Alkyne derivatives mechanisms

Scheme 17. DFT-derived mechanism for the alkyne hydrogenation by a neutral palladium(0)-bisphospine complex. ...

The polymerization of substituted alkynes is postulated to proceed either by the metathesis mechanism or by an insertion mechanism (18). Numerous alkyne derivates have been shown to polymerize in the presence of group V, VI, and VIII transition metal catalysts. 

Scheme 34. Carbene-alkyne polymerization mechanism to generate polyacetylene derivatives.

Tin alkynes rearrange to yield allene products in much the same way as do lithium alkynes, except that the reaction involves a radical mechanism. It is very similar to the reaction of allyl stannanes with alkyl halides, which substitutes the allyl group. Similar reactions are reported for allyl derivatives of cobalt, rhodium and iridium, but this work has not been extended to alkyne derivatives. 

SCHEME 12. Postulated mechanism for heme alkylation by terminal alkynic derivatives. Reproduced with permission from Ref. 74... 

The novel phospha-alkyne system (303) is formed in the reaction of lithium bis(trimethylsilyl)phosphide with 0,0 -diethyldithiocarbamate. C-chlorophosphaethyne, ClCsp, has been generated by the pyrolysis of trichloromethyldichlorophosphine over granulated zinc at 550°C, and characterised by infrared spectroscopy. A bimolecular proton transfer mechanism has been suggested for the base-promoted isomerism of alkynyl- and alkenyl-phosphines to the phospha-alkynes (304). The potential of phospha-alkynes as novel building blocks in heterocyclic chemistry has been reviewed. New examples of phospha-alkyne-derived... 

In contrast to the common methods described above, no sophisticated or powerful dehydration reagent is needed and the reaction can easily be carried out on a large scale. A reaction mechanism has been suggested, in which the aldehyde serves as a relay for the water transfer from the amide to the acetonitrile solvent. The aldehyde may be varied, but formic acid is essential for the reaction. Alkyne derivatives decompose under the reaction conditions, and both THP ethers and TBDMS groups are unstable. 

The rates of bromination of dialkylacetylenes are roughly 100 times greater than for the corresponding monosubstituted alkynes. For hydration, however, the rates of reaction are less than 10 times greater for disubstituted derivatives. Account for this observation by comparison of the mechanisms for bromination and hydration. 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Moulijn et al. (33) studied the reactions of some linear alkynes over a W08-Si02 catalyst in a fixed-bed flow reactor. Besides metathesis, cyclotrimerization to benzene derivatives occurred. Thus, propyne yielded, in addition to metathesis products, a mixture of trimethylbenzenes. From this an indication of the mechanism of the metathesis of alkynes can be obtained. 

It has been shown how alkenylcarbene complexes participate in nickel(0)-me-diated [3C+2S+2S] cycloaddition reactions to give cycloheptatriene derivatives (see Sect. 3.3). However, the analogous reaction performed with alkyl- or aryl-carbene complexes leads to similar cycloheptatriene derivatives, but in this case the process can be considered a [2S+2S+2S+1C] cycloaddition reaction as three molecules of the alkyne and one molecule of the carbene complex are incorporated into the structure of the final product [125] (Scheme 82). The mechanism of this transformation is similar to that described in Scheme 77 for the [3C+2S+2S] cycloaddition reactions. 

The regioselective preparation of 2-substltuted naphthalenediol derivatives having the diols differentially protected in a predictable and straightforward manner, previously not directly attainable, is readily accomplished using chromium carbene complexes. First prepared by E. O. Fischer, chromium carbene complexes react readily with alkynes (extensively investigated by K. H. D6tz, and others).3 Steric effects dictate the substitution pattern observed2-4 and the reaction mechanism has been widely studied.2... 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

Depending on the nature of the substrates, selectivity could be completely reversed between the two isomeric products. For example, switching R1 group between Buc and Ph gave high yields of the first and second product structures, respectively. The authors noted that the reaction did not proceed if the imine contained an ortho-MeO group at R2 or if the imine was replaced with an aldehyde, oxime, or hydrazone. The catalytic cycle is initiated by C-H activation of the imine, that is, the formation of a five-membered metallocycle alkyne insertion affords the intermediate drawn in Scheme 69. It is noteworthy that this is the first report of catalytic synthesis of indene derivatives via a C-H insertion mechanism (C-H activation, insertion, intramolecular addition). 

---