Allenyl iodides

In situ magnesiation of an allenyl iodide with isopropylmagnesium bromide gives rise to a transient allenyl Grignard reagent, which adds to aldehydes and ketones to afford mainly homopropargylic alcohol adducts (Table 9.8) [18]. The anti diastereo-mers are favored, especially with sterically demanding aldehydes. Additions to ketones are less selective. 

A direct zinc-halogen exchange has been used to convert allenyl iodides into alle-nylzinc reagents (Table 9.47) [98]. These react with aldehydes and ketones to yield mainly anti homopropargylic adducts in modest yield. The selectivity is highest with aliphatic aldehydes. 

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... 

Wang s approach for the synthesis of enyne-allenes focused on ene-allenyl iodide 45 (Scheme 14.12) [24]. Palladium-catalyzed Sonogashira reaction of 45 with terminal alkynes 46 (R= Ph or CH2OH) proceeded smoothly under mild reaction conditions in the presence of the cocatalyst cuprous iodide and n-butylamine. The initially formed enyne-allene 47b with substituent R= CH2OH cyclized spontaneously to the corresponding a-methylstyrene derivative 48. 

The small number of electrophiles that react well with Af-Boc-2-hthiopyrrolidine limits applications of the asymmetric deprotonation of Af-Boc-pyrrolidine. In a significant development. Dieter and coworkers have shown that transmetalation of the enantioen-riched 2-lithiopyrrolidine with CuCN-2LiCl forms a cuprate that reacts enantioselectively with vinyl and allenyl iodides (Scheme 34). Enantioselectivities in the 90-95% range were achieved, although the reaction is very sensitive to reagent purity. ... 

To probe the nature of the allenyl iodide-diethylzinc exchange process, Utimoto and coworkers monitored the 1II NMR spectrum of a 1 1 mixture of iodoallene and Et2Zn in . 11 It was found that within 2 hours, the olefinic protons of the allenyl iodide were... 

Allenyl iodides, in Grignard reagent preparation, 9, 39 Allenyl ligands... 

The homologous (5-allenyl iodide 36 was transformed by t-BiiLi at — 78 C to the corresponding organolithium 37Li. Warming 37Li to room temperature for Ih prior to hydrolysis gave a 27 20 50 3 mixture of allene 38, acetylene 39, vinylcyclopentane 40, and cyclohexene 41 in 86% yield [27] (Scheme 7-29). 

Iodine-magnesium exchange reaction of allenyl iodides occurs by the action of PrMgBr in Et2O. Subsequent reaction with aldehydes or ketones provides homopropargyl alcohols with high regioselectivity (Scheme 3.96). 

Reduction of 68a with L-selectride gave a 6 1 mixture of cyclopropyl carbinols in 92% with the (R)-alcohol predominating. Highly stereoselective hydroxyl-directed epoxidation from the a-face of the cyclopentane ring followed by silylation of the alcohol gave 69 (contaminated with a small amount of the product derived from the S-alcohol) in 84% yield. This intermediate was then coupled with the allenyl iodide 63 via the cuprate of 69 to afford an 86% yield of the diyne 70. Partial reduction of the alkynes followed by desilylation and chromatography afforded 71a and 71b in 79% and 13% yields, respectively. Conversion of the undesired major (R)-isomer 71a into the minor (5)-compound was accomplished via an oxidation-reduction sequence to provide 71b in 75% yield contaminated with 16% of the (R)-71a. Orthoester 71b was then cleaved... 

Marshall JA, Grant CMJ. Preparation of chiral allenylmetal reagents from enantioenriched allenyl iodides and propar-gyUc mesylates. A comparison of indium, bismuth, and tin derivatives. Org. Chem 1999 64 8214-8219. 

Note 2. Traces of unreacted triethylamine might cause partial isomerization of the allenyl sulfoxide into the propargyl sulfoxide. The methyl iodide is added to ensure that no triethylamine remains. 

A related method was reported by Katritzky et al. [25], who prepared 1-alkoxy-l-(l,2,4-triazol-l-yl)allenes from the corresponding triazole-substituted alkynes, e.g. the reaction of 18 to 19 in Eq. 8.2. In this case the generated allenyl anion was trapped with methyl iodide. 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

Table 9.6 Generation of allenyl-Cu(I) reagents and their reaction with alkynyl iodides.

A related method of preparation involves the oxidative addition of a tin(II) salt to propargylic iodides, which yield mixtures of allenyl- and propargyltin halides on treatment with SnCl2 in DMF-DMI (l,3-dimethylimidazol-2-one) (Eq. 9.75) [68], These intermediates react in situ with aldehydes to afford mixtures of propargylic and allenic carbinols via a cyclic SE2 process (Eqs. 9.76 and 9.77). As explained in the Introduction, the ratio of these two products reflects the relative transition-state energies of the addition reactions. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

SCHEME 60. Selective formation of either of the two possible regioisomers of allenes via Pd-catalyzed allenylation of aryl iodides... 

The acid portion of 1 was assembled by enantio-and diastereocontrolled addition of Z-crotyl borane to the aldehyde 8, following the Brown protocol. Hydroboration and oxidation led to 9, which was condensed with the allenyl silane 10 to give 11 with high diastereocontrol. Conversion of the alcohol to the iodide followed by three-carbon homologation by the Myers procedure then led to 1, which was cyclized with > 10 1 regio- and diasterocontrol to give 12. Ozonolysis and methylenation of the less hindered ketone then delivered 3. 

Linkers that enable the preparation of y-lactones by cleavage of hydroxy esters from insoluble supports are discussed in Section 3.5.2. Resin-bound y-lactones have been prepared by Baeyer-Villiger oxidation of cyclobutanones [39], by intramolecular addition of alkyl radicals to oximes [48], by electrophilic addition of resin-bound sele-nenyl cyanide or bromide to 3,y-unsaturated acids (Figure 9.2 [100]), and by palladium-mediated coupling of resin-bound aryl iodides with allenyl carboxylic acids (Entry 10, Table 5.7 [101]). 

In the case of the allenyl copper intermediate 724, its alkylation with (iodomethyl)zinc iodide and carbonyl compounds afforded dienes 7251032 (Scheme 189). 

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