Allylstannanes transmetallation

Carbonyl Allylation and Propargylation. Boron complex 8, derived from the bis(tosylamide) compound (3), transmetalates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78 °C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent as)mimetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. 

The a-bromo-7-lactone 901 undergoes smooth coupling with the acetonyltin reagent 902 to afford the o-acetonyl-7-butyrolactone 903[763j. The o-chloro ether 904, which has no possibility of //-elimination after oxidative addition, reacts with vinylstannane to give the allyl ether 905, The o -bromo ether 906 is also used for the intramolecular alkyne insertion and transmetallation with allylstannane to give 907[764],... 

The allylstannane 474 is prepared by the reaction of allylic acetates or phosphates with tributyltin chloride and Sml2[286,308] or electroreduction[309]. Bu-iSnAlEt2 prepared in situ is used for the preparation of the allylstannane 475. These reactions correspond to inversion of an allyl cation to an allyl anion[3l0. 311], The reaction has been applied to the reductive cyclization of the alkenyl bromide in 476 with the allylic acetate to yield 477[312]. Intramolecular coupling of the allylic acetate in 478 with aryl bromide proceeds using BuiSnAlEti (479) by in situ formation of the allylstannane 480 and its reaction with the aryl bromide via transmetallation. (Another mechanistic possibility is the formation of an arylstannane and its coupling with allylic... 

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. 

Both allylstannane transmetalation and thermolysis of homoallyl stannoxanes have been used to prepare 2-butenyltin halides as (E)j(Z) mixtures44-45. The reaction between 2-butenyl-(tributyl)stannane and dibutyltin dichloride initially provides dibutyl(l-methyl-2-propenyl)tin chloride as the kinetic product by an SE2 process, but this isomerizes under the reaction conditions to give a mixture containing the (Z)- and (E)-2-butenyl isomers46. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

As organotin compounds (organostannanes) undergo smooth Pd-catalysed transmetallation, aryl halides react with a wide variety of aryl-, alkenyl- and alkylstannanes [139]. Coupling with allylstannane is the first example [140]. The reaction is called the Migita-Kosugi-Stille or Stille coupling. Aryl, alkenyl, allyl, alkynyl and benzyl... 

Allylstannane 176 is formed by the reaction of allyl acetate with distannanes [97,98], In this reaction, umpolung of the electrophilic 7r-ally] palladium to the nucleophilic allylstannane occurs. Allylation of bromoindole 198 to give allylindole 199 involves the oxidative addition of 198 to Pd, transmetallation with the allylstannane 176, and final reductive elimination [99],... 

SnCU is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored because of steric interactions. Furthermore, the six-coordinate 2 1 complex is most probably the reactive intermediate in these systems. The use of crotylstaimanes provides evidence for competing transmetalation pathways (Eq. 35) [60]. TiCU results in superior selectivity. 

Alkoxy-substituted allylstannanes are transmetalated on treatment with Sn(IV) halides to give allyltin trihalides which react with aldehydes with effective remote asymmetric induction [62]. For example, the 5-benzyloxy-4-methylpent-2-enylstan-nane gives predominantly the 1,5-anti products with useful stereoselectivity (Eq. 37) [62b]. Allylstannanes and 5-benzyloxy-4-methylpent-2-enylsilanes also react with aldehydes with modest levels of 1,5-induction after treatment with SnCU. Trans-metallation to give intermediate allyltin trihalides might be involved (Eq. 38) [63]. 

The transmetallation or the metal-metal exchange reaction of an allylic tin species with an electrophile was first observed in 1970 [77]. The possibility that transmetallation may play a role in the Lewis acid-promoted reaction of allylstannanes with aldehydes was initially discussed by Tagliavini [78], Keck [79], Yamamoto [71b, 80], and Maruyama [81]. It is believed that upon transmetallation with either SnCU or TiCl4, the addition of an allylstannane and aldehyde will occur via a cyclic, six-membered transition structure. The reaction occurs by coordination of the aldehyde carbonyl with the Lewis acidic trichlorotin or trichlorotitanium reagent, thus affording the anti homoallylic alcohol (Scheme 10-42). 

Optically active allylstannanes, R3SnC HR (CH=CHR"), are formed from active secondary propargyl phosphates from the sequence of reactions shown in equation 9-4,8 using the Ti(OPr1)4/Pr1MgBr reagent.9 Presumably the titanium/tin transmetallation involves an Sn2 process. 

Allylstannanes rapidly undergo the Kocheshkov comproportionation reaction with SnCU, even at -50 °C. The initial product is apparently formed with allylic inversion, but further transmetallation may give other allylic isomers.35 The SnCl3 group which is formed enhances the electrophilic power of the allyl group. This has been exploited in aromatic allylation (equation 9-13),36 and transmetallations of this type provide the basis for some, but not all, of the examples of the catalysis of the allylation of carbonyl compounds by allylstannanes in the presence of Lewis acids37,38 (Section 9.1.3.4). 

Many of the applications of organotin compounds are specific to one class of compounds (e.g. tin hydrides or allylstannanes) and these topics are covered in the corresponding chapters. On the other hand, reactions involving tin/lithium transmetallation, or palladium-catalysed coupling, can be carried out with a variety of types of organostannanes, and these reactions are collected together in this chapter. These topics have an extensive literature that is beyond the scope of this book, and the treatment here is limited to an outline of the fields, and a guide to the literature. 

The bis(trimethylstannyl) derivative of triquinacene, which can be regarded as a bis(tri-allylstannane), was used for preparing the triquinacene dianion (equation 22-5).22 A secondary benzyl system provides an exception to the rule that the transmetallations involve configurational retention.23... 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

The initial transmetalation of 249 leads to 250 via anti-SE substitution and additional isomerizations provide 251 and 252. The closed, six-membered transition state 253 from 250 is used to rationalize formation of the E-anti-alcohol 254 as the major product, whereas the minor -xyn-adduct 256 is generated from 252 via the closed arrangement 255. The stereochemical consequences for InCh trans-metalations are complimentary to the syn outcome of reactions for y-(alkoxy)allylstannanes. As a result, the technique has been utilized in organic synthesis, with the illustration of several applications in Scheme 5.2.56. ... 

In the Lewis acid-promoted allylation with allylsilanes, the promoter is usually regarded as activating electrophiles. Thomas [374] and Dias [375] have, however, described SnCLj-promoted allylation reactions proceeding via a transmetalation mechanism (Scheme 10.130). In the latter equation of Scheme 10.130, the formation of the allylstannane species has been observed by NMR spectroscopy [375 b]. 

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