Ring closure chromium complexes

An electrocyclic ring closure then leads to a cyclohexadienone complex 7, which upon migration of a proton, yields the chromium tricarbonyl-hydroquinone complex 3. 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

Among the synthetically useful reactions of Fischer carbenes, the benzannulations are certainly the most prominent. In particular, the so-called Dotz reaction, first reported by Dotz in 1975 [3], is an efficient synthetic method that starting from aryl- or alkenyl-substituted alkoxycarbene complexes of chromium affords p-alkoxyphcnol derivatives by successive insertion of the alkyne and one CO ligand in an a,/Tunsaturated carbene, and subsequent electrocyclic ring closure (see Figure 1). 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

Spiropyrans SP-2, reported by Miyashita et al., forming chromium complexes on their indoline phenyl ring, can possibly take two diastereomeric spiro forms >[47]. Because of the favorable interaction of the pyran oxygen atom and the chromium atom, the thermal ring closure occurred diastereomerically to give only one diastereomer from the planar merocyanine form generated by photoirradia-... 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Electrocycylic ring closure combined with Diels-Alder addition has been employed to derive all four rings of the tetracyclic ring system in the steroids by means of a one pot synthesis (ref.198). Thus, from the triflate of hex-4-ynol and the readily available hexa-1,5-diyne a triyne intermediate was obtained which with chromium or tungsten hexacarbonyl formed a triyne carbene complex. This with a 1,3-dialkoxybuta-1,3-diene in acetonitrile with carbon monoxide 0 atmosphere) initially at ambient temperature and then at 110°C over 24 hours afforded a 62% yield of the two products shown [(1) R = TBS (2) R = H, (1) (2) 5 2]. 

Cyclobutenones are fairly common side-products in the reaction of chromium arylalkoxy-carbenes with internal alkynes. As indicated in Scheme 5-1, the branch point in the formation of cyclobutenone versus naphthol products is believed to be vinylketene intermediate 4, which may undergo electrocyclic ring closure to 9, followed by reductive elimination to the product [7 a]. Cyclobutenone formation occurs only in the presence of internal or external ligands that can coordinate to unsaturated chromium species sufficiently well to prevent complexation to an internal n-system and thus divert the system toward 9. Depending on the alkyne and aryl substitution patterns and the reaction conditions, cyclobutenone formation can be made to predominate. Thus, solvents of good coordinating ability such as acetonitrile, o-OMe aryl substitution (which allows internal coordination to chromium), and bulky alkynyl substituents all favor cyclobutenone formation [Eq. (23)] [13]. In fact, the effect of solvent alone can be even more dramatic for the reaction partners in Eq. (21), a 0.5 M concentration of the carbene complex in acetonitrile gives instead a 78 % yield of cyclobutenone and only a combined 17 yield of quinone and indene products [9]. 

Observations on the reaction of ethylmalonatopentaamminecobalt(III) with Cr(H20)g + introduce a new element of interest. The chromium(III) products are the chelated malonate (67%) with a corresponding amount of free alcohol and the monodentate ester complex (33%). Since ester hydrolysis in the latter species is slow, we conclude that hydrogen results from the ester in the chelate form. Again, since ring closure of the monodentate product complex is slow, chelation must have occurred before Cr is oxidized to Cr . It is possible that formation of the chelate as primary product is complete, and that this product reacts in part to yield the monodentate product before hydrolysis occurs. Activation based on electron transfer to trap a function which is sensitive to a substitution-inert metal ion acting as a Lewis acid could presumably be extended to other more interesting situations. 

The acid-catalyzed cyclization of Af -phenethyl-2-phenyl-2-hydroxyethylainines is an established method for the preparation of benz[d]azepines <84CHEC-l(7)49l>, but none of the methods permit the direct synthesis of chiral products. If, however, the starting material is converted into the chiral chromium tricarbonyl complex prior to cyclization, the ring closure proceeds in a stereospecific... 

A new synthetic route to j8-lactams has recently been reported " involving the photolytic reaction of chromium carbene complexes with imines. Detailed analysis now establishesa mechanism involving the initial photolytic formation of a ketene complex (24). This is followed by nucleophilic attack by the imine at the ketene carbonyl to produce a zwitterion intermediate, which undergoes a conrotatory ring closure to form the j8-lactam as shown in Scheme 15,... 

The synthesis of monocyclic 3-amino-P-lactams by the photolytic reaction of imines with pentacarbonyl[(dibenzylamino)carbene]chromium(0) was developed by Hegedus and co-workers [74]. These reactions are closely related to the previously described [2 -h 2]-cycloaddition reactions in that they are thought to involve attack of the imine nitrogen on a photogenerated, metal-bound ketene, followed by ring closure (Scheme 15). In a synthesis of a nocardicin precursor, optically active imine trimer 122 was photolyzed with carbene complex 123 providing a 46% yield of a 1 1 diastereomeric mixture of lactams... 

Reduction of the cobalt complex (10) is much faster than reduction of (9) and leads to a chelate chromium(m) product, identified as (11) on the basis of its visible-u.v. spectrum. It is presumed that the reaction involves attack of chromium(ii) at the formyl group of (10), followed by rapid ring closure. 

The sugar 2-amino-2-deoxy-D-gluconic acid inhibits reduction of iron(III) by D-galacturonic acid. Decarboxylation of oxalacetic acid is the final step in a sequence of reactions with the C-bound chromium(III) complex, [(H20)5CrCH2CN] ". The initial steps are coordination and ring closure of the oxalacetate, both by displacement of coordinated H2O with rate constants 0.16 M s and 1 x 10 s respectively at 25.0 °C and 1.0 M ionic strenght. Decarboxylation takes place after protonation of the coordinated substrate. 

Cr(H20)4(SCH2C02H)] and [Cr(H20)2(SCH2C02H)a] are predominant with a species [Cr(H20)2(SCH2C02)2] detectable at neutral values of pH. The rates of ring opening and closure for the chromium(III) mercaptoacetate complex [Cr(H20)4SCH2C02] have been measured. 

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