Allylstannanes formation

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... 

Allylstannanes can be prepared by treatment of allyl halides with trialkyl- or triaryltin lithium reagents. Displacement of primary allyl halides tends to be regioselective for formation of the less substituted allylstannane, and takes place with useful retention of double-bond geometry14-16. 

Displacement of secondary allyl halides is satisfactory for the formation of symmetric allylstannanes, e.g., ( )-tributyl(l-mcthyl-2-butcnyl)stannane17, but can give 1,3-rearranged products in other cases, e.g., from 3-chlorobutene and trimethyltin reagents14 lS. Allylic cyclohexenyl chlorides react with triphenyltin lithium with clean inversion and little 1,3-transformation19,20. 

Finally, the in situ formation and thermolysis of trialkyltin ethers derived from tertiary homoal-lylic alcohols has been developed into a useful allylstannane synthesis39,40. This fragmentation is the reverse of the addition of an allylstannane to a ketone41. 

Formation of a vinyl-substituted pyrrolizidine derivative is also observed in case of an allylstan-nane cyclization94. Since the allylstannane moiety is acid sensitive, the iV-acyliminium ion is generated by exposure of the hydroxylactam to methanesulfonyl chloride and triethylamine in dichloromethane. The very rapid cyclization produces the endo-vinyl compound with very high stereoselectivity. 

A further application of ring-closing metathesis in seven-membered heterocyclic ring formation is in the synthesis of the trans-fused oxpane systems. This process involved tandem RCM/allylstannane-aldehyde cyclizations and interaction of the process provides access to trans-fused polyoxepanes <00S883>. 

The palladium-catalyzed multicomponent coupling reactions have attracted considerable interest.12,12a 12e A reaction using allylstannane 39 and allyl chloride 40 was applied to the three-component diallylation of benzylidenema-lonitrile and its congeners by Yamamoto et al 2 Analogous diallylation of isocyanate 41 was studied by Szabo et al. (Scheme 7).12a The reaction mechanism can be explained by formation of an amphoteric bis-allylpalladium intermediate 43 which undergoes an initial electrophilic attack on one of the allyl moieties followed by a nucleophilic attack on the other. 

A case of the addition of an allylstannane to aldehydes has been reported by Tagliavini to proceed with appreciable enantioselectivity (Scheme 6.15) [40]. A notable feature of the Zr-catalyzed transformations is that they proceed more rapidly than the corresponding Ti-catalyzed processes reported by the same research team (see Scheme 6.16). Furthermore, C—C bond formation is significantly more efficient when the reactions are carried out in the presence of 4 A molecular sieves the mechanistic rationale for this effect is not known. It should be noted that alkylations involving aliphatic aldehydes are relatively low-yielding, presumably as the result of competitive hydride transfer and formation of the reduced primary alcohol. 

Most of the current interest, however, is directed toward the use in synthesis of the formation of a carbon-carbon bond by the addition of an allylstannane to carbonyl groups (Equation (114)). These reactions have been thoroughly reviewed,16 18 19 299-301 and are covered in Section 9.08.3 of this series, and will not be considered in detail here. 

Alkyl halides possessing / -hydrogens are usually poor substrates for carbonylative cross-coupling due to competitive / -hydride elimination/ Allyl chlorides can be used in carbonylative cross-coupling with allylstannanes/ phenyl-, 3-furyl, or vinylstannanes " to afford allylketones in modest to good yields. Divinylketones can be accessed through the reaction of vinylstannanes with vinyl iodides or vinyl triflates, with the latter requiring the addition of LiCl. Synthetic potential of this method has been proved in the formation of macrocyclic ketone jatrophone. In the reaction of vinyl triflates with tetramethyltin or aryltrimethylstannanes the additional activation by ZnCle is required. 

Derivatives of trifluoroethanethiol have limited though interesting chemistry. Unfortunately, metallated difluorothioenol chemistry has not been reported, because rapid nucleophilic attack occurs even by hindered bases such as LDA. Nakai et al. exploited this high electrophilicity in a tandem addition/elimina-tion-rearrangement sequence [146], but more recent applications have concerned free radical chemistry (Eq. 46). Chlorination of trifluoroethyl phenyl sulfide followed by exposure to tin hydride in the presence of an allylstannane resulted in C-C bond formation with a reasonable level of stereocontrol [147]. 

In situ generation of an allylstannane from an allylic acetate and hexamethyldistannane results in five-membered ring formation in preference to the allyl alternative seven-membered ring (equation 243).140... 

Scheme 5.10 Formation of tin carboxylates with a dual allylstannanes/Pd (PPh3)4 catalytic system [45].

The reaction is explained by the in situ formation of bis-7r-allylpalladium (172), which is amphiphilic and reacts with 177 [88]. It is known that the reaction of the allylstannane 176 with aldehydes proceeds under rather severe conditions. Facile formation of the homoallyl alcohol 179 at room temperature in the presence of a Pt or Pd catalyst is explained by the nucleophilic nature of bis-7c-allylplatinum or -palladium generated in situ as an intermediate [89]. 

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. 

Finally, ruthenium-catalyzed carbocyclization by intramolecular reaction of allylsilanes and allylstannanes with alkynes also led to the formation of vinyl-alkylidenecyclopentanes [81] (Eq. 60). This reaction is catalyzed by RuC13 or CpRuCl(PPh3)2/NH4PF6 in methanol. The postulated mechanism involves the coordination of the alkyne on the ruthenium center to form an electrophilic /f-alkyne complex. This complex can thus promote the nucleophilic addition of the allylsilane or stannane double bond. 

Umani-Ronchi et al. [56] investigated the asymmetric allylation of aldehydes with allylstannane in the presence of chiral bis(oxazoline) ligands and several metal salts (Sch. 31). Combination of zinc halides and the bis(oxazoline) ligand 70 gave the ally-lated product 71 with moderate enantioselectivity (40 % ee), while other metal salts afforded either no product or racemic products. Because the formation of an allyl-zinc-bis(oxazoline) species was excluded on the basis of the NMR experiments, the reaction was considered to proceed by a Lewis acid-mediated pathway. 

Kobayashi et al. also reported interesting chemoselectivity of aldehydes and imines in the Yb(OTf)3-catalyzed addition reactions of silyl enol ether, allylstannane or trimethylsilyl cyanide [12]. In the competitive reactions between aldehydes and imines, the imines reacted faster than the aldehydes (Tables 4-6). This tendency is not unique to Yb as catalyst selectivity is similar for other Ln(OTf)3. Nuclear magnetic resonance (NMR) studies revealed selective formation of an imine-Yb(OTf)3 complex in the presence of an aldehyde. This preference was reversed when conventional Lewis acids (SnCE, TiCU, TMSOTf, and BF3 OEt2) were used. 

Organotin compounds may be synthesized by the cathodic reduction of organic compounds in the presence of tin halides. For example, the reduction of allylic halides in the presence of chlorostannanes gives the corresponding allylstannanes in good yields [69]. Combination of this reaction with in situ palladium-catalyzed reaction with allylic halides leads to effective formation of the head-to-tail homo coupling products as shown in Eq. (17). 

The addition of allyltributylstannane to a chirally modified aldehyde 111 proceeds in high yield and moderate diastereoselectivity to give the horaoallylic alcohols 112 and 113 (Scheme 10-46) [86]. When the methyl group on the sugar is not present, the reaction proceeds in higher diastereoselectivity (95%) and excellent yield. The formation and reaction of a 2/1 complex (aldehyde/Lewis acid) with the allylstannane can account for the observed selectivity. 

The diastereoselectivity of these reactions is consistent with product formation occurring through transition state 137, where the reactive conformation of the aldehyde in the transition state (corresponding to the normal Felkin-Anh model) minimizes steric interactions with the allylstannane as well as the 1,3-dipole interactions of the aldehyde and the /(-alkoxy group. The allylation reaction of the 2,3-syn aldehyde 138, however, with allyltri-n-butylstannanes 98, generates the anti-Felkin adducts 139 preferentially (Eq. (11.9)) [93], The stereochemistry of these reactions is consistent with product formation occurring preferentially through transition state 140, in which 1,3-dipole interactions of the aldehyde and the P-... 

A number of investigations have explored the reactions of ally lie stannanes containing a y-alkoxy substituent. A direct preparation of these substances utilizes the kinetic deprotonation of an allyl ether followed by alkylation with tri-n-butylstannyl chloride. In a typical experiment, the deprotonation of 101 with 5-butyllithium leads to internal coordination of lithium cation and provides formation of the Z-allylstannane 102. The behavior of y-alkoxyallylstannanes is similar to the corresponding Z-alkylstannanes, and as a result, the reaction provides a stereoselective route for the synthesis of complex diol derivatives. In the allylation of chiral aldehyde 80 with stannane 102, /l-chelation dictates face selectivity. The expected. yyn, anti-product 104 is obtained with high diastereoselection via the antiperi-planar 103, which accommodates the sterically demanding silyl (TBS) ether (Scheme 5.2.23).23... 

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