Aluminum complexes silyl

Alkali metal derivatives of 2-(trimethylsilyl)aminopyridines can be further derivatized by insertion of 1,3-dicyclohexylcarbodiimide. Functionalized guani-dinates are formed in this reaction via a 1,3-silyl shift. Scheme 170 illustrates the reaction sequence as well as the preparation of an aluminum complex of the modified ligand, which exhibits pseudo jS-diketiminate binding of the metal center, thus exemplifying the coordinative versatility of this new multi-N-donor system. ... 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

The highly electrophilic cationic bis(8-quinolinolato)aluminum complex 407 enabled Yamamoto and coworkers to perform Mukaiyama-Michael additions of silyl enol ethers to crotonylphosphonates 406. The procedure was not only applicable to enol silanes derived from aryl methyl and alkyl methyl ketones (a-unsubstituted silicon enolates) but also to several cycfic a-disubstituted silyl enol ethers, as illustrated for the derivatives of a-methyl tetralone and indanone 405 in Scheme 5.105. Despite the steric demand of that substitution pattern, the reaction occurred in relatively high chemical yield with varying diastereoselectivity and excellent enantiomeric excess of the major diastereomer. The phosphonate residue was replaced in the course of the workup procedure to give the methyl esters 408. The protocol was extended inter alia to the silyl enol ether of 2,6,6-tetramethylcyclohexanone. The relative and absolute configuration of the products 408 was not elucidated [200]. 

Scheme 5.105 Conjugate addition of silyl enol ethers 405 to crotonylphosphonates 406, mediated by aluminum complex 407.

To avoid the intrinsic instability of cyanohydrins and their silyl ether, Saa and coworkers reported catalytic asymmetric cyanophosphonylation reaction of aldehydes with commercially available diethyl cyanophosphonate [58]. In these works, Lewis acid-Lewis base bifunctional catalyst (65) prepared by mixing BI-NOLAM ligand with amino arms as Lewis base and Et2AlCl was found to work nicely (Scheme 6.46). Since a strong positive nonlinear effect was observed in this reaction, actual catalyst is in equilibrium with some oligomeric species of the aluminum complexes. Bifunctional catalyst (65) could also catalyze cyanosilylation of... 

Shibasaki has described the use of bifunctional catalysis in asymmetric Strecker reactions, using BlNOL-derived Lewis acid-Lewis base catalyst 160 (Equation 24) [114]. The aluminum complex had previously been shown to catalyze enantioselective cyanohydrin formation (Chapter 2, Section 2.9) [115]. In the proposed catalytic cycle, the imine is activated by the Lewis acidic aluminum while TMSCN undergoes activation by association of the silyl group with the Lewis basic phosphine oxide. Interestingly, the addition of phenol as a putative proton source was beneficial in facilitating catalyst turnover. The nature of the amine employed for the formation of the N-substituted aldimine proved to be vital for enantioselectivity, with optimal results obtained for N-fluorenyl imines such as 159, derived from aliphatic, unsaturated, and aromatic aldehydes (70-96% ee) [114],... 

Oxidative addition of the silyl species to nickel is followed by insertion of unsaturated substrates. Zero-valent nickel complexes, and complexes prepared by reducing nickel acetylacetonate with aluminum trialkyls or ethoxydialkyls, and in general Ziegler-Natta-type systems, are effective as catalysts (244, 260-262). Ni(CO)4 is specific for terminal attack of SiHCl3 on styrene (261). 

With Chiral Al Complexes Chiral bis(silyl)binaphthol-modified aluminum catalyst, which is originally developed for asymmetric hetero-Diels-Alder reaction [50], is successfully applied to asymmetric Diels-Alder reaction of cyclopentadiene with methyl acrylate or methyl propio-late [51 ] (Eq. 8A.28). The latter is a rather rare example in the literatures. 

Ethyl aluminum dichloride mediates a formal [5 + 2] cycloaddition of complex (164) and (166) with enol silyl ethers to produce the highly strained seven-membered rings (165) and (167) respectively (Schemes 239 -240). Excellent stereoselectivity is observed in both cases. A related double alkylation affords complexed seven-membered rings via a formal [4 - - 3] cycloaddition. Incorporation of fluorine is observed using boron trifluoride etherate (Scheme 241). 

The silylated aminoiminophosphine (MesSi)2N P=N SiMe3 reacts with anhydrons alnmtnum chloride at 40 °C with elimination of Me3SiCl to form a dicoordinate phosphorus-nitrogen ligand, in a four-membered chelate ring complex. Anhydrous aluminum chloride extracts a chlorine ion from [C1PNR]2 to form a cyclic cation with a dicoordinated... 

Although the Michael addition of metal ynolates to a,/ -unsaturated carbonyl compounds is expected to give six-membered cycloadducts, 1,2-addition to carbonyl groups usually precedes 1,4-addition. The cycloaddition of the lithium-aluminum ate complex of silyl-substimted ynolate 112 with ethyl benzylideneacetoacetate (113), which is doubly activated by the ester and keto functions, gives the y-lactone 114 via a [4 4- 2] type cycloaddition (equation 46). Diethyl benzylidenemalonate (115) affords the uncyclized ketene 116 by reaction with 112 (equation 47). This could be taken as evidence for a stepwise mechanism for equation 46. ... 

The scope of the reaction was examined with a catalyst prepared from the benzene sulfonamide and DIBAL, because it was found that essentially the same induction could be obtained as with those obtained from tri-/so-butyl aluminum. Two years earlier the authors had reported that this Simmons-Smith reaction could also be catalyzed by the aluminum-free sulfonamide 132 (optimum with Ar = /7-NO2C6H4) the induction obtained is listed in the far right column of Table 8 [34]. It was proposed that a zinc complex of 132 is generated in-situ. Surprisingly, with the exception of the silyl-substituted allyl alcohol (the last entry in the table) [35], almost identical asymmetric induction obtained by use of the aluminum-containing and aluminum-free catalysts. The main advantage of the diazaaluminolidine catalyst is that it is apparently more soluble than the aluminum-free bis-sulfonamide catalyst, with the result that a tenfold increase in concentration (0.1 m) can be used this might explain the increased rate observed for the diazaaluminolidine catalyst. Finally, it has recently been reported that a catalyst formed from the Ci symmetrical sulfonamide 135 and DIBAL will induce the formation of 131 from cinnamyl alcohol in 68 % ee [36]. 

The silyl organoaluminum reagent (52) was prepared either by the addition of activated aluminum to a tetrahydrofuran solution of chlorotrimethylsilane, or by the treatment of sodium tetrakis(trimethyl-silyl)aluminate with aluminum chloride. Alternatively, ate complex (53) may be prepared by the addition of methyllithium to tris(trimethylsilyl)aluminum. ... 

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