Manganese complexes carbene

Scheme 34 Manganese carbene complexes in benzannulation reactions...

Carbonylation of the manganese carbene complex (tj5-C5Hs) (CO)2Mn=CPh2 to an analogous ketene complex has been reported (107). Compound 76 is also formed directly in the reaction of ketene with 45 (39). 

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. 

Further studies on the reaction of CH3Mn(CO)s with MnfCOls indicated that these species are in equilibrium with an anionic acyl complex which can be trapped as a carbene complex by O-alkylation (Anderson and Casey, 1971). The corresponding reaction of Re(CO)5 with CH3Mn(CO)s did not produce the expected manganese-carbene complex (VIII), but gave the rhenium-... 

This photodriven benzannulation was used in the synthesis of indolocar-bazoles (Eq. 22) [96] and calphostins (Eq. 23) [97]. The thermal insertion of isonitriles into these same classes of carbenes provided a complementary approach to similar benzannulations [98-100]. Manganese alkoxycarbene complexes underwent both inter- [101] and intramolecular [102] photodriven benzannulation reactions with alkynes (Eqs. 24 and 25). 

It is not known whether there is any carbenoid chemistry of end-on coordinated manganese-diazoalkane complexes 420 410). However, it is known that diaryldiazo-methanes 408,411 and azibenzil411 yield carbene rather than diazoalkane complexes with (n5-C5H5)Mn(CO),THF. 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

The manganese derivative 84, generated from the reaction of the uranium-carbene complex [UCp3(=C(H)PMe2Ph ] with [MnCp(CO)3], via initial... 

Bis(butadiene) complexes, with tantalum, 5, 173 Bis(z-butanethiolato) complexes, with bis-Cp Ti(IV), 4, 601 Bis(calixarene) complexes, as organic molecule hosts, 12, 799 Bis(carbene) complexes with gold(I), 2, 287-288 with manganese, 5, 780, 5, 826 with mercury, 2, 429 with palladium, 8, 230 with silver , 2, 206... 

Manganese carbenes, preparation, 5, 825 Manganese carbonyl complexes with 776-arene complexes, 5, 787 cyclopentadienyl complexes, 5, 783 with hydrocarbon ligands, complexes, 5, 776 synthesis and characteristics, 5, 761 Manganese carbonyl halides, applications and reactivity,... 

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

Summary The synthesis of various Fischer carbene complexes of manganese bearing a diazomethylsilyl group in a-position to the carbene carbon atom is described. 

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]. 

All carbene complexes were characterized by their h, C NMR, and IR spectra. In the C NMR spectra the resonances of the earbene C-atoms are found at very low field between 334.0 ppm (2f) and 348.3 ppm (2g). Most of the signals are broadened or occur twice. This phenomenon is due to the quadrupole moment of manganese and hindered rotation around the 0-C(earbene) bond and O-Si bond as often described [6]. The IR spectra show the eharaeteristic absorption bands of the diazo group (v = 2100 cm ) and the carbonyl ligands (v = 1850-2000 cm ). 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

Transition metal carbenes constitute a very important class of molecules that have found a multitude of applications. With reference to manganese, a number of new carbene complexes have been reported recently. The ) -alkyne complex (18) was found to undergo oxidation by dimethyldioxirane to afford the a-keto carbene (19). The fascinating aspect of this reaction is the likely existence of an oxirene intermediate (20). Stable oxirene complexes have never been reported, imdoubtedly because of the extreme instability of the antiaromatic oxirene ring. The possibility of trapping an oxirene by epoxidation of a coordinated alkyne is intriguing. Scheme 10 summarizes the chemistry involved. 

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). ... 

Diethyl(ethylene)tellurourea formed complexes with chromium, molybdenum, tungsten, and manganese carbonyls, in which the tellurium is coordinated to the transition metal The solid complexes are moderately stable in air. They do not decompose when stored in the dark at 20° under an inert atmosphere. A toluene solution of the chromium complex at 20° deposited tellurium forming the chromium-carbene complex. ... 

The electrophilicity of the alkylidyne carbon in cationic complexes of manganese and rhenium such as 154 or 157 is well established. A study by Chen et al. established that the carbonyl ligands are also potential sites for nucleophilic attack (157,158). Reaction of 154 with the bulky carborane anion LiCjBioH, results in the formation of two products the carbene complex 156, resulting from attack at the alkylidyne carbon, and the alkylidyne acyl complex 155, resulting from attack at a carbonyl ligand [Eq. (135)]. At room temperature complex 155 transforms into complex 156. 

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