Allenylstannanes, reactions with

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],... 

It has already been mentioned that allenylstannanes undergo addition-substitution reactions with nitriles (Table 1, entry 7). Prop-2-ynylstannane derivatives also have similar reactivity with nitriles and give addition-substitution products [6f[. As an example, 3-phenylprop-2-ynyl(tributyl)stannane 42 reacts with ethyl-2-... 

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. 

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. ... 

The ramifications of Se transmetalations for chiral allenylstannanes are briefly summarized in Scheme 5.2.72 in which the common electrophile, (2f )-3-benzyloxy-2-methylpropanal, serves to selectively generate two distinct diastereoisomers 336 and 337. The reaction with (M)-286 and SnCU proceeds via anti-SE transmetalation followed by stereospecific isomerization to give the (P)-trichloroallenylstannane... 

In this case, the reversible coordination of tin with aldehyde favors the closed transition state for Felkin-Anh addition in 339, whereas the jS-chelation model of 340 introduces destabilizing steric interactions owing to placement of the methyl substituent of the chiral allene. Transmetalations of chiral allenylstannanes with InBrs occur with net retention of allene geometry (Scheme 5.2.64). Thus, the starting (P)-286 can also be utilized for a stereoselective reaction with the corresponding (5 )-aldehyde (Scheme 5.2.72, bottom). The enantiomeric alcohol ent-337 is produced via the closed transition state... 

Allylzirconocene chlorides. These re l-alken-4-ols by reaction with aldehydes h derived from allenylstannanes condense i... 

Allylzirconocene chlorides. These reagents are formed from allenes. A route to l-alken-4-ols by reaction with aldehydes has been developed. 1,1-Bimetallic species derived from allenylstannanes condense with aldehydes to form 1,3-dienes. ... 

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. 

Using the enantiomeric allenylstannane (S)-218a with MgBf2 as the promoter, the anft>yn-dipropionate 398 is obtained in high yield and diastereoselectivity, where the diastereoselection is consistent with reaction occurring through the chelate antiperiplanar transition state 399 (Eq. (11.32)). 

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. 

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. 

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). 

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. 

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)... 

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

In a related example, Hegedus has incorporated chirality into the functionalization of the allenylstannane. Lewis acid-promoted additions of stannylallenamide 359 to simple aldehydes and imines are high i yn-selective processes (Scheme 5.2.76)." The reaction is presumed to occur via antiperiplanar 362, illustrating the anti-SE characteristics of stannyl substitution with a minimization of non-bonding interactions. 

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). 

The additions of chiral nonracemic allenylmetal reagents to chiral a-methyl propanal derivatives have been proven useful for the assembly of polypropionate fragments. These reagents rely on allene chirality to favor one of the two possible diastereomeric transition states in the addition and, thus, differ in a fundamental way from the aforementioned methods in which a chiral auxiliary or catalyst provides the control element. For example, a chiral allenylstannane 246 is added to a chiral aldehyde (S)-230, derived from the Roche ester, in the presence of various Lewis acid promoters to afford any of the four diastereo-mers with excellent diastereo- and enantioselectivity, depending on the reaction conditions. Representative results are depicted in Scheme 10.48. From the stereocontrol point of view, these transformations follow Cram-fike open transition state models without or with chelation, respectively. If InBr3, SnCLi, BuaSnCl, or other additives... 

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