Chromium complexes acetylene

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

Chromium carbene complexes containing chromenes undergo benzannulation reactions with acetylenes to give 3//-naphtho[2.1 -//]ehmmcncs as products (Equation 12 163—> 164). The intermediate naphthols are air sensitive and are best trapped by protecting groups or further functionalization <2006JA11044>. 

Sugar acetylene reacted with a Fisher chromium carbene complex to give a phenol derivative, a possible intermediate for naturally occurring aryl-C-glycoside antibiotics (O Scheme 34) [55]. 

P. Quayle and co-workers utilized the Dotz benzannulation reaction for the synthesis of diterpenoid quinones." The authors developed a novel synthetic approach to 12-O-methyl royleanone using a simple vinyl chromium carbene complex along with a disubstituted oxygenated acetylene. The bicyclic hydrazone was converted to the corresponding vinyllithium derivative by the Shapiro reaction and then functionalized to give the desired crude Fischer chromium carbene complex. The benzannulation took place in refluxing THF with excellent regioselectivity, and the natural product was obtained in 37% overall yield from the hydrazone. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

From an investigation of various Michael acceptors, it has emerged that ethoxymethylidene malonates are suitable for addition-elimination reactions with acetylides to give diacids (426) after hydrolysis and elimination. Upon heating in a-dichlorobenzene, these acids cyclise to give ylidenebutenolides (427)Chromium carbonyl complexes of acetylenes have been used to prepare 2-methoxyfurans and... 

Some carbohydrate acetylenic alcohols have been converted to metal carbene spirocyclic derivatives, e.g. 22 (Scheme 4) Acetobromoglucose has been converted into tri-O-acetyl-D-glucal in 90% yield by way of a glycosyl chromium-(III) complex, and an organomercury intermediate featured in a synthesis of tri-O-benzyl-2-C-methyl-D-glucal covered in Chapter 14. 

The EDA results for the metal-ligand interactions in the metallacyclic compounds CI4TM-C2H, (TM = Mo, W), which are shown in Table 7.7, are very different from the data for the ethylene and acetylene complexes. There are no results for the chromium compounds because of SCF problems [34]. The electron-sharing... 

Figure 6 Synthesis of naphtoquinone antibiotics from a chromium carbene complex and an acetylenic substrate. ...

In attempting to extend this reaction to derivatives of chromium and molybdenum, completely different results were obtained under comparable reaction conditions. Reaction of 3-hexyne with (CH CN) Mo(CO) did not appear to give a compound corresponding to the tungsten acetylenic complex, Hexaethylbenzene isolated from the reaction mixture indicates that the molybdenum complex causes cyclotrimerization of the acetylenic molecule. 

As stated above, cyclopentanones, cyclobutenones, and indenes have been observed as by-products in the DBR. Wulff has studied the effect of solvent, chelation, concentration, and alkyne substitution on the product distribution. He reported that simple a,(3-unsaturated chromium carbene complexes typically show excellent selectivity for the benzannulated product. This selectivity is not sensitive to changes in solvent or substituents on the acetylene. However, the reactions of aryl complexes with acetylenes are very sensitive to the nature of both the solvent and the acetylene. For aryl chromium complexes, the highest selectivities and yields for the benzannulated product arise with solvents of low coordinating ability hexane and benzene. Solvents with intermediate coordinating ability and small size... 

Carbonylation of the chromium carbene complex (CO)5Cr=C(OCH3)Ph at 150bar pressure in the presence of l-vinyl-2-pyrrolidone resulted in Cr(CO)6 and organic products that were rationalized by the intermediate formation of methoxyphenylk-etene [33]. The formation of a free vinylketene and vinylketene chromium complexes were found in the reaction of (CO)5Cr=C(OCH3)Ph and bis(trimethylsilyl)acetylene at 50 °C (reaction 8.19) [34-36]. 

Figure 50. Proposed mechanism of formation of arene-chromium derivatives from acetylenes and chromium-aryl complexes.

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

Similar pyrone complexes were isolated by Semmelhack97a as the products of the reaction between tetracarbonyl[ethoxy(alkyl)carbene]iron(0) complexes and various acetylenes. Vinylketene complexes are proposed as key intermediates in the mechanism of this conversion, which closely matches analogous reactions with cobalt carbenes51 (see Section V,B), while showing crucial differences with the analogous reaction of a chromium carbene (see Section II,B). 

Density functional calculations on the Dotz reaction leading from chromium carbene (75) with acetylene to give the phenol (78) suggested a new mechanism involving the formation of a chromahexatriene complex (77) from the initially formed vinylallyl-idene complex (76). " Complex (77) then collapses to the phenol complex (78). 

The starting Fischer-type carbene complexes 1 were obtained by Michael addition of dimethylamine to the carbon-carbon triple bond of the corresponding ethoxy-(phenylethynyl)carbenes. In this regard, de Meijere and co-workers observed that the reactions of several primary and secondary amines with this sort of carbenes, in particular chromium derivatives 3 containing bulky substituents at the terminal carbon of the acetylenic unit, result in formation of the aminoallenylidene derivatives 5 as by-products of the expected Michael adducts 4 (Scheme 2) [20-24]. 

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. 

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