriPr4}2 (4) was also generated from this new method.

Structure and Luminescence Studies of Salts of the Helical Dication, [Au2(µ-bis(diphenylphosphine)ethane)2]2+ and Comparison with Salts of [Au2(µ-bis(diphenylphosphine)propane)2]2.

Six salts ([Au2(µ-dppe)2](BF4)2·CHCl3, [Au2(µ-dppe)2](BF4)2·1,2-Cl2C2H4, [Au2(µ-dppe)2](PF6)2·CHCl3, [Au2(µ-dppe)2](PF6)2, [Au2(µ-dppe)2](SbF6)2, and [Au2(µ-dppe)2](OTf)2·2CHCl3), (dppe is bis(diphenylphosphine)ethane) containing the dication, [Au2(µ-dppe)2]2+, have been prepared and structurally characterized by single-crystal X-ray crystallography. Unlike the three-coordinate dppe-bridged dimers, Au2X2(µ-dppe)2 (X = Br, I), which show considerable variation in the distance between the gold(I) ions over the range 3.0995(10) to 3.8479(3) Å in various solvates, the structure of the helical dication, [Au2(µ-dppe)2], in the new salts is remarkably consistent with the Au···Au separation falling in the narrow range 2.8787(9) to 2.9593(5) Å. In the solid state, the six crystals display a green luminescence both at room temperature and at 77 K, which has been assigned as phosphorescence. However, solutions of the dication are not luminescent. Salts containing the analogous dication [Au2(µ-dppp)2](PF6)2 (dppp is bis(diphenylphosphine)propane) have been prepared to determine whether the longer bridging ligand might also twist into a helical shape. These salts include [Au2(µ-dppp)2](OTf)2 (OTf is triflate) and three crystalline forms of [Au2(µ-dppp)2](PF6)2: the solvate [Au2(µ-dppp)2](PF6)2·(CHCl3) and two polymorphs of the unsolvated salt. None of these crystals are luminescent, but all contain a similar dication, [Au2(µ-dppp)2]2+, that contains two nearly parallel, linear P-Au-P groups and a long separation between the gold ions that varies from 5.3409(4) to 5.6613(6)Å.

Correction to "Synthesis, Structure, and Spectroscopy of the Biscarboranyl Stannylenes (bc)Sn·THF and K2[(bc)Sn]2 (bc = 1,1'(ortho-Biscarborane)) and Dibiscarboranyl Ethene (bc)CH=CH(bc)".

[This corrects the article DOI: 10.1021/acs.organomet.3c00190.].

Reactions of Thianthrene and 10-Phenylphenothiazine with the Lewis Acids─Titanium Tetrachloride, Titanium Tetrabromide, Tin(IV) Tetrachloride, or Antimony(V) Pentachloride: The Competition between Coordination and Oxidation.

The oxidation of thianthrene and 10-phenylphenothiazine into cation radicals has been examined using redox-active Lewis acids. The reaction of titanium(IV) tetrachloride with thianthrene in toluene produces a solution with an EPR spectrum indicative of oxidation of thianthrene to a cation radical, but the molecular compound (1) (µ-thianthrene)Ti2(µ-Cl2)Cl6 crystallized exclusively. Red crystalline (2) (µ-thianthrene)Ti2(µ-Br2)Br6 formed similarly from titanium(IV) tetrabromide. In contrast, the reaction of antimony(V) pentachloride with thianthrene in toluene yielded crystalline (3) (thianthrene·+)2(Sb2(µ-Cl)2Cl62-)·(SbCl3), while the same reaction in acetonitrile produced crystals of (4) (thianthrene·+)(SbCl6-). The two paramagnetic salts differ in that in (3), the folded (thianthrene·+) cation radicals self-associate, whereas in (4), the (thianthrene·+) cation radicals are isolated from one another and are planar. The reaction of 10-phenylphenothiazine with titanium(IV) tetrachloride in toluene solution resulted in the formation of crystalline paramagnetic (5) (10-phenylphenothiazine·+)(Ti(µ-Cl)3Cl6-) and the reaction of 10-phenylphenothiazine with tin(IV) tetrachloride produced paramagnetic (6) (10-phenylphenothiazine·+)(SnCl5-).

Group 13 ion coordination to pyridyl models NAD+ reduction potentials.

N-alkylation and N-metallation of pyridine are explored herein to understand how metal-ligand complexes can model NAD+ redox chemistry. Syntheses of substituted dipyrazolylpyridine (pz2P) compounds (pz2P)Me+ (1+) and (pz2P)GaCl2+ (2+) are reported, and compared with (pz2P)AlCl2(THF)+ and transition element pz2P complexes from previous reports. Cyclic voltammetry measurements of cationic 1+ and 2+ show irreversible reduction events ∼900 mV anodic those for neutral pz2P complexes of divalent metals. We proposed that N-metallation using Group 13 ions of 3+ charge provides an electrochemical model for N-alkylated pyridyls like NAD+.

Synthesis, Structure, and Spectroscopy of the Biscarboranyl Stannylenes (bc)Sn·THF and K2[(bc)Sn]2 (bc = 1,1'(ortho-Biscarborane)) and Dibiscarboranyl Ethene (bc)CH=CH(bc).

Two compounds containing a Sn(II) atom supported by a bidentate biscarborane ligand have been synthesized via salt metathesis. The synthetic procedures for (bc)Sn·THF (bc = 1,1' (ortho-carborane) (1) and K2[(bc)Sn]2 (2) involved the reaction of K2[bc] with SnCl2 in either a THF solution (1) or in a benzene/dichloromethane solvent mixture (2). Using the same solvent conditions as those used for 2 but using a shorter reaction time gave a dibiscarboranyl ethene (3). The products were characterized by 1H, 13C, 11B, 119Sn NMR, UV-vis, and IR spectroscopy, and by X-ray crystallography. The diffraction data for 1 and 2 show that the Sn atom has a trigonal pyramid environment and is constrained by the bc ligand in a planar five-membered C4Sn heterocycle. The 119Sn NMR spectrum of 1 displays a triplet of triplets pattern signal, which is unexpected given the absence of a Sn-H signal in the 1H NMR, IR spectrum, and X-ray crystallographic data. However, a comparison with other organotin compounds featuring a Sn atom bonded to carboranes reveal similar multiplets in their 119Sn NMR spectra, likely arising from long-range nuclear spin-spin coupling between the carboranyl 11B and 119Sn nuclei. Compound 3 displays structural and spectroscopic characteristics typical of conjugated alkenes.

Rearrangement of a Ge(II) aryloxide to yield a new Ge(II) oxo-cluster [Ge6(µ3-O)4(µ2-OC6H2-2,4,6-Cy3)4](NH3)0.5: main group aryloxides of Ge(II), Sn(II), and Pb(II) [M(OC6H2-2,4,6-Cy3)2]2 (Cy = cyclohexyl).

The new Ge(II) cluster [Ge6(µ3-O)4(µ2-OC6H2-2,4,6-Cy3)4](NH3)0.5 (1) and three divalent Group 14 aryloxide derivatives [Ge(OC6H2-2,4,6-Cy3)2]2 (2), [Sn(OC6H2-2,4,6-Cy3)2]2 (3), and [Pb(OC6H2-2,4,6-Cy3)2]2 (4) of the new tricyclohexylphenyloxo ligand, [(-OC6H2-2,4,6-Cy3)2]2 (Cy = cyclohexyl), were synthesized and characterized. Complexes 1-4 were obtained by reaction of the metal bissilylamides M(N(SiMe3)2)2 (M = Ge, Sn, Pb) with 2,4,6-tricyclohexylphenol in hexane at room temperature. If the freshly generated reaction mixture for the synthesis of 2 is stirred in solution for 12 h at room temperature, the cluster [Ge6(µ3-O)4(µ2-OC6H2-2,4,6-Cy3)4](NH3)0.5 (1), which features a rare Ge6O8 core that includes ammonia molecules in non-coordinating positions, is formed. Complexes 3 and 4 were also characterized via119Sn{1H} NMR and 207Pb NMR spectroscopy and feature signals at -280.3 ppm (119Sn{1H}, 25 °C) and 1541.0 ppm (207Pb, 37 °C), respectively. The spectroscopic characterization of 3 and 4 extends known 119Sn parameters for dimeric Sn(II) aryloxides, but data for 207Pb NMR spectra for Pb(II) aryloxides are rare. We present also a rare VT-NMR study of a homoleptic 3-coordinate Pb(II) aryloxide. The crystal structures of 2, 3, and 4 feature interligand H⋯H contacts that are similar in number to those of related transition metal derivatives despite the larger size of the group 14 elements.

Catalytic generation of ortho-quinone dimethides via donor/donor rhodium carbenes.

Substrates engineered to undergo a 1,4-C-H insertion to yield benzocyclobutenes resulted in a novel elimination reaction to yield ortho-quinone dimethide (o-QDM) intermediates that undergo Diels-Alder or hetero-Diels-Alder cycloadditions. The analogous benzylic acetals or ethers avoid the C-H insertion pathway completely and, after hydride transfer, undergo a de-aromatizing elimination reaction to o-QDM at ambient temperature. The resulting dienes undergo a variety of cycloaddition reactions with high diastereo- and regio-selectivity. This is one of the few examples of catalytic generation of o-QDM without the intermediacy of a benzocyclobutene and represents one of the mildest, ambient temperature processes to access these useful intermediates. This proposed mechanism is supported by DFT calculations. Moreover, the methodology was applied to the synthesis of (±)-isolariciresinol in 41% overall yield.

Mn(II), Fe(II), and Co(II) Aryloxides: Steric and Dispersion Effects and the Thermal Rearrangement of a Cobalt Aryloxide to a Co(II) Semiquinone Complex.

A series of Mn(II), Fe(II), and Co(II) bisaryloxide dimers ([M(OC6H2-2,4,6-Cy3)2]2 {M = Mn (1), Fe (2), and Co (3)} were synthesized by the addition of 2,4,6-tricyclohexylphenol (HOC6H2-2,4,6-Cy3) to the silyl amido dimers [M(N(SiMe3)2)2]2 (M = Mn, Fe, Co; Cy = cyclohexyl). An unexpected and unique Co(II) phenoxide derivative (4), [Co(OC6H2-2,4,6-Cy3)(O2C6H-3,5,6-Cy3)]2, was obtained via ligand rearrangement of 3 at ca. 180 °C. This yielded 4 in which there are two unchanged, bridging phenoxide ligands as well as a terminal bidentate semiquinone ligand bound to each cobalt. Complexes 1 and 2 did not undergo such a rearrangement under the same conditions; both are thermally stable to temperatures exceeding 250 °C and feature numerous short-contact (<2.5 Å) H···H interactions consistent with the presence of dispersion stabilization. Use of the aryloxide ligand -OC6H3-2,6-Pri2 (Pri = isopropyl), which is sterically similar to -OC6H2-2,4,6-Cy3 but produces fewer close H···H interactions, gave the trimeric species [M(OC6H3-2,6-Pri2)2]3 {M = Fe (5) or Co (6)} which feature a linear array of three metal atoms bridged by aryloxides. The higher association number in 5 and 6 in comparison to that of 1-3 is due to the lower dispersion energy donor properties of the -OC6H3-2,6-Pri2 ligand and the lower stabilization it produces.

Disproportionation of Sn(II){CH(SiMe3)2}2 to ˙Sn(III){CH(SiMe3)2}3 and ˙Sn(I){CH(SiMe3)2}: characterization of the Sn(I) product.

Half a century after the photolytic disproportionation of Lappert's dialkyl stannylene SnR2, R = CH(SiMe3)2 (1) gave the persistent trivalent radical [˙SnR3], the characterization of the corresponding Sn(I) product, ˙SnR is now described. It was isolated as the hexastannaprismane Sn6R6 (2), from the reduction of 1 by the Mg(I)-reagent, Mg(BDIDip)2 (BDI = (DipNCMe)2CH, Dip = 2,6-diisopropylphenyl).

London Dispersion Effects in a Distannene/Tristannane Equilibrium: Energies of their Interconversion and the Suppression of the Monomeric Stannylene Intermediate.

Reaction of {LiC6 H2 -2,4,6-Cyp3 ⋅Et2 O}2 (Cyp=cyclopentyl) (1) of the new dispersion energy donor (DED) ligand, 2,4,6-triscyclopentylphenyl with SnCl2 afforded a mixture of the distannene {Sn(C6 H2 -2,4,6-Cyp3 )2 }2 (2), and the cyclotristannane {Sn(C6 H2 -2,4,6-Cyp3 )2 }3 (3). 2 is favored in solution at higher temperature (345 K or above) whereas 3 is preferred near 298 K. Van't Hoff analysis revealed the 3 to 2 conversion has a ΔH=33.36 kcal mol-1 and ΔS=0.102 kcal mol-1 K-1 , which gives a ΔG300 K =+2.86 kcal mol-1 , showing that the conversion of 3 to 2 is an endergonic process. Computational studies show that DED stabilization in 3 is -28.5 kcal mol-1 per {Sn(C6 H2 -2,4,6-Cyp3 )2 unit, which exceeds the DED energy in 2 of -16.3 kcal mol-1 per unit. The data clearly show that dispersion interactions are the main arbiter of the 3 to 2 equilibrium. Both 2 and 3 possess large dispersion stabilization energies which suppress monomer dissociation (supported by EDA results).

Tuning the Intermediate Valence Behavior in the Zintl Compound Yb14ZnSb11 by Incorporation of RE3+ [Yb14-xRExZnSb11 (0.2 ≤ x ≤ 0.7), RE = Sc, Y, La, Lu and Gd].

The solid solutions of Yb14-xRExZnSb11 (RE = Sc, Y, La, Lu, and Gd; 0.2 ≤ x ≤ 0.7) were prepared to probe the intermediate valency of Yb in Yb14ZnSb11. The substitution of Yb with RE3+ elements should reduce or remove the intermediate valency of the remaining Yb ions. Large crystals are grown from Sn-flux, and the structure and magnetic susceptibility are presented. All compounds crystallize in the Ca14AlSb11 structure type and the RE3+ ions show Yb site substitution preferences that correlate with size. Two compositions of Yb14-xYxZnSb11 were investigated [x = 0.38(3), 0.45(3)] by temperature-dependent magnetic susceptibility and the broad feature in magnetic susceptibility measurements at 85 K in pristine Yb14ZnSb11 attributed to valence fluctuation decreases and is absent for x = 0.45(3). In compounds with nonmagnetic RE3+ substitutions (Sc, Y, La, and Lu), temperature-dependent magnetic susceptibility shows a transition from intermediate valency fluctuation toward temperature-independent (Y, La, and Lu) or Curie-Weiss behavior and possibly low temperature heavy Fermion behavior (Sc). In the example of the magnetic rare earth substitution, RE = Gd, the Curie-Weiss-dependent magnetic moment of Gd3+ is consistent with x. Hall resistivity of Yb14-xYxZnSb11 showed that the carrier concentration decreases with x and the signature of the low-T intermediate valence state seen for x = 0 is suppressed for x = 0.38 and gone for x = 0.45.

Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight.

We leveraged the recent increase in synthetic accessibility of SF5 Cl and Ar-SF4 Cl compounds to combine chemistry of the SF5 and SF4 Ar groups with strain-release functionalization. By effectively adding SF5 and SF4 Ar radicals across [1.1.1]propellane, we accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, we measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C⋅⋅⋅C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect observed in bicyclopentanes.

Preparation of 3-Substituted Isoindolin-1-one, Cinnoline, and 1,2,4-[e]-Benzotriazine Derivatives.

Herein, we report a new approach to synthesize a series of 1,2,4-[e]-benzotriazine and cinnoline derivatives from 3-substituted isoindolin-1-one. All the reported products are obtained through an economical two-step synthetic procedure resulting in fair-to-high yields. Cinnolines (a) and 1,2,4-[e]-benzotriazines (b) result from an intramolecular cyclization of the corresponding 3-substituted isoindolin-1-ones, which, in turn, are prepared by an addition reaction from 2-cyanobenzaldehyde and 2-(2-nitrophenyl) acetonitrile (a) or 2-nitroaniline derivatives (b). A proposed mechanism for this transformation is presented.

One-Pot Assembly and Synthetic Applications of Geminal Acyl/Alkoxy Tetrasubstituted Allenes.

Polysubstituted allenes are useful synthetic intermediates in many applications, offering structural complexity, modularity, and their axial chirality in further transformations. While acyl and alkoxy-substituted allenes are known, there are currently few examples of allenes containing both functionalities and no reports of geminally substituted acyl/alkoxy allenes being isolated and characterized. Herein, we report the synthesis of tetrasubstituted allenes featuring a novel geminal acyl/alkoxy substitution. These unique "push-pull" allenes are bench-stable and exhibit interesting reactivity in several applications.