L-Alkynes

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

The synthesis of 10 features the SN2 displacement of the allylic acetate with migration of R2 from the ate complex6. Precursors 9 are prepared by the hydroboration of 3-acetoxy-l-alkynes that are available with very high enantiomeric purity via the asymmetric reduction of the corresponding l-alkyn-3-ones, and a substantial degree of asymmetric induction occurs in the conversion of 9 to 10. Best results, based on the enantioselectivity of reactions of 10 with aldehydes, are obtained when R2 is a bulky group such as isopinocampheyl (79 85 % ee)6. The yields of reactions of 10 with aldehydes are 62-76%. 

Addition of trifluoroacetic acid to 5-substituted-l-alkynes of the type... 

ALKYL-l-ALKYNES, 58, 1 ALKYL ARYL SULFIDES, 58,143 Alkyl aryl thioethers, 58,145 Alkylation, enolates, 56, 52 C-ALKYLATION, phase transfer catalysis... 

Alkenyl sulfoxides 177 and 178, which can be readily prepared from l-alkynes provide synthones for the carbocations 179 and 180. These synthones are useful for the simple construction of cyclopentenones and also in providing an electrophilic precursor for the -side-chain on prostanoids ". ... 

Methylenetetrahydrofurans can also be realized by an intramolecular radical cyclization of bromoalkynes utilizing indium(I) iodide as a radical initiator <06TL2859>. 3-Diiodomethylenetetrahydrofurans were also prepared from l,to-diiodo-l-alkynes in the presence of l-hexynyllithium<06CC638>. 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

Terminal allenes.1 A synthesis of 1,2-dienes (3) from an aldehyde or a ketone involves addition of ethynylmagnesium bromide followed by reaction of the adduct with methyl chloroformate. The product, a 3-methoxycarbonyloxy-l-alkyne (2), can be reduced to an allene by transfer hydrogenolysis with ammonium formate catalyzed by a zero-valent palladium complex of 1 and a trialkylphosphine. The choice of solvent is also important. Best results are obtained with THF at 20-30° or with DMF at 70°. 

The organozinc reagents 123, which were generated from 1-phenyl-l-alkyne 122 with nBuLi in the presence of 1.5 mol% of HgCl2 followed by addition of 1.5 equiv. of ZnBr2, exists as a tautomeric mixture of an allenylzinc and a propargylzinc species (Scheme 3.62) [103]. 

Interesting highly reactive 1-azidoallenes such as 263 and 265 are accessible by a [3 + 3]-sigmatropic isomerization of 3-azido-l-alkyne precursors 262 (Scheme 8.71) [152]. Due to the synthetic potential of this class of allenes, the scope and limitations of their transformations were explored in detail by Banert and co-workers [153],... 

Scheme 3.21. Regioselectivity in the stannylcupration of l-alkynes, -hydroxy-l-alkynes, enynes, and propargyl alcohols.

IrCl(cod)]2 leads to three-component coupHng products of aldehydes 21, primary amines 22 and l-alkynes 23 to afford 24 via an activation of the C—H bond adjacent... 

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

Preparation of 1 -Bromo-l-alkynes from AlkynyUfrhium and Cyanogen Bromide... 

Stannylalumination of l-alkynes proceeds smoothly[7l], Silylstannation of l-alkynes with 144 proceeds to give 145 with high rcgio- and stereoselectivity. The reaction is cis addition and Sn always adds to the internal position. The addition products are converted into either a vinylsilane or a 2-stannylalk-ene[72-76]. The 2-stannylalkenc 146 is prepared by silylstannation of alkynes, followed by treatment with tributylammonium fluoride[77]. Rapamycin has been synthesized applying the silylstannation as a key step. The ew-adduct... 

---