The nature of the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene: sigma-allyl-insertion mechanism versus pi-allyl-insertion mechanism

We present a theoretical investigation on the nature of the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene that employed a gradient-corrected DFT method. We have explored critical elementary steps of the whole polymerization cycle for the trans-1,4 regulating cationic allylnickel(II) [RC3H4NiII(C4H6)L]+ catalyst. These steps are i) cis-butadiene insertion into either the eta 1-sigma-butenyl-NiII bond (sigma-allyl insertion mechanism) or the eta 3-pi-butenyl-NiII bond (pi-allyl insertion mechanism) along with competing pathways for generation of trans-1,4 and cis-1,4 polymer units, and ii) anti-syn isomerization. Based on the analysis of geometric and electronic structures of key species involved and the energetics, we present a detailed insight into the different nature of the monomer insertion step according to the two mechanistic alternatives. An understanding of why the pi-allyl insertion mechanism is favored over the sigma-allyl insertion mechanism is provided. eta 1-sigma-butenyl-NiII Species are predicted to be sparsely populated and also distinctly less reactive than eta 3-pi-butenyl-NiII species. Although they are commonly believed to be reactive intermediates, eta 1-sigma-butenyl-NiII species are, therefore, not likely to be involved along viable pathways for cis-butadiene insertion into the butenyl-NiII bond. The present investigation corroborates our previous conclusion that the pi-allyl insertion mechanism represents the preferred mechanism for the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene. On the other hand, the suggested alternative sigma-allyl insertion mechanism has to be considered to be not operative, for both thermodynamic and kinetic reasons. Furthermore, the sigma-allyl insertion mechanism would neither provide a rationalization of cis-trans selectivity nor of chemoselectivity in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene.     

Reductions of acetylenes and conjugated diolefins by lithium aluminum hydride

A convenient method is described for the stereospecific reduction of acetylenes by LAH. For example, 2-pentyne, 3-hexyne, 1-phenylpropyne and diphenylethyne in an ether solvent have been reduced to the corresponding trans olefin in high yield 2,4-Hexadiene was reduced in a similar manner to give a mixture of 2- and 3-hexenes. With toluene as the solvent, the main products from acetylenes are the saturated hydrocarbons. Whereas a trans addition of the hydride to the triple bond occurs in ether solvents, it appears that cis addition occurs in toluene.     

Well-defined phosphate yttrium dialkyl complexes for catalytic stereo-controllable 1,4-polymerization of isoprene

Oxygen ligation is envisioned to provide a stable and distinctive coordination environment to the strongly oxophilic rare-earth metals. However, the well-defined dialkyl complexes bearing oxyanion ancillary ligand had been rarely addressed for the instability of the complexes and the shortage of easily available ligands. Herein, we report the synthesis of phosphate ligated dialkyl yttrium complexes（PYR2） featuring a high stability and a tunable ligand. Treated with the borate reagent, the phosph...     

Circular dichroism and stereochemistry of chiral conjugated cyclohexenones

An empirical analysis of CD spectra of variety of cyclohexenones, including variable temperature and solvent studies, reveals the presence of up to three Cotton effects in the 260-185 nm region, in addition to the well- recognized n-π^* Cotton effect of 300–350 nm. The long-wavelength band I and band II Cotton effects are ascribed respectively to the first (allowed) and second (formally forbidden) π-π^* transitions. The third (band III) Cotton effect at below 200 nm is of different nature and is attributed to the n-σ^* transition in the CO group.It is shown that CD spectra of cyclohexenones not only are influenced by the conformation of the chromophore but also reflect the presence of allylic axial substituents (n-π^* and band I), the absolute configuration of the cyclohexenone ring (band II), and the presence of axial alkyl groups in α′ or β′ positions (band III).     

On the stereochemistry of the aldol-addition

NOE-experiments and STO-3G calculations suggest that E-enol borates exist in an U-conformation $\text{7a}$, whereas the Z-enol borates should prefer the extended conformation $\text{8b}$. These observations have been linked to the stereoselectivity of the aldol addition. The resulting predictions are in line with the stereoselectivity observed on addition of cyclohexenol borates to benzaldehyde.     

On the stereochemistry of palmerolide A

Degradative studies of the anticancer macrolide palmerolide A have resulted in re-assignment of the C-7, C-10, and C-11 stereocenters.     

Remote OH-group directing effect in the stereochemistry of intramolecular alkoxide addition to conjugated double bond

Preferential formation of less stable (endo) product in intramolecular Michael reaction is explained by the participation of structurally remote OH group in the transition state.     

On the stereochemistry of the Pictet-Spengler reaction

A pathway for the Pictet-Spengler reaction is proposed herein, which is consistent with all of the known mechanistic and sterochemical observations concerning the formation of tetrahydro-β-carbolines by this method.     

On the stereochemistry of the bis-nor-wieland-miescher ketome

The chemo- and stereoselective reduction and the absolute configuration of 5-methylbicyclo[3.3.0]oct-1-en-3,6-dione is reported.     

Preparation of the conjugated polyene chains with the 1,4-dimethyl substitution

Allylic sulfones undergo the conjugate addition to diethyl chloroisopropylidenemalonate, followed by intramolecular cyclopropanation. DBU-promoted ring opening and subsequent desulfonation reactions of the resulting adduct produce the conjugated polyene chains with the 1,4-dimethyl substitution.     

On the stereochemistry of 2-hydroxyethylphosphonate dioxygenase

Stereochemical investigations have shown that the conversion of 2-hydroxyethylphosphonate to hydroxymethylphosphonate by the enzyme HEPD involves removal of the pro-S hydrogen at C2 and, surprisingly, the loss of stereochemical information at C1. As a result, the mechanisms previously proposed for HEPD must be re-evaluated.     

On the stereochemistry of 1,4-diheterocyclanes. IV—carbon-13 NMR spectra and structural properties of 1,4-dioxane-2,3-diols and their methyl-substituted derivatives

The 1,4-dioxane-2,3-diols and a number of their methyl-substituted derivatives were synthesized and their ¹³C NMR spectra measured. The configurational properties were studied on the basis of the spectral data and chemical equilibration. For the parent compound the trans configuration is slightly more stable. The hydroxyl groups are predominantly axial. Introduction of a methyl group at the 2 position causes the cis configuration of the hydroxyl substituents to become more stable. The substituent effects on the chemical shifts were calculated and used to test the conformational homogeneity of the compounds. The results indicate a biased (chair) conformation for all the methyl-substituted derivatives which were studied. Methyl groups have strongly predominating equatorial orientation in each case studied.     

Copolymerization of styrene and conjugated dienes with half‐sandwich titanium(IV) catalysts: The effect of the ligand structure on the monomer reactivity,monomer sequence distribution,and insertion mode of dienes

The copolymerization of styrene and 1,3‐butadiene (Bd) or isoprene (Ip) was carried out with half‐sandwich titanium(IV) Cp′TiCl₃ catalysts (where Cp′ is cyclopentadienyl 1 , indenyl 2 , or pentamethylcyclopentadienyl 3 ) with methylaluminoxane as a cocatalyst. For the copolymerization with Bd, catalyst 3 gave the copolymers containing the highest amount of Bd among the catalysts used. The resulting copolymers were composed of a styrene–Bd multiblock sequence. High melting points were observed in the copolymers prepared with catalyst 1 . The structures of hydrogenated poly(styrene‐co‐Bd) were studied by ¹³C NMR spectroscopy, and the long styrene sequence length was detected in the copolymers prepared with catalyst 1 . For styrene/Ip copolymerization, random copolymers were obtained. Among the used catalysts, catalyst 1 gave the copolymers containing the highest amount of Ip. The copolymers prepared with catalyst 1 showed a steep melting point depression with increasing Ip content because of the high ratio of 1,4‐inserted Ip units and/or the low molecular weights of the copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 939–946, 2003     

Application of carbenoid N-H insertion in the synthesis of the tricyclic 1,4-dihydropyrazines

The N-H insertion of the substituted anilines with 2-diazo-1,3-cyclohexanedione or 2-diazo-1,3-cyclopentanedione and subsequent cyclization enabled an efficient synthesis of a novel series of tricyclic 1,4-dihydropyrazines.     

On the determination of monomer reactivities in copolymerization

Existing methods to determine reactivity ratios in copolymerizations are critically discussed; particular emphasis is placed on the difference between exact and approximate procedures. Among the latter, the “intersection” method (Mayo and Lewis) and the “linearization” method (Fineman and Ross) are still preferred. They are compared from the point of view of projective geometry, and a “duality” is shown to exist between them.A new procedure, based on the so-called “method of grouping”, is then suggested: it allows ready evaluation of reactivity ratios with a minimum of computational difficulties. The results obtained by this procedure are, as such, good estimates for systems reported in the literature without the information necessary for a careful statistical treatment, and may best be used as the starting data for subsequent and more sophisticated elaborations.     

Synthesis of a ferrocenyl uracil PNA monomer for insertion into PNA sequences

The deprotection of the tert-butyl group of a ferrocenyl uracil Peptide Nucleic Acid (PNA) monomer, Fmoc-aeg(R)-O^tBu (1) was achieved using a two step synthesis involving hydrolysis in basic conditions to give first the zwitterion of ⁺NH₃-aeg(R)-O⁻ (7). Compound 7 was reacted in situ with N-(9-fluorenylmethoxycarbonyloxy)succinimide to obtain the expected compound Fmoc-aeg(R)-OH (2) (Abbreviations: Aeg = (2-aminoethyl)-glycine; Fmoc = 9-fluorenylmethoxycarbonyl; O^tBu = tert-butyl; R = 5-(N-ferroce-nylmethylbenzamido)uracyl).