Reactions 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],... 

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. 

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. 

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. 

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. 

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. 

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)... 

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

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 5.2.70 Examples of conjugate addition reactions of allenylstannanes...

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... 

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. 

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 recently disclosed facile generation of benzyne 61 from 2-(trimethylsilyl)phenyltriflate 60, by fluoride ion-initiated elimination has generated renewed interest in this reactive species. For example, Cheng et al. have reported a palladinm-catalyzed three-component reaction involving benzyne, allyl chloride, and an allenylstannane to afford l-allyl-2-allenyl benzene 62 (Scheme 5.6.21). 

A radical chain reaction is again available in the tin series but not in the silyl, and it has been used in the synthesis of the parent allenylstannane (Scheme 86). ... 

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