Propargylic homopropargylic alcohol

A similar allyl [91] or propargyl [92] Reformatsky reagent has been used to prepare fluormated homoallylic or homopropargylic alcohols, respectively [91, 92] (equations 60 and 61)... 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Allenic alcohols,3 In the presence of Sml2 and Pd[P(C6H3),]4, sec- and tert-propargylic acetates add to ketones to give allenic alcohols as the only or major product. A mixture of allenic and homopropargylic alcohols is formed from reactions of primary propargylic acetates. 

Table 9.12 Conversion of lithio propargyl chloride to allenic carbinols and homopropargylic alcohols.

Propargylic ethers undergo directed lithiation and subsequent transmetallation to afford oxygenated allenyl titanium reagents. Subsequent addition of aldehydes gives rise to various homopropargylic alcohol adducts as mixtures favoring the anti dia-stereomers (Tables 9.18 and 9.19) [29, 30]... 

The preference for a syn elimination pathway is higher for tertiary versus secondary carbonates, as illustrated in Eq. 9.31. The two-step sequence of allenyltitanium formation followed by addition to benzaldehyde affords the homopropargylic alcohol adduct with essentially complete retention of configuration. In contrast, the phosphate derivative yields a benzaldehyde adduct in which the propargylic center is... 

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

Allenyltrichlorosilanes can also be prepared by Sn2 displacement of propargylic chlorides with a Cu or Ni complex of HSiCl3 [56]. The reaction requires an amine base and a donor solvent such as THF or propionitrile (Table 9.32). Conditions can be adjusted to favor the propargylic or allenic silane, which is not isolated, but treated directly with various aldehydes to afford allenylcarbinols (A) or homopropargylic alcohols (B). These reactions presumably proceed by an SE2 pathway, such that the allenyl products arise from the propargylic silane and vice versa. 

The regioselectivity is maintained with mono- and even disubstituted propargylic chlorides (Table 9.33) [56], The copper complex affords allenylcarbinols (A) and the nickel complex favors homopropargylic alcohols (B). In the latter case, the syn adducts are predominant, suggestive of an acylic transition state. 

Allenyltin halides are not isolable but can be prepared in situ through reaction of a propargylic halide with a mixture of tin and aluminum metal, usually in powdered form. For example, propargyl bromide is converted to diallenyltin dibromide (Eq. 9.71). This intermediate reacts with aldehydes to produce homopropargylic alcohols in high yield. Allenyl adducts are not formed in this reaction [67]. 

Allenyl iodides can be prepared from propargylic mesylates by Sn2 displacement with LiCuI2 (Eq. 9.143) [118]. The reaction proceeds primarily by an anti pathway with slight racemization. Metallation of these iodides with powdered indium in various donating solvents leads to transient allenylindium intermediates which react in situ with aldehydes to afford anti homopropargylic alcohols (Table 9.52). Additions... 

Propargyl bromide reacts with tributylstibine to form an intermediate allenylstibi-nate (A) (Table 9.62) [124], This intermediate fails to react with aldehydes at temperatures as high as 120 °C. However, addition of BuMgBr produces a reactive alleny-lantimony reagent (B), which reacts with aldehydes to yield homopropargylic alcohols. 

Ketones and aldehydes possess the highest a-acidity, many examples being known. In fact, it is usually very difficult to keep a terminal propargyl ketone from isomerizing to the allenyl ketone. Thus the oxidation of a homopropargylic alcohol 54, a typical precursor, in most of these cases directly delivers the allenyl ketones 56 rather than the propargyl ketones 55 in high yields [92-109] (Scheme 1.23). 

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

An alternative preparation of enantioenriched anti-homopropargylic alcohols along similar lines uses EtjZn to effect in situ transmetallation of an allenylpalladium intermediate from a propargyl mesylate and 2.5 mol % of Pd OAc)2 PPh3 in the presence of an aldehyde (Table II). The two methods are comparable. 

Interestingly, silylated propargylic zinc reagents, such as 342, may be better viewed as the allenic zinc reagent 343 which reacts with an aldehyde via a cyclic transition state, affording only the a f/-homopropargylic alcohol 344 with 90% yield (Scheme 91) 22. Alkylzinc halides react only sluggishly with aldehydes or ketones. This reactivity can be... 

The same group also achieved the regioselective propargylation of aldehydes and ketones affording the corresponding homopropargyl alcohols (equation 5)14. 

Allenie alcohols (9, 119).1 The reaction of propargylic bromides (1) with aldehydes or ketones can result in allenie alcohols (3) and/or homopropargylic alcohols (4). The selectivity depends on the substitution in 1 and 2 and is also... 

Zn, BrCH2CH2Br, CuBr, LiBr, EtOH (propargylic and homopropargylic alcohol and amine, propargyhc ether, enyne, diyne)... 

Umpolung of propargyl compounds occurs in the presence of excess Et2Zn, and homopropargyl alcohols 187 are obtained by the reaction of propargyl benzoates (186) with benzaldehyde, although the exact mechanism is not known. Ethylallene, which is expected to be formed by transmetallation of the allenylpalladium with Et2Zn and reductive elimination, is not formed [41],... 

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