Insertion acetylene

71-Allyl complexes sometimes undergo acetylene insertion reactions, as given in equation (6-83). 

However, the insertion probably proceeds through a tj-allyl derivative since n-a conversion is anticipated upon coordination of an acetylene group to the metal, equation (6-84). 

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Formation of 1,3,2-diazagermine was identified57 by a crystal study of the novel eight-membered 1,5,2-diazagermocine ring which resulted from acetylene insertion. 

A boron-containing analog of cyclooctatetraenide dianion (CgHg2-), the C7H7BMe3 trianion, has been obtained via acetylene insertion into the boratabenzene ring, affording complex 126 and other products, e.g., 127.1... 

Complex D4 is considered as the active species for both alkyne and olefin coordinations. Starting from the olefin coordinated complex (D5 ), the olefin insertion into the Pt-B bond is unfavorable because of a high activation barrier (22.9 kcal/mol). On the contrary, the acetylene insertion from the acetylene coordinated complex (D5) occurs easily with a small reaction barrier (9.0 kcal/mol). This significant difference in the reaction barriers has been used to explain the inertness of olefins for diborafion reactions. The smaller barrier from D5 to D6 coincides with the highly stable insertion product D6. In contrast, the olefin insertion product D6 is relatively unstable with respect to the olefin coordinated species D5 . 

Another clear example of an acetylene insertion reaction was reported by Chiusoli (15). He observed that allylic halides react catalytically with nickel carbonyl in alcoholic solution, in the presence of CO and acetylene, to form esters of cis-2,5-hexadienoic acid. The intermediate in this reaction is very probably a 7r-allylnickel carbonyl halide, X, which then undergoes acetylene insertion followed by CO insertion and alcoholysis or acyl halide elimination (35). Acetylene is obviously a considerably better inserting group than CO in this reaction since with acetylene and CO, the hexadienoate is the only product, whereas, with only CO, the 3-butenoate ester is formed (15). [See Reaction 59]. 

Reaction 59 differs from the cobalt-acetylene insertion mentioned above because the cobalt prefers to insert CO before the acetylene, and the nickel the reverse. Whether or not this difference results from specific eflfectsof the 7r-allylnickel system is not known but it is a good possibility since the allylic double bond is probably coordinated to the nickel throughout the reaction. 

Under more vigorous conditions, complex XII can apparently add more acetylene and carbon monoxide, forming a bifurandione, XIII (/, 79, 82). A reasonable mechanism for the dione formation would be a CO insertion, then an acetylene insertion, and another CO insertion, followed by cyclization by ketone insertion, and finally a Co2(CO)3 elimination. 

The reported addition of triphenylaluminum to diphenylacetylene to form 1, 2, 3-triphenylbenzaluminole 22) is another clear example of an acetylene insertion, this one being followed by a cyclization reaction. 

The polymerization of acetylene by Ziegler catalysts very likely involves metal alkyl-acetylene insertion reactions also 26). 

Figure F shows some acetylene insertion reactions. These, too, are similar to the olefin insertion reactions. The manganese and cobalt hydrocarbonyls again add. Chloronickelcarbonyl hydride, which I believe is an intermediate in many of the nickel carbonyl-catalyzed reactions, adds to olefins. Diborane and the aluminum hydrides also add.

The extent of acetylene insertion can be limited by the control of reaction conditions, such as the ratio of reactant acetylene to the number of Si-Si bonds or reaction time. In addition, if an organic substrate with two alkyne functional groups is used, cross-linked polymers are formed. Other palla-... 

One of the first transition metal-catalyzed ring-expansion reactions of SCBs with the formation of new C-C bonds involved the insertion of acetylenes catalyzed by Pd-complexes to furnish silacyclohexenes (Scheme 46) <1975CL891, 1991BCJ1461>. In addition to the acetylene-insertion products (silacyclohexenes), ring-opened allyl-vinylsilane products that also incorporate the acetylene moieties were observed. The ratio of the two types of the products depends heavily on the nature of acetylenic compounds. 

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

The radical or ionic character depends on the identity of the metal, the effective metal oxidation state, and the auxiliary ligands. These combined effects determine the reactivity, stereo- and regioselectivity toward acetylene insertion. For example, the reaction of CpCo(PPh3) (RC=CR ) with asymmetric acetylenes, RC=CR, gives a mixture of isomeric products ... 

Bis(triphenylphosphine)palladium dichloride [(Ph3P)2PdCl2] can also be used as a catalyst for the phase-transfer carbonylation of halides. However, considerably more drastic conditions [95°C, 5 atm] are required when compared with Co2(CO)8 (44). The carbonylation of allyl chlorides can be catalyzed by nickel tetracarbonyl, giving isomeric mixtures of bu-tenoic acids. There is evidence for the intermediacy of polynuclear nickel-ates in this phase-transfer process (45). Acetylene insertion did not occur... 

Intermolecular olefin and acetylene insertion into zirconaaziridines has been studied by Buchwald and coworkers. High regio- and diastereoselectivity is observed for the formation of the 3,4-disubstituted product 23 in Eq. 10 [20]. Only a single diastereomer of the resulting chiral amine is isolated upon acidic workup. 

Another rapidly progressing field is that of multistep reactions which occur in ordered sequences chemo-, regio- and stereo-selectively on a transition metal species. To this end, it is necessary to delay release of the desired product until the whole series of steps has been completed competitive terminations (such as hydride elimination) must be prevented or must only occur at low rates compared to the main sequence. An example, reported by Chiusoli in the late 50s, is offered by the nickel-catalyzed synthesis of methyl 2,5-heptadienoate from 2-butenyl chloride, acetylene, CO and methanol. The reaction is chemo-, regio- and stereo-selective the four molecules react in the order shown in Equation A3.4 (chemoselectivity) the butenyl group attacks the terminal allylic carbon rather than the internal one (regioselectivity) and acetylene insertion leads to a Z double bond (stereoselectivity). 

The five-coordinate Ru-boryl complex, Ru(Bcat)Cl(CO)(PPh3)2, inserts ethyne mto the Ru-B bond forming the borylvinyl-compound (13) (Scheme 8).11 This reaction and product models the acetylene insertion step to give intermediate C in the metal-catalysed... 

In addition to isolation and characterization of the ruthenacycle complexes 18 or 32, the detailed reaction mechanism of the [2 + 2 + 2] cyclotrimerization of acetylene was analyzed by means of density functional calculations with the Becke s three-parameter hybrid density functional method (B3LYP) [25, 33]. As shown in Scheme 4.12, the acetylene cyclotrimerization is expected to proceed with formal insertion/reductive elimination mechanism. The acetylene insertion starts with the formal [2 + 2] cycloaddition of the ruthenacycle 35 and acetylene via 36 with almost no activation barrier, leading to the bicydic intermediate 37. The subsequent ring-... 

Various vinylsilanes, olefins or acetylenes insert into the ortho C-H bond of aromatic ketones in the presence of catalytic amount of ruthenium complexes in high yields [21,22], The C-H bond cleavage reaction of aromatic ketones also involves orthometallation which is promoted by prerequisite coordination of the carbonyl group to ruthenium (Scheme 14.9) [21], This type of reaction has a wide generality for aromatic and alkenyl ketones with a variety of alkenes. 

According to the first information on the catalysis of hydrosilylation by orga-noactinide complexes Cp 2A Me2 (where A = Th, U), they are efficient for hydrosilylation of terminal alkynes [82]. All catalytic and kinetic examinations of catalysis by early (d°) transition metal complexes (also by metal complexes with non-Cp ligands, e.g., [83]) support the generally accepted mechanism involving rapid olefin (acetylene) insertion into an M-H bond followed by a... 

Acetylene insertion reactions have been the subject of theoretical studies . 

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