TMEDA reagents

This Zn-CH3CHBr2-TiCl4-TMEDA reagent can also be used in the ethyl-idenation of thioesters and amides, as shown in Scheme 5.19 [27]. 

This Zn-CHjCHBrj-TiCU-TMEDA reagent can also undergo the ethylidenation of thiolester and amide as shown in Scheme 8.21 [29]. 

The method does not require optically pure a-pinene because 100% enantiomeric excess (ee) is achieved by crystallisation of the intermediate TMEDA-2IpcBH2 adduct, where TMEDA = (CHg )2NCH2CH2N(CH3 )2 (tetramethylethylenediamine). Other chiral monoalkylboranes derived from 2-alkyl- and 2-phenylapopinene are slightly more selective reagents as compared to monoisopinocampheylborane (66—68). 

BzOCF(CF3)2, TMEDA, 20°, 30 min, 90% yield. This reagent also reacts with amines to form benzamides in high yields. 

Ni(CO)4, TMEDA, DMF, 55°, 4 h, 87-95% yield. Because of the toxicio associated with nickel carbonyl, this method is rarely used and has largeb been supplanted by palladium-based reagents. 

The milder metal hydnde reagents are also used in stereoselective reductions Inclusion complexes of amine-borane reagent with cyclodexnins reduce ketones to opucally active alcohols, sometimes in modest enantiomeric excess [59] (equation 48). Diisobutylaluminum hydride modified by zmc bromide-MMA. A -tetra-methylethylenediamme (TMEDA) reduces a,a-difluoro-[i-hydroxy ketones to give predominantly erythro-2,2-difluoro-l,3-diols [60] (equation 49). The three isomers are formed on reduction with aluminum isopropoxide... 

Trialkylsilanes are usually formed by the addition of a lithium or Grignard reagent to the silyl chloride, and thus, discussions related to the formation of the silyl acetylene bond will be kept to a minimum. Silyl acetylenes are prepared from the alkynylcopper(I) reagents in the presence of PPh3, Zn or TMEDA in CH3CN at 100°, 36-98% yield. It is interesting to note that the... 

Several reviews cover hetero-substituted allyllic anion reagents48-56. For the preparation of allylic anions, stabilized by M-substituents, potassium tm-butoxide57 in THF is recommended, since the liberated alcohol does not interfere with many metal exchange reagents. For the preparation of allylic anions from functionalized olefins of medium acidity (pKa 20-35) lithium diisopropylamide, dicyclohexylamide or bis(trimethylsilyl)amide applied in THF or diethyl ether are the standard bases with which to begin. Butyllithium may be applied advantageously after addition of one mole equivalent of TMEDA or 1,2-dimethoxyethane for activation when the functional groups permit it, and when the presence of secondary amines should be avoided. 

The reagents prepared by lithiation (see Section 1.3.3.3.1.2.) and titanium exchange of (S)-(Z)-l-methyl-2-butenyl diisopropylcarbamate106 show a diminished reactivity when compared with those derived from the ( -isomer, indicating that in both metalation steps the doublebond geometry is retained16. After treatment of the lithium -TMEDA complex with chlorotris-(diethylamino)titanium and 2-methylpropanal, the homoaldol adduct (3S,47f)-(Z)-4-hydroxy-1,3,5-trimethyl-l-hexenyl diisopropylcarbamate [( + )-4], is formed with 88% ee16. 

A solution of s-BuLi (1.05 mmol, 1.1 m in cyclohexane) was added dropwise to a stirred solution of chloromethyltrimethylsilane (lmmol) in THF (to give a 1 m solution of reagent) at -78 °C. After 5 min, TMEDA (1.05 mmol) was added, and the stirred solution was allowed to warm to -55 °C over 0.5 h, by which time it was pale yellow (the presence of TMEDA increases the rate of formation of the reagent, but has no other advantages). 

It has been reported that the zinc hexasulfido complex, Zn(S6)(TMEDA) (TMEDA=] r,] T,N, N -tetramethylethylenediamine) acts as a sulfur transfer reagent in an analogous manner as the above-mentioned titanium polysulfido complexes. The sulfur transfer reaction between Zn(S6)(TMEDA) and TiCp2Cl2 cleanly proceeds at room temperature to afford ZnCl2(TMEDA) and TiCp2S5 together with Ss (Scheme 51) [132]. This result suggests that... 

Reagents (i) sBuli, TMEDA, EtzO (ii) Ti(OiPr)4, R1CHO, Et20 (iv) R2CHO, BF3.OEt2, CH2CI2... 

Titanium-based reagents generated by the reduction of gem-dihalides with low-valent metal species are widely used for the alkylidenation of carbonyl compounds (Scheme 14.28). As in the case of methylidenation, the system gem-dibromide/TiCl4/Zn/TMEDA... 

Peterson methylenation (10, 433 11, 581). Methylenation with trimethyl-silylmethyllithium, (CH3),SiCH2Li, is not widely used in synthesis because of lack of selectivity and moderate yields. However, a modified reagent prepared from (CH3)3SiCH2Li and CeCl, adds to aldehydes or ketones (even enolizable ones) to form adducts in generally high yield, particularly in the presence of TMEDA. The 2-hydroxysilanes are converted into methylene compounds by aqueous HF (with or without pyridine).4... 

---