A-heteroatom-substituted organolithiums

Alternative strategies for the generation of a-heteroatom-substituted organolithiums were then developed (Scheme 7-61), such as the reductive lithiation of a-phenylthio ethers or phenylthio amines [67]. 

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

Some heteroatom-substituted or chelate-stabilized organolithium compounds, on the other hand, can be sufficiently stable toward racemization to enable their use in stereoselective reactions with electrophiles [223, 225, 271, 531, 543, 552-554] (Scheme 5.75). This increased configurational stability of a-heteroatom-substituted carbanions might be due to the stronger pyramidalization of such carbanions [261,555] and fixation of the metal by chelate formation. 

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]. 

Heterosubstituted allenes are versatile synthetic intermediates. Direct substitution of a heteroatom on the allene confers upon the system an electronic bias that allows the molecule to be deprotonated easily using an organolithium reagent, and also permits regioselective reaction with carbonyl compounds. 

Thioacetal anions and related anions of type (3 X, Y = SMe or SiMea) can be obtained by addition of organolithiums (RLi) to the corresponding compounds (4)." On the other hand, compounds of type (4 X = Br, SePh, or SnBua) react with Bu"Li to give o-heteroatom-substituted vinyl-lithiums (5), which in cases where Y = SPh or SePh are themselves acyl carbanion equivalents. Interestingly, the vinyl-lithium (5 Y = SPh) can also be used as a source of compounds (4 Y = SPh,... 

A key driver for the development of the DBR has been the increased availability of the requisite chromium carbene. Fischer carbenes undergo a wide variety of useful reactions and a significant effort has been devoted to their synthesis. These carbenes undergo many of the same reactions as esters. The a-hydrogens in 13 are quite acidic, with a pKa of approximately 8, that allows for application of the Aldol condensation to form the vinyl-substituted carbene 14. Of course, alkynes insert into these carbenes to form new vinyl substituted carbenes 15. However, the absence of a heteroatom on the carbene center makes these poor substrates for the DBR. The classical route to Fischer carbenes is the Fischer route addition of an organolithium to hexacarbonyl chromium and alkylation with a hard electrophile. Hoye has also shown that alkyl iodides under phase-transfer conditions can be used to alkylate the lithium alkoxide. Thus reaction of vinyl lithium 16 provides the carbene 17 in 53% over two steps. 

Strong nucleophiles such as organolithium or organomagnesium derivatives do not react with substituted or unsubstituted phosphabenzene or arsabenzene (39, Y = P or As) by nucleophilic substitution as in the case of pyridines, but by addition to the heteroatom forming intermediate anions 40. These can then be converted into nonaromatic compounds by reaction with water to yield 1-alkyl-1,2-dihydro-derivatives 41, or they can be alkylated by an alkyl halide with the same or a different alkyl group, when two products may result a 1,2-dialkyl-1,2-dihydro 40-derivative 42, or a -derivative 43. The former products are kinetically controlled, whereas the latter compounds are thermodynamically controlled, so that one may favor the desired product by choosing the appropriate reaction conditions. 

The ability of sulfur to stabilize an adjacent anion will be discussed in Chapter 46, and it means that sulfur heterocycles are much easier to deprotonate than THF. The most important of these contains two sulfur atoms dithiane. Deprotonation of dithiane occurs in between the two heteroatoms, and you can see some chemistry that arises from this on p. 1234. For tire moment, we will just show you series of reactions that illustrate nicely both dithiane chemistry and the ring opening of oxygen heterocycles in the presence of BF3, This substituted derivative of dithiane is deprotonated by BuLi in the same way to give a nucleophilic organolithium that will... 

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