Carbene complexes dihalocarbene

A final class of carbene complexes includes vinylidenes (E in Figure 2.14). Vinylidenes possess one mediylene substituent at the carbene carbon instead of two substituents connected by single bonds. Free vinylidene is electrophilic, and the singlet state is calculated to lie below the triplet state by 43.1 kcal/mol. The electrophilicity of free vinylidene is similar to that of a dihalocarbene. It is highly reactive toward rearrangement to the much more stable alkyne tautomer. 

More useful for synthetic purposes, however, is the combination of the zinc-copper couple with methylene iodide to generate carbene-zinc iodide complex, which undergoes addition to double bonds exclusively to form cyclopropanes (7). The base-catalyzed generation of halocarbenes from haloforms (2) also provides a general route to 1,1-dihalocyclopropanes via carbene addition, as does the nonbasic generation of dihalocarbenes from phenyl(trihalomethyl)mercury compounds. Details of these reactions are given below. 

Dihalocarbene complexes are useful precursors to new carbenes by nucleophilic displacement of the chlorine substituents. This has been nicely illustrated for Fe(TPP)(=CCl2) by its reaction with two equivalents of Re(CO)5J to give the unusual /t-carbido complex Fe(TPP)=C=Re(CO)4Re(CO)5 which also contains a rhenium-rhenium bond. " The carbido carbon resonance was observed at 211.7 ppm in the C NMR spectrum. An X-ray crystal structure showed a very short Fe=C bond (1.605(13) A, shorter than comparable carbyne complexes) and a relatively long Re=C bond (1.957( 12) A) (Fig. 4, Table III). " ... 

Although the C=C bond of allyl alcohols is frequently less susceptible to reaction with dihalocarbenes, insertion of the carbene into the C=C bond invariably occurs (Table 7.4) to the exclusion of reaction at the hydroxyl group (see Section 7.5) [98]. A complex mixture of products is obtained from the reaction of dichlorocarbene with allyl alcohol, but the cyclopropane can be obtained in high overall yield (>70%) via... 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol-1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ah initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=SC>2) are best viewed as dihalocarbene-S02 complexes with a carbon-sulfur bond order of approximately zero.14 In a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]1. ... 

The formation of dihalocarbene is one of the most useful phase-transfer processes developed in organic chemistry. The dihalocarbenes, once generated from haloform and base, can undergo addition or insertion reactions (3, 11, 12). If the double bond is complexed to a metal, addition by the carbene will not occur, but rather insertion into a saturated carbon-hydrogen bond will take place. 

Grubbs and coworkers used (4a) to perform olefin metathesis with 1,1-difluoroethelyene a metathesis partner that had theretofore been unavailable for reaction consideration. While metathesis did occur, the difluoro carbene Ruthenium complex (4m) (Figure 2) was not an effective catalyst itself thus, little if any turnover was observed, although this did provide a potential convenient pathway for constructing dihalocarbene-metal complexes. 

Although the reaction of dihalocarbenes with alkenes gives good yields of halogenated cyclopropanes, this is not usually the case with methylene, tCH2, the simplest carbene. Methylene is readily formed by heating diazomethane, CH2N2, which decomposes and loses N2, but the reaction of CH2 with alkenes often affords a complex mixture of products. Thus, this reaction cannot be reliably used for cyclopropane synthesis. 

Crown ethers, cryptands, quaternary salts bonded to a polymer (Sects. 5.1, 5.2, 6) and finally free amines have been successfully used in carbene generation in two-phase systems. In the latter case, the mechanism is rather complex, and different interpretations have been advanced. However, the last step of the generation of the dihalocarbene probably does not differ substantially from the reaction of CHCI3, NaOH and quaternary salt In Table 16 are compiled some examples of addition and insertion of carbenes. 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

---