Reaction with Isolated Carbenes

Cyclopentadienyl ligands (Cp) of metallocenes could also be sacrificed in the deprotonation of imidazolium salts generating [(NHC)M(Cp)] complexes and 1,3-cyclopentadiene. 

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As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

In isolated examples, reactions of specific amides and thioamides with dihalo-carbenes can take unusual pathways. Thus, for example, using procedure 7.1.1, A,A-dialkylamides are converted into a-chloromethylene derivatives of the amides [48]. The initial step in which the carbene attacks the carbonyl oxygen atom is the same as for the dehydration of the A-alkyl amides, but subsequent steps, for which there is evidence from 2H/ H labelling experiments, lead to the formation of an enamine and further reaction with the carbene (Scheme 7.34). 

Allylic ethers, particularly cyclic allylic ethers, may enter a competitive reaction with dibromo-carbene of insertion into a C H bond. In the example of the formation of 3 and 4, a suitable workup of the reaction mixture allows isolation of pure cyclopropane 3.71... 

Yet another kind of alkene addition is the reaction with a carbene to yield a cyclopropane. A carbene, R2C , is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and is generated only as a reaction intermediate, rather than as an isolable molecule. Because they re electron-deficient, carbenes behave as electrophiles and react with nucleophilic C=C bonds. The reaction occurs in a single step without intermediates. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

Trichloromethyl lithium (generated from BrCCl3 and CH3Li at —100 °C) adds to dialkyl acetylenes and to monoalkyl acetylenes23, thus monoalkyl cyclopropenones became accessible which could not be obtained from terminal acetylenes by reaction with the above carbene sources. The 3,3-dihaIogeno-A1,2-cycIopropenes formed as primary products in the dihalocarbene reactions are usually not isolated, but are hydrolyzed directly to cyclopropenones. 

Interaction of alkadienylidenecarbenes, R2C=C=C=C, with group 14 hydrides results in the formation of Si, Ge and Sn functionalized butatrienes R2C=C=C=CHER3 (Figure 3)24. The reaction is general for alkyl- and aryl-substituted carbenes with isolated... 

The thermal reaction of matrix-isolated lh with acetylene leads to the formation of the intensely red-colored vinylcarbene 18 as the primary product.62 This carbene, which can also be formulated as a 1,3-diradical, has a triplet ground state and is thus formed in a spin-allowed reaction from triplet carbene lh (Scheme... 

The carboxylic acid derivatives li-lm can only be matrix-isolated if the corresponding quinone diazides 2i-2m are irradiated with monochromatic blue light (k = 436 nm).81 91 92 UV or broad-band visible irradiation rapidly results in the decarboxylation of the carbenes. As expected, the IR and UV/vis spectra of the carbenes are very similar to that of la. Oxygen trapping results in the formation of the photolabile carbonyl oxides 7. Thus, the carbenes li-lm were identified both spectroscopically and by their characteristic reaction with molecular oxygen. 

The anthronylidenes lp-lr are easily matrix-isolated by photolysis of the corresponding diazo compounds 2 (Scheme 17).99 The reaction of the carbenes with molecular oxygen results in the formation of anthraquinone 0-oxides 7,... 

Carbenes are such highly reactive intermediates that their direct observation requires extraordinary efforts. One set of conditions that has proved quite valuable is low temperature isolation. Carbenes can be generated by irradiation of an appropriate precursor within a glass or more ordered inert matrix at very low temperatures. The low temperature of the experiment stops or slows reactions of the carbene with the matrix material. Also, the rigidity of the medium prevents diffusion and the dimerization of the carbene is stopped. Many carbenes can be stabilized at the boiling point of nitrogen (77 K) others require liquid helium temperatures (4 K). 

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

The major product isolated from the reaction of secondary amines with dichloro-carbene the reaction under phase-transfer conditions is the A-formylamine [12-14], The isolated yields are considerably higher (Table 7.14) than those recorded for classical procedures and are not inhibited by steric effects [15]. Diphenylamine is converted into its A-formyl derivative in low yield by conventional procedures and, although application of the phase-transfer catalysed procedure increases the yields, the E- and Z-l,2-bis(diphenylamino)-l,2-dichloroethenes are also obtained as byproducts of the reaction [13],... 

Mori has reported that in the reaction of chromium carbene 25 with an alkyne containing a tethered 4-amidobutyne unit (26), a postulated vinylketene complex (27) is intercepted by nucleophilic amide attack, yielding a mixture of lactams (28 and 29).15 The expected naphthol 30 was also isolated in low yield. 

Similar pyrone complexes were isolated by Semmelhack97a as the products of the reaction between tetracarbonyl[ethoxy(alkyl)carbene]iron(0) complexes and various acetylenes. Vinylketene complexes are proposed as key intermediates in the mechanism of this conversion, which closely matches analogous reactions with cobalt carbenes51 (see Section V,B), while showing crucial differences with the analogous reaction of a chromium carbene (see Section II,B). 

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). 

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