Reactions of Allenylstannanes

The syn stereochemistry of adducts 383, the major products of the BF3-OEt2-catalyzed reactions of allenylstannane (5)-218, indicates that these reactions proceed preferentially either through the synclinal transition state 385 or the antiperiplanar transition state 386 (Scheme 11-27). However, when the aldehyde R substituent is unbranched (e.g. -hexCHO, Table 11-21), the antiperiplanar transition state 387, which leads to the anti adduct 384, becomes more favorable, presumably due to a diminished steric interaction between the aldehyde R sub.stituent and the allenylstannane Me substituent. 

An important section of the chapter has been dedicated to the Sr reactions of allenylstannanes, and the Sr reactions of propargylic organostannanes are briefly reviewed as a transformation, which efficiently enables the preparation of allenes. The chapter concludes with a summary of promising opportunities for enantiocontrolled processes. 

Bidentate Lewis acids result in a-chelation, and the (P)- and (M)-stannanes 286 approach from the less hindered face of the carbonyl, with the small substituent (H) over the metallocycle in the transition states 299 and 300, respectively. In the case of BF3 OEt2, reactions of allenylstannanes (P)-286 and (M)-286 create a matched/mismatched scenario with the (5)-aldehyde (Scheme 5.2.66). Addition of (M)-286 gives the syn,anti diastereomer 301 via the Comforth or polar Felkin-Anh transition state 303, whereas allenylstannane (P)-286 results in a diastereomeric mixtnre of syn,syn-2>fi4 (via 306) and antfanti-305. 

Scheme 5.2.69 Reactions of allenylstannanes with N-acyliminium ions...

Scheme 5.2.70 Examples of conjugate addition reactions of allenylstannanes...

In 1994, Badone et al. reported that the Stille coupling of allenylstannane 77 and aryl triflates 78 resulted in formation of various aryl-substituted allenes 79 in moderate to good yield (Scheme 14.18) [39]. The choice of catalyst was certainly a crucial issue in this process for optimizing yield and rate. The best results could be obtained employing a catalyst cocktail of Pd2(dba)3-TFP-LiCl-CuI. Similar Stille coupling reactions with stannylated allenes and aromatic iodides as substrates were described by Aidhen and Braslau [40a] and Huang et al. [40b],... 

The addition of allenylstannanes to aldehydes bears many similarities to the allyl-stannane reactions. All of the same stereochemical issues are relevant and have been addressed. Mechanistically, there are also strong re.semblances and finally these reactions have been used successfully as a key transfonnation in the total synthesis of several natural products [104]. The reaction proceeds in high yield and excellent regio- and stereoselectivity to afford the desired homopropargylic alcohols. 

Keck and Yu have reported the enantioselective synthesis of homopropargylic alcohols in the reactions of aldehydes with allenylstannane 456, promoted by the BINOL-Ti(IV) catalyst 451. The allenylstannane 456 provided enantiomerically... 

Allenylstannanes can be prepared by the reaction of allenyl Grignard reagents with tin halides (equation 9-36), or of stannylmetallic reagents (e.g. R3SnLi, R3SnCu) with propargyl halides or sulfonates (equation 9-37) 88,89 reactions of this latter type occur with complete inversion of structure. 

Reactions of enantiomeric allenylstannanes (P)-286 and (M)-286 with (5)-2-benzyloxypropion-aldehyde, in the presence of MgBr2 OEt2, are highly substrate-controlled processes, giving the syn,syn-297 and anti,syn-298 diastereomers, respectively (Scheme 5.2.65). 

Scheme 5.2.66 Reactions of enantiomeric allenylstannanes 286 in the presence of BF3 OEt2...

Reactions of aiienyistannanes with oxocarbenium cations are also facile processes. Williams, and subsequently Nelson,have demonstrated the use of glycal acetate 313 in the course of the total synthesis of laulimalide, to generate a reactive oxocarbenium species in the presence of Lewis acids. Axial addition of aiienyistannanes 314 and 316 resulted in tranx-2,6-disubstituted dihydropyrans 315 and 317, respectively (Scheme 5.2.68). Similarly, the propargylation of lactol 318 is affected with allenylstannane 319 in the presence of BFjoOEta (Eq. 24)... 

Finally, the Lewis acid activation of a,)6-unsaturated carbonyl compounds and Q -nitroalkenes is sufficient to induce productive Sr reactions with allenylstannanes (Scheme 5.2.70). Haruta and Kita have successfully achieved condensation reactions with 326 and 328 in the presence of TiCU, and cyclo-hexenones 330 and 332 also serve as synthetically effective substrates for the 1,4-conjugate addition. In the case of enone 332, activation with TBSOTf led to the isolation of silyl enol ether 333. ... 

Scheme 5.2.77 Diastereoselection in reactions of nonracemic allenylstannanes leading to chiral allenylal-cohols...

Recently, Akiyama has reported the catalytic enantioselective propargylation of a-imino esters 389 with allenylstannane, in presence of 1 mol% [Cu(MeCN)4]C104/(/ )-tol-BrNAP to generate amine 390 in good yield and high enantioselectivity, 22 and related efforts by Jorgensen and coworkers 22 have examined the reaction of 389 with ( )-crotylstannane (Scheme 5.2.86). 

The palladium-catalysed hydrostannylation of the thioacetylenes 317 (R = alkyl or HOCH2) with tributyltin hydride affords selectively the ( )-alkenes 318. Allenylstannanes 320 (R = Me or Bu R =H, Et, PhCH2 or PhCH2CH2) are formed in the reaction of lithium tributyltin with the acetylenes 319. ... 

The use of allenylstannanes in this type of reaction furnishes homopropargylic alcohols. Reaction of 464 with (5)-496 under Lewis acid catalysis affords 497 with excellent diastereoselectivity [158]. Boron trifluoride etherate-promoted addition produces a 68 32 mixture of syn- and anri-alcohols 497 and 498 (95% yield), while MgBr2 Et20-promoted addition produces 497 with virtual exclusion of the a /-isomer ( > 99 1). 

Conjugate propynylation of enones results from TiCU-mediated addition of allenylstannanes. Other Lewis acids are ineffective in this reaction (eq 37). ... 

Allenylstannanes of 90 % ee are obtained from the corresponding chiral propargylic mesylates 15 (available in ca. 90% ee) in the reaction with the reagent prepared from tributylstannyllithi-um and dimethylsulfide19. 

Allenylstannanes are readily prepared by Sn2 displacement of propargylic mesylates by stEmnyl cuprates (Table 46) [82]. When the reaction was performed on an enan-tioenriched (/ )-mesylate the allene was deduced on the basis of the optical rotation and by use of Brewster s rules to be of the (M) configuration (Eq. 66). Thus it was surmised that the displacement had proceeded with inversion of stereochemistry. 

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