KaKing Yan

Lewis Acid-Mediated β-Hydride Abstraction Reactions of Divalent M(C(SiHMe2)3)2THF2 (M = Ca, Yb)

The divalent calcium and ytterbium compounds M(C(SiHMe(2))(3))(2)THF(2) contain beta-agostic SiH groups, as determined by spectroscopy and crystallography. Upon thermolysis, HC(SiHMe(2))(3) is formed. However, the SiH groups are hydridic. The compounds M(C(SiHMe(2))(3))(2)THF(2) react with 1 and 2 equiv of the Lewis acid B(C(6)F(5))(3) to form MC(SiHMe(2))(3)HB(C(6)F(5))(3))THF(2) and M(HB(C(6)F(5))(3))(2)THF(2), respectively. These species contain the anion [HB(C(6)F(5))(3)](-) from hydride abstraction rather than [(Me(2)HSi)(3)CB(C(6)F(5))(3)](-) from alkyl abstraction. The 1,3-disilacyclobutane byproduct initially suggested beta-elimination [as the dimer of the silene Me(2)Si horizontal lineC(SiHMe(2))(2)], but the other products and reaction stoichiometry rule out that pathway. Additionally, Yb(C(SiHMe(2))(3))(2)THF(2) and the weak Lewis acid BPh(3) react rapidly and also give the H-abstracted products. Despite the strong hydridic character of the SiH groups and the low-coordinate, Lewis acidic metal center in M(C(SiHMe(2))(3)THF(2) compounds, beta-elimination is not an observed reaction pathway.

Nonclassical β-Hydrogen Elimination of Hydrosilazido Zirconium Compounds via Direct Hydrogen Transfer

Salt metathesis reactions of Cp(2)(NR(2))ZrX (X = Cl, I, OTf) and lithium hydrosilazides ultimately afford hydride products Cp(2)(NR(2))ZrH that suggest unusual β-hydrogen elimination processes. A likely intermediate in one of these reactions, Cp(2)Zr[N(SiHMe(2))t-Bu][N(SiHMe(2))(2)], is isolated under controlled synthetic conditions. Addition of alkali metal salts to this zirconium hydrosilazide compound produces the corresponding zirconium hydride. However as conditions are varied, a number of other pathways are also accessible, including C-H/Si-H dehydrocoupling, γ-abstraction of a CH, and β-abstraction of a SiH. Our observations suggest that the conversion of (hydrosilazido)zirconocene to zirconium hydride and silanimine does not follow the classical four-center mechanism for β-elimination.

Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI3THFn and 3 equiv of KC(SiHMe2)3. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C3-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe2)3}3 and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR time scale at room temperature, but the two motifs are resolved at low temperature. Variable-temperature NMR studies provide activation parameters for the exchange process in 1a (ΔH⧧ = 8.2(4) kcal·mol-1; ΔS⧧ = -1(2) cal·mol-1K-1) and 1a-d9 (ΔH⧧ = 7.7(3) kcal·mol-1; ΔS⧧ = -4(2) cal·mol-1K-1). Comparisons of lineshapes, rate constants (kH/kD), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d9 reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five β-Si-H⇀Ln and one γ-C-H⇀Ln. The calculations also suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe2)3 ligand that is coordinated to the Ln center through the Ln-C bond and one secondary interaction. These robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe2)3}3 and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(OxMe2)2) give {MeC(OxMe2)2}Ln{C(SiHMe2)3}2, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

A trid(alkyl)yttrium compound cotaining six beta-agostic Si-H interactions

The new homoleptic rare earth compound [Y(C(SiHMe(2))(3))(3)] () is prepared in 82% yield by salt metathesis of YCl(3) and 3 equivalents of [KC(SiHMe(2))(3)] (); two beta-agostic Y(H-Si) interactions are observed for each C(SiHMe(2))(3) ligand in , giving six agostic interactions per yttrium(iii) center.

Chiral Crystalline Sponges for the Absolute Structure Determination of Chiral Guests

Chiral crystalline sponges with preinstalled chiral references were synthesized. On the basis of the known configurations of the chiral references, the absolute structures of guest compounds absorbed in the pores of the crystalline sponges can be reliably determined without crystallization or chemical modification.

Chiral crystalline sponges: absolute structure determination of chiral guests

Enantiomers of optically active molecules often display vastly different chemical and biological activities. This is particularly important for pharmaceutical industry where nearly half of the active pharmaceutical ingredients (APIs) are chiral compounds.1 As a result, reliable analysis of their handedness has paramount importance to establish structure-activity relationship. Several chromatographic, and spectroscopic methods have been implemented in the literature for absolute structure determination of chiral compounds. However, single crystal X-ray diffraction (SC-XRD) remains one of the most dependable methods.2 Unfortunately, this method compels the pre-existence of heavy atoms in molecular scaffold and also a pre-condition of crystallization of the chiral compound for reliable analysis. To overcome these problems, Fujita group recently developed the ‘crystalline sponge method’, which is an analytical tool for the micro-to-nanogram scale X-ray structure analysis of non-crystalline compounds. Additionally, presence of heavy atoms in the parent framework enhances the likelihood of crystalline sponges for absolute structure determination.3 In this work, we designed new crystalline sponges with pre-installed chiral internal references either as chemical substituents or guest molecules within their porous network and further these ‘chiral crystalline sponges’ were utilised for the reliable structure elucidation of chiral guest compounds. References 1. McConathy, J. & Owens, M. J. (2003) Stereochemistry in Drug Action. Primary Care Companion to The Journal of Clinical Psychiatry, 5, 70–73. 2. Parsons, S., Flack, H. D., Wagner, T. (2013). Acta Cryst. B69, 249– 259. 3. Inokuma, Y., Yoshioka, S., Ariyoshi, J., Arai, T., Hitora, Y., Takada, K., Matsunaga, S., Rissanen, K., Fujita, M. (2013) Nature 495, 461−466.