Allenylstannanes, reactions with aldehydes

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. ... 

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... 

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... 

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. 

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. 

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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