Alkylation rhenium complexes

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

Alkylations of Chiral a-Alkoxy Vinyl-Rhenium Complexes... 

Chiral rhenium complexes, such as 1 and 4, are isoelectronic to the a-alkoxy vinyl-iron complexes discussed above and they exhibit analogous chemistry in many respects. Like the iron complexes, they are prepared as the Z-isomer and are readily alkylated by primary iodoalkanes and (bromomethyl)benzene with efficient 1,3-asymmetric induction97. Subsequent, spontaneous loss of halomethane produces the elaborated rhenium-acyl complexes. Two examples of the stereocontrolled preparation of diastereomeric rhenium-acyl complexes via this methodology are illustrated. 

Alkylation via approach of the electrophile from the least hindered side of an a-alkoxy vinyl -rhenium complex of antiperiplanar (alkoxy oxygen anti to the carbon monoxide oxygen) geometry has been proposed. The NMR spectroscopic data are consistent with the transient presence of rhenium-carbene complexes, such as 2 and 597. 

One important example of this class of rearrangement was reported for a rhenium complex in 199367 and, in retrospect, took advantage of the lower barrier of hydrogen migrations as compared to alkyl migrations. Mayer and co-workers prepared the tris-acetylene hydroxide complex 11, as well as the oxo hydride 12. Upon standing at room temperature for 5 days in benzene solution, the hydroxide spontaneously forms the oxo hydride with loss of one equivalent of acetylene, as shown in Eq. (8). 

Complexes of the general formula /ac-Re(a-diimine)(CO)3X and Re(a-diimine)(CO)2XX (where ot-diimine = bpy, phen, substituted bpy or phen, etc. and X, X = halide, solvent, alkyl, benzyl, monodentate phosphine, CO, etc.), have attracted interest since the mid-1970s [51-53]. Many of these complexes show emission from their lowest long-lived MLCT state at room temperatme in solution. Their catalytic properties for CO2 reduction have also been investigated. Electrolysis of a solution containing/uc-Re(bpy)(CO)3 Cl and 0.1 M BU4NPF6 in freshly distilled C02 saturated MeCN at —1.5 V (vs. SCE) produces both CO and C03 with cmrent efficiencies of 98 and 110 %, respectively [54]. Further, yhc-Re(bpy)(CO)3X (X = Cl, Br ) has been used successfully as a photocatalyst for CO2 reduction to CO with TEOA in DMF [55-58]. When X = Cl , a quantiun yield of 0.14 has been measured in the presence of excess Cl". A formato-rhenium complex,/ac-Re(bpy)(C0)3(02CH), has been isolated in the absence of excess Cl". 

One of the characteristic features of rhenium is its tendency to form stable complexes in a high oxidation state such as Re(VI) and Re(VII) and a series of rhenium 0x0-alkyls and a variety of polyhydrides of this type have been extensively studied. 

Acids, acyl, and aroyl halides all react with [ReCl(N2)(PMe2Ph)4]. However, protonation occurs at the metal to give the hydride [ReClH(N2)-(PMe2Ph)4], whereas slow acylation and aroylation occur at the end nitrogen atom. Interestingly, this latter reaction is the reverse of the preparation of the rhenium dinitrogen complexes. Alkyl halides do not react with rhenium complexes of dinitrogen. 

The reactions of rhenium(V) oxo-alkyl-phosphine complexes with various alkynes were studied and the X-ray crystal structure of [Re(=0)Mc3(PhC>CPh)] was reported OO. The synthesis of a number of ibenium(I) tris-alkyne complexes was reported and [Re(OSiMe3)(MeCjCMe)3] characterised by X-ray diffraction Oi. The synthesis of a series of bis-imido alkyne and imido bis-alkyne rhenium complexes of the type [Re(NAr)2Cl(RCSCR)] and [ReCl(NAr)(NpGCNp)2] has been reported302 and the electrtxiic structures of the compounds investigated by Extended Huckel molecular mbital calculations. 

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