Homoallylic linear

Butyltin halides have also been used to mediate this process. One of the first examples involved addition of a 3 1 mixture of tram- and ds-crotyl tributyltin and a variety of conjugated aldehydes to Bu2SnCl2 without solvent to form (Z) homoallylic linear adducts (Table 22) [39], In this reaction, addition of the initially formed secondary allylic dibutylchlorostannane to the aldehydes must be faster than that of the tributyl crotylstannanes, and faster than 1,3-isomerization of the chlorostannane. Formation of the (Z) isomer is consistent with a chair transition state in which the allylic methyl group of the stannane adopts an axial orientation to avoid steric interactions with the adjacent stannane substituents (Eq. 23). 

Figure 24. Phosphite directed hydroformylation of homoallylic linear alkenes...

One limitation of these noncatalyzed allyl(trialkyl)- and allyl(triaryl)stannane-aldehyde reactions is the high temperature required unless the aldehyde is activated towards nucleophilic attack. Allyltin halides are much more reactive because of their enhanced Lewis acid character however 2-butenyltin halides show reduced syn I anti selectivity45, and give other products including linear homoallylic alcohols and tetrahydropyrans47. 

Transmetalation to give l-methyl-2-propenylaluminum followed by isomerization to 2-butenyl isomers may be involved in reactions between aldehydes and 2-butenyl(tributyl)-stannane induced by aluminum(III) chloride in the presence of one mole equivalent of 2-propanol. Benzaldehyde and reactive, unhindered, aliphatic aldehydes give rise to the formation of linear homoallyl alcohols, whereas branched products are obtained with less reactive, more hindered, aldehydes66,79. 

Cyclohydrocarbonylation of unsymmetrical amidodiene 29 catalyzed by Rh-BIPHEPHOS complex yielded dehydropiperidine aldehyde 31 as the sole product (Scheme S). The fact that no pyrroline was formed indicates that this reaction was extremely chemo- and regioselective so that the hydroformylation took place at the homoallylic olefin moiety exclusively, yielding the linear aldehyde intermediate 30. 

In all cases, the ratio of branched (7) to linear alcohols (8) was 95/5 or higher. The homoallylic alcohols were formed in 1-2 h at room temperature. In addition to the homoallylic alcohol, it has illustrated the formation of an unsaturated ketone coming from the reaction of the organozinc intermediary with the solvent acetonitrile (equation 17)... 

Significantly better results in addition of non-stabilized nucleophiles have come from hydrogenolysis reactions using formate as a hydride donor as shown in Scheme 8E.46. The racemic cyclic acetate and prochiral linear carbonates were reduced in good enantioselectivities by monophosphine ligands (/ )-MOP (16) and (Zf)-MOP-phen (17), respectively [195]. The chirality of the allylsilane can be efficiently transferred to the carbinol center of the homoallylic alcohol by the subsequent Lewis acid catalyzed carbonyl addition reaction 1196], The analogous... 

CoClr-Controlled addition to aldehydes.1 This reaction, when catalyzed by a Lewis acid, TiCl4 or BF3 etherate, involves an allylic rearrangement, regardless of the order of addition of reactants (12, 146). In contrast, if CoCl2 (1 equiv.) is present, the addition produces mainly or even exclusively the linear homoallylic alcohol (a-adduct). 

Haloalken-l-yl)dialkylboranes, obtained by hydroboration of propargyl halides with dialkylboranes, smoothly rearranged to ( )-allyboranes, which can be trapped with aldehydes. This one-pot three-component sequence provided linear homoallyl alcohols (Equation (145)) or //-homoallyl alcohols (Equation (146)).610-612... 

Figure 8. Metathesis and addition of crotyl tributyltin and BuSnCb leading to branched and linear homoallylic alcohols.

The reaction of ( )-crotylsilane with RCHO in the presence of TiCl4 proceeds with high 5yn-selectivity. On the other hand, the corresponding (Z)-silane furnishes syn-anti mixtures of homoallylic alcohols. It has been shown that the reaction proceeds via an acyclic linear and not via a cyclic six-member transition state. As shown below for the (E)-crotylsilane, transition state A leading to the jyn-alcohol is sterically favored over the diastereomeric transition state B, which has a gauche Me-Et interaction. ... 

Tab. 8.29 Rearrangement of branched homoallylic alcohols to the linear isomers...

Reactions of crotyl organometallics with aldimines can give linear (4) and/or branched (5) homoallyl-amines, a result of addition at the a- and y-crotyl carbon atoms (equation 1). Although branched products usually dominate, the metal (M) and the imine substituents (R and R ) can strongly influence the regiochemical outcome of the reaction similar effects are observed with crotyl in additions to iminium salts and gem-amino ethers. 

Linearly fused lactone rings are constructed by tandem radical cyclization from homoallylic xanthates using thionolactone formation as the key step87. The 5-e.Yo-cyclized product is obtained as a single diastereomer. 

Loh et al. found a tritlic acid-catalyzed 2-oxonia Cope rearrangement, which was used in the stereocontrolled synthesis of linear 22i -homoallylic sterols (eq 48). Interestingly, poor stereoselectivity was observed when In(OTf)3 was employed as the catalyst for this reaction. Stereoselective Mannich-type reaction of chiral aldimines with 2-silyloxybutadienes in the presence of triflic acid gives the corresponding products with 70-92% de in 62-74% chemical yield, which are not obtained by general Lewis acid-promoted methods (eq 49). ... 

Lee CK, Lee CA, Tan K, Loh T (2004) An Unusual Approach to the Synthesis of Enantiomerically Cis Linear Homoallylic Alcohols Based on the Steric Interaction Mechanism of Camphor Scaffold. Org Lett 6 1281... 

Homoallylic alcohols such as 107 (R=H) give mixtures of regioisomers 108 and 109. Both can be cycled to provide tetrahydrofiiran or tetrahydropyran derivatives (Figure 31). As in the case of allyhc alcohols, substitution in the double bond or in the allyhc carbon favors the formation of the linear aldehyde. The linear aldehyde 109 provides the hemiacetal 110, from which acetals [64], lactones [65, 77] and also enolethers such as 111, can be formed [70, 71, 72, 73]. 

Seok and colleagues investigated the hydroformylation of allyl alcohol with paraformaldehyde in the presence of HRh(CO)(PPh3)3 (Scheme 3.4) [14]. Similar to that found in the reaction with syngas (see Section Allyl and homoallyl alcohols in Chapter 4), the functional group in the olefinic substrate directed the regiochemistry of the reaction and a cyclic transition state between catalyst and substrate was assumed. A maximum isomeric product ratio of lib = 21 was achieved. The addition of syngas to the reaction with paraformaldehyde or an excess of phosphine inhibited the formation of the linear aldehyde. 

Palladium-catalyzed retro-allylation of homoallylic alcohols is extended to retro-propargylation of homopropargylic alcohols [20]. Despite the rigid and linear structure of the triple bond, the reactions of aryl halides with homopropargylic alcohols necessitate slightly modified conditions to afford allenylated arenes (Scheme 5.25). Not only gewi-disubstituted allenes but also tri- and tetrasubstituted ones are accessible. The reactions with diastereomerically pure homopropargylic alcohols threo- and erythro-17 result in stereospecific transfer to afford anti- and syn-18, respectively (Scheme 5.26). 

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