Rotation and inversion barriers in 2-lithio-2-phenyl-1,3-dithianes

The barriers to phenyl rotation in 2-lithio-2-phenyl-cis-4,6-dimethyl-, 2-lithio-2-phenyl-4,4,6-trimethyl- and 2-lithio-2-phenyl-trans-4,6-dimethyl-1,3-dithiane are compared in tetrahydrofuran (THF) and hexamethylphosphortriamide (HMPA). In the first two cases, the barriers in THF are lower than those in HMPA, presumably because the lithio compound exists as a tight ion pair in THF but as a solvent-separated ion pair (with more delocalization of charge into the phenyl ring) in HMPA. However, in the trans-4,6-dimethyl compound the barriers are the same in the two solvents and nearly equal to the barriers for ring reversal. It is concluded that in this compound the rate-determining step for phenyl rotation may actually be ring reversal, at least in solvent HMPA.     

Evidence for ionic samarium(II) species in THF/HMPA solution and investigation of their electron-donating properties

The fundamental nature of samarium(II) complexes in THF/HMPA (HMPA = hexamethylphosphoramide) solutions containing SmI2 has been clarified by means of cyclic voltammetry, conductivity measurements, UV spectroscopy, and kinetic measurements. The principal species is not [SmI2(hmpa)4] as previously suggested, but either the ionic cluster [Sm(hmpa)4(thf)2+2I- if four equivalents of HMPA is present in the THF solution or [Sm(hmpa)6]2+ 2I- in the presence of at least 10 equivalents of HMPA. The formal potential of the [Sm(hmpa)4(thf)2]3+ 2I-/[Sm(hmpa)4(thf)2]2- 2I- redox couple determined by cyclic voltammetry was -1.79 +/- 0.08 V versus SCE. The order of reactivity of the samarium(II) complexes was found to be [Sm(hmpa)6]2+2I- > [Sm(hmpa)4(thf)2]2+2I- > SmI2 in their respective reactions with 1-iodobutane and with benzyl chloride. Very high rate enhancements, of the order of 1,000-15,000-fold, were observed upon addition of HMPA to the THF solution containing SmI2, Comparison of these rate constants with the corresponding rate constants for electron transfer (ET) reactions involving aromatic radical anions revealed that none of the reactions studied can be classified as outer-sphere ET processes and that the inner-sphere electron-donating abilities of the [Sm(hmpa)4(thf)2]2+ 2I- and SmI2 complexes are comparable. The inner-sphere ET character of the transition state increases on going from 1-iodobutane and benzyl bromide to benzyl chloride and acetophenone.     

Synthesis, reactivity, and structural characterization of sodium and ytterbium complexes containing new imidazolidine-bridged bis(phenolato) ligands

A new imidazolidine-bridged bis(phenol) [ONNO]H2 ([ONNO]H2=1,4-bis(2-hydroxy-3,5-di-tert-butyl-benzyl)imidazolidine) was prepared in relatively high yield by Mannish reaction of 2,4-di-tert-butylphenol, formaldehyde, and ethylenediamine in a 2:3:1 molar ratio. Reaction of the bis(phenol) with NaH in THF, after workup, afforded the sodium bis(phenolate) {[ONNO]Na2(THF)2}2.2THF (1) as a dimeric tetranuclear complex in an almost quantitative yield. Reaction of YbCl3 with complex 1 in a 2:1 molar ratio in THF, in the presence of HMPA, produced the desired bis(phenolate) ytterbium dichloride as bimetallic complex [ONNO]{YbCl2(HMPA)}2.2.5C7H8 (2). Complex 2 can be used as a precursor for the synthesis of ytterbium derivatives by salt metathesis reactions. Reaction of complex 2 with NaOiPr in a 1:2 molar ratio in THF led to the formation of bimetallic alkoxide [ONNO]{Yb(mu-OiPr)Cl(HMPA)}2.THF (3). However, the residual chlorine atoms in complex 3 are inactive for the further substituted reaction. Further study revealed that the bulkiness of the reagent has profound effect on the outcome of the reaction. Complex 2 reacted with bulky NaOAr (ArO=2,6-di-tert-butyl-4-methylphenoxo) or NaNPh2 in a 1:2 molar ratio under the same reaction conditions, after workup, to give the ligand redistributed products, (ArO)2YbCl(HMPA)2 (4) and [ONNO]YbCl(HMPA)2 (5) for the former and complexes 5 and (Ph2N)2YbCl(HMPA)2 (6) for the latter. If the molar ratio of complex 2 to NaNPh2 decreased to 1:4, the expected ligand redistributed products [ONNO]YbNPh2(HMPA) (7) and (Ph2N)3Yb(HMPA)2.C7H8 (8) can be isolated in high yields. All of the complexes were well characterized, and the definitive molecular structures of complexes 1-4, 7, and 8 were provided by single-crystal X-ray analysis.     

Synthesis, characterization, and structural determination of polynuclear lithium aggregates and factors affecting their aggregation

The reaction of [(mu3,mu3-EDBP)Li2]2[(mu3-nBu)Li(0.5Et2O)]2 (1) [EDBP-H2 = 2,2'-ethylidenebis(4,6-di-tert-butylphenol)] with 1 equiv of ROH in toluene gave [(mu3,mu3-EDBP)Li2]2[(mu3-OR)Li]2 [R = Bn (2), CH2CH2OEt (3), and nBu (4)]. In the presence of 3 equiv of tetrahydrofuran (THF), the hexanuclear compound 1 slowly decomposed to an unusual pentanuclear Li complex, [(mu2,mu3-EDBP)2Li4(THF)2][(mu3-nBu)Li] (5). Further reaction of 5 with ROH gave [(mu2,mu3-EDBP)2Li4(THF)3][(mu4-OR)Li] [R = Bn (6), nBu (7), and CH2CH2OEt (8)] without a major change in its skeleton. Treatment of 2 with an excess of hexamethylphosphoramide (HMPA) yields [(mu2,mu2-EDBP)Li2(HMPA)2][(mu3-OBn)Li(HMPA)] (9). Compounds [(mu2,mu3-EDBP)2Li4(THF)][(mu4-OCH2CH2OEt)Li]2 (10) and [(mu2,mu2-EDBP)2Li4(mu4-OCH2CH2OEt)(HMPA)]-[Li(HMPA)4]+ (11) can be obtained by the reaction of 3 with an "oxygen-donor solvent" such as THF and HMPA, respectively. Among the compounds described above, 8 has shown great reactivity toward ring-opening polymerization of L-lactide, yielding polymers with very low polydispersity indexes in a wide range of monomer-to-initiator ratios.     

A first oxalamidino complex of samarium via reduction-coupling of carbodiimine: synthesis and molecular structure of [eta4-C2(NR)4][(MeC5H4)2Sm(HMPA)]2.2THF (R = Pr i, Cy)

Treatment of the THF solution of (MeC5H4)2Sm(THF) with an equivalent of carbodiimine [RN=C=NR](R = Pr(i) or Cy; Cy = cyclohexyl) in the presence of an equivalent of hexamethylphosphoric triamide (HMPA) at room temperature gives, via a reduction-coupling reaction of carbodiimine, the corresponding bimetallic oxalamidino complex of samarium [eta4-C2(NR)4][(MeC5H4)2Sm(HMPA)]2.2THF.     

Comparative study of TmI2, SmI2, and SmI2/HMPA in the cross-coupling reactions of 2-acetylthiophene and thiophene-2-carboxylate with carbonyl compounds

The cross-coupling reactions of acetylthiophene or ethyl thiophene-2-carboxylate with aldehydes or ketones were achieved in a regioselective manner by using thulium diiodide in THF solution. Similar coupling reactions were also realized by using samarium diiodide together with excess amounts of hexamethylphosphoramide (HMPA). However, ethyl thiophene-2-carboxylate was inert in SmI₂/THF solution, and acetylthiophene was simply reduced to thienylethanol by SmI₂ in the absence of HMPA.     

Lithium diisopropylamide-mediated reactions of imines, unsaturated esters, epoxides, and aryl carbamates: influence of hexamethylphosphoramide and ethereal cosolvents on reaction mechanisms

Several reactions mediated by lithium diisopropylamide (LDA) with added hexamethylphosphoramide (HMPA) are described. The N-isopropylimine of cyclohexanone lithiates via an ensemble of monomer-based pathways. Conjugate addition of LDA/HMPA to an unsaturated ester proceeds via di- and tetra-HMPA-solvated dimers. Deprotonation of norbornene epoxide by LDA/HMPA proceeds via an intermediate metalated epoxide as a mixed dimer with LDA. Ortholithiation of an aryl carbamate proceeds via a mono-HMPA-solvated monomer-based pathway. Dependencies on THF and other ethereal cosolvents suggest that secondary-shell solvation effects are important in some instances. The origins of the inordinate mechanistic complexity are discussed.     

Alkali and alkaline-earth metal ketyl complexes: isolation, structural diversity, and hydrogenation/protonation reactions

The use of hexamethylphosphoric triamide (HMPA) as a stabilizing ligand allowed successful isolation of a series of structurally characterizable alkali metal and calcium ketyl complexes. Reaction of lithium and sodium with one equivalent of fluorenone and reaction of sodium with one equivalent of benzophenone in THF, followed by addition of two equivalents of HMPA, yielded the corresponding ketyl complexes 1, 2, and 11, respectively, as microketyl-bridged dimers. If one equivalent of HMPA was used in the reaction of sodium with fluorenone, a further aggregated complex, the mu3-ketyl-bridged tetramer 3, was isolated, whereas analogous reaction of benzophenone with sodium afforded the trimeric ketyl complex 13, rather than a simple benzophenone analogue of 3. In the reaction of potassium with fluorenone, the use of two equivalents of HMPA gave the tetramer 4, rather than a dimeric complex analogous to 1 or 2. Compared to the tetrameric sodium complex 3, there is an extra HMPA ligand that bridges two of the four K atoms in 4. When 0.5 equiv of HMPA was used in the above reaction, complex 5, a THF-bridged analogue of 4, was isolated. In the absence of HMPA, the reaction of sodium with an excess of fluorenone yielded the tetrameric ketyl complex 6, in which two of the four Na atoms are each terminally coordinated by a fluorenone ligand, and the other two Na atoms are coordinated by a THF ligand. Two bridging THF ligands are also observed in 6. Reaction of 1,2-bis(biphenyl-2,2'-diyl)ethane-1,2-diol (7) with two equivalents of LiN(SiMe3)2 or NaN(SiMe3)2 in the presence of four equivalents of HMPA easily afforded 1 or 2, respectively, via C-C bond cleavage of a 1,2-diolate intermediate. The reaction of calcium with two equivalents of fluorenone or benzophenone in the presence of HMPA gave the corresponding complexes that bear two independent ketyl ligands per metal ion. In the presence of 3 or four equivalents of HMPA, the fluorenone ketyl complex was isolated in a six-coordinate octahedral form (10), while the benzophenone ketyl complex was obtained as a five-coordinate trigonal bipyramid (13). The radical carbon atoms in both benzophenone ketyl and fluorenone ketyl complexes are still in an sp2-hybrid state. However, in contrast with the planar configuration of the whole fluorenone ketyl unit, the radical carbon atom in a benzophenone ketyl species is not coplanar with any of the phenyl groups; this explains why benzophenone ketyl is more reactive than fluorenone ketyl. Hydrolysis of 2 or 11 with 2N HCI yielded the corresponding pinacol-coupling product, while treatment of 2 or 11 with 2-propanol, followed by hydrolysis, gave the pairs fluorenone and fluorenol or benzophenone and benzhydrol, respectively. A possible mechanism for these reactions is proposed.     

Novel lanthanide(II) complexes supported by carbon-bridged biphenolate ligands: synthesis, structure and catalytic activity

[Ln[N(SiMe3)2]2(THF)2](Ln = Sm, Yb) reacts with 1 equiv. of carbon-bridged biphenols, 2,2'-methylene-bis(6-tert-butyl-4-methylphenol)(L1H2) or 2,2'-ethylidene-bis(4,6-di-tert-butylphenol)(L2H2), in toluene to give the novel aryloxide lanthanide(II) complexes [[LnL1(THF)n]2](Ln = Sm, n = 3 (1); Ln = Yb, n = 2 (2)) and [[LnL2(THF)3]2](Ln = Sm (5); Ln = Yb (6)) in quantitative yield, respectively. Addition of 2 equiv. of hexamethylphosphoric triamide (HMPA) to a tetrahydrofuran (THF) solution of 1, 2 and 5 affords the corresponding HMPA-coordinated complexes, [[LnL1(THF)m(HMPA)n]2(THF)y](Ln = Sm, n = 2, m = 0, y = 2 (3); Ln = Yb, m = 1, n = 1, y = 6 (4)) and [[SmL2(HMPA)2]2](7) in excellent yields. The single-crystal structural analyses of 3, 4 and 7 revealed that these aryloxide lanthanide(II) complexes are dimeric with two Ln-O bridges. The coordination geometry of each lanthanide metal can be best described as a distorted trigonal bipyramid. Complexes 1-3, 5 and 7 can catalyze the ring-opening polymerization of epsilon-caprolactone (epsilon-CL), and 1-3, along with 5 show moderate activity for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC) and the copolymerization of epsilon-CL and DTC to give random copolymers with high molecular weights and relatively narrow molecular weight distributions..     

Aggregation and reactivity of the dilithium and dicesium enediolates of 1-naphthylacetic acid

UV-vis spectra of the dilithium, 1-Li, and dicesium, 1-Cs, enediolates of alpha-naphthylacetic acid show no systematic change with concentration in dilute THF solution, but addition of small amounts of HMPA causes a bathochromic shift in the spectrum of 1-Li. These results indicate that these salts are aggregated and that HMPA breaks up the aggregates of 1-Li. The quantitative effect of small increments of HMPA indicates that 1-Li is a dimer. Alkylation reactions of 1-Cs show half-order kinetics in enediolate indicating that this salt is also dimeric but that the small amount of monomer in equilibrium is the actual reactant. Alkylation of 1-Li, however, is much slower and shows first-order kinetics interpreted as a direct reaction of the dimer; the amount of monomer in this case is too small to compete. A solution of 1-Li in THF containing 10% HMPA is much more reactive in alkylation than 1-Li alone and the first-order dependence in 1-Li is now interpreted as reaction of the monomer. Compound 1-Li is found to form a mixed aggregate with LDA, a finding that has possible synthetic significance since enediolates used in syntheses are frequently prepared using LDA. Structures of these compounds are suggested based on model ab initio computations.     

A Facile and Direct Conversion of Aryl-Acetate to Alkyl-Aryl Ether

Arylacetate was directly converted to the corresponding alkyl-aryl ether by treatment with sodium hydride and alkyl halide in THF and HMPA.     

Synthesis and molecular structure of piperazidine-bridged bis(phenolate) samarium(II) complex and its reactivity to carbodiimides

The reaction of Sm[N(TMS)(2)](2)(THF)(2) with H(2)L (L = 1,4-bis(2-hydroxy-3-tert-butyl-5-methyl-benzyl)-piperazidine) afforded [SmL(HMPA)(2)](4)·8THF 2 upon treatment with 2 equivalents of HMPA (hexamethyl phosphoric triamide). X-ray crystallographic analysis of 2 reveals a tetrametallic macrocyclic structure, which represents the first example of a crystal structure of a Sm(II) complex stabilized by heteroatom bridged bis(phenolate) ligands. Reduction of carbodiimides RNCNR (R = (i)Pr and Cy) by [SmL](2)(THF) 1, which was formed in situ by the reaction of Sm[N(TMS)(2)](2)(THF)(2) with H(2)L in THF, yielded the Sm(III) complex with an oxalamidinate ligand [LSm{(N(i)Pr)(2)CC(N(i)Pr)(2)}SmL]·THF 3 for (i)PrNCN(i)Pr and the Sm(III) complex with a diamidocarbene ligand [LSm(μ-CyNCNCy)SmL]·5.5THF 4 for CyNCNCy.     

Effet de solvant sur support solide:régiosélectivité de l'alkylation des énolates de l'acétylacétate d'éthyle par le sulfate d'éthyle en présence de “hmpt solide”

The reaction of Li, Na, K enolates of ethylaceto acetate with diethylsulfate in THF at room temperature gives 60–70% O-alkylation when performed in the presence of one molar equivalent of solid HMPA. Without HMPA, in the same conditions, Li enolate docs not react, Na enolate only gives C-alkylation while K enolate leads to 90% C-alkylation, the reactions being quite slower in the two latter cases. The effects of one molar equivalent of solid and liquid HMPA are compared: for the K enolate the reaction is faster and the degree of O-alkylation higher with solid HMPA; practically no difference is seen for Na enolate, while the reverse is observed with the Li enolate. The cooperative effect of the polymer should thus only work for a sufficiently large cation, as the Li enolate is fixed on the solid HMPA in a larger amount than the K enolate. The study of this reaction is proposed to test the efficiency of other “solid solvents” interacting strongly with cations.     

Substitution of a bridgehead bromide by primary organolithium reagents

The reaction of ketone 1 with primary organolithium reagents in THF/HMPA affords not the expected carbonyl addition product but the product of bromide substitution, in reasonable yield.     

Anion reaction of 1,3-benzoxathiole-3-oxide with several electrophilic compounds

The reaction of benzoxathiole-3-oxide with LDA in THF or THF/hexane or THF/HMPA gave a carbanion which was reacted with methyl iodide, aromatic aldehydes or carbon dioxide. The conformational stability (-diastereoselectivity) of the carbanion and the asymmetric induction due to the prochiral electrophiles (β-diastereoselectivity) was studied. The temperature and the solvent effects on the - and β-diastereoselectivity are discussed.     

Reactivity of the triple ion and separated ion pair of tris(trimethylsilyl)methyllithium with aldehydes: a RINMR study

Low-temperature rapid-injection NMR (RINMR) experiments were performed on tris(trimethylsilyl)methyllithium. In THF/Me2O solutions, the separated ion (1S) reacted faster than can be measured at -130 degrees C with MeI and substituted benzaldehydes (k >/= 2 s -1), whereas the contact ion (1C) dissociated to 1S before reacting. Unexpectedly, the triple ion reacted faster with electron-rich benzaldehydes relative to electron-deficient ones. The addition of HMPA had no effect on the rate of reaction of the triple ion with p-diethylaminobenzaldehyde, and the immediate product of the reaction was the HMPA-solvated separated ion 1S, with the Peterson product forming only slowly. Thus, the aldehyde is catalyzing the dissociation of the triple ion. HMPA greatly decelerated the reaction of 1S (<10 -10), providing an estimate of the Lewis acid activating effect of a THF-solvated lithium cation in an organolithium addition to an aldehyde.     

Rheological Properties of Hydrophobically Modified Poly(acrylic acid) in Mixed Solutions

Hydrophobically modified poly(acrylic acid) (HMPA) with single-tailed pendant side groups was prepared by precipitation polymerization. The effects of polymer concentration, surfactant and co-solvent on the solution properties of HMPA were investigated. HMPA solutions showed good viscosity enhancement and typical shear thinning behavior with increasing concentration. The surfactant TX-10 and co-solvent ethylene glycol gave rise to factors that changed the hydrophobic interactions and in turn the rheology behavior of the solutions. The transient associative network of HMPA in ethylene glycol + water mixed solutions was retained as the temperature was decreased to below 0 °C.     

Reduction of ketones and alkyl iodides by SmI(2) and Sm(II)-HMPA complexes. Rate and mechanistic studies

The effect of HMPA on the electron transfer (ET) rate of samarium diiodide reduction reactions in THF was analyzed for a series of ketones (2-butanone, methyl acetoacetate, and N,N-dimethylacetoacetamide) and alkyl iodides (1-iodobutane and 2-iodobutane) with stopped flow spectrophotometric studies. Activation parameters for the ET processes were determined by temperature-dependence studies over a range of 30-50 degrees C. The ET rate constants and the activation parameters obtained for the above systems in the presence of different equivalents of HMPA were compared to understand the mechanism of action of HMPA on various substrates. The results obtained from these studies indicate that coordination or chelation is possible in the transition state geometry for SmI(2)/ketone systems even in the presence of the sterically demanding ligand HMPA. After the addition of 4 equiv of HMPA the ET rate and activation parameters for ketone reduction by Sm is unaffected by further HMPA addition while a linear dependence of ET rate on the equivalents of HMPA was found in the SmI(2)/alkyl iodide system. The results of these studies are consistent with an inner-sphere-type ET for the reduction of ketones by SmI(2) (and SmI(2)[bond]HMPA complexes) and an outer-sphere-type ET for the reduction of alkyl iodides by SmI(2) or SmI(2)[bond]HMPA complexes.     

Reaction of Metal-Free Triorganogermyl Anions with Aldehydes

Hexaorganosilylgermane (2) reacted with aldehydes and ketones (1) in the presence of catalytic amounts of fluoride ion in THF or HMPA to give 1-(triorgano-germyl)alkyl alcohols (3).     

Addition of lithiated 5-hydroxymethyl-1,3-dithiane to benzaldehyde: HMPA-controlled trans stereoselectivity

Addition of 5-substituted dithianyl anions to carbonyl compounds normally produces trans adducts. The presence of a nucleophilic hydroxymethyl group in position 5 dramatically decreases the trans stereoselectivity of the reaction in THF. The trans/cis ratio shows a bell curve dependence on HMPA, fitted to a quantitative model involving a series of equilibrated ion pairs, of which an intermediate contact ion pair possessing three (effective) HMPA molecules yields the trans adduct with much higher stereoselectivity. [structure: see text]