Lithium adduct formation

Since the nitrogen in pyridine is electron attracting it seemed reasonable to predict that the trihalopyridynes would also show the increased electrophilic character necessary to form adducts with aromatic hydrocarbons under similar conditions to those employed with the tetra-halogeno-benzynes. The availability of pentachloropyridine suggested to us and others that the reaction with w-butyl-lithium should lead to the formation of tetrachloro-4-pyridyl-lithium 82 84>. This has been achieved and adducts obtained, although this system is complicated by the ease with which pentachloropyridine undergoes nucleophilic substitution by tetrachloro-4-pyridyl lithium. Adducts of the type (45) have been isolated in modest yield both in the trichloro- and tribromo- 58) series. 

When the chiral molybdenum Ti-allyl-substituted enone 147 was treated with lithium dimethylcuprate, formation of adduct 148 with fair selectivity was observed (Scheme 6.29) [69]. Interestingly, higher selectivities were obtained in the presence of boron trifluoride etherate. It is assumed that Lewis acid coordination induces the s-trans reactive conformation 149 [64]. Consequently, nucleophile attack anti to the molybdenum fragment should afford the major diastereomer 148. 

The stereochemical outcome of the Wittig reaction can depend on the presence or absence of lithium salts. This may be due to a betaine intermediate stabilized by lithium cation. A stable adduct of this type has now been observed during a Wittig reaction. When Ph3P=CH2 is treated with 2,2 -dipyridyl ketone, P NMR shows the formation of an oxaphosphetane (72) and addition of lithium bromide gives the chelation-stabilized betaine lithium adduct (73). 

An aldol reaction of a trimethoxysilyl enol ether, catalysed by a lithium binaphtholate, shows anti diastereoselectivity and modest ees under dry conditions, but addition of water brings about syn adduct formation, with higher ee.131... 

The reactions of strong carbon nucleophiles with arene oxides 1 and 11 leads to rapid adduct formation.Methyl lithium and dimethyl magnesium react with arene oxide 1 by 1,6-addition to give ds-adducts. Trans-adducts were also obtained from reaction of dimethyl magnesium and methyl lithium with arene oxides 1 and 11, respectively, by the more usual 1,2-trans addition mechanism. ... 

Formation of 2-halothiophenes, via quenching of the corresponding lithium adduct, is also achieved with 1,2-dihaloethanes [34], For instance, thienopyran 31 was converted to its 2-lithiothiophne species with -BuLi, and subsequent quenching of this anion with 1,2-diiodoethane delivered iodothiophene 32 in 80% yield [34a]. These authors took the iodination one step further by performing an iodine rearrangement with LDA to deliver the 3-iodothiophene 33 in 50% yield. 

A chlorine substituent in the highly electrophilic 2-position is very reactive towards nucleophiles. With lithium or sodium organics the reported reactions seem to indicate adduct formation and hydrogenolysis in addition to substitution <81RCR816>. Metal-catalyzed coupling reactions, as discussed for pyrimidines, are expected to provide for versatile carbosubstitutions. 

Not a superbase, but closely related, is the adduct of w-butyllithiiun with lithium t-butoxide, 206 [179]. Evidence for adduct formation between the two components came initially from NMR studies [180], and it was shown that in benzene solution tetrameric associates are present [181, 182]. Crystallization from hexane gave a well-defined self-assembled tetramer containing two types of differently coordinated Li atom. One set of lithium atoms is bonded to two a-C atoms of n-butyl groups and one oxygen, with additional close contact with a / -carbon atom the other lithium atoms are connected to two oxygens and only one a-carbon [183]. 

Alkaline Adducts PC species can readily form alkaline (Aik = Li, Na, K) adducts. The formation of alkaline adducts depends largely on the availability or the concentration of the alkaline ions in the lipid solution. Therefore, if lithium adducts would like to be used for the analysis of PC species, lithium hydroxide or a lithium salt should be used as a modifier in the lipid solution. A sodium adduct is always displayed in a mass spectrum if there is no any other modifier added to the lipid solution since sodium ions are essentially present everywhere. ... 

The type of modifier also affects IMS analysis in general as discussed in Chapter 4. For example, mass spectra can be simplified by addition of potassium acetate [57] or LiCl [58] to the matrix solution, which results in the formation of exclusively potassium or lithium adducts of lipids, respectively. By changing the concentration of alkali metal salts in the matrix solution, it is also possible to selectively ionize either polar or nonpolar lipids [59]. 

Figure 2.3. Electrospray mass spectrum of sucrose obtained with a single quadrupole mass spectrometer (Thermo Electron) operated in the positive-ion mode. Fragmentation was achieved by in-source collisionally induced dissociation. The post-column addition of submillimolar LiCl to the analyte solution facilitated the formation of lithium adducts. (Reprinted from Ref. 28, with permission.)...

Stereoselectivities of aldol reactions of trimethoxysilyl enol ethers catalysed by lithium binaphthoate are greatly affected by the presence of water, which may induce a change from anti- to iyn-adduct formation for those derived from cyclohexanone, for example. " Direct anti- and regio-specific aldol reactions of cyclododecanone with 0 benzaldehyde in NaOH/MeOH have provided building blocks for helical construction of supramolecules. ... 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

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