Chromium complexes olefin

Mulzer J, Ohler E (2004) Olefin Metathesis in Natural Product Syntheses. 13 269-366 Muniz K (2004) Planar Chiral Arene Chromium (0) Complexes as Ligands for Asymetric Catalysis. 7 205-223 Murai S, see Kakiuchi F (1999) 3 47-79... 

Other half-sandwich Cr complexes which show good activities for olefin polymerization include those with ether and thioether pendant arms (93) and (94) which show activities of 1,435gmmol-1 h-1 bar-1 and 2,010 gmmol-1 h 1 bar-1 respectively.252 The half-sandwich phosphine complex (95) affords a-olefins arising from chain transfer to aluminum,256,257 while the related boratabenzene chromium(III) complex (96) generates linear PF.258,259 Cationic species have also been investigated, and (97) polymerizes ethylene with an activity of 56 gmmol-1 h-1 bar-1.260-263... 

Mukai et al.85 reported an asymmetric 1,3-dipolar cycloaddition of chromium(0)-complexed benzaldehyde derivatives. As shown in Scheme 5 52, heating chiral nitrone 171a, derived from Cr(CO)3-complexed benzaldehyde, with electron-rich olefins such as styrene (173a) or ethyl vinyl ether (173b) generates the corresponding chiral a.v-3,5-disubstitutcd isoxazolidine adduct 174 or... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Chelating polyolefins displace carbon monoxide from chromium hexa-carhonyl to form stable olefin complexes. Thus cyclo-octa-1,5-diene gives the yellow complex [Cr(CO)4(C8H12)] for which the cfs-structure (VI M = Cr) is proposed (79). 

Both conjugated and nonconjugated olefins form complexes with the transition-metal carbonyls. Despite the fact that the first known complex, Zeises salt K(PtC2H4Cl3), discovered in 1827, was that of a simple olefin, complexes of monoolefins are rather limited in number. However, nonconjugated diolefins (L) react with group-VI carbonyls to form complexes of the type LM(CO)4 an example is provided by tetracarbonyl-bicyclo-(2,2, l)hepta-2,5-diene chromium (2) (Fig. 1). In contrast, the iron carbonyls... 

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... 

Consequently, the elements to the left of the noble metals show strongest (ft)-character in their zero-valent oxidation state. Thus iron(O), cobalt(O) and nickel(O) are typically (b), forming inter alia strong carbonyl complexes, while the higher oxidation states of these elements have no marked ( )-character at all. Elements in zero-valent state in fact display (b) -character as far left in the periodic system as chromium, or even vanadium, which in higher oxidation states behave as very typical (a)-acceptors. To the right of the noble metals, on the other hand, the metals in their zero-valent states do not show any marked (6)-character they do not form e.g. carbonyl or olefin complexes. 

Nowadays, it is an accepted mechanistic model [5, 6] that the photolysis step (which proceeds under thermo-reversible CO insertion) leads to species best described as chromium ketene complexes of type 7 (Scheme 2). Indeed, these intermediates exhibit a ketene-like reactivity they undergo [2 + 2] cycloaddition reactions with olefins, imines and enol ethers, whereas reaction with nucleophiles leads to carboxylic acid derivatives. 

Peterson olefination involving cyclopropabenzenyl and cyclopropa[Z ]naphthalenyl anion intervention, respectively. Particularly noteworthy is the fact that subjection of disilyl-containing chromium(O) complexes (104, or its anthracene analogue) to r-BuOK provides C(i) anion without the loss of chromium in the case of 104 Peterson olefination has provided the corresponding complexed alkylidene derivative in high yield (equation 25) ", but the compound is unstable and decomposes upon attempted purification. 

The intramolecular reaction of pentacarbonyl[phenyl(3-hexenyloxycarbene)]chromium(0), a chromium carbene complex bearing an alkenyloxy substituent, provides 6-ethyl-5-phenyl-2-oxabicyclo[3.1.0]hexane with moderate exo preference19. This example shows that carbene transfer to electronically neutral olefins can proceed nonstereospecifically. 

Iron carbene complexes bearing chirality at the carbene ligand can be generated from optically pure bimetallic (chromium-iron) complexes by the addition of 1 equiv of TMSOTf in the presence of an olefin, which in situ undergoes an asymmetric cyclopropa-nation. Excellent ee s are obtained when the reaction is carried out with em-disubstituted olefins (eq 122). 5... 

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