Allylic indium

Allenic alcohols couple with allyl indium reagents at 140°C to give allylic alcohol products. Similarly, (o-hydroxy lactones couple with organoindium reagents. 

Gunter Helmchen of the Universitat Heidelberg took advantage (Chem. Comm. 2004, 896) of the substitutional flexibility of It-allyl indium complexes to develop enantioselective cyclizations such as 3 to 4. Six-membered rings are also formed efficiently (88% ). 

The most widely used preparative method of allylindium(m) or propargylindium(lll) compounds is the oxidative addition of metallic indium or indium(l) halides to allylic or propargyl substrates.4 26 27 Allylic bromides and iodides serve as good allylic sources without any other activation. In the case of allylic chlorides, a proper additive such as lithium iodide is required to promote the oxidative addition. Allylic indium compounds prepared by oxidative addition of metallic indium are considered to exist as the sesquihalide structure (allyl jImXj), which has been... 

A new preparative method for allylic indium(m) reagents via a reductive transmetallation of 7r-allylpalladium(n) or 7T-allylnickel(n) complexes with indium(i) salts is reported. This method enables the use of a wide variety of allylic compounds, such as allylic chlorides, acetates, and even allylic alcohols, in combination with Pd or Ni catalysts.43-50 7r-Allylpalladium(ii) resulting from the addition of arylpalladium(n) to allene is also transformed by metallic indium to the corresponding allylindium.51-54 Similarly, propargylindium(m) can be prepared from the corresponding propargyl alcohol derivatives.55-58... 

The control of diastereoselectivity in the allylation reaction of carbonyl compounds with allylic indium reagents has been an important issue since the discovery of the indium-mediated carbonyl allylation. As earlier discussions have been summarized in the precedent reviews,6-24 only relatively recent references are cited below. 

Indium trichloride-mediated addition of (i )-a-(methoxymethoxy)allylic stannane (>95% ee) to cyclohexanecarbox-aldehyde affords the anti-adduct predominantly (anti syn = 98 2) and stereoselectively (>95%ee) (Equation (12)). Production of a transient allylic indium reagent is postulated via a stereosepecific anti-Se2 transmetallation. This a-(methoxymethoxy)allylic stannane reacts without allylic inversion, whereas the reaction of crotylstannane in Equation (5) (Section 9.14.3.3.1) proceeds with net allylic inversion. <5-Oxygenated allylic stannane also undergoes transmetallation with InCl3, and in situ addition to a-ODPS acetaldehyde leads mainly to the //-adduct, which is a potential precursor to D-(+)-altrose (Scheme 31).149,150... 

Scheme 9 Pd-catalyzed oxidative carbonylation of allyl indiums.

The high anti selectivity observed in these additions with both branched and unbranched aldehydes makes the oxygenated allylic indium reagents well suited for applications related to annonaceous acetogenins [74]. The following examples are illustrative (Eqs 55-58). 

Figure 16. Reaction pathways for Se2 addition of enantioenriched y-OMOM allylic indium chlorides to aldehydes.

Table 44. Additions to aldehydes of transient allylic indium dichlorides from a,d-dioxygenated allylic stannanes.

A possible explanation might lie in the experimentally determined relatively poor reactivity of enals, compared with ynals, toward these allylic indium reagents. The slower rate of addition would enable equilibration of the kinetic (E) allylic indium intermediate to a mixture of E) and (Z) isomers. The latter would afford the syn adduct. 

Addition of the foregoing dioxygenated allylic indium intermediates to a-ODPS acetaldehyde then dihydroxylation of the double bond led to a potential precursor of the hexose D-(+)-altrose (Eq. 61). The overall transformation effects a four-carbon homologation of carbohydrates. 

Other organometallic compounds, including aUylic stannanes, allylic samarium, allylic germanium, and allylic indium compounds add to aldi-mines in the same manner. Aryltrialkylstannanes also add the aryl group to Al-tosyl imines using a rhodium catalyst and sonication. Catalytic enantioselective addition reactions are well known, including reactions in an ionic liquid.Allylic... 

The preparation and subsequent reaction of allylic indium reagents with aldehydes or ketones was first reported by Butsugan in 1988 [190]. The reaction of the 2-butenylindium reagents with aldehydes produced homoallylic alcohols in high yield albeit low selectivity. 

Achiral allylic indium reagents and chiral aldehydes. Allylindium reagents generated in water react smoothly with aldehydes and ketones (Scheme 10-99) [196], The reaction of achiral aldehydes and a-oxygenated aldehydes 290 with the allyl indium reagents proceeds smoothly to homoallylic alcohols without the need for external promoters. It is interesting to note that the a-hydroxyl aldehyde was se-leetive for the syn (chelation-eontrolled) product even in water. 

Chiral allylic indium reagents and achiral aldehydes. Chiral indium reagents have been generated by the oxidative metallation of allylic bromides, which bear remote stereogenic subunits. For example, the 1,4-asymmetric induction in the indium-mediated coupling of allyl bromides 298 with benzaldehyde affords the syn-adducts as the major products (Table 10-35) [199]. The stereoselectivity of the reaction improves with more sterically encumbered allyl bromides. 

Figure 10-10. Transition structure models for substituted allylic indium reagents.

The generation of allylic indium reagents by transmetallation of allylic stannanes with InCl3 and subsequent reaction of these with aldehydes represents an important advance for diastereoselective synthesis of homoallylic alcohols [203]. In these reactions, the stannane is added to a premixed solution of the aldehyde and InCl3 in acetone. In this way, the reaetion of cyclohexanecarboxaldehyde with the 2-butenylindium reagent affords a 98/2 antilsyn mixture of homoallylic alcohols (Scheme 10-104). 

The relative diastereoselectivity in the indium trichloride-mediated allylation with allylic stannanes in water is for the anti homoallylic alcohol [204]. The reactions are anti selective regardless of the geometry of the starting 2-butenylbro-mide. The in-situ-generated allylic indium species undergoes reaction with the... 

The L-talo and L-gulo adducts 447 and 449 were obtained with very high stereoselectivity (no other diastereomers reported) from the reaction of aldehyde 444 with the [y-(alkoxy)allyl]indium reagents generated from (5)-230a and (R)-230a, respectively. In these double asymmetric reactions, reagent control is clearly dominant. The stereochemistry of adduct 447 is rationalized by the Felkin transition state 448 while the stereochemistry of adduct 449 is rationalized by the anti-Felkin transition state 450 [275]. 

The use of InCls for Lewis acid activation of aldehyde substrates leads to rapid transmetalation of the allylic stannane, followed by carbonyl addition reactions of an allyl indium reagent. Premixing of InCb and the allylic stannane in the absence of aldehyde often produces precipitation and poor results. On the other hand, allyl indium reagents have been independently prepared by several procedures, including reductive metalations. Several important reviews describe the methods of preparation and the reactivity of allyl indium compounds. This discussion will be limited to key factors regarding the transmetalation of allylic stannanes in the presence of aldehydes. Stereochemical events leading to the production of anti adducts as major products are illustrated in Scheme 5.2.55. 

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