Carbenes, amino, chromium complexes

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. 

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]. 

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). 

The stepwise coupling of two cis ligands as depicted in Scheme 3 has been verified as involving a sequence of three discrete steps at low temperatures, allowing the isolation of the relevant intermediates as individual compounds [18]. When a chelated tetracarbonyl amino-vinyl carbene complex (chelated analogue of intermediate B in Scheme 3) was reacted with an electron-deficient alkyne under controlled conditions, a l,4,5- 3-dienylcarbene tetracarbonyl chromium complex (corresponding to intermediate D in Scheme 3) was formed. It underwent thermal decomposition to give phenol derivatives as the final products. 

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

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Since transition-metal-carbene complexes with amino substitution at the carbene carbon atom are thermally very stable, their application as aminocarbene transfer reagents necessitates the use of high decomposition temperatures which, in many cases, is disadvantageous for the isolation of cyclopropylamines. Thus, for example, the cyclopropylamine 13 was not isolated from the reaction of an aminocarbene-chromium complex 12 with acrylonitrile in cyclooctane at 140°C instead, the respective decomposition product 15 was obtained in low yield.It is well known that cyclopropylamines readily undergo thermal ring opening so this result is not surprising. 

Pentacarbonyl(carbene)chromium complexes with a primary amino group at the alkenyl terminus 32, readily obtained by addition of ammonia to alkynyl-substituted complexes, initially rearranged upon heating to pentacarbonylchromium-coordinated 1-azabuta-l,3-dienes 33, which subsequently underwent [4 + 2] cycloaddition of an alkyne to give coordinated 4-ethoxy-1,4-dihydropyridines the corresponding pyridines, e.g. 34, were formed by 1,4-elimination of ethanol. ... 

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. 

Organometallic reagents were used for the synthesis of bicyclic aromatic compounds via Moore-type cyclization. Rahm and Wulff described the new synthesis of 5-hydroxyindolines with the use of a chromium carbene complex bearing alkynyl substituent 22." The amino-tethered bis-alkynyl carbene complex 22 was transformed into indoline 23 by thermolysis in the presence of a hydrogen source. The low yield of product 23 was improved when the reaction was carried out in the presence of the electrophile, added to protect the phenol function. This process involves the insertion of one carbon monoxide group from the chromium complex into the skeleton of an eneyne compound 24. The resulting enyne-ketene 25 undergoes a cycloaromatization reaction to afford the 1,4-diradical intermediate 26. Subsequent demetalation yields product 23."... 

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... 

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... 

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>. 

L. S. Hegedus, Synthesis of Amino Acids and Peptides Using Chromium Carbene Complex Photochemistry, Acc. Chem. Res. 28, 299-305 (1995). 

Reactions of 4-amino-l-metalla-l,3-dienes, generated from (1-alkynyl) carbene complexes 1 by addition of secondary amines (Scheme 32), with alkynes may lead to production of different compounds, depending on the substitution pattern as well as on the reaction conditions employed.3r Whereas successive addition of a secondary amine and one molecule of a terminal alkyne to a (l-alkynyl)carbene complex lj affords cyclopentadi-enes (Scheme 35), addition of two molecules of a terminal alkyne to complex lj yields cyclopenta[h]pyrans (Scheme 36). In an extension of this reaction sequence, (methylene)cyclopent-2-enones 103 are obtained from (1-alky-nyl)carbene chromium compounds 1 (R = rc-Pr, Ph, CH2CH2CH2OTBDS) by addition of secondary amines (NR) = NMe2, morpholino) and disubsti-tuted alkynes (Scheme 37).9e... 

Thermolysis of the pentacarbonyl chromium compounds 151 affords tet-racarbonyl chelate complexes by loss of a cis CO group,20 in the same fashion that has been found for ortho-alkoxy phenylcarbene complexes181 and similar products.182 The C=C triple bond of alkoxy(l-alkynyl)carbene complexes 1 is more reactive than that of amino(l-alkynyl)carbene complexes. (CO)5Cr = C(NHMe)—C=CPh was shown to add ethanol very slowly to give (CO)sCr = C(NHMe)-CH=C(OEt)Ph in 79% yield.21... 

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... 

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. 

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