Aryl stannanes

The alkaloid dubamine contains a single bond between the two heteroarene units. This lond was formed in 79% yield by the generally valuable palladium-catalyzed eoupling of an ryltrimethylstannane with an aryl triflate (see section 1.6). The requisite stannane was pre-ared from l,3-benzodioxol-5-yl triflate and hexamethyldistannane with the same palladium atalyst, the triflate ester was obtained from 2(1 f/)-quinolinone and trifluoromethanesulfonic jihydride (A.M. Echavarren, 1987). An earlier attempt to perform this aryl coupling by dassical means gave a yield of only 1 %. 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

The Suzuki coupling of arylboronic acids and aryl halides has proven to be a useful method for preparing C-aryl indoles. The indole can be used either as the halide component or as the boronic acid. 6-Bromo and 7-bromoindolc were coupled with arylboronic acids using Pd(PPh3)4[5]. No protection of the indole NH was necessary. 4-Thallated indoles couple with aryl and vinyl boronic acides in the presence of Pd(OAc)j[6]. Stille coupling between an aryl stannane and a haloindole is another option (Entry 5, Table 14.3). 

Synthesis of aryl ketones or aldehydes from aryl tnllates or iodides and organo stannanes in ihe presence of CO and a palladum catalyst... 

A number of StHle coupling reactions have been reported by Handy et al. [95]. With PdCl2(PhCN)2/Ph3As/Cul in [BMlM][Bp4], good yields and good catalyst recyclability (up to five times) were reported for the reaction between a-iodenones and vinyl and aryl stannanes (Scheme 5.2-19). However, the reported reaction rates were significantly lower than those obtained in NMP. 

Scheme 5.2-19 Pd-catalyzed Stille coupling of a-iodoenones with vinyl and aryl stannanes in...

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. 

The first example of a NHC-Pd catalysed Stille reaction between aryl bromides and aryl stannanes was reported by Herrmann in 1999 [120]. Summarised in Scheme 6.36 are the best results obtained when the weU-defined pre-catalyst 22 was employed. Unfortunately, the coupling of aryl chlorides was not possible. 

Another major route for synthesis of stannanes is reaction of an organometallic reagent with a trisubstituted halostannane, which is the normal route for the preparation of aryl stannanes. 

If the stannane bears substituents at the position geminal to tin, the reaction requires activation by Cu1 (Section 9.6.3.2.1). The cross-coupling of aryl (or heteroaryl) stannanes with aryl (or heteroaryl) halides or triflates is a general route to various biaryls and their analogues, particularly useful in those cases when the Suzuki-Miyaura reaction is less effective for... 

COUPLING OF ARYL HALIDES WITH SILANES, STANNANES, GERMANES, AND BORANES 388... 

The analogous formation of aryl stannanes and aryl germanes from aryl halides occurs in the presence of catalytic amounts of palladium complexes of triphenylphosphine. Hexabutyl-... 

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... 

Substitution at the 2-position of carbapenems by use of stannanes of the heterocycle has been explored and found to have some advantages over the conventional Stille cross-coupling which employs aryl stannanes and carbapenem triflates <00TL2995>. 

In the total synthesis of moracin M (61), a phytoalexin isolated from infected white mulberry, Widdowson et al. first prepared 2-stannylated benzofuran 58 from benzofuran 57 via direct metalation and treatment with Me3SnCl [57], Stannane 58 was then coupled with aryl iodide 59 to afford adduct 60, which was desilylated to moracin M (61). 

Like most aryl halides, furyl halides and furyl triflates have been coupled with a variety of organostannanes including alkenyl, aryl, and heteroaryl stannanes in the presence of catalytic palladium. Carbamoylstannane 66 was prepared by treating lithiated piperidine with carbon monoxide and tributyltin chloride sequentially. The Stille reaction of 66 and 3-bromofuran then gave rise to amide 67 [61]. In another example, lithiation of 4,4-dimethyl-2-oxazoline followed by quenching with MesSnCl resulted in 2-(tributylstannyl)-4,4-dimethyl-2-oxazoline (68) in 70-80% yield [62], Subsequent Stille reaction of 68 with 3-bromofuran afforded 2-(3 -furyl)-4,4-dimethyl-2-oxazoline (69). 

The Stille reaction of 2-chloro-3,6-diisopropylpyrazine (7) and 2-chloro-3,6-diisopropylpyrazine 4-oxide (9) with tetra(p-methoxyphenyl)stannane (readily prepared in situ from the corresponding Grignard reagent and SnCU) led to the corresponding arylation products 8 and 10, respectively [9]. Additional Stille coupling reactions of chloropyrazines and their N-oxides have been carried out with tetraphenyltin [10] and aryl-, heteroaryl-, allyl- and alkylstannanes [11]. 

The poly(dialkyl)stannanes absorb at Amax ca. 390-400 nm, and poly(diaryl)stannanes (with substituents in the aryl rings to increase the solubility) absorb at ca. 435 nm. This absorption is due to a transition, although the apparent... 

---