Phenyl carbene, reactions

The photofragmentation of easily accessible 2,2,2-triethoxy-4,5-diphenyl-l,3,2-dioxaphos-pholane-4,5-dicarbonitrile (1) results in the formation of cyano(phenyl)carbene. Reaction of the latter with alkenes leads to 1-phenylcyclopropane-l-carbonitriles 2 in high yield/ ... 

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... 

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

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.

The formation of side products depends on the choice of substituents and solvent [21]. The role of the solvent is illustrated by the reaction of phenyl carbene complex 1 with diphenylethyne (Scheme 7). An ethereal solvent such as THF leads exclusively to the benzannulation product isolated as quinone 7 after oxidative work-up, while use of the noncoordinating solvent hexane results in comparable amounts of cyclobenzannulation and cyclopentannulation products 7 and 8a. Strongly coordinating acetonitrile suppresses benzannulation product 7 in favor of the cyclobutenone 9, which is accompanied by minor amounts of cyclopentannulation products 8a and 8b. Indene 8a is obtained exclusively if the polar solvent DMF is employed. 

The benzannulation reaction tolerates phenyl carbene complexes bearing both electron-donating [35a] and -withdrawing [35b] substituents. Additionally, carbene complexes containing condensed systems such as naphthalene and other carbocyclic hydrocarbons as well as heterocydes have also been successfully submitted to the reaction (see Section 8.5). 

Scheme 8.4 Stepwise synthesis of the tetrapeptide 18, containing the sequence 14-17 of the human proinsulin C-peptide, from the methoxy(phenyl)carbene complex 16 via the carbene chromium derivative 17 as the intermediate (Gly-OMe and Pro-OMe were the methyl esters of glycine and proline). The C—N coupling reactions in steps (1)—(4) were carried out using the DCCD/HOSU method, explained in [35]...

Recently, Schulze and coworkers synthesised a group of (2-aryl-4,5-diphenyl)isothiazol-3-ylidenes (see Figure 6.11) by deprotonation of the respective isothiazolium perchlorates with Bu OK as base [37], The compounds are yellow solids that dimerise readily and show typical carbene reactions like insertion into NH bonds. The tendency to dimerise follows the known electronic properties of substituents on the phenyl ring on nitrogen (< -methyl [Pg.317]

However, 56 disappears at 11 K with a half-Ufe of 2.46 h, while the deuterated analog is stable. The WKB-computed half-Ufe (using the path computed at MP2/aug-cc-pVDZ and energies at CCSD(T)/cc-pVTZ) of 56 is 3.3 h and 8700 years for d-56. This fast tunneling rate also acconnts for the lack of rearrangement of 56 into the tropone 58, a reaction analogons to the chemistry of phenyl carbene and phenyl nitrene discussed earlier in this chapter (Section 5.2). 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

The photolysis of phenyldiazomethane in the presence of olefins leads to the formation of cyclopropanes the addition is largely stereospecific. The reactivity of phenyl carbene has also been examined with respect to insertion reactions . The ESR spectrum of triplet phenylcarbene has been observed in the photolysis of phenyldiazomethane at 77 °K . 

Scheme 14 Generation of chloro(phenyl)carbene (64) and its subsequent reactions.

Reaction of bromofluorophenylmethane (for a preparation see ref 118) and potassium tert-butoxide with an alkene afforded l-fluoro-l-phenylcyclopropanes (Houben-Weyl, Vol.4/3, p233 Vol. E19b, p980). To identify the reacting species, e.g. carbene or carbenoid, the reaction was carried out without and with an equimolar amount of 18-crown-6. ° Alternatively, chlorofluorophenylmethane, in place of the bromo derivative, can be used. Reactions with bromofluorophenylmethane were performed at 25 °C (sealed tube), whilst those with chlorofluorophenylmethane at 60-80 °C. Addition of fluoro(phenyl)carbene to alkenes is at least 98% stereospecific e.g. formation of . ... 

Dichloro(phenyl)methyl](trimethyl)tin is more stable, and forms chloro(phenyl)carbene at 140-160"C only. This reaction carried out with 2,3-dimethylbut-2-ene afforded diene 13 via 1-chloro-l-phenylcyclopropane, which did not survive the reaction conditions. ... 

The reaction of an alkene with bromo(phenyl)carbene (or carbenoid), generated from di-bromo(phenyl)methane (benzal bromide) and potassium ter -butoxide, afforded 1-bromo-l-phenylcyclopropanes (Houben-Weyl, Vol. 4/3, pp232-233, and Vol. E19b, p998). The reactions were carried out by shaking all components (10- 12 equivalents of an alkene) in a sealed tube at ca. 25°C for 50-70 hours. ... 

The cyclopropanes thus formed are conveniently analyzed by NMR spectroscopy since they are unstable to various gas chromatographic conditions. The stereospecific addition of bromo(phenyl)carbene to an alkene (at least 95%) has been reported. It should be stressed that the reaction of alkenes with aryldibromomethanes and alkyllithiums gave arylcyclop-ropanes, rather than l-aryl-l-bromocyclopropanes. - ... 

Thus, the measurement of the relative activity of organo(organooxy)carbenes toward alkenes shows that methoxy(phenyl)carbene is ambiphilic with an extraordinarily strong nucleophilic component,while methoxy(methyl)carbene shows nucleophilic properties.Therefore, these two carbenes react readily with electrophilic alkenes. On the other hand, methoxy(trifluoromethyl)carbene, in spite of possible push-pull stabilization, is highly reactive and electronically indiscriminant in its reactions with alkenes.The absolute rate constants of its addition to alkenes confirm such a reactivity pattern. 

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