Organolithium reagents, addition compounds

A method for oxidative cleavage of cyclic ketones involves a four-stage process. First, the ketone is converted to an a-phenylthio derivative (see Section 4.7). The ketone is then converted to an alcohol, either by reduction or addition of an organolithium reagent. This compound is then treated with lead tetraacetate to give an oxidation... 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

Organolithium compounds can add to a, (3-unsaturated ketones by either 1,2- or 1,4-addition. The most synthetically important version of the 1,4-addition involves organocopper intermediates, and is discussed in Chap 8. However, 1,4-addition is observed under some conditions even in the absence of copper catalysts. Highly reactive organolithium reagents usually react by 1,2-addition, but the addition of small amounts of HMPA has been found to favor 1,4-addition. This is attributed to solvation of the lithium ion, which attenuates its Lewis acid character toward the carbonyl oxygen.111... 

Organolithium compounds are highly reactive and have been used in a variety of organic transformations. A major problem in the development of catalytic asymmetric conjugate additions of organolithium reagents to a,/3-unsaturated carbonyl compounds is that the high reactivity of RLi may cause both low chemoselectivity (1,2- vs. 1,4-addition) and low enantioselectivity. 

The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a,(3-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [151], additions of nucleophilic radicals [231], 1,3-dipolar cycloadditions [232,233], inter-[234-241] or intramolecular [220,242] Diels-Alder reactions, as well as Simmons-Smith- [243], sulfur ylide- [244] or diazomethane-mediated [151] cyclopropanati-ons of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents ean lead via conjugate addition to substituted 1,4-cyclohexadien-6-ylidene complexes [245]. 

The most important application of organolithium reagents is their nucleophilic addition to carbonyl compounds. One of the simplest cases would be the reaction with the molecule CO itself, whose products are stable at room temperature. Recently, it was shown that a variety of RLi species are able to react with CO or f-BuNC in a newly developed liquid xenon (LXe) cell . LXe was used as reaction medium because it suppresses electron-transfer reactions, which are known to complicate the reaction . In this way the carbonyllithium and acyllithium compounds, as well as the corresponding isolobal isonitrile products, could be characterised by IR spectroscopy for the first time. 

The asymmetric addition of organomagnesium and organolithium reagents to a,P-unsaturated carbonyl compounds and especially imines can be achieved in situations where rigid chelation controls the geometry of the transition state. Stereospecific alkyl addition occurs in the case of a chiral leucine-derived imine to provide overall asymmetric alkyl addition to an a,P-unsaturated aldehyde (Scheme 107).380 381... 

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