Alkenylstannanes coupling

Oxidative coupling of alkenylstannanes.3 1-Alkenylstannanes undergo homocoupling to 1,3-dienes when treated with f-butyl hydroperoxide in the presence of catalytic amounts of Pd(OAc)2. Under these conditions 1-alkenylstannanes couple with 2-aIkenyIstannanes to give 1,4-dienes. 

Aryl halides react with a wide variety of aryl-, alkenyl- and alkylstan-nanes[548-550]. Coupling of an aryl tritlate with an arylstannane is a good preparative method for diaryls such as 688. The coupling of alkenylstannanes with alkenyl halides proceeds stereospecifically to give conjugated dienes 689. The allylstannane 690 is used for allylation[397,546,551-553]. Aryl and enol triflates react with organostannanes smoothly in the presence of LiCl[554]. 

The intramolecular coupling of organostannanes is applied to macrolide synthesis. In the zearalenone synthesis, no cyclization was observed between arylstannane and alkenyl iodide. However, intramolecular coupling take.s place between the alkenylstannane and aryl iodide in 706. A similar cyclization is possible by the reaction of the alkenylstannane 707 with enol triflate[579]. The coupling was applied to the preparation of the bicyclic 1,3-diene system 708[580]. 

Alkenylation of cyclopentenone with the alkenylstannane 719 has been used for the introduction of an a,>-chain into a prostaglandin derivative[590]. Even the vinyl mesylate (methanesulfonate) 720 can be used for coupling with alkenylstannanes[59l]. 

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]. 

One of the first cross-coupling reactions performed on solid supports was the Stille reaction [250] which is a paUadium-catalyzed reaction of a trialkylaryl or trialkylalkenyl stannane with an aromatic iodide, bromide or triflate. In contrast to the process in liquid-phase, the organotin reagent is easily removed from the solid-phase because of the subsequent washing processes. Immobilized aryl halides have been frequently coupled with aryl and alkenylstannanes, whereas stan-nanes attached to the solid support have been used less frequently for the StiUe reaction. An example is the synthesis of a benzodiazepine library by EUman et al. Recently, a Stille cross-couphng reaction has been employed in the synthesis of al-kenyldiarylmethanes (ADAM) series of non-nucleoside HlV-1 Reverse Transcriptase Inhibitors (Scheme 3.14) [251]. 

Achiral 2-stannyloxazolines are known to undergo Shlle coupling with aromahc halides. However, Stille couplings with chiral 2-halooxazolines were not reported until Meyers and Novachek prepared the requisite 2-bromooxazoline 365 (see Scheme 8.116) and successfully coupled 365 with a variety of alkynyl and alkenylstannanes to afford chiral 2-substituted oxazolines 635 in reasonable yields (Scheme 8.200). ... 

Coupling of aryl halides with alkenyl stannanes promoted through palladium metal catalysis, otherwise known as the Stille coupling, has many applications,22 including solid-phase variants (see Chapter 2).3-5 One of these is featured in Nicolaou and co-workers solid-phase synthesis of (5)-zearalenone wherein a resin-bound alkenylstannane undergoes a Stille cycli-... 

The palladium/copper-cocatalyzed coupling of the readily available trisubsti-tuted alkenyl(phenyl)iodonium triflates 77 with alkynyl- and alkenylstannanes proceeds under exceedingly mild conditions with retention of geometry of the alkenyl ligand of the iodonium salt (Scheme 35) [60]. 

The construction of the highly functionalized cyclic polyether framework 339 in maitotoxin has been achieved by efficient coupling of the alkenylstannane 338 with the enol triflate 337 [159]. The smooth coupling occurs in the presence of CuCl (2 equivalents) and K2CO3 in THF. 

Oxidative couplings. The synthesis of biaryls from aiylboronic acids and 1,3-dienes from alkenylstannanes- under mild conditions involves the Pd(0Ac)2-02 system. 

Alkenylstannanes. Hydrostannylation of 1-bromoalkynes by employing BUjSnH, (dba)jPd2, and PhjP provides ( )-alkenylstannanes. It is convenient to effect a Stille coupling immediately because the proper Pd catalyst is already present. From 1 -alkynes and 1-bromoalkenes the synthesis of 1,3-dienes requires only catalytic amounts of the tin reagent. 

Figure 2.1.10 76.4 MHz " Snp H NMR spectrum of a mixture of isomers of alkenylstannanes, in which a boryl group is either in cis- or in trans-position relative to the stannyl group. The line widths of the " Sn NMR signals are a function of the residual broadening owing to partial relaxation of three-bond " Sn— 6 spin-spin coupling. ...

The palladium-catalyzed stannylation of allenes is an efficient approach for the preparation of y-substituted allylic stannanes. williams has recently described the bis-stannylation of 1-methoxymethyleneoxyallene yielding ( )-105, as well as the use of this functionalized stannane in allylation reactions. Pre-organization of the aldehyde by a-chelation provides for the synclinal transition state 108 leading to ant/,yyn-adduct 107 (Scheme 5.2.24). The mild conditions of the reaction retain the alkenylstannane of the product for further elaboration via cross-coupling reactions. 

The stereospecific Stille coupling of alkenyl bromides or iodides with alkenylstannanes is one of the most widely applied methods for the synthesis of dienes. Thus, a slight modification of the original method was applied in the coupling of alkenyl iodide 1 with (Z)-alkenylstannane 2, stereospecifically building a polyene as the last step in the synthesis of (—)-stipiamide (Scheme 5.4.1).25... 

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