Stannyl metals, addition

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

It is believed that this reaction involves the formation of the a-stannyl ester. Metals such as lithium that form ionic enolates would be more likely to reverse the addition step. 

In an interesting contrast to the analogous silicon and germanium systems, catalytic addition of the unactivated hexamethyldistannane to unsaturated substrate readily occurs with the common palladium catalysts Pd(PPh3)4 or Pd(dba)2. In fact, prior to 1991 this was the only distannane investigated for addition reactions. The first example of transition metal-catalyzed double stannylation of unsaturated molecules, palladium-catalyzed cis addition of hexamethyldistannane to 1-alkynes, was reported in 1983 by Mitchell and co-workers.66 More recently, the system has been extended to include other substrates and both the hexaethyl- and the hexabutyldistannanes. All examples reported involve the use of a Pd(0) complex as the catalytic species. 

Several publications about oxidative addition of metal or organometal derivatives to stannylenes describe a new and efficient way to stannyl anions38,70-74. The reaction of CpLi with Cl2Sn resulted in a mononuclear complex as colorless cubic crystals obtained in 48% yield (equation 53)70. Its structure was resolved by X-ray diffraction and shows a complete separation of the ion-pair70. 

Intermolecular free-radical additions of stannyl radicals to multiple bonds have emerged as important methods for the preparation of tetraorganostannanes which can be reacted further to afford new C—C bonds through transition metal mediated coupling processes (e.g. Stille coupling). There are numerous examples of this chemistry715-737, and this treatise will focus on a few selected examples. 

The classical routes to 2//-pyran-2-ones (-pyrones) are illustrated in Scheme 124 . Additional examples include various transition metal-catalyzed cyclizations , e.g., the palladium-catalyzed reaction of -chloroacrylates with internal alkynes (Scheme 125) <2001NJC179> and the coupling of tributyl-stannyl allenes with (Z)—iodoacrylic acid (Scheme 126) <2005JOC6669>. 

Hexamethylditin is a suitable reagent for Pd-catalyzed metallation of aryl-halides to furnish aryltin compounds that in turn can react in Stille couplings with aryl halides to form biaryl derivatives. Hitchcock and coworkers [ 126] have shown that 2-pyridyl triflate and (hetero)aryl bromides can be coupled to 2-(hetero)aryl pyridines 174 in moderate to good yields (Scheme 66). The underlying principle of this heterocoupling of two aryl (pseudo)hahdes is the selective stannylation of the triflate that is undergoing a faster oxidative addition to the Pd(0) complex than the bromide. 

Intermediates 663 can be prepared by tin-lithium transmetallation with w-BuLi from a-stannylated vinyl sulfides974. Starting from l,l-bis(arylsulfanyl)ethenes, a reductive metallation with lithium naphthalenide at —70°C is a very efficient approach to lithiated vinyl sulfides975,976. Other methods involved bromine-lithium exchange977 or addition of methyl or phenyllithium to thioketenes978. A convenient method for the preparation of l-(methylsulfanyl) and l-(phenylsulfanyl) vinyllithiums was the treatment of 2-methoxyethyl sulfides with 2 equiv of w-BuLi-TMEDA at — 30 °C979. 

Metalated ferrocenes also serve as convenient precursors to ferrocenylboranes. Lithiated ferrocenes have been ntilized for the preparation of ferrocenylboronates FcB(OR)2 and l,T-fc(B(OR)2)2 and are especially suitable in the presence of ort/ o-directing donor-substituents. The borylation of disilylated ferrocenes with excess BCI3 on the other hand was reported to yield varying amounts of 1,3-diborylated prodnct (82 M = Fe, X = Cl) in addition to the l,F-diborylated species (83 M = Fe, X = Cl). In contrast, when 1,F-distannylated ferrocenes were treated with equimolar amounts of boron halides, l-stannyl-2-borylferrocenes (86) were formed as the major product rather than the expected... 

In addition to the functionalization protocols of Schemes 13 and 14, vinylic substitution reactions involving metalation and palladium-mediated carbon-carbon bond formation have been formulated," which further broaden the variety of stmctural types available from the Patemo-BUchi reaction. For example, deprotection and stannylation of photoaldol (133), followed by refimctionalization of the a-enol ether position of vinylstannane (134), gave the substituted oxetane (135) in good overall yield. Similar functionalization of bicyclic oxetane (136) via exo-face dihydroxylation and acid-catalyzed reorganization of the acetal to the protected 3-deoxy-( )-streptose (137) has been reported, which illustrates the synthetic utility of such processes in the synthesis of polyoxygenated materials." ... 

Mixed bimetallic reagents possess two carbon-metal bonds of different reactivity, and a selective and sequential reaction with two different electrophiles should be possible. Thus, the treatment of the l,l-bimetailic compound 15 with iodine, at — 78"C, affords an intermediate zinc carbenoid 16 that, after hydrolysis, furnishes an unsaturated alkyl iodide in 61% yield [Eq. (15) 8]. The reverse addition sequence [AcOH (1 equiv), —80 to — 40 C iodine (1 equiv)] leads to the desired product, with equally high yield [8]. A range of electrophile couples can be added, and the stannylation of 15 is an especially efficient process [Eq. (16) 8]. A smooth oxidation of the bimetallic functionality by using methyl disulfide, followed by an acidic hydrolysis, produces the aldehyde 17 (53%), whereas the treatment with methyl disulfide, followed by the addition of allyl bromide, furnishes a dienic thioether in 76% yield [Eq. (17) 8]. The addition of allylzinc bromide to 1-octenyllithium produces the lithium-zinc bimetallic reagent 18, which can be treated with an excess of methyl iodide, leading to only the monomethylated product 19. The carbon zinc bond is unreactive toward methyl iodide and, after hydrolysis, the alkene 19... 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

Although a great number of cyclopropanes bearing a trialkylsilicon, -germanium, -tin, or -lead substituent are known, few are prepared by an addition reaction of a metal-substituted carbene to a C-C double bond. The syntheses and reaction pathways of silyl-, germyl-, stannyl-, and plumbylcarbenes have been described in detail in Houben-Weyl, Vol. E19b, p 1410. 

Diazo(trialkylsilyl)acetates 1 can be prepared conveniently from an alkyl diazoacetate and a trialkylsilyltriflate in the presence of a tertiary amine (path A). Furthermore, metalation of ethyl diazoacetate with butyllithium at — 110 °C, followed by addition of chlorotrimethylsilane provides ethyl diazo(trimethylsilyl)acetate. Ethyl diazo(trimethylsilyl)acetate and the corresponding trimethylgermyl- and trimethylstannyl- substituted diazoacetate have been prepared from mercury bis(ethyl diazoacetate) and bis(trimethylsilyl-, germyl- or stannyl)sulfide (path B). Ethyl diazo(trimethylgermyl)- and ethyl diazo(trimethylstannyl)acetate were obtained from ethyl diazoacetate and diethylaminotrimethyltin or -germanium (path C). Similarly, the ethyl (trimethylplumbyl)diazoacetate was obtained from bis(trimethylsilyl)aminotrimethyl-lead. 2... 

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