Carbene complexes as intermediates

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Complexes of Pd, Pt and Ru catalyse enyne metathesis, giving similar products to those obtained by the Ru-carbene complex 22 as described in this chapter. These enyne metatheses are discussed in Section 7.2.6. Other mechanisms, without involving carbene complexes as intermediates, have been proposed. 

The two syntheses of 11-deoxydaunomycinone are summarized in Schemes 38 and 39 as they are illustrative of the viability of carbene complexes as intermediates in organic synthesis. The synthesis by Dotz and Popall begins with the chelated o-methoxyphenyl carbene complex (275) and the alkyne (276). A single regioisomer of the phenol (277) was obtained in 76% yield in the benzannulation. After protection of the phenol and carbon homologation of the ketone to the acid, an intramolecular Frie-del-Crafts reaction gave the tetracyclic dione (279) in 45% overall yield from (277). This represents a formal synthesis of 11-deoxydaunomycinone (269) since it has been reported that (279) can be converted to (269) by the procedures of Sih and Johnson. ... 

Kinetic analysis shows that exchange is multiple, so the substrate does not dissociate until several H/D exchange steps have taken place. This was initially interpreted in terms of alkene or, for CH4, carbene complexes as intermediates, but with the discovery of the stability of agostic complexes (Section I.B) an alternative explanation is that the alkane remains bound to the platinum via the C—H bond as L3Pt(HR). One end of an... 

Chauvin and Herisson found in 1970, that the initial product distribution in the cross metathesis of cyclopentene and 2-pentene is not in accordance with such a simple pairwise mechanism [30,50]. Therefore, they proposed a novel non-pairwise mechanism with metal carbene complexes as intermediates (5) [50]. 

The proposed mechanism of the ring cleavage reaction of HCI (and other protic acids) with cyclopropyl carbynyl complexes involves addition of HQ across the carbyne triple bond to give a carbene complex as key intermediate. In the absence of a carbonyl ligand this is followed by ring expansion to a metallacyclopentene complex, /J-hydrogen elimination and reductive elimination to the diene complex (equation 111)164. 

Treatment of diphenylketene (407) with a catalytic amount of Co2(CO)8 produces tetraphenylethylene (410,) involving a carbene complex as an intermediate. In this reaction carbene complex 408 is formed from 407 and Co2(CO)8, the cobaltacyclo-butanone 409 is generated by cycloaddition of 407 and 408, and cleaved to give 410 [130],... 

Treatment of the vinylcarbene chromium complex 97 with /-BuC=P affords the dihydrophosphetylketene complex 98 (Scheme 26). This transformation is believed to proceed via r -phosphaalkyne carbene-, phosphaalkenylcar-bene-, and phosphaalkenylketene complexes as intermediates. An intramolecular [2+2] cycloaddition completes the reaction sequence.53 Different carbene/carbon monoxide/phosphaalkyne cycloaddition products (e.g., 1,3-oxaphospholes, phosphaphenanthrenes) are obtained depending on substitution at the carbene ligand (vide infra). 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

Casey, C. P. Metal-carbene complexes [as reactive intermediates]. Reactive Intermediates (Wiley) 1981, 2, 135-174. 

Generation of 1-stannaallene 35 was suggested as an intermediate in the synthesis of the first stable distannirane 36, by Escudie et al. (Scheme 2.9.12). Because of the probable lability of the Sn—C double bond, 35 could behave as a stannylene-vinylidene carbene complex, as observed in the related 1-stannaketenimine 31, which behaves as a stannylene-isocyanide complex. Therefore, the generation of the final product of this reaction, 36, should be most likely interpreted in terms of the [2+ l]-cycloaddition of 35 with the stannylene 37, which should be generated in an equilibrium amount. 

Historically one of the first asymmetric methods to be explored, cyclopropanation came of age32 with box and salen ligands on Cu(I). Diazo compounds, particularly diazoesters 138, react with Cu(I) to give carbene complexes 140 that add to alkenes, particularly electron-rich alkenes to give cyclopropanes 141. The reaction is stereospecific with respect to the alkene -1runs alkenes giving trans cyclopropanes - and reasonably stereoselective as far as the third centre is concerned. Any enantioselectivity comes from the chiral ligand L. You have already seen the Ru carbene complexes are intermediates in olefin metathesis (chapter 15). 

As previously described, RCM is a powerful tool for the preparation of useful carbo- and heterocyclic intermediates. Almost all of the recent RCM reactions have been performed homogeneously with well-defined metal carbene complexes as catalysts. An example of the use of a heterogeneous catalyst in RCM is the synthesis of hydroazulenes such as 6 via RCM of highly functionalized diolefins (Eq. 17) catalyzed by CH3Re03/Si0i-Al703 and with heating under reflux at 80 °C [41]. 

The hydrolysis of Fischer carbene complexes such as 11,13,42,87 and 112 has been discussed in the section Hydroxide ion and water as nucleophiles and proceeds by a standard nucleophilic addition-elimination mechanism (equation 74). On the other hand, carbene complexes with ionizable a-carbons are hydrolyzed by a different mechanism which involves the deprotonated carbene complex as the key intermediate. This conclusion is based on a detailed kinetic investigation of the hydrolysis of 66, 68, 144 and 8 in 50% MeCN-50%... 

It was concluded that the addition of pyrrolidine follows a two-step process with a polar transition state leading to a zwitterionic intermediate. The addition of a series of p-substituted anilines to a Fischer carbene complex as shown in Scheme... 

Che and coworkers [152] were able to isolate and characterize a pure bis-carbene (TPFPP)Os(CPh2)2 (Fig. 3). The bis-carbene species represents the first structurally characterized fra s-bis-carbene metal complex whose carbene groups are not stabilized by heteroatoms. The related pentacoor-dinated mono-carbene complex was also prepared and characterized by an X-ray structure. A comparison of the reactivity of these complexes with olefins suggests that the bis-carbene species acts as an intermediate in cy-clopropanation. Thus, the inertness of the mono-carbene complex towards stoichiometric styrene cyclopropanation and the observation of an efficient cyclopropanation of styrene in the presence of the bis-carbene complex as a catalyst support this suggestion [152]. A recent X-ray structure determination for (TPFPP)Os(CPh2)(MeIm) revealed an Os = C distance of 1.902(3) A (Table 3) [141]. Recently, Che and coworkers [153] and Miyamoto and coworkers [154] reported oxo-bridged carbene complexes of osmium porphyrins (see Table 3). These compounds are rare examples of oxo-binuclear carbene complexes. 

Reactions of soft carbon nucleophiles derived from active methylene compounds such as /3-keto esters or malonates proceed by attack of the nucleophiles at the central sp carbon of the allenyl complexes. The attack of the nucleophiles generates cr-allyl anion intermediates, which are regarded as palladium-carbene complexes. These intermediates pick up a proton from the active methylene compound to form 7r-allylpalladium complexes, which undergo further reaction with the nucleophile as expected, and hence the alkenes are formed by introduction of two molecules of the carbon nucleophiles (Scheme 21). 

Transition metal salts or complexes are known to catalyze effectively the cyclopropanation of olefins with diazoalkanes. Asymmetric synthesis with chiral copper catalysts (Nozaki et ai, 1966, 1968 Noyori et al., 1969 Moser, 1969), as well as a detailed kinetic study (Salomon and Kochi, 1973), has suggested the intervention of copper-carbene complexes as reactive intermediates. Recently synthesis of crysanthemic acid (CCXXXIV) (R = H) with high optical yield (60-70 %) has been achieved by applying this asymmetric catalysis (Aratani et al., 1975). The camphorglyoxime-cobalt(I) complex is also effective for the enantioselective reaction (Tatsuno et al., 1974). 

As shown in eq. (15.27), vinylketene reacts with isonitrile to afford the iron-carbonyl complex of vinylketeimine. If the reaction is stopped at the initial stage (3—3.5 h), the intermediate product in which one carbonyl group is replaced with isonitrile forms with the same amount of the final product. The intermediate is rearranged to a keteimine via an iron carbene complex as shown in eq. (15.27) [80]. 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

At this point the catalytic process developed by Dotz et al. using diazoalkanes and electron-rich dienes in the presence of catalytic amounts of pentacar-bonyl(r]2-ds-cyclooctene)chromium should be mentioned. This reaction leads to cyclopentene derivatives in a process which can be considered as a formal [4S+1C] cycloaddition reaction. A Fischer-type non-heteroatom-stabilised chromium carbene complex has been observed as an intermediate in this reaction [23a]. 

---