Aryl carbene complexes

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

It has been shown how alkenylcarbene complexes participate in nickel(0)-me-diated [3C+2S+2S] cycloaddition reactions to give cycloheptatriene derivatives (see Sect. 3.3). However, the analogous reaction performed with alkyl- or aryl-carbene complexes leads to similar cycloheptatriene derivatives, but in this case the process can be considered a [2S+2S+2S+1C] cycloaddition reaction as three molecules of the alkyne and one molecule of the carbene complex are incorporated into the structure of the final product [125] (Scheme 82). The mechanism of this transformation is similar to that described in Scheme 77 for the [3C+2S+2S] cycloaddition reactions. 

A greatly enhanced chemoselective formation of phenol is observed for alkoxy(alkenyl)carbene complexes compared to alkoxy(aryl)carbene complexes. This behaviour reflects the ease of formation of the rf-vinylketene complex intermediate E starting from alkenylcarbene complexes for aryl complexes this transformation would require dearomatisation. 

Amino(aryl)carbene complexes prefer cyclopentannulation over benzannulation. Amino(alkenyl)carbene complexes may react in a benzannulation reaction. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Experimental Procedure 2.2.6. Cyclopentannulation with a Molybdenum Aryl-carbene Complex 3-Hexyl-5-methyl-l-indanone 346]... 

If the Ddtz benzannulation reaction is conducted with ori/zo-disubstituted aryl-carbene complexes, the final aromatization step is blocked and cyclohexadienones can be isolated (Figure 2.34) [356,378,379]. 

Non-heteroatom-substituted carbene complexes of almost all transition metals are known. Depending on the oxidation state of the metal, the overall charge of the complex, and the properties of the additional ligands, the reactivity of alkyl or aryl carbene complexes can vary greatly. Some examples of compounds with strikingly different chemical properties are shown in Figure 3.1. 

In contrast to aryl carbene complexes, vinyl carbene complexes are known to yield only the benzannulation products [37]. For instance, carbohydrates [38], tetramethyl ketals of qui-nones [39], heterocycles, and oxacycloalkenylidene carbene complexes [40] have been used as part of a (cyclic) vinyl carbene complex. For example, complex 29 and diphenylethyne were converted to the acyl hydroquinone 30. Thus, 29 serves as a synthon for the (electron-poor) benzoyl vinyl carbene complex (Scheme 14) [40]. 

For example, in attempts to realize benzannelation reactions, alkyloxy aryl carbene complexes of manganese failed to react with alkynes even in refluxing toluene, and the starting compounds could be recovered [4]. The documented low reactivity of the Mn as opposed to Cr and Mo carbene complexes may in part explain why the electrophilic carbene C-atom and the nucleophilic diazo C-atom tolerate each other in the same molecule. Besides, the bulky substituents at the silicon atom protect it fi"om being attacked by nucleophiles leading to desilylation as reported for trimethylsilyl substituted Cr carbene complexes [5]. 

Rearrangement. Allyloxy(aryl)carbene complexes are converted to aUyl aryl ketones on Pd(0) catalysis. ... 

Aryl carbene complexes with electron-donating or electron-withdrawing substituents in the ortho-, para-, or meta-positions participate in the DBR. The aryl group can also be naphthyl and heteroaryl such as furan, thiophene, pyrrole, pyrazole, and indole. Simple alkyl substituted vinyl carbene complexes have been extensively examined. The double bond can be in either a cyclic or an acyclic system. 

The [3+2+1] cycloaddition of an a, 3-unsaturated or aryl carbene complex of chromium, an alkyne, and carbon monoxide, that is named the Dotz benzannulation, is a useful method for the synthesis of a phenol or naphthol derivative, although this reaction requires a stoichiometric amount of the chromium carbene complex of chromium [35]. A mechanism of this reaction is shown in Scheme 21.31. When unsymmetrical alleynes are used, the regioselectivity is determined in the alkyne insertion step by the steric effect. 

Acyclic aminocarbenes have not been used widely as ligands for transition metal catalysts. The amino(aryl)carbene complexes 26 and 27 are rare examples that showed moderate activity in the Suzuki coupling of aryl iodides and bromides with phenylboronic acid (Figure 5.7). ... 

Transition-metal carbene complexes have proven especially useful in the construction of six-membered rings, primarily in the synthesis of naphthalenes and higher aromatic ring systems. The major method employed is the Dbtz benzannulation reaction (depicted in Scheme 17.12), where an a,p-unsaturated or aryl carbene complex of chromium (78) couples with an alkyne to afford a phenol derivative (76). This reaction has been demonstrated for a vast array of molecular architectures and functionalities and has proven to be a very reliable and efificient process. Perhaps the major limitation on the synthetic utility is the construction of the carbene complex for systems that require structural complexity in the carbene complex. In addition to the Dotz reaction itself, several efforts to develop alternative procedures have been inspired by a detailed understanding of the mechanism of the Dotz reaction. 

The thermal [3-i-2-i-l]benzannulation reaction of a,/3-unsaturated alkenyl- or aryl-carbene complexes with alkynes represents the unique and most synthetically valuable reaction of chromium carbenes [20]. A variety of densely substituted benze-noid compounds is accessible by this Cr(CO)3-templated one-pot cyclization reaction. Experimental and theoretical studies support a stepwise C-C bond formation according to a mechanism (Scheme 11.6) that starts with a reversible decarbonyla-... 

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