Chromium amino-carbenes

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

Asymmetric synethesis of dipeptides. The chromium amino-carbene 1, pentacar-bonyl [(R-l-aza-2,2-dimethyl-3-oxa-5-phenylcyclopenty )(methyl)carbene]chromium(0), derived from (R)-alanine, undergoes carbonylative coupling with amino esters under irradiation (visible light) at 0° to form (S,S)-dipeptides with high diastereoselectivity. 

Scheme 5.36 Stereocontrolled dipeptide formation by chromium amino carbene photolysis.

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

Otherwise, while the reactions of chromium mono(amino)allenylidene complexes 5 with dimethylamine afforded the aIkenyl(amino)carbene derivatives 49 by means of the expected N-H addition across the C (=Cp double bond (Scheme 13)... 

The relatively sensitive acylamino chromium complexes (e.g., 43) can be prepared in situ from stable amino carbene complexes (e.g., 42) as shown for the generation of miinchnone 44 and conversion to pyrrole 45 with DMAD (Scheme 10.5) (21). 

To obtain amino acids, amino carbenes (X = NH2 in Fig. 6) must be used, and if chirality is required, the asymmetry of the new a-carbon center must be controlled. Higedus and co-workers have achieved good stereochemical control by using a chiral oxazolidine chromium carbene. 

M. 1-Aza-t,3-butadiene Complexes via 1,4- and N-Atkenyl(amino)carbene Chromium Complexes via 1,5 Hydrogen Shifts... 

Merlic, C. A., Xu, D., Gladstone, B. G. Aminobenzannulation via photocyclization reactions of chromium dienyl(amino)carbene complexes. Synthesis of o-amino aromatic alcohols. J. Org. Chem. 1993, 58, 538-545. 

Similarly, the [cyclopropylmethyl(methyl)amino]carbene system reacted to give a cyclopropyl-methyl-substituted pyrrolinone. This cyclopropane-containing aminocarbene complex of chromium underwent alkyne insertion and formation of a heterocyclic ylide. Further rearrangements gave dihydro pyrrolones 17 and 18 with cyclopropylmethyl substituents. ... 

The competition of benzannulation and pentannulation significantly depends on the donor ability of the carbene ligand. Substitution of alkoxy for amino groups in the carbene ligand increases the thermal stability of the metal-carbonyl bond which hampers both the primary decarbonylation and the CO incorporation into the final product. As a consequence, annulation of chromium amino(aryl)carbenes requires elevated temperatures (> 90°C) and affords cyclopentannulation products (indenes). A-Acylation e.g. by Boc) reduces the donor properties of the amino substituent and thus favours again the... 

The cycloaddition reactions of chiral 2-amino-l, 3-dienes with Fischer-type carbene complexes have been examined by Barluenga et al. Reactions with tungsten vinyl carbene complexes A and with boron-nitrogen-chelated chromium complexes B lead very selectively to cyclohexanone derivatives. However, when employing chromium vinyl carbene complexes in acetonitrile at room temperature in these reactions, cycloheptadienes were obtained selectively by cyclopropanation and subsequent Cope rearrangement. 

Irradiation of chromium dienyl(amino) carbene complexes produces o-amino aromatic alcohols. An intermediate in this reaction is presumably a photogenerated dienylketene which undergoes electrocyclisation and aromatisation. Irradiation of optically active chromium aminocarbenes,... 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

C)5W=C(0Et)CH=C(0Et)CH=CR(02CR)]. Aldehydes or ketones react with the [diosphine complexes, [(OC)4(PR3)Cr=C(OMe)Me] to give aldol condensation products. Reaction with the amino carbene complexes [(OC)sCr=C(NMe2)Me] results in aldol addition products. Aldol reactions of optically active aminocarbene complexes proceed with moderate to high diastereoselectivity. Photolysis of chromium alkoxycaibene complexes with aldehydes in the presence of Lewis acids produces -lactones in low yield. Intramolecular reaction by incorporation of the aldehyde into either side chain of the carbene complex is considerably more efficient. ... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols upon treatment with alkynes, but form indene derivatives. Vinyl(dialkylamino)car-bene complexes, however, react with alkynes to yield aminophenols as the main products if solvents of low nucleophilicity are used [335]. (2-Amino-1-vinyl)carbene complexes do not undergo benzannulation when treated with alkynes, but form cyclopentadienes or heterocycles instead [251]. 

The synthesis of unnatural chiral amino acids is a considerable challenge to the organic chemist. One approach is via chiral organochro-mium(0) carbenes (56). The starting material is easily prepared from chromium hexacarbonyl and organolithium reagents (Fig. 5), and the resulting carbenes are air-stable yellow to red solids. 

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