Sparteine complex

Solvent extraction techniques are useful in the quantitative analysis of niobium. The fluoro complexes are amenable to extraction by a wide variety of ketones. Some of the water-insoluble complexes with organic precipitants are extractable by organic solvents and colorimetry is performed on the extract. An example is the extraction of the niobium—oxine complex with chloroform (41). The extraction of the niobium—pyrocatechol violet complex with tridodecylethylammonium bromide and the extraction of niobium—pyrocatechol—sparteine complex with chloroform are examples of extractions of water-soluble complexes. Colorimetry is performed on the extract (42,43). Colorimetry may also be performed directly on the water-soluble complex, eg, using ascorbic acid and 5-nitrosahcyhc acid (44,45). 

IS,2E)-[1-(Diisopropylammocarbonyloxy)-l-meth)i-2-butenyl lilhium-( — )-Sparteine Complex. 

Primary 1-lithio-2-alkenyl diisopropylcarbamates are not configurationally stable in solution. However, under properly selected conditions, the ( )-sparteine complex of the 5-enantiomer crystallizes, leading to a second-order asymmetric transformation6 77-78 132. The suspension is converted to the tri(isopropoxy)titanium derivative with inversion of the configuration, which is shown to have enantiomeric purities up to 94% (Section D.l.3.3.3.8.2.3.). 

Diisopropylaminocarbonyloxy)-2-alkenyl]litkium -(- )-Sparteine Complexes General Procedure 78 ... 

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

The lithium-(-)-sparteine complex, generated by deprotonation of 1-methylindene, does not lose its configuration in diethyl ether solution even at room temperature80 presumably, the observed major diastcreonier is the thermodynamically determined product. Substitution with carbonyl compounds leads to 1-substituted (fl)-l-methyl-l//-indenes with >95% ee in high yields81. 

The a-substitution of enantiomerically enriched (-)-sparteine complexes of lithioalkenyl carbamates with methyl chloroformate76 or carbon dioxide77, in a manner contrary to a former assumption 76, proceeds with inversion of the configuration 131 131, leading to optically active 3-alkenoic acid esters. 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

Enantiomerically and diastereomerically enriched lithium-(-)-sparteine complexes of primary 2-alkenylcarbamates, which are configurationally stable as solids (Section 1.3.3.3.1.2.), are transmetalated stereospecifcally by tetraisopropoxytitanium. The resulting titanates are stable in solution and give rise to homoaldol adducts with enantiomeric purities up to 94 % ee107,107a. 

To a suspension of lithium-(-)-sparteine complex, prepared from 4.0 mmol of ( )-2-butenyl diisopropylcarbamate (Section 1.3.3.3.1.2.), at —78 °C are added rapidly 8.0 mL (28 mmol) of tetraisopropoxytitani-um with intensive magnetic stirring. After 5 min, 980 mg (10 mmol) of 3-methyl-2-methylenebutanal are added and stirring is continued below — 70 °C for 3 h. The mixture is allowed to warm to r.t. and poured into 20 mL of 2N hydrochloric acid and 20 mL of diethyl ether. The aqueous layer is extracted with three 20-mL portions of diethyl ether, washed with sat. aq NaHCO, and dried over Na2S04. Chromatography on 50 g silica gel (diethyl ether/pentane) affords a colorless oil yield 840 mg (78%) 90% ee [a]n° —0.8 (c = 4.4, CH2C12). 

By careful optimization, Widdowson and coworkers were able to show that methoxy-methyl ethers of phenols are better substrates for alkyllithium-diamine controlled enan-tioselective deprotonation, and (—)-sparteine 362 is then also the best ligand among those surveyed the BuLi-(—)-sparteine complex deprotonates 447 to give, after electrophilic quench, compounds such as 449 in 58% yield and 92% ee (Scheme 180) . Deprotonation of the anisole complex 410 (see Scheme 169) under these conditions gave products of opposite absolute stereochemistry with poor ee. 

Early investigations on organometallic (—)-sparteine complexes concern the nucleophilic addition of lithium, magnesium and zinc complexes onto carbonyl compounds, utilization in polymerization and also NMR spectroscopic investigations ... 

The (—)-sparteine-complex of l-(2-pyridyl)-l-(trimethylsilyl)methyllithium is monomeric and shows, according to an X-ray analysis, an almost planar methine group. The lithium cation is placed in a -fashion above the plane of the azaallyl anion moiety. 

These results were recorded for the TMEDA complexes, but there is little doubt that the sparteine complexes exhibit similar reactivity, presumably with a further shift towards inversion. 

The TMEDA complex of a-lithiobenzyl iV,iV-diisopropylcarbamate was found to be configurationally stable on the microscopical scale in the Hoffmann test . The (—)-sparteine complex 222 has moderate configurational stability on the macroscopic scale, which could not been brought to useful selectivities in substitution reactions . As... 

Beak and coworkers found the (—)-sparteine-complex of iV-Boc-Af-(p-methoxyphe-nyl)benzyllithium 244, obtained from 243 by deprotonation with n-BuLi/(—)-sparteine (11) in toluene, to be configurationally stable (equation 57) . On trapping 244 with different electrophiles, the substitution products 245 are formed with high ee. Efficient addition reactions with imines and aldehydes have also been reported. The p-methoxyphenyl residue is conveniently removed by treatment with cerinm ammoninm nitrate (CAN). 

An interesting stereochemical situation was found for the lithium-)—)-sparteine complex derived from o-ethyl-A,A-diisopropylcarboxamide 267 (equation 65) . Control experiments, involving lithiodestannylation experiments and the Hoffmann test, led to the conclusion that 268/ep/-268 are configurationally unstable at —78 °C and the e.r. in 269 is determined by a dynamic kinetic resolution of the rapidly interconverting intermediates . It is noteworthy that the configuration is inverted by using tosylates . 

The lithium-(—)-sparteine complexes, derived from primary 2-alkenyl carbamates, are usually configurationally labile even at —78 °C. During the investigation of the (ii)-crotyl carbamate 301, the (—)-sparteine complex (5 )-302 crystallized in a dynamic thermodynamic resolution process (equation 76) and stereospecific substitutions could be performed with the slurry An incorrect assignment of the configuration of the lithium inter-... 

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 lithium-titanium exchange in the allyllithium-sparteine complexes 317 by tetraisopropoxytitanium (TIPT) or chloro-triisopropoxytitanium, resulting in titanates 318, proceeds with strict stereoinversion (equation 84). We assume that—contrary to the lithium-TMEDA complexes—the lithium-(—)-sparteine complexes are weaker Lewis acids and are no longer capable of binding the TIPT in the transition state of the exchange reaction. 

The titanium compounds, derived from the configurationally stable lithium-(—)-sparteine complexes 349a,b prepared from primary allyl carbamates, undergo lithium-titanium exchange with chlorotriisopropoxytitanium to form the allyltitanates... 

The asymmetric lithiation/substitution of Af-Boc-Af-(3-chloropropyl)-2-alkenylamines 395 by w-BuLi/(—)-sparteine (11) provides (5 )-Af-Boc-2-(alken-l-yl)pyrrolidines 397 via the allyllithium-sparteine complexes 396 (equation 106) . Similarly, the piperidine corresponding to 397 was obtained from the Af-(4-chlorobutyl)amine. Intramolecular epoxide openings gave rise to enantioenriched pyrrolidinols. Beak and coworkers conclude from further experiments that an asymmetric deprotonation takes place, but it is followed by a rapid epimerization a kinetic resolution in favour of the observed stereoisomer concludes the cyclization step. 

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... 

The addition of alkyllithium-(—)-sparteine complexes to C=C bonds can lead to chiral carbanions and may be an interesting alternative to deprotonation. 

Organolithium-(—)-sparteine complexes have been used to initiate the anionic polymerization of acrylates and styrenes. ... 

Cinnamate salts and cinnamic amides react with low regioselectivity to yield mixtures of the 2- and 3-alkylated products . orflzo-Substituted aryllithium-(-)-sparteine complexes 468 add with good enantiofacial discrimination to orf/zo-substituted ferf-butyl cinnamates 467 to give 469 (equation 128) ". The ehiral additive (i ,i )-l,2-dimethoxy-1,2-diphenylethane in some cases gave improved ee values. 

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