Acetylenic carbon, substitution

Iodoallenes can also be prepared in reasonable yields by treatment of secondary acetylenic alcohols with triphenylphosphite-methyl iodide, using DMF as a solvent. One of the -OPh groups is probably first replaced by the propargyloxy group. This intermediate subsequently undergoes attack by iodide on the terminal acetylenic carbon atom, affording the iodoallene in a 1,3-substitution ... 

The /3-trimethyltin-substituted alkenylborane 36 was prepared in situ by treatment of triethylborane with 1-lithio-l-hexyne to form the lithium 1-alkynyltriethyl-borate 35 followed by trimethyltin chloride (Scheme 20.9) [37]. The trimethyltin chloride-induced migration of an ethyl group from boron to the adjacent acetylenic carbon is stereoselective with the boron and the tin substituents in 36 cis to each... 

The assignment of the monomer solution spectrum was performed by using an NMR spectral database system (SDBS-NMR)54. The signals of the six acetylene carbons from 60.34 to 81.91 ppm in the solution spectrum indicated the monomer structure of a dodec-ahexyne derivative substituted symmetrically by alkyl groups. Since the spectral patterns in Figure 36A are almost the same as those of the monomer, only a small extent of polymerization had occurred during the 30 min after recrystallization. The signal at about... 

Mo and W hexacarbonyls, Mo(CO)6 and W(CO)6, alone do not induce polymerization of acetylenic compounds. However, UV irradiation toward these catalysts in the presence of halogenated compounds can form active species for polymerization of various substituted acetylenes. Carbon tetrachloride, CCI4, when used as the solvent for the polymerization, plays a very important role for the formation of active species, and thus cannot be replaced by toluene that is often used for metal chloride-based catalysts. Although these metal carbonyl-type catalysts are less active compared to the metal halide-based counterparts, they can provide high MW polymers. It is a great advantage that the metal carbonyl catalysts are very stable under air and thus handling is much easier. 

In an internal alkyne such as pent-2-yne, however, the acetylenic carbon atoms are equally substituted, and a mixture of products results. 

In 2001, Gimbert, Greene and co-workers investigated, using DFT calculations, whether it is possible that electronic effects resulting from the substitution of the acetylene could affect the position from which CO is lost and, in turn, determine the regiochemistry of the reaction.90,99 It was possible to calculate the atomic charges of the acetylenic carbons CR and CR in... 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

C NMR is generally applicable to organometallic and organo cluster compounds, but has the advantage of giving a direct probe on the acetylenic carbon atoms in alkyne-substituted clusters. With the development of more powerful NMR instruments, this technique has been used extensively to characterize organo-substituted cluster complexes in solution. For smaller clusters, the combination of 13C NMR with 31P NMR and resonance studies from metallic nuclei, where appropriate, frequently leads to complete structure elucidation. 

It is interesting to note that the rate ratios follow the spectroscopic A h (chemical shift difference of the acetylene hydrogen on the substituted vs. parent phenylacetylene) and the A8Cp (chemical shift difference of the terminal acetylene carbon in the substituted vs. parent phenylacetylene) (12). 

The acetylide ion is a strongly basic and nucleophilic species which can induce nucleophilic substitution at positive carbon centres. Acetylene is readily converted by sodium amide in liquid ammonia to sodium acetylide. In the past alkylations were predominantly carried out in liquid ammonia. The alkylation of alkylacetylenes and arylacetylenes is carried out in similar fashion to that of acetylene. Nucleophilic substitution reactions of the alkali metal acetylides are limited to primary halides which are not branched in the -position. Primary halides branched in the P-position as well as secondary and tertiary halides undergo elimination to olefins by the NaNH2. The rate of reaction with halides is in the order I > Br > Cl, but bromides are generally preferred. In the case of a,o)-chloroiodoalkanes and a,to-bromoiodoalkanes. 

Table 28. Synthesis via nucleophilic substitution at an acetylenic carbon ... 

Many other addition reactions of olefins, dienes, and acetylenes are known, which are catalyzed by metal carbonyls including Ni(CO)4, Fe(CO)5, and Co2(CO)8 and by carbonyl derivatives such as hydrocarbonyls or phosphine-substituted carbonyls. Among these are the hydro-carboxylation, hydroesterification, and hydrocyanation of olefins the synthesis of hydroquiniones from acetylenes, carbon monoxide, and water ... 

Additions to Unsaturated Phosphonium Salts. In a review of nucleophilic substitution at acetylenic carbon, attention has been drawn to the synthetic potential of alkynyl-phosphonium salts, e.g. (140), which have received little study so far. ... 

Fujii, A., Dickstein, J.I., and Miller. S.L. Nucleophihc substitution at an acetylenic carbon. Carbon vs. halogen attack by phosphorus nucleophiles. Tetrahedron Lett.. 11, 3435, 1970. 

Carbon-13 chemical shifts of the acetylenic carbons in 1-substituted hexynes, p.p.m. 

The question of the mechanism(s) of nucleophilic substitution at the acetylenic carbon atom is an important one and has been investigated and discussed for a long time. For leading references, see [4] and A. Fujii, J. I. Dickstein, S. 1. Miller, Tetrahedron Lett. 1970,39, 3435-3438 and references cited therein R. Tanak, M. Rodgers, R. Simonaitis, S. I. Miller, Tetrahedron 1971,27, 2651-2659 A. Commercon, J. F. Normant, J. Villiers, Tetrahedron 1980, 36, 1215-1221. 

The unsubstituted carbon-carbon triple bond, by virtue of its tt-bonds, is electron-rich and hence generally not disposed toward interaction with other electron-rich species. Therefore, even acetylenes bearing a leaving group, such as haloalkynes, do not undergo the direct S -l or Sn-2 type of nucleophilic displacement reactions. In fact the parent alkynyl cation, HC/, is estimated to be some 60 kcal/mol less stable than the methyl cation [1]. As a consequence the vast majority, if not all, nucleophilic substitutions at an acetylenic carbon occur via some type of addition-elimination process [2, 3]. 

Nucleophilic substitution of halogen at an acetylenic carbon in principle represents another general approach to acetylenic ethers (equation 45). 

The oxygen transfer from an ortho nitro group to a carbon-carbon triple bond has been studied by high-resolution mass spectrometry, linked scan techniques and chemi-cal substitution. Oxygen transfers to both acetylenic carbons were detected as parallel fragmentation pathways. ... 

How to control the regioselectivity of the PKR is another topic of fundamental and practical importance. For a long time, it was supposed that the steric effects of the acetylenic substituents are mainly responsible for the observed regioselectivity [140]. In the intermolecular PK reaction with the larger acetylenic substituent, olefin coordinates adjacently to the less bulky acetylenic substituent and that the subsequent C-C bond formation occurs between an olefinic carbon and this closer acetylenic carbon eventually leading to the a-substituted cyclopentenone. Indeed, ethyl propiolate under PK conditions yields mainly an a-carbethoxy-substituted cyclopentenone [143]. However, ethyl butynoate under similar conditions reacts to form only a S-carbethoxy-substituted derivative ([144] for other reactions with 2-aUcynoates, see [145]). To rationalize this, a detailed DFT B3LYP study has been carried out by Robert et al. [140] in 2001 on alkynedicobalt hexacarbonyl complexes (33 in Scheme 23) to probe for the first time whether electronic... 

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