Propargyl tins

Bacteria, antimicrobials against, 12, 456 Baeyer-Villiger oxidation, via tin amides, 9, 370 Barbier-Grignard-type reactions, and sonochemical metal insertions, 1, 315 Barbier-type reactions allenyl and propargyl tins, 9, 358 with allylic tins, 9, 357 with antimony(III) compounds, 9, 426 with bismuth(III) compounds, 9, 433 with cerium reagents, 10, 409 with indium compounds, 9, 685... 

Propargylic halides are converted to allenyl and propargylic tin halides upon exposure to SnCl2 in a mixture of A(,A(-dimethylformamide (DMF) and l,3-dimethyl-2-imidazo-lidinone (DMI) [87]. Subsequent addition of aldehydes leads to homopropargylic and/or allenic carbinols (Table 47). The ratio of the two regioisomeric adducts depends on the nature of and R. Alkyl substitution on the alkyne (R = Me) strongly favors the allenic adduct. On the other hand, the ratio of adducts from the TMS substituted alkyne (R = TMS) is dependent on the aldehyde substituent. 

Table 49. In situ addition of allenyl/propargyl tin halides to aldehydes in water.

Propargylic acetate adds to aldehydes with good anti selectivity in the presence of Et2Zn and a palladium catalyst. Propargylic bromide add to ketones in the presence of Nal/Dy, In, or Mn/Cr catalyst/TMSCl. ° Propargylic tin compounds react with aldehydes to give the alcohol, with good antiselectivity. 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

The allylic, allenic, propargylic, 2,4-dienylic, cyclopentadienylic, and related tin compounds present special, structural features and show special reactivity by both heterolytic and homolytic mechanisms. 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

Allylations, allenylations, and propargylations of carbonyl compounds in aqueous media can also be carried out with preformed organic tin reagent, rather than the use of metals.86,87,88 For example, the allylation reaction of a wide variety of carbonyl compounds with tetraal-lyltin was successfully carried out in aqueous media by using scandium trifluoromethanesulfonate (scandium triflate) as a catalyst (Eq. 8.40).89 A phase-transfer catalyst (PTC) was found to help the allylation mediated by tin at room temperature without any other assistance.90... 

Chiral allenylstannanes can be prepared by Sjv2 displacement of propargylic halides sulfinates or sulfonates with tin cuprates (Table 14)78. The nonracemic propargylic mesylate (74) afforded a nonracemic allene, [a]D —570, whose configuration was assigned by application of Brewster s rules (equation 38)78. Displacements on the steroidal mesylates 75 and 76 afforded the allenic products with complete inversion of configuration (Scheme 32)78. 

An alternative route to allenyl stannanes involves organocuprate displacements on propargylic chlorides bearing an alkynyl PhsSn substituent (equation 39)79. Interestingly, transmetallation by attack of the cuprate on the tin substituent is not observed in these systems. A parallel strategy can be employed for allenylgermanes (equation 39)79. The... 

Organogermanium compounds can be prepared by transmetallation reactions with tin reagents. Examples include Me2PhGeCl (Equation (66)),89 the alkene-functionalized species 26-28, (Equations (67) and (68)),90 and the allenic (Equation (69)) and propargylic (Equation (70)) species 29 and 30.91 A series of aryltrichlorogermanes was prepared from the corresponding tin reagents (Equation (71), Table 9).92 Transmetallation with zirconium species can also be used (Equation (72), Table 10).93... 

In this context, albeit not real isomerizations, the [2,3]-Wittig rearrangements induced by a tin-lithium exchange must also be mentioned. Starting from enantio-merically pure propargylic alcohols, high ee values for the axial chiral allenes could be observed as shown for 153 (Scheme 1.69) [505, 506],... 

Allenyltin halides are not isolable but can be prepared in situ through reaction of a propargylic halide with a mixture of tin and aluminum metal, usually in powdered form. For example, propargyl bromide is converted to diallenyltin dibromide (Eq. 9.71). This intermediate reacts with aldehydes to produce homopropargylic alcohols in high yield. Allenyl adducts are not formed in this reaction [67]. 

A related method of preparation involves the oxidative addition of a tin(II) salt to propargylic iodides, which yield mixtures of allenyl- and propargyltin halides on treatment with SnCl2 in DMF-DMI (l,3-dimethylimidazol-2-one) (Eq. 9.75) [68], These intermediates react in situ with aldehydes to afford mixtures of propargylic and allenic carbinols via a cyclic SE2 process (Eqs. 9.76 and 9.77). As explained in the Introduction, the ratio of these two products reflects the relative transition-state energies of the addition reactions. 

Whereas tin cuprate displacements of terminal propargylic mesylates lead exclusively to allenic stannanes (Eqs. 9.82 and 9.83), the reaction of an internal propargylic mesylate with Ph3SnCu afforded a mixture of allenic and propargylic isomers (Eq. 9.84) [71]. 

Reaction of the unsaturated bromoethyl glycoside 216 in the foregoing manner gives the bicyclic product 217,218 and similar treatment of the propargyl ether 219 with a tributyltin radical results in carbon-radical generation and cyclization to afford the tin-containing adduct 220 in 90% yield. On oxidation with osmium tetraoxide and periodate ion, the Sn-C bond is cleaved, and the corresponding ketone 221 is produced in excellent yield.219... 

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