Re alkyl complexes

Compounds of the formulas Re(CR]), ReO(CH3)4, Li2[Re2(CH3)g] [60975-25-9], Re02(CH3)3 [56090-011-8], and Re03CH3 [70197-13-6] have been prepared. The first two compounds were obtained from reaction of rhenium hahdes or oxyhahdes and methyllithium the last three were formed from the species by oxidation or reduction reactions. The use of these hydride and alkyl complexes as catalysts is under investigation. 

Structure determinations of secondary metal alkyl complexes are relatively rare, yet they provide an opportunity to assess interactions of the metal with the /3-atoms of the alkyl. The angles (excluding hydrogen) about C(24) all exceed 109°, ranging from 111.7° to 115.1°. There is no evidence for any Re 0 interaction (compare V), this distance exceeding 3 A. Both the /3-carbon, C(25), and its attached hydrogens are over 3 A from rhenium. The hydrogen on the a-carbon, C(24J, is 2.76 A from rhenium. 

Alkyl radicals have also been prepared by reaction of alkylbromides with photolytically generated Re(CO)5 (from Re2(CO)io) [17], photolysis of cobalt-alkyl complexes [20], photolysis of AIBN [17, 21, 22] or thermolysis of TEMPO adducts [23]. 

Similarly to the hydroformylation, under certain reaction conditions the formation of the intermediate palladium-alkyl complex can be practically irreversible as shown by the different prevailing chirality of the 2-methylbutanoic acid ester obtained from 1-butene and (Z)-2-butene, as well as from ( )- and (Z)-2-butene. Therefore, re-gioselection and enantioface selection must occur, as in hydroformylation, during or before the formation of the postulated palladium-alkyl intermediate (see Scheme IV). 

It has been also reported that Re—R bond homolysis can be promoted by the photoexcitation of mononuclear rhenium(I) complexes with a Re—alkyl bond, /ac-[Re(CO)3(a-diimine)R] (R = alkyl ligand). This type of the reaction also proceeds via a state, which is produced by the relaxation from the MLCT excited state. In the cases of R = ethyl (Et) and benzyl (Bz), the homolysis reaction proceeds efficiently. However, the reaction efficiency is much lower in the case R = methyl (Me) because the transition from MLCT to requires extra energy (Scheme 1). Using transient absorption spectroscopy, the state was... 

The CpRe-Lewis acid shown in Sch. 25 forms stable complexes with cyclohexenone and cyclopentenone. The enones coordinate to the metal forming an Re-O cr-bond. Addition of organocuprates followed by treatment with HI yield enantiomerically enriched 3-alkylated cycloalkanones and an optically active Re-I complex. MeMgBr-ether and MeLi-ether gave low yields in these reactions. The data shown in Sch. 25 reflect the problem that the yields and enantioselectivity of the organocuprate additions vary widely with the reaction conditions used and the preparation of the organocuprate [122], and must be optimized for every reaction. 

Similar considerations may be applied to j8-hydride transfer processes. For example, °ox(ReCH2CHMe2) = +0.06 V relative to Ag/Ag+ (reversible) [52a] and red(Re(CH2=CMe2)+) = —1.34 V (irreversible) which translates to a 135 kJ mol bond weakening of the f C-H bond upon oxidation. In this case, H loss from the Re-alkyl cation radical generates an alkene complex in which a new Re C bond has been formed and this contributes substantially to the thermodynamic weakening of the C H bond. 

Irradiation of the title complex in pentane solution was reported (71) to lead to decarbonylation and formation of the rj -alkyl complex Re(CO)4 (v -C.Hs) in 55% yield. No other details were provided. 

Photolysis of diethylthallium bromide in cyclohexane is a radical process involving cleavage of the thallium-carbon bond, which yields ethylcyclohexane and dicyclohexyl, as well as other products. Photoelectron transfer from benzyltributylstannanes to 10-methylacridinium ion results in cleavage of the metal-carbon bond, to give the corresponding benzyl radicals, rather than benzyl cations. Photochemical homolysis of Re- and Ru-alkyl bonds in Re(alkyl)-(CO)3(diimine) and Ru(I)(alkyl)(CO)2(diimine) complexes has been studied by Fourier transform ESR. In related manganese complexes, Mn(R)(CO)3(di-imine), elimination of CO is the predominant pathway when R = methyl, but Mn-alkyl homolysis occurs when R = benzyl. 

In Scheme 9.4, various oxo-alkyls of Re(Vl) and Re(VII) starting from rhenium( V) chloride are shown [27]. The binuclear oxo-alkyl complexes of formula [ReR202]2 (R = neopentyl), have been prepared by the reaction between ReOCl3(PPh3)2 and dineopentylzinc [28]. 

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