Carbyne complexes, reactions with alkyne

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

A series of general headings can be applied to the synthetic routes used for generating vinylidene complexes (i) reactions of alkynes with labile and/or coordinatively unsaturated species, (ii) reactions with alkynes in the presence of a halide abstracting agent, (iii) formation from alkynyl complexes, and (iv) formation from carbyne complexes. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Fig. 3.44. Reactions of carbyne and carbene complexes with alkynes.

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... 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

A general approach to metallacyclobutadienes (90) is the reaction of carbyne complexes with alkynes. These four-membered rings are intermediates in the metathesis of acetylenes. Compounds 90 (M = W) offer a possi-... 

We first encountered alkyne metathesis in Chapter 10 in connection with reactions of metal-carbyne complexes. The mechanism of alkyne metathesis, first proposed by Katz,64 is analogous to that for alkenes, and it is shown in Scheme 11.9. 

Phosphines, as nucleophiles, add to many unsaturated substrates giving metallated ylides. Scheme 17 collects some representative examples of the addition of phosphines to carbyne complexes, giving (57) [132], to allenylidenes (58) [133], rr-alkenyls (59) [134], or CT-alkynyls (60) [135]. Moreover, reaction of phosphines with jt-aUcenes [136] and n-alkynes (61)-(64) [137-140] have also been reported. It is not possible to explain in depth each reaction, but the variety of resulting products provides an adequate perspective about the synthetic possibilities of this type of reactions. 

The molecular orbital analysis of the nucleophilic addition at the carbyne C atom infers the orbital control of the reaction since the C atom undergoing attack is the most negative one in the carbyne complex. [2 + 2] cycloadditions of [ReCp(CO)2(CPh)]+ with MeN=C(Ph)H, f-BuN=0, and ArN=NAr (Ar = aryl) but not with alkenes or alkynes, give the metallacycles. These reactions are driven by the nucleophilic attack of the lone pairs of the N atom at the electrophilic carbyne carbon atom. These metallacycles are... 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

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