Alkynes transition metal-promoted

Catalytic quantities of transition- or non-transition metals promote the cyclization of 2-alkynynylbiphenyl analogs to phenanthrene or fulvene analogs. The mechanism is thought to involve activation of the alkyne by metal coordination, prior to cyclization (Equations (179) and (180)).146... 

Transition-metal-promoted cycloaddition is of much interest as a powerful tool for synthesis of carbocyclic stmcture in a single step. Utilization of carbon monoxide as a component of the cycloaddition reaction is now widely known as the Pauson-Khand reaction, which results in cyclopentenone formation starting from an alkyne, an alkene, and carbon monoxide mediated by cobalt catalyst. Although mechanistic understanding is limited, a commonly accepted mechanism is shown in Scheme 4.16. Formation of dicobalt-alkyne complex followed by alkene... 

Transition metal-promoted allylzincation of disubstituted alkynes. . 901... 

Main Croup Element- and Transition Metal-Promoted Carboxylation of Organic Substrates (Alkanes, Alkenes, Alkynes, Aromatics, and Others)... 

To date, the most commonly used transition metal-promoted cycloaddition in organic synthesis is the Pauson-Khand reaction. First reported by Pauson and Khandin 1973 [9],this transformation is the cobalt-mediated [2+2+1] cycloaddition of an alkyne, an alkene and carbon monoxide to form a cyclopentenone, Eq. (1). Although mechanistic understanding is Hmited, the accepted mechanism for the transformation is depicted in Fig. 2. Loss of two equivalents of CO followed by complexation of an alkyne produces 1. Subsequent loss of CO from... 

Stille coupling of stannylthiophenes with 3-(4-methoxyphenyl)-3-(5-bromothien-2-yl)naphtho[2,l-6]pyran has been used to form photochromic ter- and quaterthiophenes 15, the open forms of which show enhanced conductivity <01SM(124)23, 02APL4297>. Transition metal promoted coupling also features in the synthesis of naphtho[2,l-6]pyrans 16 linked to thiophene units through an alkyne function. Apart from a shift in the absorption maximum,... 

Wilczynski, R. Sneddon, L. G. Transition-metal-promoted Reactions of Boron Hydrides. 3. (R2C2)Co2(CO)6-catalyzed Reactions of Alkynes and Small Carboranes Synthesis of Boron-substituted Alkenylcarboranes. Inorg. Chem. 1982,21, 506-514. 

Zevaco T, Dinjus E (2010) Main group element- and transition metal promoted carboxyla-tions of organic substrates (alkanes, alkenes, alkynes, aromatics, and others). In Aresta M (ed) Carbon dioxide as chemical feedstock. Wiley-VCH Verlag GmbH Co, KGaA, Weinheim... 

In 2010, Toste s group reported the first example of a highly enantioselective polyene cyclization reaction in which transition metal-promoted alkyne activation serves as the cyclization initiating event [34], The (MeO-DTBM-BIPHEP)gold(I)-catalyzed reaction offers an efficient method for the stereoselective synthesis of polycyclic compounds... 

The enantioselective transition-metal-promoted skeletal rearrangements of polyunsaturated substrates possessing olefin, alkyne, or allene functions have been reviewed (Scheme 97). ... 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

Due to the low reactivity of alkyl and arylorganozinc reagents towards alkenes and alkynes, it appears clear that the carbozincation chemistry for this class of reagents is intimately associated with transition metal catalysts. Some of the metal-catalyzed/promoted reactions do indeed produce organozinc reagents as the final organometallic species that can further react with an appropriate electrophile, whereas other processes lead to highly functionalized products by an entirely different pathway. 

The vapors of the transition metals are effective in promoting the catalytic trimerization of alkynes, e.g.,... 

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... 

The intramolecular Alder-ene reaction (enyne cydoisomerization reaction) with alkynes as the enophiles has found wide application compared with diene systems. The reason may be the ready chemo-differentiation between alkene and alkyne functionality and the more reactive alkyne moiety. Furthermore, the diene nature of the products will promote further applications such as Diels-Alder reactions in organic synthesis. Over the past two decades the transition metal-catalyzed Alder-ene cycloisomerization of l,n-enynes (typically n= 6, 7) has emerged as a very powerful method for constructing complicated carbo- or heterocydic frameworks. The transition metals for this transformation indude Pd, Pt, Co, Ru, Ni-Cr, and Rh. Lewis acid-promoted cydoisomerization of activated enynes has also been reported [11],... 

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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