Phenyl carbene

In 1986 Yamashida et al. found that the reaction of the (morpholino)phenyl-carbene complex 46 with symmetric alkynes 47 gave the morpholinylindene derivatives 48 and 49, as well as the indanones 50 derived from the latter by hydrolysis, in excellent yields (Scheme 9) [54]. This contrasts with the behavior of the corresponding (methoxy)phenylcarbene complex, which solely undergoes the Dotz reaction [55]. This transformation of the amino-substituted complex 46 apparently does not involve a CO insertion, which is an important feature of the Dotz benzannelation. 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

B. C-H Insertion Reaction with Chloro(phenyl)carbene. 292... 

Chloro(phenyl)carbene or carbenoid generated from benzal chloride reacts with potassium salts of benzylic, allylic, and other alkoxides to produce phenyl-substituted oxiranes 9 in high yields, as an approximately 1 1 mixture of cis... 

Table 2. Reaction of Chloro(Phenyl)Carbene with Alkoxides °...

The latter method was utilized in the synthesis of ruthenium vinylalkylidene complexes of the type RuCl2(=CHCH=CPh2)(PR3)2 (R = Ph [29] and R=Cy [30]. The phenyl carbene species was isolated when diazomethylbenzene was used [31], Figure 16.17. 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

A solution of [methoxy(phenyl)carbene]pentacarbonylchromium (2.03 g, 6.50 mmol) in diethyl fumarate (10.7 mL, 11.3g, 65.4 mmol) is heated to 110°C for 2.5 h. Precipitated chromium hexacarbonyl is removed by filtration and excess diethyl fumarate is evaporated at 45 °C under reduced pressure. Column chromatography of the residue (alumina, hexane/benzene 2 1) yields 925 mg (49%) of the title compound as a colorless solid, mp 37 °C (pentane). 

Dimethyl-1-butyne (0.92 mL, 0.62 g, 7.52 mmol) is added to a solution of l methoxy(phenyl)carbene]pentacarbonylchromium (1.56 g, 5.00 nimbi) in tert-butyl methyl etoer (25 mL), and the resulting mixture is stirred at 45 °C under argon for 2 h. Concentration under reduced pressure and column chromatography... 

Pentacarbonyl(methoxyphenylmethylene)tungsten(0), [Pentacarbonyl[methoxy(phenyl)carbene] tungsten(O)]... 

Prompted by W. M. Jones work on the formation of heptafulvene from phenyl-carbene in the gas phase,which implied a seven-membered ring intermediate, and by the implications of the possible reversibility of the process, Baron et al. rediscovered Vander Stouw s cryptic work and showed that all three possible tolyl-carbenes gave benzocyclobutene and styrene, albeit in different ratio from the ortho isomer than from the meta or para species.Baron et al. ° proposed a mechanism in which a methylcycloheptatrienylidene (CH3-7I) was the active seven-membered species, and which additionally feamred the intermediacy of methylbicyclo[4.1.0]-heptatrienes 73 and 73 (Scheme 7.35). 

As shown in Figures 1.6b,c, Z and 44C substituents either lower the 2p-spn gap or leave it about the same. In either case, the ground state for Z- or 44C -substituted carbenes is expected to be T. See recent investigations of phenyl carbene, C6H5CH, [164] ethynyl carbene, HC=C—CH [165] carboxylate-substituted carbenes, R—C—C02R [166] and formyl carbene, H—C—C(0)H [167]. 

The rearrangement of phenyl carbene to cyclohepta-1,2,4,6-tetraene has been detected at 10 K (Westetal., 1982 cf. Fig. 3.3), but the relevance of this to photochemistry in solution at higher temperatures is not yet clear. Ortho-substituents that might react with photogenerated aryl carbenes should be avoided as they are with arylazides (Section 3.2.4 Fig. 3.4). 

The first X-ray crystal structure determination, carried out by Mills in cooperation with us 31) on pentacarbonyl[methoxy (phenyl) carbene ]-chromium(0), confirmed our originally postulated bonding concept. According to this concept, the carbene carbon atom is sp1 hybridized. It should therefore possess an empty p-orbital and be electron-deficient. 

Fig. 2. The structure of pentacarbonyl [methoxy(phenyl)carbene]chromium (0) bond lengths in angstroms.

In 1971 we succeeded, again for the first time, in transferring a carbene ligand from one metal atom to another (26, 47). For example, if one irradiates a solution of cyclopentadienyl(carbonyl) [methoxy (phenyl) carbene]-nitrosylmolybdenum(O) in the presence of an excess of pentacarbonyliron, one obtains tetracarbonyl[methoxy (phenyl) carbene]iron(0) with the simultaneous formation of cyclopentadienyl (dicarbonyl) nitrosylmolyb-denum ... 

Since the shift of a hydrogen atom is not possible with the methoxy-(phenyl) carbene ligand, it can only dimerize in reactions with bases and in thermal liberations (75). 

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

As expected, there arise two isomers [(a) and (b) in Eq. 153 whose proportions depend on the choice of the central metal atom under otherwise similar reaction conditions. This seems to us to be a rather important hint that the reaction does not proceed via a free methoxy(phenyl)-carbene but that the metal atom participates in the decisive reaction step. 

Fig. 6. Hypothesis concerning the course of the reaction during the treatment of pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with JV-vinyl-2-pyrrolidones under a CO pressure of 150 atm.

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

We could show further that not only methoxy(organo)carbene complexes react with boron trihalides in the above-mentioned sense. For instance, it was found that b aws-(bromo)tctracarbonyl(phenylcarbyne)chromium(0) and -tungsten(0) are also accessible from pentacarbonyl[hydroxy (phenyl)-carbene [[chromium (0) (02) and from pentacarbonyl[methoxycarbonyl-... 

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