W(CO)5]-catalyzed endo- or exo-cycloisomerization reactions of 1,1-disubstituted 4-pentyn-1-ols: experimental and theoretical studies

The [W(CO)5]-catalyzed cycloisomerization reaction of 1,1-disubstituted 4-pentyn-1-ol derivatives has been studied from both, an experimental and theoretical point of view. Three different catalytic systems have been evaluated {preformed [(thf)W(CO)5], [W(CO)6]/excess Et3N, and [W(CO)6]/2 mol % Et3N]. We have found that the reaction proceeds to give the formal endo- or exo-cycloisomerization products depending on the amount of Et3N used and on the substitution along the alkyl chain of the starting alkynol. The theoretical study allowed us to find the mechanisms of the reactions which explain the formation of the formal endo- or exo-cycloisomerization products.     

Hybrid Pd-Nanoparticles within Polymeric Network in Selective Hydrogenation of Alkynols: Influence of Support Porosity

This work is addressing the selective hydrogenation of alkynols over hybrid catalysts containing Pd-nanoparticles, within newly synthesized hyper-cross-linked polystyrenes (HPS). Alkynols containing C₅, C₁₀, and C₂₀ with a terminal triple bond, which are structural analogues or direct semi-products of fragrant substances and fat-soluble vitamins, have been studied. Selective hydrogenation was carried out in a batch mode (ambient hydrogen pressure, at 90 °C, in toluene solvent), using hybrid Pd catalysts with low metal content (less than 0.2 wt.%). The microporous and mesoporous HPS were both synthesized and used as supports in order to address the influence of porosity. Synthesized catalysts were shown to be active and selective: in the case of C₅, hydrogenation selectivity to the target product was more than 95%, at close to complete alkynol conversion. Mesoporous catalysts have shown some advantages in hydrogenation of long-chain alkynols.     

Gold-Catalyzed Iminations of Terminal Propargyl Alcohols with Anthranils with Atypical Chemoselectivity for C(1)-Additions and 1,2-Carbon Migration

This work reports gold-catalyzed iminations of terminal propargyl alcohols with anthranils or isoxazoles to yield E-configured α-amino-2-en-1-ones and -1-als with complete chemoselectivity. These catalytic iminations occur exclusively with C(1)-nucleophilic additions on terminal alkynes, in contrast to a typical C(2)-route. For 3,3-dialkylprop-1-yn-3-ols, a methyl substituent is superior to long alkyl chains as the 1,2-migration groups toward α-imino gold carbenes. For secondary prop-1-yn-3-ols, phenyl, vinyl, and cyclopropyl substituents are better than hydrogen as the migrating groups, obviating typical gold carbene reactions. DFT calculations have been performed to rationalize the observed C(1)-regioselectivity and the preferable cyclopropyl migration based on gold carbene pathways.     

Catalyst‐Dependent Selectivity in the Relay Catalytic Branching Cascade

The synthesis of small organic molecules as probes for discovering new therapeutic agents has been an important aspect of chemical biology. One of the best ways to access collections of small molecules is to use various techniques in diversity‐oriented synthesis (DOS). Recently, a new form of DOS, namely “relay catalytic branching cascades” (RCBCs), has been introduced, wherein a common type of starting material reacts with several scaffold‐building agents (SBAs) to obtain structurally diverse molecular scaffolds under the influence of catalysts. Herein, the RCBC reaction of a common type of substrate with SBAs is reported to give two different types of molecular scaffolds and their formation is essentially dependent on the type of catalyst used.     

Tunable Cascade Reactions of Alkynols with Alkynes under Combined Sc(OTf)3 and Rhodium Catalysis

Two tunable cascade reactions of alkynols and alkynes have been developed by combining Sc(OTf)₃ and rhodium catalysis. In the absence of H₂O, an endo‐cycloisomerization/C? H activation cascade reaction provided 2,3‐dihydronaphtho[1,2‐b]furans in good to high yields. In the presence of H₂O, the product of alkynol hydration underwent an addition/C? H activation cascade reaction with an alkyne, which led to the formation of 4,5‐dihydro‐3H‐spiro[furan‐2,1′‐isochromene] derivatives in good yields under mild reaction conditions. Mechanistic studies of the cascade reactions indicated that the rate‐determining step involves C? H bond cleavage and that the hydration of the alkynol plays a key role in switching between the two reaction pathways.     

1H NMR study of the hetero‐association of unsaturated alcohols with pyridine

The ¹H NMR titration method is used to investigate the association of some unsaturated alcohols with pyridine in benzene. The association constant for allyl alcohol (2‐propen‐1‐ol) is slightly higher than for alkanols, but putting one methylene group between the OH and vinyl group completely eliminates this enhancement. In alkynols both the OH and ≡CH protons associate with pyridine. Here, the effects of chain‐lengthening on the association constant are irregular. Values for alkynols and some alcohols with hetero‐atom substituents are lower than expected on the basis of a Taft polar constant (σ*) correlation of alkanol association constants and of a correlation with the pK_a’s of the corresponding carboxylic acids. It is suggested that stabilization of the ground state by OH/π interactions is responsible for these low association constants. Small increases in the NMR shifts of the OH protons in the 3‐carbon and 4‐carbon alkenols and alkynols can also be attributed to OH/π interactions, but the 5‐carbon analogues have shifts as low as alkanols. Copyright © 2011 John Wiley & Sons, Ltd.