Alkylidenes triple bond

From Carbyne Complexes. Addition of HC1 across the metal-carbon triple bonds of Ru and Os d8 arylcarbyne complexes yield stable, neutral secondary alkylidene complexes ... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

On the basis of the reaction of conjugated dienes with unsaturated halides in the presence of external nucleophiles, an elegant intramolecular version leading to a-alkylidene-y-lactams, has been developed (Scheme 8.19). Starting with an aryl halide, the regioselective insertion of an arylpalladium halide to the triple bond of acyclic compound 42 gives the c-vinylpalladium intermediate 43. Subsequent intramolecular carbopalladation of the diene affords a re-allyl palladium intermediate... 

Vinyl radicals also add to carbon-carbon double bonds intramolecularly to give 2,6-cw-disubstituted cyclic ethers (Equation (5)).41 In the tin hydride-mediated cyclization of the substrates including alkynes, alkyl radicals attack to carbon-carbon triple bonds leading to uco-alkylidene allylic alcohols (Equation (6)).42 The coupling reaction between alkyl radicals may afford cyclization products. Thus, the reduction of 1,3-diiodopropane derivatives with a tin hydride provides substituted cyclopropanes.4... 

For the corresponding cyclization of acetylenic amides 418, Nagasaka etal.347,347a used a mixture of silver(i) triflate and lithium hexamethyldisilazide as the precatalyst. The process is probably initiated by the coordination of AgN(SiMe3)2 (generated in situ from AgOTf and LHMDS) to the triple bond, followed by nucleophilic attack of the lithium amide at the activated alkyne which affords (ZVy-alkylidene-y-butyrolactams 419 in high yields (Scheme 122). 

The rather unusual precatalyst silver(i) isocyanate was found to efficiently catalyze the cyclization of propargyl carbamates 420 to 4-alkylideneoxazolidin-2-ones 421 in good to high yields (Scheme 123).348 The presence of a base such as potassium /-butoxide or triethylamine is required for formation of the amide nucleophile which undergoes a stereoselective intramolecular attack at the activated triple bond (/rstereo-isomer 421 exclusively. Likewise, homopropargyl carbamates are converted into six-membered (Z)-4-alkylidene-l,3-oxazinane-2-ones under the same conditions. 

Diazoalkanes are important sources of reactive CR2 fragments in alkylidene chemistry. Bridging alkylidenes are extensively reviewed (116-118). A number of diazoalkanes add across the metal-metal triple bond associated with 2. The reactions are, however, highly complex (as shown by the formation of 30 above), with reaction products heavily dependent on the R substituents of R2CN2 as well as the substituents on the cyclopentadienyl ring. While many interesting products arise in these reactions, the consequence is a minimum ability to predict reaction products. 

Section III of the review is concerned with alkylidene and alkylidyne halocarbonyl complexes, and a review on recent advances in the chemistry of metal-carbon triple bonds by Mayr and Hoffmeister in 1991 contains material of interest.6 Section IV is concerned with halocarbonyl alkyne and alkene complexes described from 1987 to mid-1995, because an exten-... 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

In addition to complexes of terminal alkylidenes, many complexes containing alkylidenes bridging two metal atoms have been made these are commonly known as p-methylene complexes. A wide range of complexes containing other metal-carbon bonds are known such as the carbyne complexes (for example, Cr = CPh(CO)5) which contain metal-carbon triple bonds. While such species may well be involved in some catalytic reactions, their importance in industrially catalyzed processes still has to be evaluated. 

Lu and Xie have reported a three-component coupling for the synthesis of Q -alkylidene-y-lactams 69 (Scheme 22) [62]. Treatment of AT-(2,4-dienyl)alkynamide 66 with an aryl iodide 67 affords a a-vinylpalladium intermediate 70 through regioselective insertion of the active ArPdX species into the triple bond. Subsequent intramolecular carbopalladation of the diene affords 7r-allylpalladium complex 71, which undergoes nucleophilic attack by amines 68 at the less hindered terminus to afford the product 69. 

The metathesis of metal-metal triple bonds with carbon-carbon triple bonds directly connects the chemistry of inorganic, organometallic, and organic multiple bonds. This reaction is synthetically very useful. Krouse and Schrock prepared the series of /t-a,[Pg.256]

A tungsten complex which possesses all three types of metal-carbon bonds, single (alkyl), double (alkylidene), and triple bonds (alkylidyne) has been prepared as shown in eq. (3) and structurally characterized [15]. 

Nickel(0)-catalyzed hydrosilylation of 1,7-diynes allows intramolecular carbosilylation of one triple bond and formation of bis-alkylidene cyclohexanes with stereoselective formation of a (Z)-vinylsilane subunit60. This reaction resembles reductive cyclization of diynes catalyzed by palladium complexes (see Section 1.5.8.2.6.). 

Acetylenes also react with alkylidene complexes to afford a new carbene derivative formed by inserting the triple bond into the alkylidene-metal bond . A metallacyclo-butene is the intermediate species ... 

On the other hand, the other direction of addition (yd-addition) produces a monosubstituted alkylidene that is sterically comparable to the initiating alkylidene, making this the desired direction of addition. Formation of a six-membered ring in a reaction involving a more reactive terminal alkylidene would transform 43b into 44b. The NMR spectrum of poly-6 reveals two carbonyl carbon resonances. Therefore it was speculated that poly-6 consist of a random distribution of five- and six-membered rings formed through what is nominally tail-to-tail and head-to-head cyclopolymerization of the two acetylenic bonds in the same monomer (Scheme 1). Other uncertainties in catalyst systems of the type are the rate at which alkylidene rotamers interconvert, the extent to which the reactivities of the two alkylidene rotamers differ, and the degree of selective formation of one rotamer when a triple bond reacts with a Mo=C bond. ... 

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