Sparteine metal complexes

BisP (87a) and analogues have been prepared in 3 steps overall from PC13 (Scheme 12.26).108109 The metal complexes are prepared by the addition of [Rh(nbd)2]BF4 to form [Rh(nbd)(BisP )]BF4 and a minor amount of [Rh(BisP )2]BF4. The use of [Rh(COD)2]BF4 produces [Rh(BisP )2]BF4 as the major product. (-)-Sparteine produces the (S,S)-isomer of BisP. Unfortunately, the preparation of (R,R)-BisP is difficult because (+)-sparteine is not readily available. (R,R)-BisP has been prepared by an alternate route from R-tert-butyl(hydroxymethyl)methylphosphine.110... 

One main challenge to extend this to the mid-IR region is the fact that the corresponding signals such as VA and VCD are orders of magnitude smaller than the electronic transition and ECD signals. These authors reported the static CH-stretching VCD spectra of the Co and Ni metal complexes, i.e., Co(sparteine)Cl2 and Ni(sparteine)Cl2 which have quite intense VCD bands due to the enhancement effect discussed in Sect. 4.3. The VCD spectra obtained compare well with the... 

K atom, acting as [as-bridges, hence supporting the formation of a polymeric arrangement. This is the first example of a metal complex which incorporates the chiral diamine (—)-sparteine as part of a polymeric framework, and also the first example of a K complex containing this chiral diamine. 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

Lithium-metal exchange in the lithium-)—)-sparteine complexes 399 or 402, respectively, by diethylaluminium chloride or triisopropoxytitanium chloride proceeds with inversion providing useful reagents for enantioselective homoaldol reactions... 

Transition metal containing chiral complexes exhibit some peculiar VCD behaviors which have fascinated researchers in inorganic chemistry and in spectroscopy. For example, an order of magnitude increase in the VCD intensity, with changes in sign for many VCD transitions, were detected for the open-shell sparteine complexes of Co(II) and Ni(II) as compared to the corresponding closed-shell Zn(II) complex... 

Physical Data bp 137-138°C/1 mm Hg d 1.02gcm [a]p° — 17.5° (c=2, EtOH). X-Ray structures of several complexes of metal salts, alkyllithium derivatives, and of allylpalladium and studies on the conformation in solution and a NMR study on the structure of the 2-propyllithium-ether-(—)-sparteine complex have been reported. 

In many of these cases, both the enolate anion and substrate can exist as (Z) or (E) isomers. With enolates derived from ketones or carboxylic esters. The (E) enolates gave the syn pair of enantiomers (p. 166), while (Z) enolates gave the anti pair. Nitro compounds add to conjugated ketones in the presence of a dipeptide and a piperazine. ° Malonate derivatives also add to conjugated ketones, and keto esters add to conjugated esters.Addition of chiral additives to the reaction, such as metal-salen complexes,proline derivatives, or (—)-sparteine, ... 

The ability of ditertiary bases of the sparteine group to complex with alkali-metals and alkaline-earth metals has been further investigated by carrying out the Reformatsky reaction in the presence of ( —)-sparteine. In all the cases examined, a partial asymmetric synthesis of (S)-hydroxy-esters was achieved, the optical purity of the products ranging from 34—98 %. 

Additional experiments were performed with the corresponding lithi-um/TMEDA complexes (S)- and (R)-255. Most of the reactions take the same sense of stereospecificity, independent from the ligands at lithiiun. An exception is the triisopropoxytitanation, it proceeds with retention of configuration with the TMEDA complexes whereas the sparteine complexes react with inversion [ 169,168]. Similar to results discussed in the benzyl section (Sect. 3), the interaction of the isopropoxy residue with the hthium cation may determine the reaction course. It seems that in the presence of the bulky (-)-sparteine as a Hgand,such a suprafacial interaction does not contribute significantly. For the metal exchange with tris(diethylamino)titanium chloride inversion was observed, too [169]. 

Finally, the saturated bispidines 36 and (—)-sparteine 34 (Fig. 5) exhibit interesting clefts for supramolecular use. Bispidines form metal salt complexes. The V,V -dimethylbispidine (36) has as its most stable conformer the twin-chair V-shape, according to ab initio DFT calculations at the B3LYP/6-31G level (5.7 or 8.4 kcal moP better than boat-chair or twin-boat conformers, respectively). 

The (-)-enantiomer of sparteine is obtained from natural sources as a commercially available and relatively inexpensive oil. The metallation of 1 with n-BuLi in hexane, in the presence of (-)"Sp, produced the a-carbanion 2 which was subsequently quenched affording the known phosphine oxide 3 in approximately 14% ee. Although it has been suggested that the low stereoselectivity is due to the disruption of the (-)-sp/BuLi complex by complexation of the oxygen atom to the lithium cation, the same reaction with unprotected dimethylphenylphosphine provides completely racemic products. Due to the low enantioselectivity, this method was initially ignored in phosphorus chemistry until a seminal paper of Evans and co-workers in which they described similar reactions with phosphine boranes and sulfides. In this case, the enan-tioselectivities were much higher and the method could be successfully applied... 

The 1 1 complex 5-BuLi/TMEDA is probably the most potent metalating organobthium reagent. With (-)-sparteine (Fig. 26.2), metalation leads to asymmetric complexes allowing the formation of enantiomerically enriched functionalized compounds [21]. 

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