Vinylidene carbon nucleophiles

In addition to alcohols, some other nucleophiles such as amines and carbon nucleophiles can be used to trap the acylpalladium intermediates. The o-viny-lidene-/j-lactam 30 is prepared by the carbonylation of the 4-benzylamino-2-alkynyl methyl carbonate derivative 29[16]. The reaction proceeds using TMPP, a cyclic phosphite, as a ligand. When the amino group is protected as the p-toluenesulfonamide, the reaction proceeds in the presence of potassium carbonate, and the f>-alkynyl-/J-lactam 31 is obtained by the isomerization of the allenyl (vinylidene) group to the less strained alkyne. 

Electrophilic vinylidene complexes, which can be easily generated by a number of different methods [128], can react with non-carbon nucleophiles to yield carbene complexes (Figure 2.9 for reactions with carbon nucleophiles, see Section 3.1). 

Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.11, Table 2.7, Entries 11, 12) [134,144]. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Carbon nucleophiles can also add to in situ generated vinylidene species. Thus, ruthenium-catalyzed cyclizations of dienylalkynes produced arene derivatives [55] (Eq. 41). 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

The palladium-catalyzed carbonylation of 4-amino-2-alkynyl carbonates 40 or 5-hydroxy-2-alkynyl carbonates 41 afforded a-vinylidene-/i-lactams 42 [60] or a-vinyl-idene-y-lactones 43 [61] in good yields (Scheme 3.25). The initially formed (allenyl-carbonyl)palladium(II) intermediates were trapped by the intramolecular amino- or hydroxy-nucleophiles to give 42 or 43. 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

This reaction is quite different from the other P-H addition reactions in that it involves external nucleophilic attack of HPPh2 on the vinylidene ligand as shown in Scheme 13. The ZIE ratio depends on the structures of the substrate and the catalyst. Ru-Cp" (Cp =77 -CsMes) species selectively forms the Z isomer while Ru-Cp (Cp r -CsHs) favors the E isomer. Since the key intermediate is the vinylidene species that has an electrophilic carbon, the reaction is applicable to other alkynes that are vinylidene precursors. Thus, phenylacetylene also reacts similarly to give Ph2PCH=CHPh ZIE=93I7), while internal alkynes are totally unreactive. 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

Scheme 4.2, we can see that for metal vinylidenes of the Fisher-type nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon [45-52]. 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

An important contribution that developed into the catalytic use of the vinylidene complexes for the construction of carbon frameworks was reported by two research groups independently for the preparation of Fischer-type carbene complexes by the reaction of terminal alkynes with pentacarbonylchromium or tungsten species in the presence of oxygen nucleophiles. 

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

Scheme 6.25 shows a plausible mechanism involving ruthenium vinylidene and ruthenium-stabilized ketene intermediates. The ketene intermediate was verified through efficient trapping of this spedes with isobutanol to produce esters [23]. Nucleophilic attack by epoxide oxygen at the Ca-carbon of ruthenium vinylidene produces the seven-membered ether spedes 64, which ultimately forms ruthenium... 

On the basis of these findings, a pathway for this cydoaddition is proposed in Scheme 7.24. The first step is the nucleophilic attack of the carbon atom in the 2-position of 1,3-cyclohexanedione on the Cy atom of the allenylidene complex to give a vinylidene complex, which is transformed into an alkenyl complex by intramolecular nucleophilic attack of the oxygen atom of a hydroxy group of an enol on the C, atom of the vinylidene complex. By the use of Ic with its bulkier alkanethio moiety as a catalyst and at lower temperature, a subsequent intramolecular cyclization may be slow enough to make isolation of the alkylated product possible. 

A proposed reaction pathway is shown in Scheme 7.29, where either the aromatic carbon or oxygen atom of naphthol may work as a nucleophile. Thus, the first step is the nucleophilic attack of the carbon atom of 1 -position of 2-naphthol on the C. atom of an allenylidene complex A to give a vinylidene complex B, which is then transformed into an alkenyl complex C by nucleophilic attack of the oxygen atom of a hydroxy group upon the Co, atom of B. Another possibility is the nucleophilic attack ofthe oxygen of 2-naphthol upon the Co, atom of the complex A. In this case, the initial attack of the naphthol oxygen results in the formation of a ruthenium-carbene complex, which subsequently leads to the complex B via the Claisen rearrangement of the carbene complex. 

The metal vinylidene intermediates discussed elsewhere in this chapter are limited to a single carbon-substituent on account of the 1,2-migration process by which they form from terminal alkynes. Alkenylidenes—vinylidenes bearing two carbon-substituents—are formed by nucleophilic addition of the (i-carbon of a metal acetylide to an electrophile (Scheme 9.16) [30]. 

The isomerization of terminal epoxyalkynes into furans catalyzed by RuCl(Tp)(PPh3) (MeCN) inthe presence of Et3N as abase at 80 °C in 1,2-dichloroethaneis explained by a related intramolecular nucleophilic addition of the oxygen atom of the epoxide to the a-carbon atom of a ruthenium vinylidene intermediate, as shovm by deuteration in the 3-position of the furan (Scheme 10.10) [45]. This reaction is specific for terminal alkynes and tolerates a variety of functional groups (ether, ester, acetal, tosylamide, nitrile). 

Treatment of some iron-acyl complexes with trifluoromethanesul-phonic anhydride (TfzO) affords vinylidene derivatives directly (5 7,38). The reaction is envisaged as a nucleophilic attack on TfzO by the acyl, followed by deprotonation to the vinyl ether complex. A combination of an excellent leaving group (TfO-) with a good electron-releasing substituent on the same carbon atom facilitates the subsequent formation of the vinylidene ... 

The electron distribution in the vinylidene ligand, which results in a high electron deficiency on the a-carbon, but with considerable electron density on the / -carbon (see Section VII,A), renders this ligand subject to nucleophilic attack on the former, and electrophilic attack on the latter. 

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

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... 

That the substitution mechanism depends on the nature of the nucleophile is shown by the formation of the ketene acetals (151) from the reaction of vinylidene chloride with alkoxide ions. It was suggested that two consecutive eliminations-additions take place, and that in both cases the alkoxide attacks the acetylene at the substituted carbon (Kuryla and Leis, 1964). Since chloroacetylene (132) is also an inter-... 

Equilibrium cycloreversion of metallacyclobutane complexes to alkylidene intermediates is similar to that of metallacyclobutene complexes (Section 2.12.6.1.3). Metallacyclobutane complexes thus provide convenient progenitors of reactive alkylidene intermediates. The a-methylenetitanacyclobutane complex 79 decomposes into the vinylidene intermediate 103 (Scheme 17), which manifests nucleophilic character at the ct-carbon, consistent with... 

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

In view of their capacity for Michael reactions with nucleophiles to give intermediate vinylidene-iodonium ylides, alkynyliodonium ions might be expected to behave as 1,3-dipolarophiles. Cycloadducts in which the nucleophilic end of the dipole is bound to the / -carbon atom of the starting alkynyliodonium ion (i.e. the / -adduct) might also be anticipated (equation 136). 

---