Chromium complexes cyclic carbenes

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

In 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

In 1994, Quayle et al. reported the application of this cyclic Fischer-carbene synthesis from 3-butynols to spirolactone synthesis, although the process was stepwise and a stoichiometric amount of the complex was employed [17]. The key transformation was the chromium or tungsten carbene complex formation followed by the CAN oxidation of the complex to give y-lactone. The reaction was further applied to the synthesis of andirolactone and muricatacin, the former being shown in Scheme 5.14. 

Insertion of isocyanide carbon atoms into the Cr—carbene bond of [(CO)5CrC(OMe)Me] gave aziridinylcarbene complexes (CIV), some reactions of which are summarized in Scheme 2 28, 198). Cyclic carbene groups (CV)-(CVIII), in which the carbene carbon atom is part of an aromatic six-electron Tr-system, have been reported to form pentacarbonyl chromium and tetracarbonyl iron complexes 383, 384). Related to carbene... 

The products of diaminoacetylene insertions into metal-carbene carbon bonds have been further examined. After initial insertion at room temperature, warming causes a subsequent displacement of carbon monoxide with formation of a new, cyclic carbene complex. The carbene-C of this species, on further heating, attacks the nr/Ao-carbon of the phenyl ring forming a substituted indene. This sequence, in which the chromium atom behaves essentially as a template, is shown in Scheme 39. A different reaction path is, however, taken when the carbene... 

Chromium aminocarbenes [39] are readily available from the reaction of K2Cr(CO)5 with iminium chlorides [40] or amides and trimethylsilyl chloride [41]. Those from formamides (H on carbene carbon) readily underwent photoreaction with a variety of imines to produce /J-lactams, while those having R-groups (e.g.,Me) on the carbene carbon produced little or no /J-lactam products [13]. The dibenzylaminocarbene complex underwent reaction with high diastereoselectivity (Table 4). As previously observed, cyclic, optically active imines produced /J-lactams with high enantioselectivity, while acyclic, optically active imines induced little asymmetry. An intramolecular version produced an unusual anti-Bredt lactam rather than the expected /J-lactam (Eq. 8) [44]. 

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

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]. 

The chemistry outlined on the next page was used to produce a cyclic arene-chromium carbene complex.100... 

In the case of using lactones as electrophiles in the presence of cerium trichloride, spiroketals are obtained after acidic work-up (90JA6389). Cyclic Fischer-type chromium carbene complexes (21) have also been prepared when hexacarbonyl chromium was added to intermediates (18), followed by treatment with trimethyloxonium tetrafluoroborate (Scheme 7) (92JCS(CC)1623). 

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. 

There is no interaction between the double bond and the metal in either complex, ascribed to the bulkiness of the diphenylmethylene group. Similarly no interaction is observed in XII obtained by the reaction of cyclic eneamine 1-pyrrolidine-1-cyclopentene with a chromium-carbene complex ... 

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