Alkyne structure

Because of the regioselectivity of alkyne hydration acetylene is the only alkyne structurally capable of yielding an aldehyde under these conditions... 

Intermoleculai cioss-coupling of (45) witfi a syntfietic alkyne structure like (46) leads to the skeleton (161,162). 

CH2 and CH3 groups. It should be noted that no bands are observed above 3000 cm indicating the absence of an aromatic, alkene or alkyne structure. 

The distinguishing feature of the alkyne structure is the carbon-carbon triple bond. The general formula for the alkynes is CnH2n 2, which is the... 

When sodium hydroxide was used as nucleophile, salt 3c was rapidly converted to alkyne 63. This alkyne now features only a single stereocenter, and this is evident from its spectra. Its structure was initially assigned incorrectly, but the alkyne structure (63) has been rigorously established by X-ray crystallography. It is curious that azide and hydroxide should attack on the periphery of the TTF ring system, while the bulkier malonate anion attacks internal carbons. It is also intriguing that none of the observed modes of attack give rise to the sulfur ylid 64, which should be more stable than 61. 

Yields of the substitution product are also suppressed when the available proton is a part of the alkyne structure. The alkynes used in equation (278) (upper branch)... 

One final structure in this series is the bis-TMEDA-solvated bis(phenylethynyl)magnesium species characterized as a monomer (111). ° Note the octahedral geometry of the central magnesium with two axial ethynyl ligands. This series of alkynic structures, (106)-(lil). serves to underscore the unpredictability of carbanion crystal structures. The alkynic carbanions have coordination numbers of one, two or three in these complexes. 

Bis-Gp titanacyclopent-3-yne complexes have been synthesized and structurally characterized (Scheme 545). Short Ti-G bonds emphasize the ring strain of the five-membered cyclic alkyne structure.1436... 

Many recent investigations in this field of chemistry have been directed toward the synthesis and study of diyne titanium complexes. The TiCp2 precursor compound Cp2Ti(Me3SiC2SiMe3) reacts with 1,4-disubstituted 1,3-butadiynes to give five-membered titanacyclocumulenes, the structures and stability of which depend strongly on the diyne substituents. Mononuclear or binuclear homobimetallic derivatives can be formed. For the mononuclear complexes, an equilibrium between the cyclocumulene and an alkyne structure is possible (Scheme 561). Binuclear complexes may exhibit different structural types (Scheme 562). 

The high regioselectivity in the formation of a-allenic alcohols from boron reagents at low temperature is markedly different from that seen with the titanium reagent derived from 1-trimethylsilyl-l-bu-tyne, in which exclusive formation of 3-alkynic alcohols is observed. This result was explained by a rapid exchange between the allenic and alkynic structures, as shown in equation (2). The alkynic structure is thermodynamically less stable and kinetically more reactive than that of the allenic form. At lower temperature, the rate of equilibrum becomes faster than the subsequent reaction with aldehydes and thus the alkynic species becomes the major reaction form. 

Transition metal-alkyne 7t-bonding interactions are similar to their alkene counterparts in that both can be considered as donation from the alkyne Jt-to-metal d orbitals, with concomitant back-donation from the metal d-to-alkyne tt orbitals leading to bent alkyne structures (Fig. 16) (304, 305) Commonly, the donor interactions are primarily ct type while the back-donation occurs mainly... 

Electrophilic additions of Brs" to alkenes and alkynes have been carried out [48-50] both in [BMIM][Br] and in other ionic liquids bearing non-nucleophilic anions (Scheme 5.1-16). The reaction is always completely anti-stereospecific, independent of alkene or alkyne structure. It follows a second-order rate law, suggesting a concerted mechanism of the type reported for Brs" addition in aprotic molecular solvents, involving a product- and rate-determining nucleophilic attack by bromide on the alkene or alkyne-Br2 jt-complex initially formed. 

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