Cyano carbene complexes

The majority of gold(I) carbene complexes are pure organometallic compounds and the are out of the scope of this work. Some halide or triphenylphosphine carbene complexes are known and they will be considered here. Nucleophilic addition of alcohols or amines to gold-coordinated isocyanides is one of the best-established methods to obtain gold carbene derivatives. The reaction of H[Au(CN)2] with propene oxide and estirene oxide yields (cyano)carbene complexes (380) avoiding the intermediate step.2257 A cyclic carbene compound is obtained by reaction of a dinuclear isocyanide with amine (Scheme 32).2258... 

In a modification of this reaction, the use of isocyanide becomes unnecessary when H[Au(CN)2] reacts with propene oxide or styrene oxide to yield (cyano)carbene complexes of the same family (Scheme 31)401. 

The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [1194], presumably formed by the mechanism sketched in Figure 4.10. 

Another large family of d complexes with MLCT absorptions which, however, appear in the UV region comprises cyano complexes [1] (e.g. [M(CN)6]4- with M = Fe, Ru and Os and metal carbonyls [1,4,43] e.g. M(C0)6 with M = Cr, Mo, W). These MLCT bands may obscure less intense bands of different origin such as LF absorption. A characteristic long-wavelength MLCT band was also identified in the abs. spectrum of the carbene complex Mn(C5H5)(CO)2(CPh2) (X iax = 380 nm. Fig. 5) [44]. 

The coordination chemistry of molybdenum reported during 1991 is reviewed, 20 as is the organometallic literature of the chromium triad reported during 1992.321 There is much material of interest in a review of chromium (II) based methods for C-C bond formation322 and a short review of recent developments in the use of chromium carbene complexes in organic synthesis.323 There is some material of interest in a review of peroxo and superoxo complexes of Cr, Mo and W324 and a review of chromium (III) cyano-am(m)ine complexes.325... 

The IR spectrum which can be measured in argon at 10 K after irradiation of diazo compound 18 with k = 313 nm is relatively complex. But the absorptions of 19 can be extracted by a subsequent irradiation with k > 570 nm. The signals of 19 decrease in intensity during this secondary irradiation. They fit much better with the bands calculated for T-19 than for S-19. The product formed under these conditions (X > 570 nm) is the ring-opened carbene 16, which in this case can directly be detected and shows an IR spectrum which is in agreement with that of S-16. Intermediate 16 can be transferred photochemically to 2-cyano-2/7-azirene (17) with X > 313 nm, which is the main product in the primary irradiation of diazocompound 18 with this wavelength. 

Song et al. extended this methodology to include cyanosilylation of aldehydes and ketones (Eq. 32) [160], They propose that NHC 276 interacts with TMSCN to form complex LXXVIII followed by cyano group transfer to the aldehyde (Scheme 48). The carbene is then regenerated and the desired product is obtained when LXXIX fragments. Concurrently, Kondo, Aoyama and co-workers describe similar reaction conditions for the synthesis of cyanohydrins in high yields [161, 162], while Suzuki and co-workers reported a cyanosilylation of aromatic and aliphatic aldehydes in good yields [163]. 

Cyclopropanation of alkenes carrying electron-withdrawing groups with dihalo-carbenes does not take place under the normal reaction conditions unless the dihalocar-bene is generated from trihalomethylphenylmercury compounds. By this procedure acrylonitrile was converted into l-cyano-2, 2-dichlorocyclopropane in 78% yield and other acrylic derivatives are transformed into dichloromethyl cyclopropane carboxylates (186) (equation 46). Treatment of electron-deficient olefins with dibromomethane in the presence of Ni(0) complex/Zinc/Lewis acid seems to be very effective for cyclopropanation. ... 

The reactions of P-halophospha-alkenes with carbenes provide a new route to 1-chloro-lH-phosphirines (181). Such compounds are highly reactive, and a study of nucleophilic displacement reactions at phosphorus has been facilitated by preparation of the related P-W(CO)5 complexes.The first pentacoordinate systems derived from phosphirenes, (182), have been formed in the reactions of P-halo- or P-cyano-phosphirenes with tetrachloro-o-quinone. Evidence has been presented for the rearrangement of P-chlorophosphirenium ions (183) to the vinylphosphenium ions (184)... 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

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