Stannanes vinylstannanes

Vinylation can also be done by Pd-catalysed cross-coupling in which one component is used as a halide or triflate and the other as a stannane (Stille reaction) or boronic acid (Suzuki reaction). Entry 9, Table 11.3, is an example of the use of a vinylstannane with a haloindole. lndole-3-boronic acids, which can be prepared by mcrcuration/boration, undergo coupling with vinyl triflates (Entry 10). 

Numerous stannanes have been coupled with halopyridines as electrophiles in the Stille coupling. One of the simplest of these is vinylstannane [82-84]. The Stille reaction of bromopyridine 95 with tributylvinyltin gave angustine (96) [84], an indolopyridine alkaloid. Bromopyridine 95 also took part in a three-component carbonylative-Stille coupling sequence to provide an entry to another indolopyridine alkaloid, naucletine (97) [84]. 

The synthesis of A-fused tricyclic (3-lactams involving a radical cascade sequence in enyne 2-azetidinones 114 and 115 bearing a methylenecyclopropane unit has been reported [82]. Slow addition of Bu3SnH/AIBN to a refluxing solution of 114 gave tricyclic vinylstannane 116 as a single stereoisomer in 42% yield, whereas cyclization of 115 under identical conditions gave fused heterocycles 117 and 118 in 73 and 11% yield, respectively, in all three cases via a 7-endo cyclization. Treatment of vinyl stannanes 117 and 118 with PPTS in dichloromethane yielded a common tricyclic product 119 (Scheme 40). 

Kobayashi et al. successfully performed asymmetric cyclopropanation using substoichio-metric amounts of catalyst 45 (Scheme 9). [32] The levels of enantioselectivity achieved are in the 70-90 % range. Both, E- and Z-allylic alcohols are readily converted. Vinylstannanes 46 are also appropriate substrates. The resulting enantio-merically pure cyclopropanated stannanes hold great synthetic potential [33]. Thus, the cyclopropanated stannane 48 can be converted into the substituted cyclopropane 49 after successful tin-lithium exchange and electrophilic substitution. 

In 1973, it was demonstrated that 1,2-epoxystannanes, produced from vinylstannanes and MCPBA, could be isolated and characterized, in comparison widi 2,3-epoxystannanes (from allylstannanes), which are extremely reactive and have not been isolated (see Section 4.2.2.3). Subsequently, useful applications of 1,2-epoxy stannanes have been reported, including the internal alkyne ketone conversion, in the caibapenem and carbacephem 0-lactam antibiotic) skeletons. Ketone (10) should be of value in the construction of the biologically interesting l-carbapen-2-ene ring system. Synthesis of ketoacetates of potential use in the carbacephem system (e.g. 11 and 12) was also achieved by similar sequences shown in Scheme 11. ... 

Equations 8-29 and 8-30 illustrate the introduction of a double bond into vinylstannanes by elimination reactions. Flash vacuum pyrolysis at 600-950 °C of the a-acetoxyalkyl-stannanes or of the corresponding thiocarbonates shows little regioselectivity,65 but... 

The projected palladium-catalyzed cross-coupling required the availability of vinylstannane 216. As shown in Scheme XXV, the preparation of this lactone was initiated by copper-catalyzed 1,4-addition of l-(trimethylsilyl)vinylmagne-sium bromide to 5(2//)-furanone (212). For this process to be successful, excess trimethylsilyl chloride had to be present from the outset in order to trap the enolate as it was formed and circumvent its polymerization. This modification gave rise to C-silylated lactone 213, which was chemoselectively desilylated and transformed via vinyl bromide 215 [120] into stannane 216. 

Recently, Corey has rekindled interest in vinylstannanes as intermediates for alkenyl lithium reagents (79,80,81), and we (52,55) and others (53,56) have utilized this method to prepare various p-chain precursors (Scheme 17). We find hydrostanna-tion of terminal acetylenes to be a facile and high yield reaction, whereas iodovinylation is a capricious and tedious transformation. Thus, treatment of 3-triethylsilyloxy-l-octyne 91 with tributyl-stannane (TBS) in the presence of azobisisobutyronitrile (AIBN) gave stereospecifically an excellent yield of l-tributylstannyl-3-triethylsilyloxy-trans-l-octene (92), which when lithiated with butyllithium and treated with pentynylcopper provided the requisite lithio cuprate 93. The use of TBS to prepare homovinylic ethers such as 96 and 97 gave a 10 1 mixture of trans and cis isomers. 

Hydrostannylations. Hydrostannanes add to alkynes in uncatalyzed reactions at 60 °C. Phenylacetylene, for instance gives a mixture of ( )- and (Z)-vinylstannanes, wherein the tin atom has added to the terminal carbons. In the presence of Wilkinson s catalyst, however, the hydrostannylation proceeds at 0 °C to give mostly the regioisomeric vinylstannanes (eq 23). Terminal stannanes in the latter process seem to result from competing free radical additions. This may not be a complication with some other catalysts the complexes PdCl2(PPh3)2 and Mo( j -allyl) (CO)2(NCMe)2 also mediate hydrostannylations of alkynes, and they are reported to be 100% cis selective. Hydrostannanes and thiols react in a similar way to silanes and alcohols (eq 24). ... 

The synthesis of enantiomerically pure dienyl sulfoxides through palladium-catalysed coupling reactions has been reported [36,37]. An efficient Stille coupling process was used to synthesize both (lE E)- and (lZ,3 )-l-sulfinyl-l,3-butadienes through the use of stereodelined vinyl stannanes [36]. For example, coupling of the P-bromo-a,P-imsaturated sulfoxide (23) with vinylstannane (24) under palladiiun catalysis leads to the enantiomerically pure (l , 3 )-l-sulfinyl-l,3-butadiene (25) (Scheme 5.8) [36]. 

Hydrostannation of AU nols. Dibutyl(trifluoromethan-sulfoxy)stannane was first introduced as a reagent for hydrostannation of alkynols other than propargyUc alcohols. The authors had previously descrihed the use of Bu2SnHCl for the Iree radical hydrostannation of propargylic alcohols (eq 1 ). A key controlling element in this methodology was coordination of the tin center to the oxygen atom, which allows for suppression of isomerization of the Z- to i -vinylstannane (eq 2). ... 

Vinylsi lanes and stannanes result from PhtlegSiBEt Li or Ph SnEJEtjUi with acetylenes, while trisubstituted vinylstannanes of high stereoselectivity are... 

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