Dienyl-carbene complex

In the total synthesis of the protein kinase C inhibitors calphostins 146, the orf/zo-substituted intermediates, which are either obtained from photolysis or from reaction of the dienyl carbene complex 144 with tert-butyl isocyanide, were oxidised to yield the 1,2-benzoquinone 145 as a common product [81] (Scheme 61). 

Pyrazolyl- [85], dihydropyridyl- [86], and pyrrolizinylcarbene complexes [87] have also been subjected to the benzannulation to give the respective oxygenated benzo-N-heterocycles. Finally, a,/ ,y,internal double bond have been utilized to prepare 2,3-dihydro-l,2-benzisoxazoles and indazoles by intramolecular benzannulation [60h]. 

In recent years the group of C. A. Merlic has reported photochemically induced cyclizations of dienyl carbene complexes of type 39 to produce phenol derivatives 40 [19]. In these very intelligently designed reactions, which are related to the Dotz reaction, the primary, photochemically generated intermediates of type 41 undergo a (formal) electrocyclic ring-closure to form linear, conjugated cyclohexadienones 42, which then tautomerize to the phenols (Scheme 12). 

The [2 + 2] cycloaddition of the propynyl tungsten complex (12b) has been found to be stereospecific with the cis and trans isomers of eAyl prc nyl ether." ITie reaction with c/s-propenyl ethyl ether gave rise to the cyclobutenyl complex (18), in addition to a minor amount of the ring-opened dienyl carbene complex (19). The stereochemistry about the double bonds in the dienyl complex (19) revealed that it was derived from a thermal electrocyclic conrotatory ring opening of the c/s-cyclobutenyl carbene complex (18) and thus the stereochemistry is completely retained in this cycloadditirxi. In support of this analysis it was found that cyclobutenyl complex (18) could be thermally opened to the dienyl complex by heating in an inert solvent at 50 C. In the cycloaddition of the propynyl complex (12) with trans-... 

The vinylcarbene complexes Cr(CX))5 =C(OEt)C=CXR) (X = NMe2, OEt R = alkyl, aryl) react with alkynes R OCH to form various cyclopenta[b]pyrans.30S In a two step process, addition of the alkynes NEt20Chfe u> the l-metalla-l,3-diene W(CX))s =C(OEt)CHsCHHi) results in metallatriene species that cyclise U) fwm cyclopentadiene complexes 40. Chromium Fischer type alkoxyalkenyl carbene complexes react with ketoalkynes to fcxm bicyclic lactones in a number of different processes involving 8 and 10-e cyclizations. Various dienyl carbene complexes whose parent in Crortho-substituted aromatic alcohols through a variation upon the more usual benzannulation reaction involving alkyne cycloaddition with carbene 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. 

Cyclopenta[fc]dioxanes (44) are accessible from the reaction of the dioxenylmolybdenum carbene complex (43) with enynes <96JOC159>, whilst an intramolecular and stereoselective cyclisation of (Ti5-dienyl)tricarbonyliron(l+) cations affords chiral frans-2,3-disubstituted 1,4-dioxanes <96JOC1914>. 2,3-Dimethylidene-2,3-dihydro-1,4-benzodioxin is a precursor of the 3,8-dioxa-lff-cyclopropa[i]anthracene, which readily dimerises to dihydrotetraoxaheptacene (45) and the analogous heptaphene <96AJC533>. 

Ruthenium vinylidene and carbene complexes containing cyclopenta-dienyl ligands such as [(i75-C5H5)(PPh3)2Ru=C=CH2]+ are known and an excellent review describing the structures and reactivities of these classes of complexes is available (103). Treatment of the -vinyl-bridged porphyrin with Ru3(CO)12 yielded (TPP)Ru C=C(p-Cl-C6H4)2, ... 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

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. 

A similar type of reaction has been observed in the reactions of iron-carbene complexes with 1,3-dienes. In this case the direction of reductive elimination in the metal hydride intermediate corresponding to (193) is constrained to that which generates a conjugated 1,3-diene however, two isomeric products are also obtained from this reaction which are epimers about the face of the diene to which the iron tricarbonyl group is attached. Tliis reaction produces highly functionalized 1,3-dienyl complexes of iron in high yield under relatively mild conditions and will likely play a role in the development of the... 

A series of stable chromium and tungsten Fischer dienyl carbenes 66 were prepared by 1,3-DC of alkenylethynyl carbene complexes 65 with nitrones. Treatment of isoxazolines with isocyanides afforded highly functionalised 2,3-dihydro-l,2-benzisoxazoles 67 in a completely selective fashion and with high yields <01CEJ5318>. 

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,... 

A [5+1] carbonylative cycloaddition of chromium Fischer carbene complexes having /ra j,/ra 5-dienyl substituents at the carbene carbon atom with nonacarbonyldiiron gives ri -2-alkoxycyclohexa-2,4-dienone(tricarbonyl)iron complexes and 2-alkoxyphenols (Scheme 4-13). Without the addition of the carbonyliron complex, this reaction works only for substrates that have a cw-disposition at the a,P-double bond. The ri -2-alkoxycyclohexa-2,4-dienone(tricarbonyl)iron complexes can be converted to the corresponding phenols by treatment with base or by stirring with silica gel in the presence of air. 

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