Carbene-like intermediates

Enamine formation occurs by the thermolysis of diazo compounds (Scheme 150)67 109,278 284 288,304 332 453 454 via a carbene-like intermediate.284 332 When R1 = Ph, it enters into competition with hydrogen migration,284,332 and the electrophilic character of the carbene enhances the migration of the dimethylaminophenyl more than the phenyl.332 When triazoline synthesis is carried out at temperatures higher than that at which thermolysis of diazo compounds occurs, enamines are obtained exclusively, as in the addition of phenyl azide to cinnamic nitriles and ketones, with phenyl migration dominating in the nitrile.284 Enamine is also formed quantitatively in the reaction of ethyl diazoacetate with benzylideneaniline at 110°C.455... 

In principle also the possibility should be considered that CO is dissociated, as in Fischer-Tropsch synthesis of hydrocarbons, and thereafter partially hydrogenated. A metal carbene would then produce, upon addition of H2O, a molecule of methanol. However, one would expect that with a metal like Rh, which can dissociate CO, the isotopically labelled atoms from C 0 would be scrambled with C 0 atoms in the methanol product but this has not been found.With metals which dissociate CO even more reluctantly, like Pd and Cu, this mechanism is even less likely than with Rh. This does not however exclude the possibility that higher alcohols can be formed by H2O addition to a carbene-like intermediate. 

Cycloisomerization of 1,5-enynes with gold complexes containing highly electron-donating Ugands proceed via carbene-like intermediates to give a bicyclo-[3.1.0]hexenes (Scheme 127). ... 

These reactions give carbenes (Chapter 11) or carbene-like intermediates. The reaction of Eq. 4.10 is particularly important because it is one of the rare ways in which the tightly bound CO can be removed to generate an open site at the metal. In this way a ligand L, which would normally not be sufficiently strongly binding to replace the CO, can now do so. 

Addition of halogens to planar d complexes is straightforward and does not deserve further comment. However, it should be noted that halogenation can also be achieved by treatment with the carbon tetrahalides (26, 41, 67). The mechanism of this reaction is obscure but a radical path may be involved (104). The fate of the carbon fragment has not been determined but it may be anticipated that carbene like intermediates could be formed in this way. A related process is the halogenation of reactive four-coordinate complexes by vicinal dihalides which are converted into olefins. Only two examples have been described (28,56a). The more interesting involves the formation of the cyclobutadiene complex (XLVI) (56a). 

Scheme 6.55. A representation of a possible pathway using metal (M = Ru or W) carbene-like intermediates to account for alkene metathesis.

Labeling studies provided some evidence that aldehydes are not intermediates in the formation of heptanol from 1-octene but a hydroxy carbene-like intermediate (Scheme 5.50) [63]. The latter derives from the protonation of the relevant Rh-acyl complex by ethanol and benefits from the high electron density at the metal center, which is caused by trialkylphosphines. The reaction with hydrogen (here D2) produces the alcohol. A similar mechanism was suggested for the tandem reaction with 2-propen-l-ol as a substrate [64]. 

The stability of an unsubstituted carbene is quite low in water. Highly correlated ab initio MO calculations have been used to study the energetics and mechanism governing the reaction between the radical CH2 and H20 in the gas phase and in solution, and it was found that methylene reacts in a barrierless fashion to produce the ylide-like intermediate methyleneoxonium, H2C-OH2, which in turn undergoes a 1,2-hydrogen shift to produce CH3OH.128 The presence of substituents appears to stabilize carbenes toward water.129... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

The only carbene-like reaction reported so far is the low-temperature addition of tert-butyl isocyanide to carbenes 2d and 2u.78 From the P-hydrogenophosphonio carbene 2d, the heterocycle 89 was isolated in high yield. It is believed that the initial coupling product 87d rapidly inserts a further equivalent of isocyanide into the P-H bond, leading to the intermediate 88, which then undergoes rapid elimination of diisopropylamine. When the same reaction was performed with the P-chloro(phosphonio)car-bene 2u, a 1/1 mixture of keteneimine 90 and phosphinonitrile was obtained. This result can be explained by the cleavage of the carbene-isocyanide coupling product 87u by residual HCN, inherently present in the f-BuNC. 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Under some circumstances, the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. When a reaction involves a species that reacts as expected for a carbene, but must still be at least partially bound to other atoms, the term carbenoid is applied. Some reactions that proceed by carbene-like processes involve transition-metal ions. In many of these reactions, the divalent carbene is bound to the transition metal. Some compounds of this type are stable whereas others exist only as transient intermediates. 

Nitrenes are neutral monovalent nitrogen analogs of carbenes. The term nitrenoid is applied to nitrene-like intermediates that transfer a mono substituted nitrogen fragment. 

When two equivalents of PPh3 were added to the cationic T7z-S = CH2 complex 51a, the yhde complex 52 together with triphenylphosphine sulfide was formed. The reaction very likely proceeds via the carbene complex intermediate [Re( CH2)(NO)(PMe3)(if-C-dL) j+ (5 ) which is rapidly trapped by PPh3. The trapping reaction is significantly faster than the for-... 

Stable transition-metal complexes of this type are known and others have been recognized as likely intermediates in a number of reactions. Rightly or wrongly, they are called carbene-metal complexes, although they also can be regarded either as metal-stabilized carbocations or as metal-stabilized ylides (Section 16-4A). 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

Carbenes and carbenoids (carbene-like reagents) are useful both for the synthesis of other compounds and for the investigation of reaction mechanisms. The carbene intermediate is generated in the presence of its target compound, so that it can react immediately, and the concentration of the carbene is always low. Reactions using carbenes are discussed in Chapter 8. 

Intermediate cases between Fischer and Schrock character are known. In the Os complex of equations (30) and (31), Roper has a carbene with intermediate character between the Fischer and Schrock extremes because it reacts both with electrophiles like SO2 (equation 30), or H+ and with nucleophiles (e.g. CO, equation 31), or CNR. This is... 

For substitutions in position 3 or 5 on the 1,2-dithiole ring various mechanisms may be considered. When R (or R ) is a good electrofuge such as H, carbenes (25) are likely intermediates. " Strong acidity slows hydrogen exchange, supporting such a mechanism. A similar mechanism is possible for the thiation of 1,2-dithiolium cations by elemental sulfur in the presence of pyridine (Eq. 8). 

There are many ways of generating what appear to be carbenes. But in some cases at least, it seems clear that no free carbene is actually an intermediate instead, a carbenoid (carbene-like) reagent transfers a carbene unit directly to a double bond. For example, in the extremely useful Simmons-Smith reaction... 

In the early 1980s, the conversion of methyl alcohol (MeOH) to gasoline over synthetic zeolite catalysts was being studied. One set of experiments using MeOH H-ZSM-5 led to the hypothesis that carbenes, like CH2, were involved in this industrially important process.87 However, a more explanatory mechanism was proposed88 and, by decade s end, a rigorous study of this reaction using MAS 13C NMR showed CO to be the key intermediate.89 Recently, the precise role of H-ZSM-5 has even been questioned.90... 

Quantum-mechanical investigation using the MINDO/3 method of the ring contraction of cyclobutylidene to methylenecyclopropane reveals that the reaction is initiated by an electrophilic attack of the empty p atomic orbital at the carbene Cl site by the methylene group at C3 giving a bicyclobutane-like intermediate. In this species, which corresponds to a non-classical carbene, electron density is shifted toward the carbene site. In a second step it can easily undergo rearrangement into methylenecyclopropane by bond fission controlled by orbital symmetry. For the total reaction an activation enthalpy of 8 kcal mol" was computed. 

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