Metal-arene complexes manganese

A variety of transition metals, for example, chromium, molybdenum, tungsten, iron, vanadium, manganese, and rhodium can be used to prepare relatively stable j -arene complexes (see Arene Complexes). Reactions of j -arene chromium tricarbonyl complexes have been extensively examined, and numerous reviews are available. Although chromium complexes are by far the most utilized in organic synthesis, complexes of iron and manganese are emerging as potentially useful alternatives. 

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

A wide range of metals form ti -arene complexes, but the best-known complexes are with chromium tricarbonyl. Complexes of iron, manganese and ruthenium have also been used. These complexes have the Cr(CO)3 moiety or other metallic fragment above the -ir-system of the aromatic ring (Figures 10.2 and 10.3), thereby making the two faces distinct. This can be exploited for stereochemical control purposes. 

Most arene complexes are hexahapto, and ternary complexes usually have 18 valence electrons. The most known and used ones for their applications in arene synthesis are the arene-metal-tricarbonyls (columns of chromium and manganese), in particular the arene-chromium-tricarbonyl and the arene-manganese-tricarbonyl cations... 

Examples of reductive cluster-opening and oxidative cluster-closing reactions are common in the chemistry of metal-hydrocarbon tt complexes. For example, bases convert nido- (hexa-hapto)arene-manganese tricarbonyl complexes into aracAno(pentahapto)-7T-cyclohexadienyl complexes 129,130, 217) ... 

General reviews have appeared in the literature [1, 2] concerning arene chromium complexes as well as iron [2] and manganese [3,4] complexes, and nucleophilic additions followed by iodine oxidation and loss of the metal have been well reported, for example by Semmel-hack [lc] and by Morris in 1995 [le]. The purpose of this chapter is to focus only on ipso,... 

Similar reaction conditions for manganese, iron and cobalt complexes lead to more stable arene systems, presumably due in part to a higher valence electron count for these metal centres. Reduction of [3,5- Pr2-2,6-Trip2C6HMX]ra (M = Mn, X = I, n= 1 M = Fe, X = C1, n= 1 M = Co, X = C1, n — 2) gives rise to the Mn(I) inverted sandwich complex [3,5- Pr2-... 

Softer nucleophiles N, for example, H or PBub, add directly (reversibly in the latter case) to the exo face of the arene to give the [Re(j7 -C6R6N-6-exo)(CO)3] (142) complexes without any participation of the metal center. Attempted extension to Re complexes of the well-established methodology for double functionalization of arenes earlier developed for the Mn analogs on the basis of [Mn(j -dienyl)(CO)2(NO)]+ (see Manganese Organometallic Chemistry) is thwarted by the limitations foimd in the preparation of the cations [Re()]5 -dienyl)(CO)2 (NO)]+. 

The cations of manganese group metals [M(arene)2] are very resistant to both oxidation and reduction. The Re(0) complex [Re(C6Me6)2] is very unstable, however. It easily undergoes dimerization. 

The Dotz benzannulation reaction (DBR) is the reaction of an a,P unsaturated Fischer carbene with an alkyne to produce a highly substituted phenol. Alternatively, the DBR can be considered a metal templated 3 + 1 + 2 cycloaddition of an allylic carbine (3 carbon unit), carbon monoxide (1 carbon unit), and an alkyne (2 carbon unit). The initial product of the reaction is the arene chromium tricarbonyl complex of the phenol as in 4. These complexes are typically unstable in air such that workup and purification of the product lead to the complete loss of the metal. Chromium is the most often used metal for the benzannulation. Molybdinum, tungsten, and manganese have been used but usually give mixtures of products and require harsh reaction conditions. 

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