Carbene complexes vinylidenes

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Although the transformation of a primary alkyne into a vinylidene complex, 2, in presence of a number of transition metal systems is well reported [2, 3], only rare examples are known for the transformation of an alkene into a carbene complex [4, 5]. Given the increased role played by vinylidene and carbene complexes as key partners in metathesis reactions and related catalytic processes [6, 7], opening up new efficient and easy synthetic routes to such complexes is an important challenge. 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

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). 

Fig. 2.9. Generation of carbene complexes from vinylidene complexes.

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). 

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]. 

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

Similar to vinylidene complexes, carbyne complexes can also react with nucleophiles to yield heteroatom-substituted carbene complexes (Figure 2.14) [122,181-184],... 

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. 3.26. Conversion of alkenyl and alkynyl complexes into carbene and vinylidene complexes.

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

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]. 

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... 

Preparation of Fischer-type Carbene Complexes through the Generation of the Vinylidene Complexes... 

Historically, vinylidene complexes of zero-valent pentacarbonyl Group 6 metals appeared as a fleeting intermediate for the preparation of Fischer-type carbene complexes. Probably the first example of the formation of such a pentacarbonyl vinylidene complex of a Group 6 metal was proposed in 1974 by Fischer et al, who examined the reaction of pentacarbonyl[hydroxy(methyl)carbene] chromium 1 with dicyclohexylcarbodiimide(DCC) [3]. Thus, treatment of 1 with DCC in CH2CI2 at —20°C rt gave a novel azetidinylidene complex 2 in 47% yield. As a possible... 

In 1979, Rudler ef al. reported another example of the presence of a vinylidene complex during the reaction of pentacarbonyl[methoxy(methyl)carbene] tungsten 5 with MeLi followed by acidification with TFA [4]. It was proposed that the vinylidene complex 7 was generated by deprotonation of the a-proton of the carbene complex followed by elimination of methoxide and reaction with the dimethylcarbene complex 8, the addition-elimination product of MeLi with the starting carbene complex, to give the dinuclear complex 6 (Scheme 5.2). 

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. 

In 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

Scheme 5.7 Formation of a cyclic Fischer-carbene complex via vinylidene by Dotz.

Scheme 5.8 Formation of a Fischer-carbene complex from vinylidene by Rudler.

Murakami et al. reported a ring-closing metathesis reaction of allenynes using Schrock s molybdenum alkylidene complex [37]. Treatment of allenynes ISl with a catalytic amount of the complex 15 2 in toluene at rt gave cyclopentene derivatives 1 S3 in good yield. Two possible reaction mechanisms were proposed, one through a vinylidene complex 154 and the other through a carbene complex, but based on several mechanistic studies, they favored the vinylidene complex pathway, which is shown here (Scheme 5.42). 

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. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Insertion can also be carried out on the C-H bonds of heteroaromatics. Masahiro Murakami of Kyoto University has described (J. Am. Chem. Soc. 2003,125,4720) a Ru catalyst that will effect rearrangement of a silyl alkyne such as 10 into the vinylidene carbene. The intermediate Ru carbene complex is then electrophilic enough to insert into the aromatic C-H bond. The insertion is highly regioselective. The Au and the Ru alkylidene insertions are geometrically complementary, as Ru gives the E-alkcne. 

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