Cationic Amphiphiles Bearing a Diacetylenic Function in the Headgroup: Aggregative Properties and Polymerization.

In the wide panorama of diacetylenic lipids, the photoresponsive conjugated 1,3-diyne function is usually encased into the hydrocarbon chain of the amphiphile at a variable distance from the headgroup. Therefore, the polydiacetylene network obtained by polymerization upon UV irradiation of the corresponding liposomes, exploited as sensing function, is embedded in the hydrophobic region of liposomes. Structurally related cationic diacetylenic amphiphiles featuring the conjugated triple bonds proximate to charged nitrogen were synthesized and evaluated in their ability to polymerize under aggregative conditions. The occurrence of polymerization only in certain aggregating conditions was rationalized by nuclear magnetic resonance (NMR) and Langmuir trough experiments.

Palladium(II) and Platinum(II) Mononuclear Complexes and Tendency to Undergo Dehydrogenation of the Multiple N-Donor Ligand Di-(2-pyridyl)dihydropyrazine.

The already known di(2-pyridyl)dihydropyrazine (dhdpp) was prepared and isolated also in the form of a bis-hydrated species, i.e., dhdpp·2H2O. As established by X-ray work, a small amount of single crystals of di(2-pyridyl)-pyrazine (dpp) was also obtained from the mother liquors, this testifying the possibility of a dehydrogenation process dhdpp → dpp in the absence of a catalyst. Using dhdpp as a ligand, mononuclear metal derivatives of formula [(dhdpp)MCl2]·xH2O (M = PdII, PtII) were obtained as stable-to-air solids, studied by X-ray powder, IR, UV-visible, and 1H NMR spectra, and proved to exhibit a N2MCl2 coordination site involving one pyridine and one pyrazine N atom ("py-pyz" coordination). An interesting relationship has been established in terms of the observed types of coordination with the analogs of di(2-pyridyl)-pyrazine (dpp) formulated as [(dpp)MCl2]·3H2O, proved also by 1H NMR spectra to exhibit the "py-pyz" mode of coordination. Attempts to isolate from the reaction of dhdpp with Pd(OAc)2 the corresponding mononuclear derivatives were shown to lead, as definitely supported by 1H NMR spectral data and crystallographic work, to the exclusive formation of the corresponding dpp complex [(dpp)Pd(OAc)2]·5H2O ("py-pyz" coordination site), this proving the tendency of dhdpp to generate dpp under different reaction conditions. The promoted conversion of dhdpp into dpp in the complex was examined by sequential NMR analysis and established to be determined by Pd(OAc)2 which plays the role of catalyst. The new salt-like species [(CH3)(dhdpp)PdI2](I)·7H2O, prepared starting from [(dhdpp)PdCl2] in its reaction with CH3I, allowed the separation from the mother liquors of small brown crystals identified on the basis of X-ray analysis as the already known complex of formula [(dpp)PdI2] ("py-py" coordination), this result once again outlining the tendency of dhdpp to be dehydrogenated to dpp.

One-step synthesis of low molecular weight poly(p-phenyleneethynylenevinylene)s via polyaddition of aromatic diynes by catalysis of the [Ru(p-cymene)Cl2]2/AcOH system.

pi-Conjugated low molecular weight polymers characterized by regio- and stereoregular alternation of phenylene and ( E)-1-en-3-yne moieties have been synthesized by polyaddition of 1,4-diethynylbenzene or of 2,5-diethynyl-1,4-alkoxybenzene monomers, employing the commercially available di-micro-chlorobis[( p-cymene)chlororuthenium(II)] complex as the metal catalyst source, under homogeneous, atom-economical, amine- and phosphine-free conditions. Bulk materials of poly( p-phenyleneethynylenevinylene) derivatives are obtained with yields larger than 80%, from which polymers readily soluble in chlorinated solvents and in tetrahydrofuran are extracted in 60-75% yields. The polymers with average degrees of polymerization in the range n AV = 4-8 display optical properties in solution similar to those of the higher molecular weights analogues.

Synthesis of alkyl-substituted six-membered lactones through ring-closing metathesis of homoallyl acrylates. An easy route to pyran-2-ones, constituents of tobacco flavor.

The ring-closing metathesis (RCM) reactions of homoallylic acrylates bearing alkyl substituents on various positions of their skeleton afford the corresponding pentenolides in the presence of carbene ruthenium catalysts. For R3 = R4 = H, or R3 = Me, R4 = H, the reactions are catalyzed by complex [RuCl2(PCy3)2(=CHPh)], while a second-generation Grubbs catalyst is required when R3 = H and R4 = Me, R3 = R4 = Me, or R3 = i-Pr and R4 = H. Alkyl substitution at the homoallylic carbon (R1, R2) increases the yield of the reaction when both the acrylic and/or homoallylic double bonds are methyl-substituted. The interaction of the catalyst with the substrate in the initiation stage involves the homoallylic double bond rather than the acrylic moiety, and the resulting alkylidene species from the first-generation Grubbs catalyst can be observed by 1H and 31P NMR. The racemic tobacco constituents 4-isopropyl-5,6-dihydropyran-2-one and 4-isopropyltetrahydropyran-2-one are prepared via a short reaction sequence, involving the RCM reaction as the key transformation.

Selective dimerization of arylalkynes to (E)-1,4-diaryl enynes catalyzed by the [Ru(p-cymene)Cl(2)](2)/acetic acid system under phosphine-free conditions.

The commercially available di-mu-chlorobis[(p-cymene)chlororuthenium(II)] complex catalyzes the dimerization of aromatic alkynes in acetic acid at room temperature to form the corresponding (E)-1,4-diarylbut-1-ene-3-yne derivatives, with high stereoselectivity. The procedure does not require the use of additives and can be carried out in the presence of water or aprotic cosolvents, under homogeneous conditions.

Synthesis of alpha,beta-unsaturated 4,5-disubstituted gamma-lactones via ring-closing metathesis catalyzed by the first-generation Grubbs' catalyst.

[reaction: see text] 4-Methyl-5-alkyl-2(5H)-furanones have been prepared by ruthenium-catalyzed ring-closing metathesis of the suitable methallyl acrylates. Despite the electron deficiency of the conjugated double bond and of the gem-disubstitution of the allylic alkene moiety in the starting acrylates, the first-generation Grubbs' catalyst I proved to be an effective promoter for the ring closure, affording the expected butenolides in good to high yields.

Formation of a cyclobutylidene ring: intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions.

Intramolecular [2 + 2] cycloaddition of two C=C bonds in vinylidene complexes [Ru(eta5-C9H7){=C=C(R)H}(PPh3){kappa1-(P)-PPh2(C3H5)][BF4] affords cyclobutylidene complexes [Ru(eta5-C9H7){kappa2-(P,C)-(=CC(R)HCH2CHCH2PPh2)}(PPh3)][BF4], which can be also obtained by reaction of terminal alkynes with [Ru(eta5-C9H7)(PPh3){kappa3-(P,C,C)-PPh2(C3H5)}][PF6]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters were determined by kinetic studies.

Mechanism of the palladium-catalyzed metal-carbon bond formation. A dual pathway for the transmetalation step.

The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [structure: see text], resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylides Bu(3)Sn-C [triple bond] C-(p-XC(6)H(4)) (11a, X = H; 11b, X = Cl) yields the products of transmetalation [structure: see text] (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR(3) (Ph > Bu > Me) and less so on the nature of the p-substituent X group, proceeds through two competing pathways, depending on the initial concentration of substrate. At high [3] (approximately equal to 10(-2) M), the transmetalation proceeds through an intermediate species (12) formed by the interaction of complex 3 with 11a. This associative complex accumulates in the presence of added PPh(3) and has been characterized spectroscopically. At low [3] (approximately equal to 10(-4) M), the reaction rate shows an inverse dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate species (13), in which PPh(3) has been substituted by a dimethylformamide (DMF) molecule, as shown by UV-vis and (31)P NMR spectroscopy. Values of k(obs) depend on the concentration and nature of the aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric iodide bridged complex [structure: see text](14) has been obtained during attempts at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give the transmetalation product.