Catalytic alkyne

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

Intramolecular [2+2+2] cyclotrimerizations of diynes and triynes possessing heteroatom tethers furnish benzoheterocycles. The cyclization of triynes 88 using the Grubbs catalyst 76 proceeds via cascade metathesis as shown in Eq. (35) to yield a tricyclic product 89 [88]. This novel type of catalytic alkyne cyclotrimerization can be applied to the cycloaddition of 1,6-diynes with monoalkynes [89]. 

The Sonogashira coupling can be considered a special case of catalytic alkyne activation. Interestingly, it is also possible to conduct alkyne activation under oxidative conditions in the presence of Pd catalysts without oxidative dimerization. Here, Costa and coworkers [139] have developed a Pd-catalyzed sequential carboxylation-alkoxycarbonylation of acetylenic amines in the presence of oxygen to give mixtures of Z- and -configurcd 2-oxo-oxazolidin-5-ylidene]-acetic acid methyl ester 193 and 194 in good to excellent yields (Scheme 75). 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

Despite the rich catalytic alkyne cyclization chemistry of Fe , there are few mononuclear ferracyclopentadienes, perhaps because of their high reactivity e.g., reduction of FeCl2[P(CH3)3]2 with Na-Hg in 2-butyne gives the blue, highly volatile and thermally unstable [(CH3)3P]4Fe[C4(CH3)4] and the cyclotrimer hexamethylbenzene . A similar blue entity IV arises when the 4-coordinate complex (dad)2Fe is exposed to dimethyl butynedioate [dad = ethanedialbis(cyclohexylimine)] . 

These inherent difficulties have elicited considerable attention, which has led to the emergence of synthetically viable protocols. The first catalytic alkyne hydrothiolation was reported by Ogawa in 1992 [151]. A number of late transition metal salts were found to be effective. In particular, Pd(OAc)2 demonstrated excellent selectivity for the branched olefin. High yields were obtained even with stericaUy demanding substrates and potentially reactive functional groups (19). 

Wu, X., Haberlag, B., Grunenberg, J., Jones, P.G., and Tamm, M. (2009) Experimental and theoretical investigations of catalytic alkyne cross-metathesis with imidazolin-2-iminato tungsten alkylidyne complexes. OrganometalUcs, 28, 1534-1545. 

Nucleophilic addition of H2O to coordinated alkyne is the key step of catalytic alkyne hydration with traditional Hg(II) or Au(I) catalysts (Eq. 8.26) that convert terminal alkynes RC=CH to the methyl ketones RCOMe. As part of a general trend associated with the rise of green chemistry (Section 1.1), toxicity and expense concerns have led to the advent of base metal catalysts, such as a water-soluble Co(III) porphyrin. ... 

We also reported on the synthesis of 2,3-dihydroquino-lin-4(lfl)-one derivatives 309, which are expected to possess attractive pharmacological properties and serve as important synthetic intermediates. Thus, the catalytic alkyne-carbonyl metathesis of arylalkyne 307 followed by intramolecular addition of heteroatom nucleophile afforded... 

Shoai S, Bichler P, Kang B, Buckley H, Love JA. Catalytic alkyne hydrothiolation with alkanethiols using Wilkinson s catalyst. Organometallics 2007 26 5778-5781. 

Sabarre A, Love J (2008) Synthesis of 1,1-disubstituted olefins via catalytic alkyne Hydrothiolation/Kumada cross-coupling. Org Lett 10 3941-3944... 

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