Arylcarbene-chromium

Activated esters for use in peptide-coupling reactions were produced by photolysis of optically active chromium aminocarbenes with alcohols which are good leaving groups, such as phenol, pentafluorophenol, 2,4,5-trichlorophenol, and N-hydroxysuccinimide (Table 17) [ 109]. Since arylcarbenes bearing the op-... 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

When the arylcarbene(pentacarbonyl)chromium complex bears a (phenyl) alkyne at the orf/zo-position of the aryl group, an intramolecular alkyne-carbe-ne chelate (126) can be formed by low temperature photodecarbonylation. The tetracarbonylchromium chelate 126 obviously dimerizes and furnishes finally a densely functionalized centrosymmetric chrysene (127, see Scheme 58 [160a, d]). 

The chromium-templated coupling of alkenyl- or arylcarbene, aUcyne and carbonyl ligands generates arene tricarbonylchromium complexes as primary benzannulation products which - based on their unsymmetric substitution pattern - bear a plane of chirality. Chiral arene complexes are powerful reagents in stereoselective synthesis however, the preparation of pure enantiomers is a lengthy and often tedious procedure, and thus diastereoselective benzannulation appears to be an attractive alternative. In order to lure the chromium fragment to one or the other face of the arene formed, chiral information may be incorporated in the carbene complex or the aUcyne. 

Synthesis of Five-Membered Carbocycles An annulation of chromium arylcarbenes by alkynes without CO-insertion leads to five-membered rings. It has been observed as a side reaction along the benzannulation which may become predominant depending on the nature of the carbene heteroatom substitution pattern, [72] the metal [40] and its coligands [73] and the solvent [43] used. In this respect, indene derivatives have been obtained from arylcarbene complexes of chromium and tungsten. [73,74]... 

The dimerization of chromium-coordinated carbene ligands typically requires temperatures above 120°C. Whereas the addition of catalytic amounts of Rh2(OAc)4 allows to only a minor decrease in temperature to 100°C, the dimerization occurs already at room temperature under palladium catalysis. [95a,96] When the reaction of chromium arylcarbene 16 was promoted by Pd(OAc)2 (10 mol-%) in the presence of NEt3 using THF as a solvent, a mixture of isomeric carbene dimers 95 was obtained with an E Z ratio of 2 1 (Scheme 42). The effects of the eatalyst load, phosphine additives, the reaction temperature, the solvent and a series of other palladium catalysts have been investigated systematically, but did not reveal significant changes. [96]... 

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