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

Abstract

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.

Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of the Heavier Alkaline Earth Metals: Synthesis, Characterization, and Efficient Catalytic Intramolecular Hydroamination

“…Interestingly, the agostic hydrogen H2a has been unambiguously localized (Ca1···H2a = 2.41(2) ), affording a nearly planar Ca1N7Si2H2a core (torsion angle 6.5 8). Similar geometric features have been noted for other heteroleptic silylamido [69] and tris(dimethylsilyl)methyl [60,70] Ca complexes that also exhibit b-SiÀH agostic interactions. The presence of this interaction is likely to confer a certain degree of stability, since 5 appeared significantly less reactive towards air and moisture than 1.…”

Optically active, bulky tris(oxazolinyl)borato magnesium and calcium compounds for asymmetric hydroamination/cyclization

“…Thus, the d SiH and 1 J SiH of values 4.89 ppm and 153 Hz from the 1 H NMR spectrum and the n SiH values of 2106 and 1877 cm À1 in the infrared spectrum indicate the presence of b-agostic SiH groups, as are observed in the starting material Ca(C(SiHMe 2 ) 3 ) 2 (THF) 2 [22] These data are consistent with a structure of To T CaC(SiHMe 2 ) 3 containing one or two b-agostic SiH interactions that are fluxional on the 1 H NMR timescale but exchange slower than the IR timescale. The three oxazoline groups are equivalent on the 1 H NMR timescale.…”

“…A related calcium complex is accessible by reaction of H[To T ] and Ca(C(SiHMe 2 ) 3 ) 2 (THF) 2 [22] in benzene, which yields To T CaC (SiHMe 2 ) 3 and HC(SiHMe 2 ) 3 as a non-coordinating byproduct that can be removed under vacuum (Eq. (6)).…”

“…In general, the calcium compound To T CaC(SiHMe 2 ) 3 catalyzes the cyclization of all three substrates at a significantly greater rate than hydroaminations catalyzed by To T MgMe. While we expected that the presumed catalyst intermediate To T MNHCH 2 CR 2 CH 2 CH]CH 2 (R 2 ¼ Ph 2 , e(CH 2 ) 5 e, Me 2 ) would be more reactive for M ¼ Ca than M ¼ Mg based on size and related rates of hydroamination/cyclizations catalyzed by diketiminate calcium and magnesium complexes, [3] initiation of To T CaC (SiHMe 2 ) 3 is surprisingly fast given the unusual properties of the -C (SiHMe 2 ) 3 group [22]. This alkyl ligand is both sterically encumbered and relatively non-basic at the central carbon, and as a result Lewis acids such as B(C 6 F 5 ) 3 react with a peripheral SiH rather than abstract the alkyl group in the typical fashion [23].…”

“…Benzene-d 6 , toluene-d 8 and tetrahydrofuran-d 8 were vacuum transferred from Na/K alloy and stored under N 2 in the glovebox. Ca(C (SiHMe 2 ) 3 ) 2 (THF) 2 [22] Me 2 Mg$(O 2 C 4 H 8 ) 2 [28] 2,2-diphenyl-4-penten-1-amine [29] 2,2-dimethyl-4-penten-1-amine [30] and C-(1-allyl-cyclohexyl)-methylamine [31] were prepared by published procedures. All the aminoalkenes were degassed and stored with 4 Å molecular sieves in a glovebox prior to use.…”