Cycloadditions chromium - carbene complexes

Silylketenes in formation of (3-lactones and (3-lactams 98JCS(P1)2105. Syntheses of (3-lactams, (3-lactones, and 1,3- and 1,4-diazetidinediones by pho-tochemically induced cycloaddition reactions of chromium carbene complexes with imines, aldehydes, and azo compounds 97T4105. 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

At this point the catalytic process developed by Dotz et al. using diazoalkanes and electron-rich dienes in the presence of catalytic amounts of pentacar-bonyl(r]2-ds-cyclooctene)chromium should be mentioned. This reaction leads to cyclopentene derivatives in a process which can be considered as a formal [4S+1C] cycloaddition reaction. A Fischer-type non-heteroatom-stabilised chromium carbene complex has been observed as an intermediate in this reaction [23a]. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

Chromium carbene complexes have also been known to react with imine equivalents to afford /3-lactam derivatives234. Furthermore, [3 + 2]-cycloaddition of an alkenylchromium carbene 133 with imines proceeded to afford 3-pyrroline derivatives 134 in the presence of a Lewis acid catalyst (Equation (21)),235 where GaClj or Sn(OTf)2 were efficient promoters. Alkenylcarbenes bearing chiral auxiliaries afforded the desired cycloadduct in optically pure form. 

Table 2.15. Cycloaddition reactions of photochemically activated chromium carbene complexes.

Initial [2 + 2] cycloaddition of the alkyne to the saturated chromium carbene complex followed by [2 + 2] cycloreversion to yield a l-chroma-l,3,5-hexatriene may be an energetically more favorable reaction pathway (Figure 2.25). However, no energy minima could be located on the path from the starting carbene complex to the chromahexatriene. Hence, the metallacyclobutene shown in Figure 2.25 does not represent an intermediate but only a transition state. 

Experimental Procedure 2.2.9. [4 + 3] Cycloaddition of a Chromium Carbene Complex to a 2-Aminodiene 6-(2-Furyl)bicyclo[5.4.0]undecan-2,4-dione [264]... 

The Dotz benzannulation reaction, based on the alkyne cycloaddition to chromium carbene complexes, is the most important application of Fischer carbene complexes. Among the various Fischer carbene complexes, alkoxy and aminocarbene complexes of chromium undergo a novel inter- and intramolecular tandem alkyne insertion/ carbene annotation sequence to give 9H-carbazoles and nf/-benzo[fl]carbazoles. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

Another more versatile method involves the photocycloaddition of chromium carbene complexes to alkenes. These transformations can be formally regarded as [2 + 1 + 1] cycloadditions since these carbene complexes are derived from [1 + 1] carbanion insertion reactions to hexacarbonylchromium. 

The use of alkenes with chiral auxiliary groups leads to chiral cyclobutanones 4. Reaction yields of 50 67% and diastereomeric excesses of 86-97% were obtained for the 3-amidocy-clobutanones which were obtained from cycloaddition of the chromium carbene complexes with chiral ene carbamates (see also Section 1.3.4.3.3.).11... 

The advantage of using the photocycloaddition of pentacarbonylcarbenechromium complexes over the ketene cycloaddition method is the absence of ketene dimerization and the avoidance of use of excess alkene in the former method. Also, the mild reaction conditions associated with the use of chromium carbene complexes avoids epimerization and thermodynamic equilibration of 2-monosubstituted cyclobutanones. 

Cycloaddition of chromium-carbene complexes with imines... 

Furthermore, the successful [3+2+1] cycloaddition of alkynes bearing a cyclopropane ring and a carbene complex unit has been reported. These benzannulations result in the formation of bimetallic naphthohydroquinone chromium tricarbonyl complexes [48]. Additionally, (non-strained) cyclic alkynes are potent reaction partners in the cycloaddition of chromium carbene complexes [49]. 

Photolysis of chromium alkoxycarbene complexes with aldehydes in the presence of a Lewis acid or base, and carbon monoxide, affords -lactones in low to moderate yield. The intramolecular variation of this [2 + 2]cycloaddition using carbene complexes having a pendant aldehyde are considerably more efficient (Scheme 36). 

The stereoselectivity of the Buchi-Paterno reaction between 3-hydroxy-2,3-dihydrofuran and benzophenone was found to be influenced by solvent, temperature and steric effect <06TL2527>. A Dotz benzannulation involving a dihydrofuran chromium carbene complex and an alkyne was employed to form the aflatoxin skeleton, providing the annulated product as the only regioisomer <06TL2299>. Cycloaddition involving 2,3-dihydrofuran, 1-aminoanthraquinone and salicy aldehyde was catalyzed by... 

Apart from numerous similar 3-lactam syntheses,the very promising photolytic [2 + 2] cycloaddition of imines (or the corresponding hexahydro-l,3,5-triazenes respectively)to chromium carbene complexes that leads to norcardicin precursors merits special interest. 

Chromium carbene complexes undergo other reactions, too. The mechanisms of these reactions consist of steps ([2 + 2] cycloadditions, electrocyclic reactions) that are similar to those seen in the mechanism of the Dotz reaction. 

Fischer-type chromium carbene complexes with 1-ethoxycyclopropylalkynyl substituents at the carbene carbon, e.g. 25, on reaction with dimethylamine and subsequent conversion of the resulting vinylcarbene with alkynes surprisingly did not give phenol derivatives, as would be expected from the known Dotz reaction, but gave cyclopenta[ )]pyrans, e.g. 26. The reaction is interpreted as a double alkyne-insertion/CO-insertion sequence with formation of a trienylketene intermediate, which undergoes intramolecular hetero-Diels-Alder cycloaddition and dimethylamine elimination. ... 

Cycloaddition of ketenes 53 and 55 with a variety of cyclic imines gives frarcs-jS-lactams exclusively with > 97% de78a. The ketenes were generated by photolysis of chromium carbene complexes generated from (R)-phenylglycinol and (S)-valinol. Poor results are obtained with the imines derived from cinnamaldehyde that react cleanly with ketene 37 as described above. This suggests that ketenes 53 and 55 are either bound to chromium or that the cycloaddition is very sensitive to substituents on the ketene. 

The preparation of cyclobutanone (123) via the [2 + 2] cycloaddition of ketene (121), and the photolytic reaction of chromium carbene complex (122) with 2,3-dihydro-1,4-dioxin (10 has also been described <92TL927>. 

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