Cycloalkyne complexes

Alkyne hydrido cluster complexes of the type [Co3(p.-H)2(/i-alkyne)Cp3] can be prepared by 1,2 double C—H activation of alkenes.132 133 For example, the p.-cycloalkyne complexes 36 can be prepared either from the respective cycloalkenes and [Co(C2H4)2Cp] or by using the reductive cleavage of cobaltocene with potassium as a source of CpCo fragments. (See Formula 36.)... 

The complexes of benzyne (an instable species in the free state) are remarkable cycloalkyne complexes whose geometry fits to the metallacyclopropenic form (see Chap. 

In a more recent and improved approach to cyclopropa-radicicol (228) [ 110], also outlined in Scheme 48, the synthesis was achieved via ynolide 231 which was transformed to the stable cobalt complex 232. RCM of 232 mediated by catalyst C led to cyclization product 233 as a 2 1 mixture of isomers in 57% yield. Oxidative removal of cobalt from this mixture followed by cycloaddition of the resulting cycloalkyne 234 with the cyclic diene 235 led to the benzofused macrolactone 236, which was converted to cyclopropa-radicicol (228). 

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. ... 

Medium-size cycloalkynes.1 In the presence of BF, etherate, a cobalt com-plexed propargylic ether can undergo an intramolecular alkylation with an allylic silane to provide six-, seven-, and eight-membered complexed cycloalkynes. This reaction is an extension of propargylation of allylsilanes to provide 1,5-enynes (10, 129-130). 

Oxidative removal of the Co2(CO)6 ligand of 2 does not result in a cycloalkyne, but 2 can be used as such for annelation to complex products. 

Although the structure of the complex arising from I52/CH2CI2 is not clear, this catalyst is excellent in terms of ease of preparation. The catalyst is very active for formation of cycloalkynes with ring sizes different from those of diynes (Table 6.5). In contrast to tungsten alkylidyne complex 150, catalyst 152/ CH2CI2 is sensitive toward an acidic proton such as amide proton and exhibited remarkable tolerance towards many polar functional groups (Table 6.5). 

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... 

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. 

Cycloalkenes, into if-allyl palladium complexes, 8, 363 Cycloalkenyl rings, metal complex conformational interconversions, 1, 414 Cycloalkynes, in nickel complexes, 8, 147 (Cyclobutadiene)cyclopentadienyl complexes, with cobalt, polymercuration, 2, 435 Cyclobutadienes... 

Nickel-cycloalkynes, synthesis and reactivity, 8, 147 Nickel dimethyl complexes, for Ni(0) complex generation, 8, 83... 

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. 

Since the Schrock carbyne is active in the transalkylidynation reaction, the possibility of ring-opening polymerisation of cycloalkynes by acetylene transalkylidynation catalysts (metal alkylidyne complexes) has been evaluated [151]. Unfortunately, cyclooctyne is one of the few relatively stable strained cyclic acetylenes, but it is not strained enough to react selectively with the catalyst and yield a polymer according to the scheme... 

To date, no mononuclear transition metal complexes of three- or four-membered cycloalkynes have been reported. A number of polynuclear complexes of cyclobutyne have been prepared.90 However, since this article is limited to complexes in which the aryne, alkyne, or cumulene is coordinated to a single metal atom, a review of this topic is not included. 

The bis(triphenylphosphine)platinum(0) complex of cyclopentyne (240) [Eq. (32)] is the only complex of a five-membered cycloalkyne that is discussed in an earlier review.2 The mechanism of its preparation is significant because 239 was isolated and shown to react with Na/Hg to give 240, thus suggesting that 7r-complexes may play a role in the formation of dw complexes of larger cycloalkynes2 36 82 and, possibly, cyclic allenes (Section IV)82 where trapping of the free intermediate had often been presumed. 

To facilitate comparison, selected examples are taken from Bennett and Schwemlein s review2 and the same format is used. The reader is referred to this review for an excellent discussion of the IR and 13C NMR spectra of metal complexes of strained cycloalkynes. 

Synthesis, reactions, and physical properties of stable mononuclear platinum and zirconium complexes of cyclohexyne reported prior to the late 1980s have been comprehensively covered in earlier reviews.2-8 More recently, reaction of the zirconocene complex of cyclohexyne with trimethyla-luminum and trimethylgallium has been reported to give 247 and 248, respectively [Eq. (36)].57-58 These products are novel because four atoms (carbons Cl and C3, the transition metal, and the main group metal) are covalently bonded to an sp2 hybridized carbon (C2) in a planar tetracoordi-nate fashion. Synthesis of this type of complex, which Erker describes as anti-van t Hoff/LeBel, does not require the strained cycloalkyne ring zirconocene complexes of acyclic alkynes react similarly.57-58... 

Stable platinum, zirconium, and cobalt complexes of seven-membered cycloalkynes have been discussed in earlier reviews.2-8... 

The first synthesis of an isolable cyclobutadiene (71) kinetically stabilized by bulky substituents was achieved starting from the angle strained cycloalkyne (31) via the PdCl2 complex (69)9 198,199). Here again the high reactivities of (10), (31) and (79)... 

The complex [(77 -cyclohexyne)Pt(PPh3)2] inserts electrophilic or strained alkenes (CH2=CHR R = COMe, CHO, COOMe, CN) into the platinum-cycloalkyne bond to yield platinacyclopentenes 310 <1997CB1029, 2000JOM(600)37>. 

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