X-Ray Structural Analyses of Organolithium Compounds

Valence-shell charge concentrations and electron delocalization in alkyllithium complexes: negative hyperconjugation and agostic bonding

In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the alpha and beta atoms reveal faithfully the delocalization of the lone pair at the Calpha atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Calpha, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li.H contacts are a consequence of this delocalization and further secondary interactions, with Li.H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Calpha provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.

NMR spectroscopic investigations of mixed aggregates underlying highly enantioselective 1,2-additions of lithium cyclopropylacetylide to quinazolinones

The solution structures of mixed aggregates derived from lithium alkoxides and lithium acetylides were investigated as part of a program to develop practical syntheses of quinazolinone-based nonnucleoside reverse transcriptase inhibitors. Low-temperature (6)Li, (13)C, and (15)N NMR spectroscopies reveal that mixtures of lithium cyclopropylacetylide (RCCLi), a (+)-carene-derived amino alkoxide (ROLi), and lithium hexamethyldisilazide (LiHMDS) in THF/pentane afford a (RCCLi)(3)(ROLi) mixed tetramer, a C(2)-symmetric and asymmetric (RCCLi)(2)(ROLi)(2) mixed tetramer, and a C(3)-symmetric (RCCLi)(ROLi)(3) mixed tetramer. Analogous mixtures of RCCLi/ROLi in Et(2)O and Me(2)NEt also provide 3:1, 2:2, and 1:3 mixed tetramers. The stereochemistry of aggregation is highly sensitive to the medium. The C(2)-symmetric (RCCLi)(2)(ROLi)(2) mixed tetramer is formed in Et(2)O, whereas the asymmetric isomer is formed in Me(2)NEt. LiHMDS in THF is shown to be an efficient proton scavenger without forming LiHMDS-RCCLi or LiHMDS-ROLi mixed aggregates. LiHMDS-RCCLi mixtures form mixed aggregates in Me(2)NEt.

A Convenient Synthetic Strategy toward Heavy Alkali Metal Bis(trimethylsilyl)phosphides: Crystal Structures of the Ladder-Type Polymers [A(thf)P(SiMe(3))(2)](infinity) (A = K, Rb, Cs)

A series of highly reactive heavy alkali metal phosphides was prepared by treating trimethylsilyl-substituted phosphines with alkali metal tert-butyl alcoholates. The compounds are formed in excellent yield and purity, and the side product can be easily removed in a vacuum. The high synthetic potential of this reaction route was further shown by utilizing excess alkali metal tert-butyl alcoholates in reaction with silyl substituted phosphines. In all cases, only monometalated products were isolated. In the course of this work the crystal structures of the bis(trimethylsilyl)phosphides [K(thf)P(SiMe(3))(2)](infinity), 1a, [Rb(thf)P(SiMe(3))(2)](infinity), 1b, and [Cs(thf)P(SiMe(3))(2)](infinity), 1c, were obtained. The compounds display polymeric ladder-type structures. Compounds 1a,b are isomorphous, while compound 1c displays a slightly altered local geometry. Despite small differences in local geometry, the coordination spheres for the phosphorus atoms and the alkali metal are fairly similar. The five coordinate phosphorus atoms are connected to three alkali metal centers in addition to two trimethylsilyl groups. The alkali metals are four coordinate with ligations to three phosphorus centers in addition to one thf oxygen donor. Compounds 1a-c were characterized using elemental analysis, NMR spectroscopy, and X-ray crystallography. Crystal data with Mo Kalpha (lambda = 0.710 73 Å) at 150 K are as follows: 1a, a = 6.4261(2) Å, b = 12.4119(2) Å, c = 21.5447(4) Å, V = 1718.41(7) Å(3), Z = 4, orthorhombic, space group P2(1)2(1)2(1), 3427 independent reflections, R1 (all data) = 0.0351; 1b, a = 6.5338(2) Å, b = 12.5664(3) Å, c = 21.5537(5) Å, V = 1769.70(8) Å(3), Z = 4, orthorhombic, space group P2(1)2(1)2(1), 4195 independent reflections, R1 (all data) = 0.0776; 1c, a = 11.3515(1) Å, b = 22.3445(3) Å, c = 7.2501(1) Å, beta = 96.017(1) degrees, V = 1828.81(4) Å(3), Z = 4, monoclinic, space group P2(1)/c, 4343 independent reflections, R1 (all data) = 0.0811.

Heterobimetallic (Imido)antimony/Lithium Cages Containing [Sb(NR)(3)](3-) Trianions

The reactions of [Sb(NMe(2))(3)] with the primary (amido)lithiums [PhCH(2)CH(2)N(H)Li](n)(), [CyN(H)Li](n)() (Cy = C(6)H(11)), [2,4-dmpN(H)Li](n)() [2,4-dmp = 2,4-(MeO)(2)C(6)H(3)], and [(t)()BuN(H)HLi](n)() give the heterobimetallic cage complexes [{Sb(NCH(2)CH(2)Ph)(3)}(2)Li(6).2THF] (1), [{Sb(NCy)(3)}(2)Li(6).2HNMe(2)].2C(6)H(5)CH(3) (2), [Sb{N(2,4-dmp)}(3)](2)Li(6).2THF.2C(6)H(5)CH(3) (3), and [{Sb(NBu(t)())(3)}(2)Li(6)] (4), respectively. The low-temperature X-ray structures of 1-4 show that they are composed of 14-membered polyhedral cages constructed from the association of two [Sb(NR)(3)](3)(-) trianions with six Li(+) cations. Crystal data; 2, triclinic, P&onemacr;, a = 12.775(6) Å, b = 13.191(9) Å, c = 11.015(5) Å, alpha = 111.55(4) degrees, beta = 95.39(4) degrees, gamma = 115.26(4) degrees; 3, triclinic P&onemacr;, a = 10.435(4) Å, b = 12.654(5) Å, c = 14.278(6) Å, alpha = 75.31(4) degrees, beta = 79.33(4) degrees, gamma = 84.33(4) degrees; 4, monoclinic P2(1)/c, a = 9.994(1) Å, b = 17.421(2) Å, c = 10.680(2) Å, beta = 111.33(1) degrees (the structure of 1 being reported previously). Lewis base solvation of the N(6)Li(6) substructures of 1-3 results in distortion and deformation of the Li frameworks. The structural variations in the cores of 1-4 indicate that the bonding in these species is dominated by the valence and bonding demands of the p block metal centers.