Introducing a Hydrogen‐Bond Donor into a Weakly Nucleophilic Brønsted Base: Alkali Metal Hexamethyldisilazides (MHMDS,M=Li,Na, K,Rb and Cs) with Ammonia

Alkali metal 1,1,1,3,3,3‐hexamethyldisilazide (MHMDSs) are one of the most utilised weakly nucleophilic Brønsted bases in synthetic chemistry and especially in natural product synthesis. Like lithium organics, they aggregate depending on the employed donor solvents. Thus, they show different reactivity and selectivity as a function of their aggregation and solvation state. To date, monomeric LiHMDS with monodentate donor bases was only characterised in solution. Since the first preparation of LiHMDS in 1959 by Wannagat and Niederprüm, all efforts to crystallise monomeric LiHMDS in the absence of chelating ligands failed. Herein, we present ammonia adducts of LiHMDS, NaHMDS, KHMDS, RbHMDS and CsHMDS with unprecedented aggregation motifs: 1) The hitherto missing monomeric key compound in the LiHMDS aggregation architectures. Monomeric crystal structures of trisolvated LiHMDS ( 1 ) and NaHMDS ( 2 ), showing unique intermolecular hydrogen bonds, 2) the unprecedented tetrasolvated KHMDS ( 3 ) and RbHMDS ( 4 ) dimers and 3) the disolvated CsHMDS ( 5 ) dimer with very close intermolecular Si?CH₃???Cs s‐block “agostic” interactions have been prepared and characterised by single‐crystal X‐ray structure analysis.     

Z/E-photoisomerizations of olefins with 4npi- or (4n + 2)pi-electron substituents: zigzag variations in olefin properties along the T(1) state energy surfaces

[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields.     

Palladium-catalyzed C,N-cross coupling reactions of 3-halo-2-aminopyridines

A simple approach toward N(3)-substituted-2,3-diaminopyridines is presented, based on Pd-catalyzed C,N-cross coupling. The use of RuPhos- and BrettPhos-precatalysts in combination with LiHMDS allows for C,N-cross coupling reactions of unprotected 3-halo-2-aminopyridines with primary and secondary amines.     

Chain‐growth condensation polymerization of 4‐aminobenzoic acid esters bearing tri(ethylene glycol) side chain with lithium amide base

Chain‐growth condensation polymerization of p‐aminobenzoic acid esters 1 bearing a tri(ethylene glycol) monomethyl ether side chain on the nitrogen atom was investigated by using lithium 1,1,1,3,3,3‐hexamethyldisilazide (LiHMDS) as a base. The methyl ester monomer 1a afforded polymer with low molecular weight and a broad molecular weight distribution, whereas the polymerization of the phenyl ester monomer 1b at ?20 °C yielded polymer with controlled molecular weight (M_n = 2800–13,400) and low polydispersity (M_w/M_n = 1.10–1.15). Block copolymerization of 1b and 4‐(octylamino)benzoic acid methyl ester ( 2 ) was further investigated. We found that block copolymer of poly 1b and poly 2 with defined molecular weight and low polydispersity was obtained when the polymerization of 1b was initiated with equimolar LiHMDS at ?20 °C and continued at ?50 °C, followed by addition of 2 and equimolar LiHMDS at ?10 °C. Spherical aggregates were formed when a solution of poly 1b in THF was dropped on a glass plate and dried at room temperature, although the block copolymer of poly 1b and poly 2 did not afford similar aggregates under the same conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1357–1363, 2010     

Lithiation at the 6-position of uridine with lithium hexamethyldisilazide: crucial role of temporary silylation

Lithium hexamethyldisilazide (LiHMDS) can mediate silylation at the 6-position of uridine, although LiHMDS alone is not able to generate the C-6-lithiated uridine. Experimental results showed that temporary silylation of O-4 (or N-3) of the uracil ring triggers the C-6 lithiation with LiHMDS. This finding allowed us to develop an efficient intramolecular alkylation of 5'-deoxy-5'-iodouridine to furnish 6,5'-C-cyclouridine. [reaction--see text]     

A multinuclear NMR study of [Pt0(PPh3)2(alkene)] compounds containing asymmetric olefins

The 1H, 13C, 31P, and 195Pt NMR spectra of [Pt0(PPh3)2(eta-ABC(1) = C(2)XY)] compounds (ABC(1)= C(2)XY (1) A = B = X = Y = H; (3) A = B = X = H, Y = CN; (4) A = H, B = p-NO2-Ph, X = COOCH3, Y = CN; (5) A = H, B = Ph, X = COOCH3, Y = CN; (6) A = H, B = Ph, X = Y = CN; (7) A = H, B = OEt, X = Y = CN), where X and Y are electronacceptor substituents, and the 1H spectrum of [Pt0(PPh3)2(eta2-C60)] (2) are reported together with extended analyses and assignments, based also on the ring current effect of the olefin phenyl in (4-6). Deviations from first order in the 13C spectra allowed the determination of the relative signs of the coupling constants J(P(1), C) and J(P(2), C) of the alkene and of the triphenylphosphine carbons. Best fit simulation of the phosphine C ipso spectrum provided also the 13C isotopic shift on phosphorus for (1). These compounds are characterised by strong differences between the two platinum-phosphorus coupling constants in the case of asymmetric olefins (3-7). The chemical shifts of alkene C(1) and C(2) indicate notable polarisation of the olefin after complexation, while the 1J(Pt, C(1)) and 1J(Pt, C(2)) values are in agreement with a stronger interaction of Pt with C(1) than with C(2). These findings together with the trend of 195Pt chemical shifts confirm the important role played by back-donation in the bonding of platinum(0)-olefin compounds.     

Beta-alkyl-alpha-allylation of Michael acceptors through the palladium-catalyzed three-component coupling between allylic substrates, trialkylboranes, and activated olefins

The palladium-catalyzed three-component beta-alkyl-alpha-allylation reaction of activated olefins has been achieved. For example, in the presence of 5 mol % of Pd(PPh3)4, the reaction of benzylidenemalononitrile 1a with Et3B and allyl acetate 2a in THF proceeded smoothly at 40 degrees C to give the corresponding beta-ethyl-alpha-allylated product 3a in 81% yield.     

Mechanistic studies of olefin and alkyne trimerization with chromium catalysts: deuterium labeling and studies of regiochemistry using a model chromacyclopentane complex

A system for catalytic trimerization of ethylene utilizing chromium(III) precursors supported by diphosphine ligand PNP(O4) = (o-MeO-C6H4)2PN(Me)P(o-MeO-C6H4)2 has been investigated. The mechanism of the olefin trimerization reaction was examined using deuterium labeling and studies of reactions with alpha-olefins and internal olefins. A well-defined chromium precursor utilized in this studies is Cr(PNP(O4))(o,o'-biphenyldiyl)Br. A cationic species, obtained by halide abstraction with NaB[C6H3(CF3)2]4, is required for catalytic turnover to generate 1-hexene from ethylene. The initiation byproduct is vinylbiphenyl; this is formed even without activation by halide abstraction. Trimerization of 2-butyne is accomplished by the same cationic system but not by the neutral species. Catalytic trimerization, with various (PNP(O4))Cr precursors, of a 1:1 mixture of C2D4 and C2H4 gives isotopologs of 1-hexene without H/D scrambling (C6D12, C6D8H4, C6D4H8, and C6H12 in a 1:3:3:1 ratio). The lack of crossover supports a mechanism involving metallacyclic intermediates. Using a SHOP catalyst to perform the oligomerization of a 1:1 mixture of C2D4 and C2H4 leads to the generation of a broader distribution of 1-hexene isotopologs, consistent with a Cossee-type mechanism for 1-hexene formation. The ethylene trimerization reaction was further studied by the reaction of trans-, cis-, and gem-ethylene-d2 upon activation of Cr(PNP(O4))(o,o'-biphenyldiyl)Br with NaB[C6H3(CF3)2]4. The trimerization of cis- and trans-ethylene-d2 generates 1-hexene isotopomers having terminal CDH groups, with an isotope effect of 3.1(1) and 4.1(1), respectively. These results are consistent with reductive elimination of 1-hexene from a putative Cr(H)[(CH2)4CH=CH2] occurring much faster than a hydride 2,1-insertion or with concerted 1-hexene formation from a chromacycloheptane via a 3,7-H shift. The trimerization of gem-ethylene-d2 has an isotope effect of 1.3(1), consistent with irreversible formation of a chromacycloheptane intermediate on route to 1-hexene formation. Reactions of olefins with a model of a chromacyclopentane were investigated starting from Cr(PNP(O4))(o,o'-biphenyldiyl)Br. alpha-Olefins react with cationic biphenyldiyl chromium species to generate products from 1,2-insertion. A study of the reaction of 2-butenes indicated that beta-H elimination occurs preferentially from the ring CH rather than exo-CH bond in the metallacycloheptane intermediates. A study of cotrimerization of ethylene with propylene correlates with these findings of regioselectivity. Competition experiments with mixtures of two olefins indicate that the relative insertion rates generally decrease with increasing size of the olefins.     

Reaction of ketones with lithium hexamethyldisilazide: competitive enolizations and 1,2-additions

Reaction of 2-methylcyclohexanone with lithium hexamethyldisilazide (LiHMDS, TMS(2)NLi) displays highly solvent-dependent chemoselectivity. LiHMDS in THF/toluene effect enolization. Rate studies using in situ IR spectroscopy are consistent with a THF concentration-dependent monomer-based pathway. LiHMDS in pyrrolidine/toluene affords exclusively 1,2-addition of the pyrrolidine fragment to form an alpha-amino alkoxide-LiHMDS mixed dimer shown to be a pair of conformers by using (6)Li, (15)N, and (13)C NMR spectroscopies. Rate studies are consistent with a monomer-based transition structure [(TMS(2)NLi)(ketone)(pyrrolidine)(3)](). The partitioning between enolization and 1,2-addition is kinetically controlled.     

Lithium hexamethyldisilazide-mediated enolizations: influence of triethylamine on E/Z selectivities and enolate reactivities

Lithium hexamethyldisilazide (LiHMDS) in triethylamine (Et 3N)/toluene is shown to enolize acyclic ketones and esters rapidly and with high E/ Z selectivity. Mechanistic studies reveal a dimer-based mechanism consistent with previous studies of LiHMDS/Et 3N. E/ Z equilibration occurs when <2.0 equiv of LiHMDS are used. Studies of the aldol condensation and Ireland-Claisen rearrangement of the resulting Et 3N-solvated enolates show higher and often complementary diastereoselectivities when compared with analogous reactions in THF. The Et 3N-solvated enolates also display a marked (20-fold) acceleration of the Ireland-Claisen rearrangement with evidence of autocatalysis. A possible importance of amine-solvated enolates is discussed.     

Thermal chemistry of C4 hydrocarbons on Pt(111): Mechanism for double-bond isomerization

The thermal chemistry of a number of C4 hydrocarbons (1,3-butadiene, 1-bromo-3-butene, 1-bromo-2-butene, trans-2-butene, cis-2-butene, 1-butene, 2-iodobutane, 1-iodobutane, and butane) was investigated on clean and hydrogen- and deuterium-predosed Pt(111) single-crystal surfaces by temperature-programmed desorption and reflection-absorption infrared spectroscopy. A combination of rapid beta-hydride eliminations from alkyls to olefins and the reverse insertions of those olefins into metal-hydrogen bonds explains the hydrogenation, dehydrogenation, and H-D exchange products that desorb from the surface. A preference for hydrogenation at the end carbons and dehydrogenation from the inner carbons also explains the extent of the isotope exchange and the preferential isomerization of 1-butene to 2-butene observed on this Pt(111) surface. The reactions of more dehydrogenated C4 species is also discussed.     

Catalytic Effect of Cobalt and Iron Complexes Incorporating Bis(2-aldiminoethyl)amine Ligands on Ethylene Oligomerization

IntroductionThe finding of a new promising family ofFe( ) - and Co( ) - based bis( imino) pyridyl cata-lysts for ethylene polymerization and oligomeriza-tion,discovered by Brookhart,Gibson andcoworkers[1— 3 ] ,has intrigued us into researchingthe preparation,the structure and the chemistry ofiron and cobalt complexes incorporating N,N ,N -tridentate ligands.The spectacular enhancement ofthe reactivity of iron and cobalt complexes towardsZiegler- Natta olefin polymerization has been re-po…     

Equilibrium constants and alkylation kinetics of two lithium enolates/LiHMDS mixed aggregates in THF

[reaction: see text] Mixed aggregates between lithium enolates and lithium hexamethyldisilazide (LiHMDS) have been studied in THF using UV-vis spectroscopy. The equilibrium constants (K(agg)) between monomeric LiEn and monomeric LiHMDS are 760 and 560 M(-1) when LiEn are LiSIBP and LiBnPAT, respectively. The alkylation kinetics of the reactions with benzyl bromide were studied at 25 degrees C. The rate constants for the mixed aggregates, k(Mixed), are substantially smaller than those of the monomeric enolates.     

Saturation transfer and chemical exchange measurements of the stereochemical drift occurring during the Wittig reaction

The Wittig reaction of butylidenetriphenylphosphorane with benzaldehyde using LiHMDS as base in THF was studied. The stereochemical drift (different ratio obtained in alkenes versus oxaphosphetane intermediates) was followed by low-temperature 1D NMR techniques. A retro-Wittig reaction is demonstrated using 13C and 31P saturation transfer experiments and homonuclear DPFGSE-ROE techniques.     

Tethered olefin studies of alkene versus tetraphenylborate coordination and lanthanide olefin interactions in metallocenes

The tethered olefin cyclopentadienyl ligand, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](-), forms unsolvated metallocenes, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Ln (Ln = Sm, 1; Eu, 2; Yb, 3), from [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))]K and LnI(2)(THF)(2) in good yield. Each complex in the solid state has both tethered olefins oriented toward the Ln metal center with the Ln-C(terminal alkene carbon) distances 0.2-0.3 A shorter than the Ln-C(internal alkene carbon) distances. The olefinic C-C bond distances in 2 and 3, 1.328(4) and 1.328(5) A, respectively, are normal. Like its permethyl analogue, (C(5)Me(5))(2)Sm(THF)(2), complex 1 reductively couples CO(2) to form the oxalate-bridged dimer [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm](2)(mu-eta(2):eta(2)-O(2)CCO(2)), 4, in which the tethered olefins are noninteracting substituents. Complex 1 reacts with AgBPh(4) to form an unsolvated cation that has the option of coordinating [BPh(4)](-) or a pendant olefin, a competition common in olefin polymerization catalysis. The structure of [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm][BPh(4)], 5, shows that both pendant olefins are located near samarium rather than the [BPh(4)](-) counterion.     

A highly regioselective amination of 6-aryl-2,4-dichloropyrimidine

[reaction: see text]. A highly regioselective amination of 6-aryl-2,4-dichloropyrimidine with aliphatic secondary amines and aromatic amines has been developed which strongly favors the formation of the C4-substituted product. The reactions with aliphatic amines are carried out using LiHMDS as the base and are catalyzed by Pd, while the aromatic amines require no catalyst.     

三烷基硼烷与4-(二甲基硫叶立德)巴豆酸乙酯的反应研究

三烷基硼烷能与4-(二甲基硫叶立德)巴豆酸乙酯反应, 给出(E/Z)-β,γ-不饱和酯(E:Z≈58:42), 条件温和, 产率高。该反应是一条使烷基延长四个碳原子得到β,γ-不饱和酯的新路线。     

The electrophilic addition of thiols to olefins: A theoretical and experimental study

Density functional theory with the B3LYP hybrid functional and 6–31G* basis set was used to study the geometric and electronic structure of H₂C = CHR (R = H, CH₃, C₂H₅, C₃H₇, C₄H₉, and C₅H₁₁) olefins, their carbocations formed in the addition of the proton to the olefins, R′-S-H aliphatic thiols (R′ = H, CH₃, C₂H₅, and C₃H₇), the products of the addition of thiols to carbocations, and the final products of the addition of thiols to olefins. The proton affinity of the olefins and the products of the addition of thiols to olefins was calculated. The conclusion was drawn that the limiting stage in the nonradical addition of thiols to olefins catalyzed by acids was proton transfer from the protonated reaction product to the olefin. The theoretical results were compared with the experimental data on the electrophilic addition of polymercaptan to heptene-1.     

Toward the synthesis of spirastrellolide B: a synthesis of the C1-C23 subunit

A synthesis of the C1-C23 subunit of spirastrellolide B is described. The synthesis features two applications of a Kulinkovich-cyclopropanol ring-opening strategy for the coupling of esters with olefins to produce ketones.