3- -sparteine complex, structure

An X-ray crystal structure analysis was obtained from the 3-(trimethylsilyl)-aUyllithium-(—)-sparteine complex (5 )-302b. It reveals the monomeric structure of these aUyllithium compounds and a -coordination of the ally lie anion to the lithium cation. The latter is tetracoordinated and takes advantage of the chelating 0x0 group. The fixation of the lithium at the a-carbon atom is supposed to be the origin of the high regioselectivity of several substitution reactions. 

The indenyllithium 50 is one of few stereochemically defined organolithiums whose configuration is known with absolute certainty in this case, from the X-ray crystal structure of its (-)-sparteine complex. It can therefore be shown to react with all acylating agents with retention of configuration52 (scheme 6.1.12). Only with acetone is there some loss of reg/oselectivity. 

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. 

The X-ray crystal structure of A -Boc-A -/7-methoxyphenyl-3-phenylallyI-lithium-(-)-sparteine complex has been reported [184], This structure differs from the previous structure in that the lithium is associated in an -fashion. The lithium-(-)-sparteine complex resides on the Re face of the ally unit. Stannylation of the lithium complex was established to occur with inversion of configuration. 

The rationale for the observed configuration (Scheme 3.29), is based on the X-ray structure of another a-carbamoyloxyorganolithium sparteine complex [185]. After deprotonation, the chelated supramolecular complex shown in the lower left is postulated. This structure contains an adamantane-like lithium-diamine chelate, and contains new stereocenters at the lithiated carbon and at lithium itself. Note that epimerization of the lithiated carbon would produce severe van der Waals repulsion between R and the lower piperidine ring, whereas epimerization at lithium produces a similarly unfavorable interaction between the same piperidine ring and the oxazolidine substituents. Thus, the carbamate is tailor-made for sparteine chelation of only one enantiomer of the a-carbamoyloxyorganolithium. These effects may provide thermodynamic stability to the illustrated isomer. To the extent these effects are felt in the transition state, they are also responsible for the stereoselectivity of the deprotonation. 

Apart from the lithium carbanions, derived from secondary 2-alkenyl carbamates, no further types of configurationally stable a-oxyallyllithium derivatives have been reported in the last 15 years. One must conclude that the five-membered lithium chelate ring plays an important role for the stereochemical integrity. This structure has been nicely demonstrated by the X-ray structure analysis of a sparteine complex [158]. 

Alkyllithiums can be turned into chiral bases in quite a simple way—by complexation with a chiral ligand. A widely used example is the tetracyclic diamine (-)-sparteine. Sparteine s structure looks complex, but it is a relatively widely available natural product which folds around the lithium atom of an alkylUthium and places the base in a chiral environment. 

In order to investigate how both components recognizes so efficiently the chirality of each other in the complex, an X-ray analysis of the structure of the (-)-sparteine complex of (-)-la was carried out,5,6... 

Structure 8.20 is noteworthy because of the high activity of this complex as a precatalyst in alcohol oxidation to the corresponding carbonyl derivative. The chelating ligand in structure 8.21 is a chiral natural product called (-) sparteine. Complex 8.21 also catalyzes oxidations of alcohols. It has been used for kinetic resolution (see Section 8.5.3) of a racemic mixture of a secondary alcohol. 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

Another result of great importance—the conformational asymmetric polymerization of triphenylmethyl methacrylate realized in Osaka (223, 364, 365)— has already been discussed in Sect. IV-C. The polymerization was carried out in the presence of the complex butyllithium-sparteine or butyllithium-6-ben-zylsparteine. The use of benzylsparteine as cocatalyst leads to a completely soluble low molecular weight polymer with optical activity [a]o around 340° its structure was ascertained by conversion into (optically inactive) isotactic poly(methyl methacrylate). To the best of my knowledge this is the first example of an asymmetric synthesis in which the chirality of the product derives finom hindered rotation around carbon-carbon single bonds. 

Figure 17.29 Structure of distorted tetrahedral copper complexes. SP is sparteine-TV./V, mint is maleonitriledithiolate, dmp is 2,9-dimethyl-1,10-phenanthroline, and phen is 1,10-phenan-throline. Reprinted with permission from Ref. 67. Copyright 2005 American Chemical Society.

The formation of diastereoisomerically pure complexes of 90 with (-)-sparteine is also controlled by crystallisation. Treatment of the indene 89 with BuLi and (-)-sparteine in ether gives, on warming, a yellow precipitate which reacts with carbonyl electrophiles to provide the products 91 typically with good regioselectivity and >95% ee.52 An X-ray crystal structure proved the stereochemistry of the intermediate complex to be that shown as 90b, and hence proved the stereochemical course of the substitution (see section 6.1). The complex is readily decomposed by THF, in the presence of which it rearranges to a racemic V allyllithium. 

The most common group of alkaloids possessing a quinolizidine nucleus is that of the lupine alkaloids which can simply be classified as bicyclic (lupinine/epilupinine type), tricyclic (cytisine type) or tetracyclic, (sparteine/lupanine or matrine type). Fig. (23). This grouping is made according to structure complexity and without considering biosynthesis, as the detailed biosynthetic pathways are still not completely understood. 

Optically active diamines (-)-sparteine (72) and (-)-isosparteine (73) form complexes with ethylmagnesium bromide which crystallize in a form suitable for diffraction analysis. In both of these structures. 

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