Fischer carbene complexes solvents

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

Reactions involving nucleophilic attack on coordinated ligands are promoted by very polar solvents that stabilize anions. Metal carbonyls are also attacked by strong nucleophiles (RLi) to give the Fischer carbene complexes qv below. Section A 1.4),... 

All Fischer carbene complexes presented in this work are air- and moisture-sensitive. The derivatives of 2 bearing electron-donating 7t-substituents and compound 5 do not decompose in substance and in dry solvents after a few weeks. The fact that corresponding Fischer carbenes of Mo and W prepared by the same method decompose even at low temperatures underlines once more the particular stability of manganese carbene complexes. 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

In acetonitrile all reactions of pyrrolidine addition exhibited AV values between -15 and -17cm3mol-1.262 However, upon decreasing the solvent polarity, AV became markedly more negative and a good correlation with the solvent parameter qv (i.e. the pressure derivative of q, the polarisability of the solvent) could be demonstrated. It was concluded that the addition of pyrrolidine followed a two-step process with a polar transition state leading to a zwitterionic intermediate. Addition of / ara-substituted anilines to a similar Fischer carbene complex in acetonitrile was characterised by more negative values of AV (-21 to -27cm3mol-1) than those for addition of pyrrolidine. 263 Second-order rate constants for the addition reaction exhibited an excellent correlation with the basicity of the aniline derivatives used. The trend in activation volumes could be correlated with an early or late transition state for the fast and slow addition reactions, respectively Scheme 12. 

As pointed out earlier, most data on pK values and rate constants for proton transfer were obtained in 50% MeCN-50% water (v/v), a solvent that is essentially aqueous but which provides better solubility for some sparingly soluble Fischer carbene complexes. Some acidity measurements were reported in other solvents as... 

Addition reactions of a, -unsaturated Fischer carbene complexes, as shown in Scheme 1.3, all exhibit AV" values of between —15 and -17 cm mol in acetonitrile. On decreasing the solvent polarity, AV becomes significantly more negative and exhibits a good correlation with the solvent parameter (i.e. the pressure derivative of q, the polarizability of the solvent) [78]. 

A number of Fischer carbene complexes of chromium and tungsten have been prepared by conventional and microwave heating (YIF = 0.79-1.1). Mono- and dimethylureas were reacted with alkynyl alkoxy Cr or W carbenes using THF as the solvent (Scheme 7.22). The resultant complexes contained a uracil-moiety. A sol-vent-free approach to the reaction was also attanpted, but this gave somewhat lower yields of the desired products. 

Solvent activation by complexes such as 194, 202 and 211 is not restricted to benzene, and has become a ubiquitous route to a variety of compounds, many of which are discussed elsewhere. Relevant to the present discussion is the thermally induced (60 °C) double C-H activation of THF by 202, to yield the Fischer-carbene complex 393 (Scheme 32) that incorporates a (7-butyl ligand. Comparable... 

One of the major applications of Fischer carbene complexes is the formation of substituted phenols, known as Dotz benzannulation. The first example of this reaction on solid phase was reported by the group of Martinez [35]. The Fischer carbene complex 16 was obtained with a loading of 95% (Scheme 10), and the reaction could be monitored by colorimetric analysis. Microwave-assisted D5tz benzannulation allowed a sixfold decrease in the reaction time. Investigations into various reaction conditions showed that the reaction was almost independent of solvent, and that elevated temperature and prolonged reaction time did not have a major influence on the yield. 

Then, in 1987, Dotz reported an improved procedure for this transformation, for which the use of Et20 as solvent improved the yield of the cyclic carbene complexes considerably [12]. For example, the five-membered Fischer-type carbene complex 29 (n=l) was prepared in 58 66% yield by the reaction of preformed M(CO)5(L) (M = Cr, W, L = Et20) and 3-butynol in Et20 at room temperature. The six-membered cyclic carbene complex could also be prepared by this method. This method has been applied to the preparation of functionalized cyclic Fischer-type carbene complexes from the corresponding alkynols. For example, Dotz et al. reported the preparation of various carbohydrate-functionalized cyclic Fischer-type carbene complexes, one of which is shovm in Scheme 5.9. 

The structure and bonding of metal silylene complexes varied from those of their carbon analogs. While Fischer-type metal carbene complexes without solvent adducts have been extensively characterized,48 most metal silylenoid complexes contain a bound solvent molecule or counterion on the silicon atom. The bond energy for donor silicon complex 22 was determined to be between 15 and 20kcal/mol 49,50... 

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