Unexpected affects of Me3Si-X on reactions of higher order cyanocuprates

The assumed compatibility of Me₃SiCl with higher order cyanocuprates has been studied by both chemical and spectroscopic (¹H, ⁷Li, ²⁹Si NMR) means. Both types of experiments confirm that Me₃Si-X significantly alters the composition of both homo (R₂Cu(CN)Li₂) and mixed (RR′Cu(CN)Li₂) reagents.     

Origin of the regio- and stereoselectivity of allylic substitution of organocopper reagents

The origin of the contrasting regioselectivities in allylic substitution found for a heterocuprate MeCu(CN)Li and a homocuprate Me2CuLi was studied using density functional calculations. The gamma-selectivity of MeCu(CN)Li is determined at the oxidative addition stage of the reaction, where the different degree of trans effect of the Me and the CN groups dictates the relative orientation of the methyl group and the leaving acetate group. As the result, the transition state where the acetate group leaves trans to the Me group on the copper atom is favored, and the gamma-selectivity results. The homocuprate Me2CuLi is symmetrical by nature and does not show such regioselectivity.     

The Use of 15N NMR Spectroscopy To Resolve the “Higher Order Cyanocuprate” Controversy: 15N, 6Li,and 13C NMR Spectroscopic Investigations of CuCN-Derived Butyl Cuprates

Structure 1 is the major component obtained by the reaction of two equivalents of RLi and one equivalent of CuCN; other proposed structures can now be ruled out on the basis of ¹⁵N NMR spectroscopic and theoretical studies. Thus, these useful synthetic reagents should be considered as cyano-Gilman reagents R₂CuLi⋅LiCN and not “higher order cyanocuprates” R₂Cu(CN)Li₂.     

Lithium Cyanide Supported by O‐ and N‐Donors

A series of adducts of LiCN, namely [Li(Me₂CO₃)CN], [Li(Et₂CO₃)CN], and [Li(NMP)CN] (NMP = N‐methyl‐2‐pyrrolidone) were prepared by treatment of solvent‐free LiCN with the appropriate donor. The starting material for these approaches, donor‐free LiCN, was quantitatively prepared from Me₃SiCN and Li[Me] in diethyl ether at 0 °C. Alternatively, [Li(NMP)CN] was synthesized by metathesis reaction of LiCl with NaCN in the presence of stoichiometric amounts of NMP. Although [Li(Me₂CO₃)CN] and [Li(Et₂CO₃)CN] are water‐sensitive compounds and decompose at the exposure to air, [Li(NMP)CN] is stable in air, even at elevated temperatures. The thermal stability of [Li(NMP)CN] was proven by differential thermal analysis (DTA). [Li(NMP)CN] shows thermal stability up to temperatures of about 132 °C. To evaluate the cyanation ability the reactions of 1‐bromooctane and 3‐bromocyclohexene with unsupported LiCN, [Li(NMP)CN], and a mixture of NaCN/LiCl/NMP were investigated. We found that [Li(NMP)CN] as well as LiCl/NaCN/NMP are efficient cyanation reagents comparable to the expensive and air‐sensitive, donor‐free LiCN. A product of the chloride‐cyanide‐bromide exchange could be isolated and structurally characterized by X‐ray diffraction.     

Cuprate-catalyzed Three-Component Couplings of Functionalized Organozinc Reagents

Organozinc reagents FG-(CH₂)_nZnX containing electrophilic centers (FG) upon conversion to their corresponding zincates (FG-(CH₂)_nZnMe₂Li) and in the presence of 5–8 mol % MeCu(CN)Li, undergo 1,4-additions to cyclic conjugated enones. The intermediate zinc enolates can be trapped with aldehydes to afford products of three-component couplings.     

Steric effects on the dialkyl substituted X2C2Si silylenes: A theoretical study

With the aim of recognizing the steric effects on the silylenic H₂C₂Si structures, ab initio and DFT calculations are carried out on 24 structures of X₂C₂Si (where X is hydrogen (H), methyl (Me), isopropyl (i‐pro), and tert‐butyl (tert‐Bu)). These species are at either triplet (t) or singlet (s) states. They are confined to the following three sets of structures ( 1 _X, 2 _X and 3 _X). Structures 1 _X include silacyclopropenylidenes ( 1 _s‐H and 1 _t‐H) and their 2,3‐disubstituted derivatives ( 1 _t‐Me, 1 _s‐Me; 1 _t‐i‐pro, 1 _s‐i‐pro; 1 _{t‐tert‐Bu}, 1 _{s‐tert‐Bu}). Structures 2 _X include vinylidenesilylenes ( 2 _s‐H and 2 _t‐H) and their 3,3‐disubstituted derivatives ( 2 _t‐Me, 2 _s‐Me; 2 _t‐i‐pro, 2 _s‐i‐pro; 2 _{t‐tert‐Bu}, 2 _{s‐tert‐Bu}). Structures 3 _X include ethynylsilylenes ( 3 _s‐H and 3 _t‐H) and their 1,3‐disubstituted derivatives ( 3 _t‐Me, 3 _s‐Me; 3 _t‐i‐pro, 3 _s‐i‐pro; 3 _{t‐tert‐Bu}, 3 _{s‐tert‐Bu}). Singlet–triplet energy separations (Δ E_{s‐t, X}) and relative energies for the above structures are acquired at HF/6‐31G*, B₁LYP/6‐31G*, B3LYP/6‐31G*, MP2/6‐31G*, HF/6‐31G**, B1LYP/6‐31G**, B3LYP/6‐31G**, and MP2/6‐31G** levels of theory. The highest Δ E_{s‐t, X} is encountered for 1 _X. All singlet states of X₂C₂Si, are more stable than their corresponding triplet states. Linear correlations are found between the LUMO–HOMO energy gaps of the singlet 1 _s‐X and 2 _s‐X with their corresponding singlet–triplet energy separations calculated at B3LYP/6‐31G**. The seven structures 2 _s‐Me, 2 _t‐Me, 3 _s‐Me, 1 _t‐Me, 1 _s‐Me, 1 _{s‐tert‐Bu}, and 3 _{t‐tert‐Bu} do not appear to be real isomers. Different stability orders are obtained as a function of the substituents (X). The order of stability for six isomers of H₂C₂Si is 1 _s‐H > 2 _s‐H > 3 _s‐H > 2 _t‐H > 3 _t‐H > 1 _t‐H. Replacing hydrogen atoms by methyl group (X = Me) presents a new stability order: 1 _s‐Me > 3 _s‐Me > 2 _s‐Me > 3 _t‐Me > 2 _t‐Me > 1 _t‐Me; and for (i‐pro)₂C₂Si is 1 _s‐i‐pro > 2 _s‐i‐pro ≈ 3 _s‐i‐pro > 3 _t‐i‐pro ≈ 2 _t‐i‐pro > 1 _t‐i‐pro. Using the larger tert‐butyl group as a substituent (X), yet it offers a more different stability order for six structures of (tert‐Bu)₂C₂Si: 1 _{s‐tert‐Bu} > 3 _{s‐tert‐Bu} > 2 _{s‐tert‐Bu} > 3 _{t‐tert‐Bu} > 1 _{t‐tert‐Bu} > 2 _{t‐tert‐Bu}. Among eight levels employed, B3LYP/6‐31G** appears as the method of choice. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:619–633, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20204     

Approaches to the synthesis of cytochalasans. Part 10. Cuprates as reagents for the formation of C?C bonds

Based upon our novel concept for the total synthesis of cytochalasans, the model lactams 2–9 were treated with Bu₂Cu(CN)Li₂. The results of these conversions vary much from those obtained with Ph₂Cu(CN)Li₂, demonstrating the uncertainty of predictions in cuprate chemistry. The bicyclic compound 20 was prepared in good yield. However, all attempts to convert p-toluenesulfonate 20 into the Ph-substituted derivative 21 , an intermediate for the synthesis of cyiochulusm B(1) , have failed so far.     

Bimolecular displacements of iodine in reactions of dimethyl[tris(trimethylsilyl)methyl]silyl iodide with salts

Direct nucleophilic displacement of iodine to give (Me₃Si)₃ CSiMe₂ Y, where Y = F, NCO, NCS, CN or N₃, takes place when (Me₃Si)₃ CSiMe₂I is treated with solutions of CsF, KOCN, KSCN, KCN, or NaN₃ in MeOH or CH₃ CN. The order of effectiveness of the nucleophiles appears to be N₃ > F > CN > NCS > NCO in MeOH and NCS > NCO > CN, F in CH₃ CN.     

Highly stereoselective addition of stannylcuprates to alkynones

The addition of stannylcuprate reagents such as (Bu3Sn)(PhS)CuLi to alkynones has been found to proceed in high yield and with excellent stereoselectivity for the Z isomer of the product (>95%). The behavior of the stannylcuprates is thus very different from that of their "carbocuprate" counterparts such as Me2CuLi or Me2Cu(CN)Li2 which are nonstereoselective. Furthermore, in contrast to the reactions of (R3Sn)(PhS)CuLi with the corresponding alkynoates, the presence of a proton source in the reaction medium has no effect on the stereoselectivity of the reaction of alkynones.     

The Crystal Structures of a Lower Order and a “Higher Order” Cyanocuprate: [tBuCu(CN)Li(OEt2)2]∞ and [tBuCutBu{Li(thf)(pmdeta)}2CN]

Different types of bonding are present in cyanocuprates 1 and 2 , whose crystal structures could be determined (the drawings below show the important structural characteristics). Accordingly, 1 is a lower order cyanocuprate of the type RCu(CN)Li, whereas 2 , which is of the type R₂Cu(CN)Li₂, does not exist as a “higher order” cyanocuprate with Cu–CN bonds, but rather as a cyano-Gilman cuprate.     

Strukturen zweifach metallierter Derivate von 3, 3‐Dimethyl‐1, 5‐bis(trimethylsilyl)‐1, 5‐diaza‐pentan mit Lithium und Aluminium sowie zweier Donor‐substituierter Digallane

Structural Characterization of Bis(metallated) Derivatives of 3, 3‐Dimethyl‐1, 5‐bis(trimethylsilyl)‐1, 5‐diaza‐pentane with Lithium and Aluminum and of two Donor‐substituted Digallanes The diaminopropane derivative Me₂C[CH₂N(H)SiMe₃]₂ is metallated with n‐butyllithium and lithium tetrahydridoaluminate to obtain Me₂C[CH₂N(Li)SiMe₃]₂ and Me₂C[CH₂N(Li)SiMe₃][CH₂N(AlH₂)SiMe₃], respectively. Both compounds exhibit a central eight‐membered ring, Li₄N₄ or Li₂Al₂N₄. Me₂C[CH₂N(Li)SiMe₃]₂ reacts with Ga₂Cl₄ · 2dioxane under formation of the corresponding tetra(amino)digallane. This is monomeric, in contrast to a dimeric tetraalkoxy‐substituted digallane, Ga₄O^tBu₈. All compounds were characterized by single crystal X‐ray crystallography.     

Reactions of pentafluorophenyltrimethylsilane and cyanomethyltrimethylsilane with carbonyl compounds catalyzed by cyanide anions

Treatment of pentafluorophenyltrimethylsilane (I) and cyanomethyltrimethylsilane (II) with enolizable ketones in the presence of a catalytic amount of potassium cyanide-18-crown-6 complex gave the corresponding trimethylsilyl enol ethers. The same dehydrogenative silylation of acetylacetone and benzoylacetone with silane I was extended to the preparation of 2,4-bis(trimethylsiloxy)-1,3-pentadiene and 1-phenyl-1,3 -bis(trimethylsiloxy)-1,3-butadiene, respectively. The dehydrogenative silylation of acetylacetone and benzoylacetone with dimethylbis(pentafluorophenyl)silane under the same conditions affords novel heterocyles 5-methylene-2,6-dioxa-1-silacylohex-3-enes. In the reaction studied the silylating ability of the silanes increases in the order Me₃SiCN ? Me₂Si(CN)₂ < Me₃SiCH₂CN < Me₃SiC₆F₅ ? Me₂Si(C₆F₅)₂. On the other hand, potassium cyanide-18-crown-6 complex catalyzed the addition of silane I or II to a carbonyl group of non-enolizable compounds such as benzaldehyde, crotonaldehyde, and methyl(triethylgermyl)ketene.     

Dilithium Hexaorganostannate(IV) Compounds

Hypercoordination of main‐group elements such as the heavier Group 14 elements (silicon, germanium, tin, and lead) usually requires strong electron‐withdrawing ligands and/or donating groups. Herein, we present the synthesis and characterization of two hexaaryltin(IV) dianions in form of their dilithium salts [Li₂(thf)₂{Sn(2‐py^Me)₆}] (py^Me=C₅H₃N‐5‐Me) ( 2 ) and [Li₂{Sn(2‐py^OtBu)₆}] (py^OtBu=C₅H₃N‐6‐OtBu) ( 3 ). Both complexes are stable in the solid state and solution under inert conditions. Theoretical investigations of compound 2 reveal a significant valence 5s‐orbital contribution of the tin atom forming six strongly polarized tin–carbon bonds.     

Theoretical study on thermal decomposition of azoisobutyronitrile in ground state

Since the first discovery of azoalkanes in 1909,the studies of the chemistry of azoalkane radicals havegone through a long history and many significativeresults have been gotten during the past 30 years[1,2].These versatile compounds lose nitrogen thermally orphotochemically under a wide variety of conditions:R─N═N─R ? 2R?+ N2; hence, they are probablythe cleanest and most convenient sources of variousradicals and biradicals of nearly any desired structure.Several reviews on the applicati…     

Synthesis and structures of silicon-, germanium-, and tin-containing imido-alkyl molybdenum complexes (ArN)2Mo(CH2EMe3)2 (E = Si, Ge, Sn)

New silicon-, germanium-, and tin-containing imido-alkyl molybdenum complexes (ArN)₂Mo(CH₂EMe₃)₂ (Ar is 2,6-diisopropylphenyl; E = Si (1), Ge (2), Sn (3)) were prepared in the crystalline state in 58–66% yields by the reactions of the (ArN)₂MoCl₂(DME) complex with alkyllithium derivatives Me₃ECH₂Li (E = Si or Ge) or the Grignard reagents Me₃ECH₂MgCl (E = Ge or Sn). The structures of complexes 1–3 and the known analog (ArN)₂Mo(CH₂Bu^t)₂ (4) were established by X-ray diffraction analysis. Complexes 1–3 were found to be isostructural. The coordination environment about the Mo atom can be described as a distorted tetrahedron. Complex 4 has a similar structure. The Mo-C distance tends to decrease with increasing electron donating ability of the EMe₃ group.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 597–600, March, 2005.     

[Li(tmeda)2][cyclo-(P5But4)]: An unusual ion-separated lithium oligophosphanide

The reaction of lithium with Bu^tPCl₂ and PCl₃ in the ratio 12:4:1 in THF gave a product mixture comprising cyclo-(P₄Bu^t₄), Li₂(P₄Bu^t₄), and lithium tetra-tert-butylcyclopentaphosphanide Li[cyclo-(P₅Bu^t₄)] (1) among other phosphanides and phosphanes. Optimization of the reaction conditions and recrystallization from THF/TMEDA (TMEDA: Me₂NCH₂CH₂NMe₂) gave [Li(tmeda)₂][cyclo-(P₅Bu^t₄)] (1b) which was characterized by multinuclear NMR spectroscopy, mass spectrometry, IR spectroscopy, and elemental analysis. Single-crystal X-ray diffraction studies showed the presence of separated [Li(tmeda)₂]⁺ cations and [cyclo-(P₅Bu^t₄)]^? anions. 1b represents the first structure of a “naked” [cyclo-(P₅Bu^t₄)]^? anion.     

Chemistry of higher order,mixed organocuprates. reactions of α,β-unsaturated ketones.

Organocuprates of general formula R₂Cu(CN)Li₂ react rapidly with α,β-unsaturated ketones at low temperatures and in high yields to deliver ligands in a conjugate manner. These reagents apparently do not require the use of additives for stabilizing or solubilizing purposes.     

Kristallstruktur eines Lithiumsilylamidbutanids

Crystal Structure of a Lithiumsilylamidebutanide Colorless single crystals of {Li₆[Me₂(H)Si—N—Si(H)—(CHMe₂)₂]₂[n‐C₄H₉]₄} ( 1 ) were obtained from a solution of Me₂(H)SiN(Li)Si(H)(CHMe₂)₂ and n‐C₄H₉Li in n‐hexane. The X‐ray analysis showed that the core of 1 is a distorted octahedron of lithium atoms with ten long and with two short LiÄLi distances. Four of the eight triangular Li₃ faces are capped by an n‐butyl group. The nitrogen atoms of the amide groups are situated about opposite edges of adjacent unoccupied Li₃ faces. (Si)H····Li interactions exist between the hydridic H atom of each Me₂(H)Si group and one Li atom.     

Order—disorder in compounds LiMe2+VO4

The crystallographic order between the Li⁺ and Me²⁺ ions in some compounds LiMeVO₄ (Me = Mg, Co, Ni, Cu, Zn) has been investigated by vibrational spectroscopy. The Li⁺ and Me²⁺ ions are disordered in the cobalt and nickel compounds and ordered in the copper and magnesium compounds.     

Structural Effects in Lithiocuprate Chemistry: The Elucidation of Reactive Pentametallic Complexes

TMPLi (TMP=2,2,6,6‐tetramethylpiperidide) reacts with Cu^I salts in the presence of Et₂O to give the dimers [{(TMP)₂Cu(X)Li₂(OEt₂)}₂] (X=CN, halide). In contrast, the use of DMPLi (DMP=cis‐2,6‐dimethylpiperidide) gives an unprecedented structural motif; [{(DMP)₂CuLi(OEt₂)}₂LiX] (X=halide). This formulation suggests a hitherto unexplored route to the in situ formation of Gilman‐type bases that are of proven reactivity in directed ortho cupration.