Lewis base promoter

Carborundum, silver on, 27 10-12 3-Carboxy - 2,2,5,5,- tetramethylpyrrolidin-1-oxy m-nitrophenyl ester, enantiomeric specificity in reactions of cyclohexaamy-lose and cycloheptaamylose on, 23 233 Carboxylate ions, 32 117-118 Carboxybc acids a, 3-unsaturated, 25 109 hydrogenation of, 25 107-115 ketonization of, 24 35-37 Lewis base-promoted ruthenium catalysis, 32 389, 400... 

The first example of a chiral Lewis base promoted allylation was given by Denmark and coworkers in 1994 [45], Stoichiometric amounts of chiral phospho-ramide R,R)-20 facilitated the enantioselective allylation (Scheme 15). There was a complete stereochemical correlation between the geometry (ElZ) of allylsilane and the diastereomeric ratio (synlanti) of the products. 

Leighton has combined this concept of strained silacycles [62-66] with the asymmetric allylation chemistry in a series of pubhcations [60, 67-70], Leighton s aUyhc silacyclopentane 29 [67] (Scheme 20) works equally for allylation of aromatic and ahphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The mechanism of... 

Studies of ruthenium-catalyzed reactions in carboxylic acid solvents have been reported by Knifton (171,172), but most of these experiments contain added salt promoters which greatly modify the catalytic behavior. These experiments will be considered in Section V, along with other Lewis base-promoted ruthenium systems. 

As described previously, carboxylic acids may be used as solvents for halide or Lewis base-promoted CO reduction. However, these solvents have the disadvantage of forming the usually less desirable carboxylate esters rather than free alcohol products. Rate or selectivity advantages are not apparent in these solvents. 

Tungsten-based catalysts with a Lewis base promoter are used in the polymerization of dicyclopentadiene also isolated from the C5 stream-cracker fraction.150,155 The product poly(dicyclopentadiene) or Metton resin has unusual physical and chemical properties. It is processed together with elastomers to increase viscosity of the end product by using reaction injection molding (RIM) technology.155... 

Lower yields of Na3[Mn(CO)4] were obtained from the direct reduction of Mn2(CO)10 in Na-HMPA, because the neutral dimer underwent slow disproportionation in this medium to form [Mn(HMPA)Jt][Mn(CO)5]2, in contrast to Re2(CO)10, which showed no tendency to react with HMPA at room temperature. Of all neutral binary carbonyl dimers known, Re2(CO)10 appears to be the most resistant toward Lewis base-promoted disproportionation reactions. The slightly lower yields of Na3[Re(CO)4], compared to those of Na3[Mn(CO)4], may have arisen from the fact that Re2(CO)10 does not cleanly reduce to [Re(CO)5] in HMPA or other solvents (22). It should... 

Asymmetric aldol additions of geometrically defined trichlorosilyl enolates of ketones to aliphatic and aromatic aldehydes have been carried out uncatalysed, and with a chiral phosphoramide as Lewis base promoter.54 Significant differences in rates and diastereoselectivities are interpreted in terms of the changeover from a boat-like transition state, with pentacoordinate siliconate, to a chair-like transition state with hexacoordination. 

Scheme 9. Lewis base-promoted catalytic asymmetric aldol reaction according to Denmark.

Pentacoordinate silicon, in the form of allyltrichlorosilane, in the presence of DMF or HMPA, also undergoes allylation with aldehyde [97,98], The Lewis base-promoted approach has the merit of operational convenience. Stereoselective formation of allylic trichlorosilanes is achieved by copper-catalyzed coupling of allylic chloride and trichlorosilane or by palladium-catalyzed hydrosilylation of 1,3-dienes (Sch. 56) [99],... 

Sugihara, T., Yamaguchi, M., Nishizawa, M. Lewis base promoted reactions of alkyne-dicobalt hexacarbonyls. Rev. on Heteroa. Chem. 1999, 21, 179-194. 

There are very few examples of Lewis base-promoted allylations of aldehydes with allylstannanes. In 1992 Baba disclosed an intriguing method for allylation of aldehydes with allyl- and 2-butenyltributylstannanes in the presence of catalytic-amounts of dibutyltin dichloride and certain coactivators such as tetrabutylammo-nium iodide, tributylphosphine oxide, HMPA or tetraphenylphosphonium iodide [76]. No definitive mechanistic information is available on the role of the co-activators the authors speculate that the ligands accelerate the metathesis to form allyldibutyltin chloride which is the actual nucleophile. The same group has recently reported the use of a lead(II) iodide/HMPA catalyst for the allylation of a,yff-epoxyketones [76bj. 

Wang, Z., Kisanga, P. and Verkade, J.G. (1999) P( PrNCH2CH2)3N an effective Lewis base promoter for the allylation of aromatic aldehydes with allyltrimethylsilane. The Journal of... 

In contrast to the polymerization of D4, the anionic polymerization of hexamethyl cyclotrisiloxane (D3) with lithium as counterion is a living polymerization which produces polydimethylsiloxanes with well-defined structures. Useful initiators include lithium silanolates or the product from the reaction of 3 mol of butyllithium with D3 in a hydrocarbon solvent as shown in Scheme 7.19. It is noteworthy that no polymerization occurs in the absence of a Lewis base promoter such as THF, glymes, DMSO, or HMPA. 

McWilliams and coworkers (03JOC467) have studied the catalytic activity and selectivity of bases as catalysts in the [l,8]naphthyridine synthesis from 2-aminonicotinaldehyde 357 (69BSB289, 74JOC720, 80T2359, OlMIl, 01SC1573) and 2-pentanone. As shown in Table 4, many different Bronsted and Lewis bases promote this reaction. The best in terms of acceleration and regioselectivity is pyrrolidine (03JOC467). 

---