Alkyl stannanes

Stille coupling was also developed in tlie early 1980s and is similar to Suzuki coupling in its sequence. It is used to couple aryl or vinyl halides or triflates with organotin compounds via oxidative addition, transmetallation, and reductive elimination. The oxidative addition reaction has tlie same requirements and preferences as discussed earlier for tlie Heck and Suzuki reactions. The reductive elimination results in formation of tlie new carbon-carbon bond. The main difference is that tlie transmetallation reaction uses an organotin compound and occurs readily without the need for an oxygen base. Aryl, alkenyl, and alkyl stannanes are readily available. Usually only one of tlie groups on tin enters into... 

Several related cross-couplings that involve both catalytic levels of Pd and Cu have also been reported, such as Hecklike reactions see Heck Reaction) of vinyl esters with arylstannanes (equation 6), aryl selenenylations between BusSnSeAr and aryl halides (equation 7), and thioarene or thioester couplings to aryl, vinyl, or alkyl stannanes (equation 8). ... 

Thioethers and selenoethers can be displaced with various nucleophiles after methylation by MeOTf. This has been used extensively in the carbohydrate synthesis field (see the section on glycosylation). A phenylselenide moiety was substituted stere-ospecifically by an alkyl stannane following MeOTf activation in the construction of a C—C bond between two quaternary carbon centers (eq 20). ... 

Ketones are also obtained in high yield from the reaction of tetra-alkyl-stannanes, carbon monoxide, and alkyl halides in the presence of a palladium(ii) catalyst." An improved route to aldehydes from olefins involves a hydro-boration-carbonylation sequence followed by oxidative work-up (Scheme 32)." ... 

Sodium in liquid ammonia may be titrated against SnH4 to produce HjSnNa and HaSnNaa- It would appear, however, that these two sodium derivatives are only stable as amines, as removal of ammonia, even at —63-5°, causes decomposition. HjSnNa reacts with alkyl iodides in liquid ammonia to produce alkyl stannanes RSnH, and HjSnNa with ammonium chloride regenerates SnH42 . 

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

Diallyldialkylstannanes with chiral alkyl substituents on the tin, show variable asymmetric induction in their Lewis acid catalyzed reactions with aldehydes. Using bis-(/f)-2-phenylbutyl-(di-2-propenyl)stannane, enantiomeric excesses of up to 54% were obtained via attack on the / e-face of the aldehyde96. 

S)-tributyl[t-alkyl-3-(methoxymethoxy)-2-propenyl stannane yield 80% 90-95% ee... 

In the frame of a medicinal project at J J Pharmaceutical Research and Development aimed at designing new potent and selective glycogen synthase kinase-3/i (GSK-3/3) inhibitors, the C-3 derivatization of the 1-methyl-4-[l-alkyl-lff-indol-3-yl]-lff-pyrrole-2,5-dione scaffold was explored [31]. Microwave-assisted Stille reaction of 3-chloro-l-methyl-4-[l-alkyl-lff-indol-3-yl]-lH-pyrrole-2,5-diones with (2,4-dimethoxy-5-pyrimidinyl)(tributyl) stannane at 200 °C yielded in 6 min the desired 3,4-diaryl-lff-pyrrole-2,5-diones in moderate yields (Scheme 12). 

In addition to the radical ipso-substitution of indolyl sulfones producing stannanes described earlier <96T11329>, Caddick has also reported an approach to fused [l,2-a]indoles based on the intramolecular cyclization of alkyl radicals. Thus, treatment of 112 with BuaSnH leads to the fused ring derivatives 113 (n = 1-4) <96JCS(P1)675>. 

To examine whether the breakdown in equilibration arose from a slow rate of equilibration or if the equilibrium does not favor the equatorial alkyllithium,the time course of equilibration was investigated (Scheme 34). The diastereomeri-cally pure a-alkoxystannanes 198 were prepared in analogy to Linderman s procedures 170], and their equilibrations were examined. Transmetallation and alkylation under kinetic control (entries 1 and 2) provided the expected equatorial and axial adducts, respectively, with no detectable minor isomer. Attempted equilibrations of the axial stannanes (entries 4 and 6) led to mixtures of axial and equatorial products. The equatorial stannane gave only the equatorial adduct under both equilibration conditions (entries 3 and 5). From these data, the authors concluded that the equilibration does take place under these conditions, and that the equilibrium lies essentially completely toward the equatorial alkyllithium. The observed product ratios in entries 4 and 6 can be attributed to a slow rate of equilibration. 

Entry Stannane Equilibration temp, (time) Protonated (%) Alkylated (%) Ratio (ax eq) ... 

Allylic silanes and stannanes react with various electrophiles with demetallation. These reactions can occur via several related mechanisms. Both types of reactants can deliver alkylic groups to electrophilic centers such as carbonyl and iminium. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Allylic stannanes are an important class of compounds that undergo substitution reactions with alkyl radicals. The chain is propagated by elimination of the trialkyl -stannyl radical.315 The radical source must have some functional group that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters316 and selenides317 are reactive. 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

Some mechanisms of radical-initiated migration copolymerization of di-alkyl(diphenyl)stannanes with non-conjugated epoxyalkadienes such as 4,4-epoxy-l,7-heptadiene (I) and 3-glycidyl-oxy-l,6-hexadiene (II) have been discussed 98). 

The reaction of a hydrosilane or a hydrostannane with alkyl(aryl)chalco-genoethers in the presence of AIBN (2,2 -azobisisobutyronitrile) gives the corresponding arylchalcogenosilane or -stannane, respectively (Scheme 6).39 41... 

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