Chromium amine complex reactions

Photolysis of chromium alkoxycarbene complexes with aldehydes in the presence of Lewis acids produced /J-lactones [83]. Intermolecular reactions were slow, low-yielding, and nonstereoselective, while intramolecular reactions were more efficient (Eqs. 19 and 20). Subsequent studies showed that amines, particularly DMAP, could also catalyze this process (Table 13) [84], resulting in reasonable yields and diastereoselectivity in intermolecular cases. 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Many preparative methods have been used, with variable success oxidative decarbonylation of Cr(CO)6 by the ligand,1093,1094 reaction of the appropriate amine with [Cr(sal)3J (salH is salicylaldehyde), reaction of the ligand, either produced in the reaction mixture or preformed,1095 with hydrated or anhydrous CrCl3, hydrated Cr111 acetate,1096 or [CrCU(THF)3],1097-1099 and aerial oxidation of the chromium(II) cpmplex produced in sifu.1099-1101 No well-established chromium(II) complexes, except of a few /3-keto amines (Section 35.3.9.1), have been isolated. 

Hydrolysis of ammonia or amines is often observed, but only in a few cases have such reactions proved to be useful synthetically. Base hydrolysis (aqueous NH3) of the so-called rhodo ion, (NH3)5Cr(OH)-Cr(NH3)55 +, yields the so-called cis hydroxo erythro ion, cis-(NH3)5-Cr(OH)Cr(NH3)4(OH)4+, and both this ion and its corresponding acid form, cis aqua erythro have been isolated as salts (227, 252, 253). The hydrolysis is complete within minutes, and unlike the hydrolysis of many other ammine chromium(III) complexes, is quite a clean reaction, at least in solutions of moderate alkalinity (225). The corresponding trans aqua isomer has been prepared by heating the solid... 

Several heteroatom nucleophiles, for example, amines, alcohols, thiols, carboxylates, and dialkylphosphines, undergo Michael addition reactions with alkene- and alkyne-substituted carbene complexes. Reaction of alkyne-substituted chromium carbenes with urea affords products derived from Michael... 

Chromium tricarbonyl-complexed aryl fluorides undergo nucleophilic substitution reactions. The substitution is not a straightforward SNAr mechanism as can be seen using, for example, 4-methoxy-l-fluorobenzene complex (71). Reaction of (71) with acetylide (72) gives a 1 2 mixture of the 1,2 and 1,3 products (73) and (74) (Scheme 115). Other leaving groups include halogens, alkoxides, and amines. Indazoles can be prepared by reaction with hydrazine followed by acidic deprotection-decomplexation (Scheme 116). 

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

Deur, C., Miller, M., Hegedus, L. S. Photochemical Reaction between Tertiary Allylic Amines and Chromium Carbene Complexes ... 

As examples of series of related reactions, compensation effects have been described [53] for the thermal decompositions of [CoXj (aromatic amine)2] type complexes (7 reactions) and also for a series of cobalt (III) and chromium (III) complexes (22 compounds studied in which two compensation trends were identified). Later work [54] examined the dehydrations and deamminations of dioximine complexes (two compensation trends identified), and [Co(NCS)2(ammine)2]-type complexes (three compensation trends identified). The systems involving larger entropy changes required less energy for activation [53]. Separate compensation plots for the dehydrations and the decompositions of eleven alkali and alkaline-earth metal dithionates were described by Zsako et al. [55]. 

The possibility of insertion of aquo- and amine-copper(II) complexes or chromium(III) complexes into a PMS matrix during the synthesis was discussed [40,41]. Tetraamino complexes of copper such as [Cuen2(H20)2] and hexa- and penta-amino complexes of chromium [Cren3] + and [Cr(NEt3)5Cl] were used. The synthesis of the adsorbents was performed in aqueous solutions at pH 9-10, in which amino complexes of transition metals undergo alkaline hydrolysis forming hydroxo-complexes. The presence of hydroxyl groups in the coordination sphere of the metal allowed a polycondensation reaction between the complex and hydrolysis products of sodium methylsiliconate (see Scheme 7). 

Polarographic reduction of chromium(iii) complexes of dithiocarbomates R2NCS J (R = Et, Mr, or Bu") and of heterocyclic amines of the type (CH2)2NCSJ (n = 4,5, or 6) in the presence and absence of the ligands bipy and phen has been studied in DMF. The addition of the first electron occurs reversibly in all cases and the rate of formation of adducts with phen is greater than with bipy. Electrolysis of a solution of Cr(Et2 NCS2)3 in MeCN at — 1.1 V in the presence of bipy gives rise to the sequence of reactions shown in Scheme 4. The Cr product species was identified by e.s.r. spectroscopy. Normal co-ordinate analysis on a series of chromiumfiii) dithiocarbamates... 

Chiral 2-substituted benzaldehyde chromium tricarbonyl complexes have been reacted with chloroacetophenone in the presence of KO-fert-Bu [544], After decomplexation, the E-epoxyketone is obtained with a high selectivity (Figure 6.88). This Darzens reaction with ClCF COO-tert-Bu is poorly stereoselective. Condensation of the same aldehydes with methyl aaylate or acrylonitrile in the presence of DABCO, followed by decomplexation, also leads highly selectively to P-hydroxyesters or -nitriles 6.105 (Y = COOMe or CN) [547] (Figure 6.88). An anti aldol product is also obtained with a high selectivity from a chromium complex and the titanium enolate of PhCF OCF COS-tert-Bu at -78°C [1281, 1282], Chiral aminals of a-ketoaldehydes react with lithium or sodium enolates of ethyl acetate. After treatment with acid, compounds 6.106 are obtained with a high enantiomeric excess (Figure 6.88). 

Thus the activation volume AV for the rate constant kp of an individual ES reaction pathway can be evaluated if the pressure dependencies of the photoreaction quantum yield, of intersystem crossing and of the ES lifetime can be separately determined. However, such parameterization becomes considerably more complex if several different excited states are involved or if a fraction of the photosubstitution products are formed from states that are not vibrationally relaxed with respect to the medium. Currently, parameterization of pressure effects on photosubstitutions has been attempted for a limited number of metal complexes. These include certain rhodium(III) and chromium(III) amine complexes and some Group VI metal carbonyls, which will be summarized here. 

---