Alkynes novel complexes

A novel complex, Mo[N( Bu) (Ar)]3 140, was prepared and shown to activate the triple bond in molecular nitrogen in a stoichiometric fashion. However, the complex, when used for alkyne metathesis, does not affect the expected transformation, but undergoes a vigorous endothermic process in a CH2CI2 solution. The resultant solution... 

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

A pathway may be considered which involves a double regioselective alkyne insertion followed by a stereoselective cyclisation to undergo a novel [3+2+2]-cyclisation. These examples illustrate the scope in which the reactivity of Fischer carbene complexes can be tuned in a qualitative manner by transmetalation. 

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. 

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

Metal-mediated and -catalyzed [3 + 2 + 2]-higher-order cycloaddition reactions have also proved to be viable and mechanistically novel methods for the synthesis of seven-membered rings. The reported [3 + 2 + 2]-cycloadditions of allyliridium (Equation (30)),139 -allylcobalt (Scheme 47),140 and allylmanganese (Equation (31 ))141 complexes with alkynes involve the reaction of preformed allylmetal complexes with two separate alkynes, leading to a cycloheptadiene-metal complex. 

Sheridan and co-workers reported a novel photo-assisted [5 + 2 + 2]-reaction based on the reactions of 77S-cyclodienyl Mn complexes160,161 or Cr complexes162 and two alkynes. Decomplexation of the metal gives cycloadducts in moderate to good overall yields (Scheme 66, Equation (42), and Scheme 67). It should be noted that the authors refer to this reaction as a [5 + 2]-, [3 + 2]- or a [5 + 2]-, homo-[5 + 2]-reaction. This reaction leads to the formation of impressively complex tricyclic products that would be otherwise difficult to prepare with step economy. 

The hydration of alkynes represents a prime example in which simple coordinative activation by transition metal complexation greatly facilitates an otherwise very slow chemical process (Equation (107)). This reaction has been a long-studied problem, but only recently have alternatives to the classical use of catalysts such as Hg(n) salts been sought. These new catalyst systems typically display much enhanced reactivity, and some can mediate an anti-Markovnikov hydration through a novel mechanism (Table 1). 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

The reaction of nBu2ZrCp2 with 2 equivalents of PhC CPh provides the novel bicyclic gem-dizirconium complex 140 [236] (Scheme 7.42). Protonolysis of complex 140 with 3 n HC1 gives bibenzyl in 88% yield, while its deuterolysis with D20 provides tetradeuterio-bibenzyl 141 with 92 % deuterium incorporation. The dual path nature (142 versus 140) of the reaction of Cp2Zr with alkynes is an important factor in designing Zr-promoted cyclizations of alkynes, enynes, and diynes. 

Dotz reaction is proposed. According to our calculations the addition of the alkyne molecule to the carbene complex takes place before CO loss in the initial steps of the reaction. Further, our study shows that a novel proposal involving a chromahexatriene intermediate entails lower energy barriers and more stable intermediates than the previous reaction mechanisms postulated by Dotz and Casey. The novel findings query revision of the classically assumed paths and put forward that additional experimental and theoretical studies are necessary to definitely unravel the reaction mechanism of this intringuing reaction. 

This section describes the main results obtained in our studies of the Dotz reaction mechanism [26-29, 39]. The section is divided as follows First, the results for the initial part of the reaction (9—>13) are presented. The central discussion will be whether the alkyne binds the carbene complex after or before CO loss. Then, the results for routes A, B and C (Figure 3) are discussed. In particular, we will examine the suitability of the novel route C involving a chromahexatriene intermediate. 

Use of Co2(CO)8 in reactions involving 1,2-propadienes remains for the most part unexplored. It has been reported that terminal 1,2-propadienes react with Co2(CO)8 to form unidentified complexes, and that excess 1,2-propadiene is polymerized concurrently [30]. It has also been reported by Nakamura that a novel dimeric complex 54, in which a carbonyl ligand is connected to the central carbon of 1,2-propadiene, is produced by the reaction of 1,2-propadiene itself with Co2(CO)8 (Scheme 23) [31]. However, unlike the well-known chemistry of alkyne-Co2(CO)6 complexes, these 1,2-propadiene-cobalt carbonyl complexes have rarely been applied in synthetic reactions, probably due to their high activity in catalyzing the polymerization of 1,2-propadienes [32]. 

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