Alkyne complexes applications

Scheme 9 Ruthenium carbene complexes from alkynes and application [17]...

A variety of aldehyde/alkyne reductive couplings involving the stoichiometric use of early transition metals (Ti and Zr) have been developed (Scheme 27) [68-70]. The low cost and ease of handling of titanium alkox-ides render these stoichiometric processes very practical despite the lack of catalytic turnover. Recent variants of stoichiometric processes involving titanium alkoxides have demonstrated impressive scope in relatively complex applications [71-73]. 

The first tt complexes of 1,3-diynes were reported by Greenfield. Shortly thereafter, Tilney-Bassett described the first heterometallic derivatives. This area has grown steadily since these initial reports and many complexes of this type are now known. Diyne complexes are often simply alkyne-substituted analogues of conventional jr-alkyne complexes. Indeed, transition metal compounds that form -complexes with mono-alkynes can be expected to form complexes with diynes. However, the thermal sensitivity of terminal diynes, especially 1,3-butadiyne, may limit the application of routine reaction conditions in some cases. Further coordination of the ynyl ligand by additional metal fragments is usually determined by the reagent stoichiometry and by steric effects. 

In 1974, Dickson and Fraser reviewed the already substantial area of cobalt-alkyne complexes.1 Since then the number of examples and applications of this class of organometallic compounds, especially dinuclear complexes, has increased enormously. This review will present the chemistry of polynuclear cobalt-alkyne complexes, with emphasis on the more recent developments. Although there have been many exciting fundamental discoveries, this is a maturing area of research that is finding many important applications in organic synthesis,2 such as the use of [Co2(jU-alkyne)(CO)6] complexes in the synthesis of enediyne antitumor antibiotics.3-4... 

Although heterobinuclear alkyne complexes have been less intensively studied, a number of routes involving cobalt as one of the metals have been developed. Compounds with two different metals are expected to have reactivity patterns different from those of the respective homonuclear species and also have applications in the preparation of chiral complexes. In a manner analogous to Eq. (1) the mixed-metal complexes [MCo (CO)7Cp] (M = Mo, W) and [CoMn(CO)9] react with a range of alkynes to afford complexes of the type [MCo(ju-alkyne)(CO)5Cp]1314 and [CoMn (jU-alkyne)(CO)7],15 as illustrated in Eq. (3). 

This review has already indicated numerous applications of dicobalt-alkyne complexes in organic synthesis. Like the Nicholas reaction (see Section II,D), the Pauson-Khand reaction has seen widespread use.175 This reaction is a three-component cycloaddition of alkynes with alkenes and carbon monoxide which occurs in the presence of octacarbonyldicobalt to afford cyclopentenones, as shown in Eq. (16). 

Continuing expansion of the chemistry of polynuclear cobalt-alkyne complexes is expected, especially in the areas of reactivity and synthetic applications, in the coming years. 

Transition metal carbenes constitute a very important class of molecules that have found a multitude of applications. With reference to manganese, a number of new carbene complexes have been reported recently. The ) -alkyne complex (18) was found to undergo oxidation by dimethyldioxirane to afford the a-keto carbene (19). The fascinating aspect of this reaction is the likely existence of an oxirene intermediate (20). Stable oxirene complexes have never been reported, imdoubtedly because of the extreme instability of the antiaromatic oxirene ring. The possibility of trapping an oxirene by epoxidation of a coordinated alkyne is intriguing. Scheme 10 summarizes the chemistry involved. 

The NLO properties of organometallic and coordination complexes are also rich (21, 184, 279-296). Metal-alkyne complexes were first reported 1960 (297) and have recently attracted significant interest because of their potential in materials applications (2, 298). Studies of these types (299) have resulted in the development of structure-NLO response relationships for quadratic optical nonlinearities (p-value), which increase with valence electron count and ease of oxidation of metal. The amplitude is also tunable by ancillary ligand modification and substitution. Select small alkynyl complexes have been shown to exhibit p values at 1064 nm > 2600 x 10 ° esu (299). 

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

Direct application of Ru3(CO)i2 in photochemical synthesis has been described in detail [120]. Thermal reactions of this cluster in presence of two-electron donors L affords [Ru3(CO)9L3]. The discovery in 1974 that irradiation of the cluster under those conditions produces mononuclear products instead of the substituted clusters initiated a wealth of research in Ru-clusters as precursors in photochemical synthesis [121]. Much research has been devoted to the preparation of mononuclear f/ -olefin complexes, as well as alkyne complexes. For example, [Ru(CO)3(PPh3)2] has been reported as an active catalyst for olefin polymerisation, and as such, many investigations have dealt with the reactivity of this compound. Other directions of research include formation of metallacycles, generation of new cluster species, and mixed transition metal/non-metal clusters. 

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

Alkyne- Co2(CO)6) complexes for application in the Pauson-JChand cyclopentenone synthesis have been prepared in simple and convenient fashion by reduction of cobalt bromide by zinc in the presence of alkynes under carbon monoxide. An intramolecular Pauson-Khand reaction has been used 172 to incorporate the proper stereochemistry at the C/D/E rings of Xestobergsterol D. The use of Co2(CO)6l-alkyne complexes as stable intermediates for the construction of the core structures of the anti-tumor enediyne agents esperamicin, calicbeamicin, dynemicin and neocarzinostatin has been reviewe(H73,... 

Since its discovery just over thirty years ago, the Nicholas reaction has become a highly useful tool for the organic chemistry community. Applications of the Nicholas reaction fall into four categories intermolecular reactions, endocyclic intramolecular reactions, exocyclic intramolecular reactions, and tandem reactions. For this discussion, endocyclic means that the cobalt-complexed alkyne is in the ring formed during the Nicholas reaction, while exocyclic indicates that the cobalt-alkyne complex is outside the newly generated ring. 

One of Green s many applications of the chemistry of cobalt-alkyne complexes in synthesis involved formation of a seven-membered ring via an endocyclic intramolecular Nicholas reaction. The key step in his successful synthesis of allocolchicine NSC 51046 (38) was production of cyclic cobalt-alkyne complex 37 from reaction of acetate 36 and boron trifluoride. Hydrosilylation of the organometallic complex followed by desilylation yielded the corresponding alkene that was ultimately transformed into the target tricycle. ... 

As noted in the Preface to Volume I, the second volume includes chapters on the applications of arene and alkyne complexes, as well as cluster compounds, in organic synthesis. Other useful synthetic transformations are discussed in the last chapter. A chapter on insertion reactions of synthetic utility was not included, due to the publication in the last few years of a substantial number of fine reviews in this area. 

Tanaka, K., Toyoda, K., Wada, A., Shirasaka, K. and Hirano, M. (2005) Chemo- and regioselective intermolecular cyclotrimerization of terminal alkynes catalyzed by cationic rhodium (I)/modified BINAP complexes application to one-step synthesis of paracyclophanes. Chemistry—A European Journal, 11(4), 1145-1156. 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

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