Vinylidenes alkynyl formations

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes.

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Movassaghi et al. [21[ reported the synthesis of substituted pyridine derivatives via ruthenium-catalyzed cycloisomerization of 3-azadienynes. To avoid the isolation of the chemically active alkynyl imines, trimethysilyl alkynyl amines served as initial substrates, as shown in Scheme 6.19. The formation of ruthenium vinylidene intermediates is accompanied by a 1,2-silyl migration according to controlled... 

Terminal alkynes can undergo several types of interaction with ruthenium centers. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. 

Carbonylation of propargyl carbonates bearing an amino group yields lactams. The a-vinylidene /1-lactams 82 are prepared by the carbonylation of 4-benzylamino-2-alkynyl methyl carbonates 81 [20], The best results are obtained by using the cyclic phosphite (4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2,2,2]octane) (83). The lactam formation is carried out in THF or MeCN as solvents at 50 °C under 1-10 atm of CO. 

Transition metal-catalysed reactions involving alkenes proceed mostly by the formation of 7r-complexes ( -complexes). Reactions of alkynes can be understood by the formation of three intermediate complexes, namely 7r-alkyne complexes (t]2-complexes) 1, alkynyl complexes 2, and i/ -vinylidene complexes (carbene complexes) 3. 

The formation of the allyl ketone 421 is explained by the following mechanism. The Ru alkynyl complex 423, formed by oxidative addition, isomerizes to the Ru vinylidene complex 424. Nucleophilic attack of allyl alcohol to the electron-deficient. vp-carbon of 424 generates the allyloxycarbene complex 425, which is converted to... 

For ruthenium catalysts a detailed study of [(PP3)RuH2] proposes a bis(alkynyl) complex as the real catalyst. The catalytic key step involves the protonation of an alkynyl ligand by external PhC=CH, allowing subsequent C-C bond formation between cis vinylidene and alkynyl groups [9]. 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

On the basis of these isotope labeling experiments, the formation of 51 has been rationalized according to Scheme 17. One equivalent of acid protonates the alkynyl group of 50 to afford a vinylidene intermediate, whereas the other equivalent catalyzes the keto-enol conversion. Finally, the addition of the enol to the carbon-carbon double bond of the vinylidene intermediate yields the unsaturated cyclic carbene ligand. 

It is important to note that some reactions leading formally to cr-vinyl- and a-a kynyl complexes of transition metals can be produced via different mechanisms. For example, the reaction of the ruthenium complex IV-9 with terminal alkynes gives rise to the formation of vinylidene-metal complexes IV-10, which can be converted to the alkynyl-ruthenium derivatives IV-11 by the action of triethylamine [23]. 

The first use of vinylidene-metal species in C-C bond formation with a ruthenium catalyst was reported by Trost in 1990. Since then, the use of vinylidene-Rh species in organic synthesis has been extended to the formation of heterocycles through cycloisomerization of alkynols and alkynyl anilines. For example, the cycloisomerization of alkynol 455, leading to the formation of pyran 456 was used for the synthesis of amino sugar 457 (Scheme 2-51). ... 

Electrophilic attack on the 3-carbon of alkynyl ligands (Equation 12.50) is common and is a route to vinylidene complexes introduced in Chapter 3. Examples of protonation and electrophilic alkylation of an anionic acetylide complex at the 3-carbon are shown in Equation 12.51. Attack of two protons on an anionic carbyne complex generates a new carbyne complex, as shown in Equation 12.52. ° This reaction, presumably, occurs by initial formation of a vinylidene complex. 

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. 

Disubstituted neutral vinylidenes 220 have been prepared by reaction of the cr-alkynyl complex [Ru(G=GPh)Gp(PPh3) P(OMe)3 ] 219 with organic halides XGH2R via electrophilic addition of the methylenic GH2R unit to the G j-atom of the phenylacetylide chain and subsequent Arbuzov-like dealkylation of the phosphite ligand (Scheme 22). The formation of minor amounts of vinylidenes 221 in the reactions of 219 with isothiocyanates has also been reported. ... 

---