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).

Dimeric Copper and Lithium Thiolates: Comparison of Copper Thiolates with Their Lithium Congeners.

The direct reactions of the large terphenyl thiols HSAriPr4 (AriPr4= -C6H3-2,6-(C6H3-2,6-iPr2)2) and HSAriPr6 (AriPr6= -C6H3-2,6-(C6H2-2,4,6-iPr3)2) with stoichiometric amounts of mesitylcopper(I) in THF at ca. 80 °C afforded the first well-characterized dimeric copper thiolato species {CuSAriPr4}2 (1) and {CuSAriPr6}2 (2) with elimination of mesitylene. The complexes 1 and 2 were characterized by NMR and electronic spectroscopy as well as by X-ray crystallography. They have dimeric Cu2S2 core structures in which the two copper atoms are bridged by the sulfurs from the thiolato ligands and feature short Cu--Cu distances near 2.4 Å as well as a weak copper-flanking aryl ring interaction from a terphenyl substituent. The structures of the planar Cu2S2 cores bear a resemblance to the CuA site in nitrous oxide reductase in which two cysteines also bridge two copper atoms. The related dimeric Li2S2 structural motif was also observed in the lithium congeners {LiSAriPr4}2 (3) and {LiSAriPr6}2 (4) which were synthesized directly from the thiols and n-BuLi in hexanes. However, despite the very similar effective ionic radii of the Li+ (0.59 Å) and Cu+ (0.60 Å) ions, the Li--Li structures display very much longer (by more than ca. 0.5 Å) separations than the corresponding Cu--Cu distances in 1 and 2, which may be due to weaker dispersion interactions.

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.

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).

Switching the site-selectivity of C-H activation in aryl sulfonamides containing strongly coordinating N-heterocycles.

The limitations of arene C-H functionalization of aryl sulfonamides containing strongly coordinating N-heterocycles were overcome using a Rh(iii) catalyst. The site-selectivity of C-H carbenoid functionalization at the ortho position relative to either the sulfonamide or N-heterocycle directing groups was elegantly switched using solvents of different polarities and different additive concentrations. Importantly, sulfonamide-group-directed ortho-C-H carbenoid functionalization tolerated strongly coordinating N-heterocycles, including pyridine, pyrrole, thiazole, pyrimidine, and pyrazine. Density functional theory (DFT) calculations were performed to rationalize the reaction mechanisms and the influence of reaction polarity.