Other Metal Allenes

Cumulenes with metal atoms in the cumulative system are of recent vintage. Although the well known carbonyl and isonitrile complexes can be written as metal cumulenes, i.e. L M=C=X, where M = metal and X = O or NR, they are only included in this book if they are involved in reactions with other cumulenes. However, metal cumulenes with one or several metal atoms in the cumulative arrangement are treated in this chapter. Examples of the one-metal complexes are listed in Table 4.23. 

bi- and trimetallic cumulene complexes, such as M=C=M, M=M=C, M=M=M and M=C=C=M are known. Cationic ruthenium allenylidene complexes are used as catalysts for ring closing metathesis reactions. Nonlinear optical properties have been measured for the Group 6 cumulenylidene complexes. Also, cationic chromium or iron vinylidene complexes undergo [2-1-2] cycloaddition reactions across imines to give fi-lactams. This reaction is useful for the synthesis of j8-lactam antibiotics.  

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes . This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

The titanium vinylidene species, Cp2Ti=C=CH2, is generated in situ from methyleneti-tanacyclobutane at 80 °C via a [2-1-2] cycloreversion reaction, or from Cp2 Il(Me)CH=CH2 by a-elimination of methane, which occurs at room temperature. The generated vinyUdene species is trapped with acetylenes, heterocumulenes, transition metal carbonyls, nitriles and phosphaalkynes to give [2- -2] cycloadducts 

Review articles on metal cumulenes appeared in 1983, in 1991 and in 1998 , while molecules with multiple bonds between transition metals and naked main group elements were reviewed in 1986Some of these molecules have a linear structure with cumulative double bonds as evidenced by X-ray crystallography. However, some triple-single bond character is also indicated. 

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The cumulenes discussed in this book are subdivided into carbon- and noncarbon cumulenes, and the 1-carbon cumulenes (sulfines, sulfenes, thiocarbonyl S -imides and thiocar-bonyl S -sulfides) are excellent dipolar species. The 2-carbon or the center-carbon cumulenes (carbon dioxide and carbon sulfides) are less reactive but their imides (isocyanates, isothiocyantes and carbodiimides) readily participate in many of the discussed reactions. The 1,2-dicarbon cumulenes (ketenes, thioketenes and ketenimenes) similarly participate in cycloaddition reactions, as well as the more exotic 1,2-dicarbon cumulenes (1-silaalene, 1-phosphaallene and other metal allenes). In contrast, 1,3-dicarbon cumulenes are only... 

In Limnodrilus sp., an oligochaete worm, copper bioavailability from surhcial freshwater sediments is associated with the amount of copper present in the manganese oxide fraction of the sediment. The redox potential and pH in the gut of Limnodrilus allows the dissolution of the manganese oxide coating, making copper and other metals available for uptake (Diks and Allen 1983). 

Tillack and co-workers developed a rhodium-catalyzed asymmetric hydrosilylation of butadiyne 258 to afford allenylsilane 260 (Scheme 4.67) [106]. Among more than 30 chiral phosphine ligands investigated, the highest enantioselectivity was observed when the catalyst was prepared from [Rh(COD)Cl]2 (1 mol%) and (S,S)-PPM 259 (2 mol%) to afford the optically active allene 260 with 27% ee. Other metals such as Ir, Pd, Pt or Ni were less effective for example, a nickel catalyst prepared from NiCl2 and (R,R)-DIOP 251 or (S,S)-PPM 259 gave the allene 260 with 7-11% ee. 

Cylizations to numerous functionalized five-membered ring systems starting from allenes have recently been described in most cases they make use of Pauson-Kand or other metal-mediated ring-forming protocols (see Chapter 16 for a discussion of these useful cyclizations) [150]. 

Since RN=S=NR and RN=S=0 are cumulated double bond systems they may be considered as heteroallene compounds. In the case of metal-allene compounds the allene may rotate about the metal- 2-allene bond and in addition the metal may jump intramolecularly from one n-C=C bond to the other and vice versa.1... 

Pauson-Khand-type reactions of enynes are mediated by other metal complexes, such as Fe(CO)5 [100], [W(CO)5THF] and Mo(CO)6 [101]. Cyclization of the allene-yne system 227 to the a-methylenecyclopentenone 228 is promoted by Mo(CO)6. The products depend on the substituents of the allene, and the cyclization of 229 afforded 230 as the main product [102]. 

It is worth mentioning that the synthesis of allenes from propargyl halides in Grignard reactions is favored by transition metal catalysis more than by the participation of allenic carbene intermediates in as much as the level of metal impurities in magnesium turnings is not reduced to a minimum, or metal salts such as ferric chloride and other metal chelates such as nickel acetoacetonate are added to the reaction mixture. ... 

Although no pK values of allenic sulfides RCH=C=CHSR/ are known, other experimental data indicate the basicity of the lithiated compounds RCH=C=C(Li)SR/ to be somewhat lower than that of 2-thienyllithium. Competition experiments with butyl bromide in THF or in a THF-HMPT mixture showed the metallated allenic sulfide to be much more reactive than 2-thienyl-lithium [9]. This difference might be due to the higher polarisability of the allenyl system. 

Dienes can bind to metals in an t) or r fashion. Many metal complexes contain t) -diene ligands, and reactions of -ri -diene complexes are described in Section 11.7.2. However, other metals, particularly Pd(II), bind dienes and allenes in an t) fashion. The reactivity of these species has been exploited to develop useful synthetic methods and is described in this section. Backvall has developed chemistry based on nucleophilic addition to palladium-diene complexes to generate allyl products, which subsequently react with a second nucleophile to generate free organic products from 1,4-addition of two nucleophiles across the diene. The palladium(O) byproduct is then re-oxidized to Pd(II) with quinone. 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

A similar transformation with iV-monosubstituted allenic carboxamides targeting lactams yields mixtures of imino dihydrofu-rans and the desired pyrrolidones whereas AgNOs-promoted cyclization of a-hydroxy allenes leads to 2,5-dihydrofurans, the same mild conditions allow conversion of allenyl ketones and aldehydes to their corresponding furan derivatives. A comparative overview concerning the reaction of these substrates with other metal salts or Lewis acid catalysts (HgClOa, PdCl2(MeCN)2, etc.) points out differences in product distribution. ... 

Allen GC, Warren KD (1974) The Electronic Spectra of the Hexafluoro Complexes of the Second and Third Transition Series. 19 105-165 Alonso JA, Baibas LC (1993) Hardness of Metallic Clusters. 80 229-258 Alonso JA, Baibas LC (1987) Simple Density Functional Theory of the Electronegativity and Other Related Properties of Atoms and Ions. 66 41-78 Andersson LA, Dawson JH (1991) EXAFS Spectroscopy of Heme-Containing Oxygenases and Peroxidases. 74 1-40 Antanaitis BC, see Doi K (1988) 70 1-26... 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

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