Metal-promoted additions, alkynes

Without question, the metal-promoted cycloaddition of three alkynes to produce benzenes is the most extensively studied organometallic cycloaddition in intramolecular versions. Early work indicated the utility of Ni° systems e.g. Ni(CO)2(PPh3)2), Ziegler catalysts and rhodacyclopentadienes in the partially intramolecular cocycloaddition of a,b>-diynes with additional alkynes. Ziegler catalysts were noteworthy in giving rise to products containing the benzocyclobutene moiety from reactions of 1,5-hex-adiyne, while the Rh systems showed considerable utility in the preparation of anthraquinone derivatives from appropriate diyne precursors (Scheme 29). 

As is clear from the introductory discussion, most, if not all, of the d-block transition metals are expected to participate in reactions that are related to those discussed here. In addition to the Co-based methodology mentioned earlier, some related reactions of Pd and are known. Also related are the cyclization reactions of metal-carbene complexes containing Cr, Mo, W and other transition metals with alkynes and alkenes and a recently reported Nb- or Ta-promoted diyne-alkyne cyclization reaction, which appears to be closely related to a number of previously developed alkyne cyclotrimerization reactions, such as those catalyzed by Co. Investigations of reactions involving other transition metals may prove to be important especially from the viewpoint of developing asymmetric and catalytic procedures. 

It is now usual to promote these cycloadditions by catalysts for example, reaction with A -tosyl-ynamides, using ruthenium or copper catalysts, giving 1-substituted 5- and 4-amino triazoles, respectively the formation of the 1,4-substitution pattern with copper catalysis and 1,5-pattem with ruthenium catalysis seems to be general. The latter metal will also promote addition to internal alkynes. ... 

In addition to transfer hydrogenation reactions, arene ruthenium complexes also display excellent activity in the catalytic hydrogenation of olefins and alkynes including asymmetric reduction [40]. Remarkably, this process occurs under milder conditions, than required for catalysis with the dissociation of arene-metal bond. Lately, arene iridium complexes have also been found to be effective hydrogenation catalysts [41 ]. It is noteworthy that iridium can also promotes addition to the carbon-nitrogen double bond. 

It has been shown that the stereochemistry of the hydrosilylation of 1-aUcynes giving 1-silyl-l-alkenes depends on the catalysts or promoters used. For example, the reactions under radical conditions give the cis-product predominantly via trans-addition , while the platinum-catalyzed reactions afford the trans-product via exclusive cts-addition. In the reactions catalyzed by rhodium complexes, thermodynamically unfavorable c/s-1-silyl-l-alkenes are formed via apparent trans-addition as the major or almost exclusive product. Since the trans-addition of HSiEts to 1-alkynes catalyz by RhCl(PPh3)3 was first reported in 1974 , there have been controversy and dispute on the mechanism of this mysterious trans-addition that is vray rare in transition-metal-catalyzed addition reactions to aUtynes. Recently, iridium 4i6 mthenium complexes were also found to give the ds-product with extremely high selectivity (vide supra). 

The transition metal-free addition of silylphosphines to styrenes, pyridines, and acrylates was promoted by a fluoride source (Scheme 4.17) [59]. The addition reaction occurred under extremely mild conditions and afforded moderate to excellent yields of the anti-Markovnikov addition prodnct. The reaction was proposed to proceed through initial formation of a phosphide anion that reacted with the alkenes through a phospha-Michael-type addition. The proton source for the generation of the final adduct was proposed to be adventitious moisture in the commercially available TBAF solution. While activated alkenes were quite amenable to this approach, unactivated alkenes were unresponsive. 2-Vmylpyridine was particularly reactive and was cleanly converted into the alkylphosphine (95%). While most of the substrates were terminal Michael acceptors, a number of internal alkenes as well as a tetrasubstituted alkene were converted into the alkylphosphines in moderate yields (Schane 4.18 and Example 4.17). The chemistry could be extended to terminal and internal alkynes, although a mixture of E- and Z-isomers were obtained with the EIZ> 1. Following this work, the authors were able take advantage of the need for another electrophile to complete the reaction and added an aldehyde to trap the intermediate carbanion [60]. This approach worked well and enabled the construction of gamma-hydroxyphosphonates in excellent yield (up to 88%). 

Our aim in this chapter is to present the most important advances in the domino reactions of carbohydrates based on the metal-promoted ring opening of cyclic sugars. These procedures, promoted by different low-valent organometaUic reagents, start with the addition of the metal to an electrophihc site (i.e., an alkyl hahde, a carbonyl, an alkene, or an alkyne), followed by reductive ehmination to the open-chain sugar. From this open-chain sugar intermediate, the next reaction takes place under the same reaction conditions. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

Addition of organometallic compounds of main group metals R—M —X (M = B, Al, Zn, Mg, Sn) to alkenes and alkynes is called carbometallation. Some reactions proceed without a catalyst, but they are promoted or accelerated by transition metal... 

Cazes et al. reported the Pd-catalyzed intermolecular hydroamination of substituted allenes using aliphatic amines in the presence of triethylammonium iodide leading to allylic amines [19]. In a way similar to the Pd-catalyzed hydrocarbona-tion reactions we reported that the hydroamination of allenes [20], enynes [21], methylenecyclopropanes [22], and cyclopropene [10] proceeds most probably via oxidative addition of an N-H bond under neutral or acidic conditions to give allylic amines. The presence of benzoic acid as an additive promotes the Pd-medi-ated inter- and intramolecular hydroamination of internal alkynes [23]. Intramolecular hydroamination has attracted more attention in recent years, because of its importance in the synthesis of a variety of nitrogen-containing heterocycles found in many biologically important compounds. The metal-catalyzed intramolecular hydroamination/cyclization of aminoalkenes, aminodienes, aminoallenes, and aminoalkynes has been abundantly documented [23]. 

Among the carbonylative cycloaddition reactions, the Pauson-Khand (P-K) reaction, in which an alkyne, an alkene, and carbon monoxide are condensed in a formal [2+2+1] cycloaddition to form cyclopentenones, has attracted considerable attention [3]. Significant progress in this reaction has been made in this decade. In the past, a stoichiometric amount of Co2(CO)8 was used as the source of CO. Various additive promoters, such as amines, amine N-oxides, phosphanes, ethers, and sulfides, have been developed thus far for a stoichiometric P-K reaction to proceed under milder reaction conditions. Other transition-metal carbonyl complexes, such as Fe(CO)4(acetone), W(CO)5(tetrahydrofuran), W(CO)5F, Cp2Mo2(CO)4, where Cp is cyclopentadienyl, and Mo(CO)6, are also used as the source of CO in place of Co2(CO)8. There has been significant interest in developing catalytic variants of the P-K reaction. Rautenstrauch et al. [4] reported the first catalytic P-K reaction in which alkenes are limited to reactive alkenes, such as ethylene and norbornene. Since 1994 when Jeong et al. [5] reported the first catalytic intramolecular P-K reaction, most attention has been focused on the modification of the cobalt catalytic system [3]. Recently, other transition-metal complexes, such as Ti [6], Rh [7], and Ir complexes [8], have been found to be active for intramolecular P-K reactions. 

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