Alkynes systems

Zirconocene dichloride has been reported to serve as catalyst for the addition of dialkyl phosphorochloridates across the triple bond of terminal alkynes.159 DIBAL is used with the terminal alkyne system to generate the dialkyl 1-alkenylphosphonate products. 

For addition reactions with unsaturated organic compounds, all the reaction products contained the —SiF2SiF2— moiety, which led to the conclusion that the most reactive species in these co-condensation reactions must be the -SiF2SiF2 diradical (65, 66). The reaction mechanism for alkyne system is illustrated in Scheme 1. 

We come now to two regioantagonistic alkyne systems in which the changes in product composition are often bewildering, if not inexplicable. In equation (52)... 

Phospha-alkyne systems have continued to attract interest. The phosphino-phospha-alkyne (324) arises as a transient intermediate in the thermolysis of a phosphino-phosphiranyl-diazo system.A convenient synthetic route to substituted phosphavinyl Grignard reagents (325) is provided by the regio- and stereo-selective addition of Grignard reagents to the phospha-alkyne... 

It is useful to explore this analogy between known alkyne systems and potential benzyne ones. For example, alkynes react with ruthenium carbonyl to form the Ru4( --alkyne)(CO)i2, which form an octahedral RU4C1 arrangement with the metal atoms in a butterfly geometry. Since the synthesis cannot be adapted to arynes, another method is needed. Oxidative addition of aryl iodides to Ru3(CO)i2 gives the aryl complexes described in Sec. 1.14.4.5. Flowever, we have... 

Some alkyne systems, with comparatively labile bonds, undergo rearrangement processes. Thus photorearrangement of the silylacetylene 152 1,3-silyl migration affording the allene 153. ... 

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... 

Conjugated alkyne systems present a unique situation where the cost of rotation about the a-bond disappears as a result of a second Jt-system and unique cylindrical synunetry of the alkyne moiety. Conjugative stabilization is not a directly measurable characteristic and it has been suggested that conjugative stabilization in diynes is zero. We will discuss this paradoxical suggestion in the following section. 

Fig. 11.9 Selected optimized geometries of (a) the Au-S-alkynes, (b) Ru-S-alkynes, and (c) CNT-S-alkynes systems. Size scales are arbitrary. Lines divide the scattering region from the eiectiode region (Reproduced from Ref. [28] with kind permission of The American Chemical Society)...

Sol 2. (c) The reactant undergoes intramolecular ene reaction under thermal conditions. In this reaction, allyl hydrogen is transferred to alkyne system instead of the transfer of propargylic hydrogen to alkene part, which may give allene derivative. 

The cuprate delivers stannyl anion into the alkyne system, and the vinylic carbanion so generated reacts with the acid to give an ester product. 

The authors also applied this reaction to terminal alkyne systems, which afforded similar yields to that shown in (Scheme 71) however, the diastereoselec-tivity was dramatically reduced affording only 8 1 dr for R2 = H and Me. 

AUylic alcohols can also be accessed from the reductive coupling of allenes and aldehydes. As with the similar aldehyde-alkyne system, trialkylsilanes can be... 

Gagosz extensively investigated on the gold-catalyzed cyclizations of ether-alkyne systems 145 and 147 bearing both terminal and internal alkynes in 2010. A wide range of structurally important spiro or fused dihydrofurans 146 and dihy-dropyrans 148 were dexterously constructed via a 1,5-hydride shift/cyclization sequence using alkyne as hydride acceptors (Scheme 12.64) [68]. This hydroalkylation process, which could be applied to the terminal as well as ester-substituted alkynes, allows the efficient conversion of secondary or tertiary C(sp )—H bonds into new C-C bonds under practical conditions. The stereoselectivity of the cycloisomerization process toward the formation of a new five- or six-membered cycle appears to be dependent on steric factors and alkyne substitution. 

SCHEME 10.9 Sequential reactions of cobalt alkyne system. 

In common with the hexacarbonyl dicobalt alkyne system discussed, the first cyclization explored was the reaction with aldehydes, in an attempt to produce tetrahydrofuran species. Optimization studies showed that strong Lewis acids only provide decomposition of the metal-diene, and the more mild reagent zinc bromide provides the best yields. The different Lewis acid also required a different solvent for maximum... 

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