Alkynyl protons

The other two compounds display no intramolecular Au- Au interactions, although the molecular packing of [Au(C6F5)(PPh2C=CH)] involves an Au- -H contact from the acidic alkynyl proton (Au- -H distance of 3.07 A, C-H-Au angle of 143°) that links the molecules by translation parallel to the a-axis.33... 

The acidity of alkynyl protons means that alkynyllithium cyclisations are rather trivial to devise - the starting alkyne can be deprotonated with LDA or LiHMDS and cyclised even onto aldehydes. An example from Kende s laboratory demonstrates the use of an alkynyllithium cyclisation in the synthesis of an enediyne 125 64... 

The parent dialkyl ethynylphosphonate was first prepared in 1960 by the addition of stoichiometric ethynylmagnesium bromide to the appropriate dialkyl chlorophosphates in THF. ° The product was isolated in low yield (12-25%), presumably because of side reactions involving the relatively acidic alkynyl proton. The yields can be increased slightly (25-35%) by the use of (toxic) dialkyl fluorophosphates. Despite advances in developing methodologies for the elaboration of dialkyl... 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

In general, the reaction can be performed only with organometallics of active metals such as lithium, sodium, and potassium, but Grignard reagents abstract protons from a sufficiently acidic C—H bond, as in R—C=C—H —> R—C=C—MgX. This method is best for the preparation of alkynyl Grignard reagents. ... 

Clearly, a large body of diverse evidence indicates that the acid-catalyzed hydration of alkynyl ethers and thioethers proceeds via a rate-determining protonation through a vinyl cation. However, these vinyl cations are unique in that they have a resonance form where the positive charge resides on the... 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

The inverse isomerization mode, providing allenyl thioethers from alkynyl thioethers, is also known [309, 310] for example, 124 is first deprotonated with n-butyllithium and then protonated with ammonium chloride [311] (Scheme 1.55). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

In the hydroboration of terminal alkenes, carrying a ketone or aldehyde group, with a variety of borane reagents, dicyclohexylborane has been identified as the most efficient reagent. Analogous hydroboration of alkynyl ketones and alkynyl aldehydes with dicyclohexylborane yields the corresponding olefinic carbonyl compounds after protonation, or dicarbonyl compounds after oxidation. ... 

A hitherto unknown type of rearrangement of l-(l-alkynyl)cyclopropanols (321) to cyclopent-2-en-l-ones (322) mediated by octacarbonyldicobalt" and hexacarbonyldicobalt" complexes has been described. A possible pathway for the transformation is outlined in Scheme 101. A -proton transfer accompanied by a metal-mediated Stevens rearrangement, which converts a coordinated dimethylsulfane... 

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