Carbyne complexes preparation

Transition metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds has proved generally applicable. In addition to making the initial characterization (723), the Fischer group has made the largest contribution to carbyne complex chemistry, with some 200 mononuclear complexes of Group 6 and 7 metals having been prepared. 

It was noted in Section V,B that the chlorophenyl carbene complex 85 can be prepared by chlorine addition to carbyne complex 80. Treatment of 85 with one equivalent of PhLi does not afford 80, suggesting that the reaction sequence is reduction/substitution rather than substitution/reduc-tion. The recent report (127) of a nucleophilic displacement reaction of the molybdenum chlorocarbyne complex 87 with PhLi to generate phenylcar-byne complex 88 suggests that the intermediacy of the chlorocarbyne complex 86 in the above mechanism is not unreasonable. 

The octahedral Ru(II) and Os(II) carbyne complexes are isoelectronic with the Group 6 compounds R—0=M(CO)4X prepared by Fischer and co-workers. Many of the Fischer compounds, particularly the cationic complexes, are susceptible to nucleophilic attack at the carbyne carbon (see Section II,B,2), and similar reactivity might be anticipated for the Group 8 carbynes. 

Formation of the thioacyl complex 109 may be contrasted with the preparation of analogous chalcoacyl compounds by electrophilic addition to the zero-valent carbyne complex 79 (see above). 

In the reaction of the cationic carbyne complex [(OC)5WCNEt2] BF4 with potassium methylphenylphosphide, the two phosphinocarbene complexes 93 and 94 have been isolated as by-products in 4 and 3.5% yield, respectively.85 Here, the phosphinocarbenes simply act as 2-electron donors via the carbenic center. It is also interesting to note that this strategy can be used to prepare the analogous arsinocarbene complexes.8513... 

Table 2.10. Heteroatom-substituted carbene complexes prepared from carbyne complexes.

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

Experimental Procedure 3.1.4. Preparation of a Molybdenum Vinylidene Complex from a Carbyne Complex Tetrabutylanunoniuih Cyano(ethoxycarbonyl) vinylidene (dicarbonyl) hydro-tris(3,5-dimethyl-1 -pyrazblyl)borato molybdenum [526] [37] pp 151 and 188... 

Closely related to the a-addition of nucleophiles is the P-deprotonation of electrophilic carbyne complexes. In many of the examples reported [143,530,531] the resulting vinylidene complexes could not be isolated but were generated in situ and either oxidized to yield stable carbene complexes [532] or used as intermediates for the preparation of other carbyne complexes [527]. Cationic carbyne complexes can be rather strong acids and undergo quick deprotonation to vinylidene complexes with weak bases [143]. An interesting example of the use of anionic vinylidene complexes as synthetic intermediates is sketched in Figure 3.24. 

Fig. 3.30. Preparation of carbene complexes by sequential [2 + 2] cycloadditions and [2 + 2] cycloreversions of carbene and carbyne complexes to alkenes and alkynes [596-598].

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

In fact, the first isolation ofthe vinylidene pentacarbonyltungsten complexes was reported by Mayr et al. in 1984 [6]. The vinylidene complexes 16 were obtained by the alkylation of anionic pentacarbonyltungsten t-butylacetylide complex 15, obtained by the reaction of [Et4N ] [W(CO)5Cl ] with lithium acetylide, with FS03Me or [Et30 ] [Bp4 ]. Further protonation with CF3SO3H in the presence of Me4N P afforded a unique method for the preparation of carbyne complexes 17 (Scheme 5.4). 

The first of these are called carbene complexes and the latter are referred to as carbyne complexes.61 The first carbene complex was reported in 1964 by Fischer and Maasbol62 and was prepared by reaction of hexacarbonyltungsten with methyl or phenyl lithium to generate an acyl anion which was then alkylated with diazomethane. 

Extension to trimetallic complexes has led to the preparation of similar bridging carbene and carbyne complexes with Pt3,421 Pt2W422 and PtWFe423 metal frameworks. 

Thus, the way appears to be open to make carbyne complexes for use in the syntheses of organic compounds. Since to our knowledge there is no specific and selective carbyne source available for preparative purposes, presumably there arises here a wide field of interesting possibilities of application, especially because of the mild conditions required to transfer the carbyne ligand. 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

In practice this approach to the synthesis of compounds with dimetallacyclo-propane or -propene ring systems has been very successful. Complexes (1) - (12) are representative of those which have been readily prepared by reacting various low-valent metal compounds either with carbene-metal species, or with the carbyne complexes [w=CR( CO) 2(11-05 )] or [w=CR(Br)(CO) ]... 

It was from such carbene complexes that, again, E. O. Fischer (1973) prepared the first carbyne complexes, as illustrated by the following reactions 132... 

Fischer first prepared carbyne complexes (1973) by electrophilic abstraction of methoxide ion from a methoxy methyl carbene. 

Similar reactions are used with Cp(OC)2MnC(OMe)Ph and M(cod)2 (M = Pt, Pd, Ni) to give heterobimetallic complexes with bridging carbenes in yields of 25-63% . Bridging carbyne complexes can be prepared, as well ... 

Following his pioneering work in the preparation of the first stable metal-carbene complexes, Fischer79 reported the first syntheses of complexes containing an M=C bond nine years after his metal-carbene articles appeared. Complexes 64, called metal-carbyne complexes or alkylidynes (if R = alkyl or aryl), were discovered serendipitously in the course of attempts to develop new methods for preparing carbene complexes. 

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