Carbyne complexes formation

We describe a further reaction channel involving CO-activation by O-attack of the silane (C) and subsequent carbyne-complex formation by electron transfer M—>C and dimerization of the formed 17e intermediate to a stable /i-biscarbyne complex 8 (Chart 1). 

This reaction is significant as it is evidence that PhLi can behave as a reducing agent in the manner proposed in the mechanism for carbyne complex formation above. 

Treatment of [Au(Cl)[C(OEt)NMe2] with Lewis acids does not lead to carbyne complex formation. Reactions with BBr3 and BI3 yield only the substitution products [Au(X) C(OEt)NMe2(]419 420. 

The related dihydride-dichloro complex OsH2Cl2(P Pr3)2 is also an active catalyst for the hydrogenation of olefins, diolefms, and a-(3-unsaturated ketones,14 but attempts to hydrogenate phenylacetylene show a rapid deactivation of the catalyst due to formation of a hydride-carbyne complex.54... 

The variation of the substituent pattern of the introduced silane provides further insight into the reaction mechanism of the CO activation process of scheme 2 (Table 1) The yield of ju-carbyne-complex (O-attack of the silane) compared to silyl hydride formation (Mn-attack of the silane) is a function of the Lewis-acidity of the silane. However, even with the strongly acidic HSiCl3 as reagent, the product ratio 12/13 is still 1 9. 

More recently, Schrock has reported the formation of coordinatively unsaturated Ta and W carbyne complexes (124). Like unsaturated carbene complexes, these carbyne compounds are now established as being active intermediates in a number of catalytic reactions. The discovery of acetylene metathesis reactions catalyzed by carbyne complexes (3), for example, has generated considerable interest in this class of compound. 

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). 

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... 

If a given vinylidene complex is not sufficiently electrophilic, protonation at Cp can promote nucleophilic addition at C by intermediate formation of an electrophilic carbyne complex [89] (Figure 2.9, Section 2.1.8). 

The reaction is thought to proceed with the dissociation of CT followed by release of the extra charge of the mthenium complex by dissociating a proton from the alkyhdene hgand. Such an exchange in itself does not lead to the decomposition of the alkyhdene complex. Nevertheless, both the formation of the charged species, both the intermediate existence of the carbyne complex (Scheme 9.5) may open new ways to the deterioration of the ROMP catalysts. 

A recent computational investigation of the [2+2] cycloaddition between a hypothetical molybdenum carbyne complex, CbMo CI I, and ethyne suggests that the formation of the molybdacyclobutadiene, CI3Mo(7/2-C3l l (), is allowed by orbital symmetry. In contrast, the direct formation of the isomeric molybdatetrahedrane (i.e., the if -cyclopropene complex), ChMoft -CsID, is symmetry forbidden <19930M1289> such complexes presumably arise by isomerization subsequent to initial molybdacyclobutadiene formation (Section 2.12.5). 

Here the conditions for the formation of a carbyne complex apparently are not available because of the fixation of the OH group by formation of a bridge to the cis-situated bromine ligand. 

Triphenylstannyl group migration also occurs on treatment of ethoxy carbene complexes of the type Cp(CO)2 PhC(OEt) MSnPh3 (M = Mo or W) with LDA/HMPA, with formation of carbyne complexes [( 5-Ph3SnC5H4)(CO)2CPh)], after treatment with PhC(0)Cl405. 

Fischer s carbyne complexes are thermally labile (facile loss of CO) and decompose bimolecularly with formation of alkynes. This carbonyl lability may be traced to the strong Tt-acidity of the carbyne ligand, which compromises retrodonation to the carbonyl ligands. On treatment with donor ligands, however, thermally stable substituted derivatives may be obtained (Figure 5.36). 

An example of this class of reactivity has already been met in the formation of halocarbenes via the reaction of cationic carbyne complexes... 

VIII. 1,2-Elimination with Formation of Alkynyl Carbyne Complexes. . . 229... 

Following the Fischer procedure, alkynyl carbyne complexes trans-X(CO)4M=C—CPh 189 have been obtained in 30-60% yields by reaction of (l-alkynyl)carbene complexes la,b (M = Cr, W) with BX3 (X = Cl, Br, I). To date, (l-alkynyl)carbyne compounds have found application as catalysts as well as stochiometric reagents in organic synthesis.205c-206 Among the transformations reported thus far is the formation of a 4-amino-l-metalla-l-yne-3-ene (= enamino carbyne complex) 190 by addition of HNMe2 to compound 189 (Scheme 79).207... 

ELIMINATION WITH FORMATION OF ALKYNYL CARBYNE COMPLEXES... 

Carbyne complexes containing the Mn=CR moiety undergo an interesting variety of reactions. " Scheme 13 provides a sampler of these reactions. The electrophilic nature of the carbyne in the cationic Cp(CO)2Mn=CR" has been exploited in cycloadditions, metatheses, and ketene formation methodologies. Extended chains containing unsaturated carbon networks that are truncated by metal... 

The reductive coupling (see Reductive Coupling) of CO ligands in M(CO)2(dmpe)2Cl (M = V, Nb, or Ta) to form M(R3SiOC=COSiR3)(dmpe)2X occurs via sequential formation of Na[M(CO)2(dmpe)2] and M(=COSiR3)(CO)(dmpe)2 complexes. Reactions of siloxy-carbynes (see Carbyne Complexes) M(=COSiR3)(CO)-... 

Silylation of Na[V(CO)2(Me2PCH2CH2PMe2)2] affords the vanadium carbyne complex see Carbyne Complexes), V(COSiPh3)(CO)(Me2PCH2CH2PMe2)2, which is supposed to be a precursor complex for the formation of a ] -acetylene... 

Similar intermediate formation of cationic carbene complexes, involving phosphine-induced a-hydrogen abstraction to afford carbyne complexes, is also possible. However, evidence of such intermediates is lacking. 

Hillhouse and co-workers discovered a new route to carbyne complexes which is based on the reverse of carbyne-carbonyl coupling (J72). Reaction of complex 200 with carbon suboxide affords the ketenyl complex 201 [Eq. (159)]. Formation of the ketenyl ligand in 201 was postulated to arise from attack of phosphine on an intermediate ketenylidene complex. Warming of complex 201 results in cleavage of the ketenyl ligand and formation of the phosphonium carbyne complex 202. This mechanism was put to work in a more direct fashion by reaction of complex 200 with PhjPCCO [Eq.(160)]. 

---