Chromium complexes reactions

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

An unexpected varying regiochemistry in intramolecular benzannulation has also been observed in the synthesis of cyclophanes. As mentioned above, there are only two possible regiochemical outcomes in the benzannulation reaction, which differ in the direction of alkyne incorporation. / -Tethered vinyl-carbene chromium complexes undergo an intramolecular benzannulation reaction with incorporation of the tethered alkyne with normal regioselectivity to give meta-cyclophanes [28]. 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

A transmetalation of the styrylcarbene chromium complex 62 in the presence of stoichiometric amounts of [Ni(cod)2] to give the nickel carbene intermediate 63 was applied to the synthesis of Cr(CO)3-coordinated cycloheptatriene 64 upon reaction with terminal alkynes [57] (Scheme 37). The formation of pen-tacarbonyl(acetonitrile)chromium is expected to facilitate the metal exchange. 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

It has, however, proved possible to synthesize the following chromium complexes via a 77 K, Cr-atom, cocondensation reaction with 2,-... 

Hg " complexes are common, but complexes of Na, Mg", or Al are rare. Chromium complexes are also common, but in such complexes the chromium is in a low or zero oxidation state (which softens it) or attached to other soft ligands. In another application, we may look at this reaction ... 

The intramolecular cyclization route to p-lactams still provides interest. P-Amino esters (obtained by a Reformatsky-type reaction of an imine and bromoacetates derived from chiral alcohols) are cyclized by the action Grignard reagents to 4-substituted P-lactams with impressive e.e. <96TL4095>. A similar approach through a Reformatsky-type reaction uses tricarbonyl(Ti -benzaldimine)chromium complexes and ultrasound <96T4849>. 3-Methyl-azetidin-2-ones (obtained from 3-amino-2-methylpropionates) have been resolved and their... 

Mikami M, Hatano M, Akiyama K (2005) Active Pd(II) Complexes as Either Lewis Acid Catalysts or Transition Metal Catalysts. 14 279-322 Minatti A, Dotz KH (2004) Chromium-TemplatedBenzannulation Reactions. 13 123-156 Mim-aM, Satoh T (2005) Catalytic Processes Involving b-Carbon Elimination. 14 1-20 Miima M, Satoh T (2005) Arylation Reactions via C-H Bond Cleavage. 14 55-84 Mizobe Y, see Hidai M (1999) 3 227-241... 

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

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