Alkyne proposed mechanism

Scheme 5.12 Alkylative or arylative carboxylation of alkynes proposed mechanism...

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

A plausible mechanism for this new alkyne aza-Prins cyclization is outlined in Scheme 27. Thus, reaction of the homopropargyl tosyl amine with an aldehyde promoted by ferric halide generates the W-sulfonyl iminium ion. This intermediate evolves to the corresponding piperidine, via the vinyl carbocation. Ah initio theoretical calculations support the proposed mechanism. 

Scheme 5.7 Proposed mechanism of [3+2] cycloaddition reaction of internal alkynes...

Scheme 5-18 Stoichiometric reactions relevant to the proposed mechanism for palladium-catalyzed hydrophosphonylation of alkynes...

Scheme5-24 Eq. (1) Proposed mechanism for Eq. (2) Stoichiometric reactions relevant to the phosphinic acid-modified palladium-catalyzed proposed mechanism hydrophosphinylation of alkynes.

The initially proposed mechanism [14], and one that continues to be considered as the likely pathway for most variants, involves the oxidative cyclization of a Ni(0) complex of an aldehyde and alkyne to a metallacycle (Scheme 18). Metallacycle formation could proceed independently of the reducing agent via metallacycle 19, or alternatively, metallacycle 20a or 20b could be formed via promotion of the oxidative cyclization transformation by the reducing agent. Cleavage of the nickel-oxygen bond in a o-bond metathesis process generates an alkenyl nickel intermediate 21. In the variants involv-... 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

Scheme 4.12 Proposed mechanism for the hydrogenation of alkynes catalyzed by [Pd4(dppm)4(H2)]2+.

The kinetic investigation of this reaction reveals the reaction is first-order in substrate, catalyst and hydrogen concentration, and thus yields the rate law r=kCat[Os][alkyne][H2]. The proposed mechanism as given in Scheme 14.6 is based on the rate law and the coordination chemistry observed with these osmium complexes. 

Scheme 5 Proposed mechanism for the catalysis of the dimerization of terminal alkynes by the mixed hafnacarborane complex. Reproduced by permission of the American Chemical Society from Organometallics 1997, 16, 4508.

Of further significance is the fact that no 1,3-pentadiene is formed This behavior is similar to that of the butynes, where also no 1,3-butadiene was observed. Furthermore, this is in complete accordance with the proposed mechanism of the potassium 3-aminopropylamide-mediated isomerization of internal alkynes to terminal alkynes by repetitive alkyne-allene-alkyne isomerizations [24]. 

Figure 2. Proposed mechanisms for the alkyne to vinylidene isomerization coordinated to a transition metal fragment...

A proposed mechanism of this reaction was reported by Magnus and Principle [10], which is nowadays widely accepted (Scheme 1). Recently, negative-ion electrospray collision experiments have confirmed this mechanism in detail [11]. Starting with the formation of the alkyne-Co2(CO)6 complex 2, olefin 3 coordination and subsequent insertion takes place at the less hindered end of the alkyne. The in situ formed metallacycle 4 reacts rapidly under insertion of a CO ligand 5 and reductive elimination of 6 proceeds to liberate the desired cyclopentenone 7. It is important to note that all the bond-forming steps occur on only one cobalt atom. The other cobalt atom of the complex is presumed to act as an anchor which has additional electronic influences on the bond-forming metal atom via the existing metal-metal bond [12]. 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

H-shift is invoked for the formation of 6 and regeneration of catalyst 8. The proposed mechanism is unusual insofar as the tt-bonds of electroneutral alkynes and arenes seldom participate in Diels-Alder reactions. The intermediacy of metal vinylidenes is supported by the failure of internal alkynes to dimerize under the reported conditions. More importantly, mechanistic restrictions imposed by the porphyrin ligand set severely restrict conceivable alternative mechanisms. 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Scheme 9.11 Proposed mechanism for hydrative dimerization of terminal alkynes.

Chatani s proposed mechanism bears some similarity to that of Jun s reaction (Scheme 9.12). They both begin with hydroamination of the C=C 7t-bond of a rhodium vinylidene. The resultant aminocarbene complexes (71 and 62) are each in equilibrium with two tautomers. The conversion of 71 to imidoyl-alkyne complex 74 involves an intramolecular olefin hydroalkynylation. Intramolecular syn-carbome-tallation of intermediate 74 is thought to be responsible for ring closure and the apparent stereospecificity of the overall reaction. In the light of the complexity of Chatani and coworkers mechanism, the levels of chemoselectivity that they achieved should be considered remarkable. For example, 5 -endo-cyclization of intermediate 72 was not observed, though it has been for more stabilized rhodium aminocarbenes bearing pendant olefins [27]. 

The reaction is considered to be initiated by Tt-complexation of Pt(ll) onto an alkyne unit in substrates (Scheme 30). A strong experimental support for the proposed mechanism comes from deuterium labeling. When geminally D-labeled 75e-2D is treated with PtCl2, 77e-2D and 77e-D are obtained. Formation of 77e-2D indicates that the reaction proceeds through XX and then XXI by hydrogen migration. 

Radical carbonylation reaction serves as a powerful tool for the synthesis of a range of carbonyl compounds. Radical carbonylation has been successfully applied to the synthesis of functionalized ketones from alkyl, aryl, and alkenyl halides.The radical aminocarbonylation reaction of alkynes and azaenynes provided efficient routes to 2-substituted acrylamides, lactams, and pyrrolidinones. For example, the aminocarbonylation of 4-pentyn-l-yl acetate 318 initiated by tributyltin hydride (Bu"3SnH) (30mol%) with AIBN (20mol%) gave acrylamide 325 in 92% yield (Scheme 43).A proposed mechanism starts from the addition of tributyltin radical 319 to alkyne... 

We found that a catalytic amount of cucurbituril markedly accelerated the reaction shown and rendered it regiospecific, yielding only the 1,3-disubstituted product. This result is explained by formation of a transient ternary complex between the reactants and the receptor. Simultaneous binding of both the alkyne and the azide, with one NH3 coordinated to each set of carbonyls and with the substituents extending into the interior of cucurbituril, results in alignment of the reactive groups within the core of the receptor so as to facilitate production of the 1,3-disubstituted triazole. The proposed mechanism may be visualized with the aid of Fig. 7 (R = H). 

The proposed mechanism involves an initial transmetallation of the dialkyl (or diphenyl) zinc with Ni(acac)2 that generates an organonickel species. The latter then achieves the carbonickelation of the alkyne leading to an alkenylnickel(II) complex. Transmetallation with RZn(acac) then regenerates the Ni11 catalyst and affords the a-arylalkenylzinc compound (equation 32)46,47. 

Linford and coworkers have shown that the attachment of alkenes to H-terminated silicon surfaces can also be initiated by direct mechanical scribing, in a process termed chemomechanical functionalization [145-147]. The reaction of 1-alkenes (as well as 1-alkynes) leads to attachment of the molecule to the surface through two new Si—C bonds. The proposed mechanism is the mechanical cleavage of Si—H and Si—Si bonds, leading to silicon radicals that then react with the reactive liquid. Interestingly, Linford and coworkers have also extended this work to show that chemomechanical functionalization can be carried out not only on H-terminated Si, but also on sihcon covered with oxide, and have shown that the process works with a variety of halides, alcohols, and epoxides in both the liquid and gas phase [146]. 

---