Alkynes cycloalkynes

Draw and name alkynes, cycloalkynes, and their derivatives. 

In fact, a wide variety of tt complexes Cp2Zr(X=Y), including complexes of alkynes, cycloalkynes, arynes, imines, and thioaldehydes, can be generated by /3-hydride abstraction from complexes of the type Cp2Zr(Me)X-Y-H. These complexes can in turn be made by hydrozirconation of X=Y with Cp2Zr(H)Cl followed by addition of MeLi, or by addition of X-YH to Cp2Zr(Me)Cl. 

The ruthenium-catalyzed hydrosilylation/protodesilylation protocol is a useful method for stereoselective internal alkyne (cycloalkyne) reduction to (E)-alkenes [(iJ)-cycloalkenes] and is a complement to the cis selectivity observed in the Lindlar reduction (eqs. (27) and (28)) (184,185). The utility of this reaction sequence is attributed to the fact that many of the knovra methods for transforming ahquies to ( )-alkenes either have poor selectivity or are incompatible with common functional groups. 

This type of alkyne dimerization is also catalyzed by certain nickel complexes, as well as other catalysts and has been carried out internally to convert diynes to large-ring cycloalkynes with an exocyclic double bond. ... 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

In situ generation of strained alkenes or alkynes requires photoelimination of a leaving group toward which the targeted product is unreactive. Thus, cycloalkynes may be generated by elimination of CO from cyclopropenones, but these must in turn be generated in situ (e.g., from diazoketones). The generation of ace-naphthyne (Scheme 17.10) may serve as an illustration of this procedure that did, however, not work for the parent cyclopentyne, a compound that still awaits spectroscopic characterization. 

Using this catalyst system for ring-closing alkyne metathesis, Fiirstner successfully synthesized ambrettolide and yuzu lactone from alcohol 125. " " Treatment of diyne 126a with Mo(CO)6 (5 mol%) and /i-chlorophenol (1 equiv.) in chlorobenzene at 140 °C leads to cycloalkyne 127a in 69% yield. Subsequent Lindlar reduction proceeds smoothly in a stereoselective manner to afford ambrettolide. Similarly, 126b affords cycloalkyne 127b in 62% yield. From this compound, yuzu lactone has been synthesized (Scheme 45). " " ... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

Although the real species of this solution for alkyne metathesis is not clear, this complex is an excellent tool from a preparative point of view. It is very active for the formation of cycloalkynes of different ring sizes from diynes. In contrast to tungsten alkylidyne complex 1, catalyst I4O/CH2CI2 is sensitive toward acidic protons such as amide proton and exhibited remarkable tolerance toward many polar functional groups (Table 6). ... 

Mechanistic studies revealed that alkyne metathesis and ring-opening metathesis polymerization of cycloalkynes proceed via metal carbyne complexes,217 218 which is also supported by theoretical studies.219 The polymerization of PhC=CMe with NbCIs or TaCIs yields a polymer that degrades to oligomers as a result of secondary metathesis reaction. A stable polymer, however, may be synthesized with TaCIs and Ph4Sn as a cocatalyst.220... 

Table 4.13. 13C Chemical Shifts of Alkynes [246, 247], Polyalkynes [248], Alkenynes [249] and Cycloalkynes [250] (<5C in ppm).

Cycloalkynes display a deshielding with increasingly strained rings, as reported for a series of cyclooctynes [250] an outstanding alkyne carbon shift is found in cyclooctadie-nyne. 

This reaction is useful in conjunction with E2 elimination to prepare alkenyl halides, allenes, and alkynes. Cycloalkenyl halides are easily prepared, but because of angle strain the cycloalkynes and cycloallenes with fewer than eight atoms in the ring cannot be isolated (see Section 12-7) ... 

Many complexes of alkenes, cycloalkenes, alkynes, and cycloalkynes with transition metals are now known. Some examples are ... 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

A clean and reagent-free generation of highly strained cycloalkynes from bi-3//-diazirin-3-yls has been described.44 A new synthesis of alkynes has been achieved by... 

The carbonyl Co2(CO)6 forms stable 7i-complexes of alkynes 0/2 complexes). Four effects on alkyne reactivity are expected from this coordination (i) protection of the triple bond (ii) stabilization of the carbonium ion on the a-carbon (or propargylic cations (iii) syntheses of common and medium-size cycloalkynes and (iv) steric effects. 

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