16015-11-5Relevant academic research and scientific papers
S-Block cooperative catalysis: Alkali metal magnesiate-catalysed cyclisation of alkynols
Fairley, Michael,Davin, Laia,Hernán-Gómez, Alberto,García-álvarez, Joaquín,O'Hara, Charles T.,Hevia, Eva
, p. 5821 - 5831 (2019/06/18)
Mixed s-block metal organometallic reagents have been successfully utilised in the catalytic intramolecular hydroalkoxylation of alkynols. This success has been attributed to the unique manner in which these reagents can overcome the challenges of the reaction: namely OH activation and coordination to and then addition across a CC bond. In order to optimise the reaction conditions and to garner vital catalytic system requirements, a series of alkali metal magnesiates were enlisted for the catalytic intramolecular hydroalkoxylation of 4-pentynol. In a prelude to the main investigation, the homometallic magnesium dialkyl reagent MgR2 (where R = CH2SiMe3) was utilised. This reagent was unsuccessful in cyclising the alcohol into 2-methylenetetrahydrofuran 2a or 5-methyl-2,3-dihydrofuran 2b, even in the presence of multidentate Lewis donor molecules such as N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA). Alkali metal magnesiates MIMgR3 (when MI = Li, Na or K) performed the cyclisation unsatisfactorily both in the absence/presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) or PMDETA. When higher-order magnesiates (i.e., MI2MgR4) were employed, in general a marked increase in yield was observed for MI = Na or K; however, the reactions were still sluggish with long reaction times (22-36 h). A major improvement in the catalytic activity of the magnesiates was observed when the crown ether molecule 15-crown-5 was combined with sodium magnesiate Na2MgR4(TMEDA)2 furnishing yields of 87% with 2a:2b ratios of 95:5 after 5 h. Similar high yields of 88% with 2a:2b ratios of 90:10 after 3 h were obtained combining 18-crown-6 with potassium magnesiate K2MgR4(PMDETA)2. Having optimised these systems, substrate scope was examined to probe the range and robustness of 18-crown-6/K2MgR4(PMDETA)2 as a catalyst. A wide series of alkynols, including terminal and internal alkynes which contain a variety of potentially reactive functional groups, were cyclised. In comparison to previously reported monometallic systems, bimetallic 18-crown-6/K2MgR4(PMDETA)2 displays enhanced reactivity towards internal alkynol-cyclisation. Kinetic studies revealed an inhibition effect of substrate on the catalysts via adduct formation and requiring dissociation prior to the rate limiting cyclisation step.
Synthesis of gem-diaurated species from alkynols
Zhdanko, Alexander,Maier, Martin E.
supporting information, p. 3932 - 3942 (2013/04/24)
A number of enol ether-derived diaurated species were synthesized directly from different alkynols and cationic gold complexes in the presence of a non-nucleophilic base (proton sponge). The reaction can be easily applied for in situ generation of diaurated species from all common types of hydroalkoxylation substrates: 5-endo, 5-exo/6-endo, 6-exo/7-endo and intermolecular types. Six examples were also synthesized in individual state as stable hexafluoroantimonate salts. Whereas diaurated species are obtained reliably from all conventional mononuclear gold catalysts, application of binuclear ones often gave diaurated species with unusual properties. The preliminary results point to complexities of behavior of binuclear gold catalysts and would require more research in future for this subclass. The formation of diaurated species from various gold-oxo compounds (LAu)2OH+, (LAu) 3O+, and LAuOH (L=phosphine ligand) was also studied. Of these three types, only (LAu)2OH+ is reactive, whereas (LAu)3O+ and LAuOH are not reactive alone but require acidic promoters to enable the reaction. These differences in reactivity were explained by ability of these compounds to generate the necessary acetylene π-complex intermediate. Catch the gold fish: A number of enol ether derived diaurated species were synthesized directly from different alkynols and cationic gold complexes in the presence of a non-nucleophilic base (proton sponge; see scheme). The reaction can be easily applied for the in situ generation of diaurated species from all common types of hydroalkoxylation substrates: 5-endo, 5-exo/6-endo, 6-exo/7-endo and even intermolecular types. The reactivity of various gold oxo compounds (LAu)2OH+, (LAu) 3O+, and LAuOH (L=phosphine ligand) towards alkynols was also examined. Copyright
Alkaline earth catalysis of alkynyl alcohol hydroalkoxylation/cyclization
Brinkmann, Christine,Barrett, Anthony G. M.,Reid, Stephanie,Hill, Michael S.,Procopiou, Panayiotis A.
, p. 7287 - 7297,11 (2020/09/02)
Heavier alkaline earth bis(trimethylsilyl)amides [Ae{N(SiMe 3)2}2]2 (Ae = Ca, Sr, Ba) are shown to act as effective precatalysts for the regioselective intramolecular hydroalkoxylation/cyclization of a wide range of alkynyl and allenyl alcohols. In the majority of cases, cyclization of alkynyl alcohols produces mixtures of the possible endo- and exocyclic enol ether products, rationalized as a consequence of alkynylalkoxide isomerization to the corresponding allene derivatives. Cyclization rates for terminal alkynyl alcohols were found to be significantly higher than for substrates bearing internal alkynyl substituents, while the efficacy of cyclization was in general found to be determined by a combination of stereoelectronic influences and the thermochemical viability of the specific alkaline earth metal catalysis, which we suggest is determined by the individual M-O bond strengths. Kinetic studies have provided a rate law pertaining to a pronounced catalyst inhibition with increasing [substrate], indicating that turnover-limiting insertion of C-C unsaturation into the M-O bond requires the dissociation of substrate molecules away from the Lewis acidic alkaline earth center.
Oxidant-free lactonization of diols using a hydrotalcite-supported copper catalyst
Mikami, Yusuke,Ebata, Kaori,Mitsudome, Takato,Mizugaki, Tomoo,Jitsukawa, Koichiro,Kaneda, Kiyotomi
experimental part, p. 855 - 861 (2010/09/16)
We newly synthesized well-crystallized hydrotalcite-supported copper nanoparticles, denoted as Cu/HT(c), which acted as a highly efficient heterogeneous catalyst for oxidant-free lactonization of various diols under liquid-phase conditions. The Cu/HT(c) catalyst could be recovered by simple filtration and reused without the significant loss of its activity and selectivity.
