Lewis Catalytic amount

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

PoIysuIfonyIa.tlon, The polysulfonylation route to aromatic sulfone polymers was developed independendy by Minnesota Mining and Manufacturing (3M) and by Imperial Chemical Industries (ICI) at about the same time (81). In the polymerisation step, sulfone links are formed by reaction of an aromatic sulfonyl chloride with a second aromatic ring. The reaction is similar to the Friedel-Crafts acylation reaction. The key to development of sulfonylation as a polymerisation process was the discovery that, unlike the acylation reaction which requires equimolar amounts of aluminum chloride or other strong Lewis acids, sulfonylation can be accompHshed with only catalytic amounts of certain haUdes, eg, FeCl, SbCl, and InCl. The reaction is a typical electrophilic substitution by an arylsulfonium cation (eq. 13). 

G in the presence of a catalytic amount of a Lewis base such as dimethylether, (GH2)20. In addition to the gas-phase pyrolysis of diborane, can be prepared by a solution-phase process developed at Union Garbide Gorp. Decaborane is a key intermediate in the preparation of many carboranes and metaHa derivatives. As of this writing, this important compound is not manufactured on a large scale in the western world and is in short supply. Prices for decaborane in 1991 were up to 10,000/kg. 

Dehydrochlorination of chlorinated derivatives such as 1,1,2-trichloroethane may be carried out with a variety of catalytic materials, including Lewis acids such as aluminum chloride. Refluxing 1,1,2-trichlorethane with aqueous calcium hydroxide or sodium hydroxide produces 1,1-dichloroethylene in good yields (22), although other bases such as magnesium hydroxide have been reported (23). Dehydrochlorination of the 1,1,1-trichloroethane isomer with catalytic amounts of a Lewis acid also yields 1,1-dichloroethylene. Other methods to dehydrochlorinate 1,1,1-trichloroethane include thermal dehydrochlorination (24) and by gas-phase reaction over an alumina catalyst or siUca catalyst (25). 

Decaborane is the most studied of all the polyhedral boranes and at one time (mid-1950s) was manufactured on a multitonne scale in the USA as a potential high-energy fuel. It is now obtainable in research quantities by the pyrolysis of B2H9 at 100-200°C in the presence of catalytic amounts of Lewis bases such as Me20. B10H14 is a colourless, volatile, crystalline solid (see Table 6.2, p. 163) which... 

Further improvements to this method have been reported by Bagley. The requirement of harsh thermal conditions to facilitate the cyclodehydration can be minimized by simply adding acetic acid or Amberlyst 15. Alternatively, one could use Lewis acids such as ZnBr2 or Yb(OTf>3 in catalytic amounts. 

Asymmetric Diels-Alder reactions using a dienophile containing a chiral auxiliary were developed more than 20 years ago. Although the auxiliary-based Diels-Alder reaction is still important, it has two drawbacks - additional steps are necessary, first to introduce the chiral auxiliary into the starting material, and then to remove it after the reaction. At least an equimolar amount of the chiral auxiliary is, moreover, necessary. After the discovery that Lewis acids catalyze the Diels-Alder reaction, the introduction of chirality into such catalysts has been investigated. The Diels-Alder reaction utilizing a chiral Lewis acid is truly a practical synthetic transformation, not only because the products obtained are synthetically useful, but also because a catalytic amount of the chiral component can, in theory, produce a huge amount of the chiral product. 

For example in the so-called Mukaiyama aldol reaction of an aldehyde R -CHO and a trimethylsilyl enol ether 8, which is catalyzed by Lewis acids, the required asymmetric environment in the carbon-carbon bond forming step can be created by employing an asymmetric Lewis acid L in catalytic amounts. 

The synthesis of the right-wing sector, compound 4, commences with the prochiral diol 26 (see Scheme 4). The latter substance is known and can be conveniently prepared in two steps from diethyl malonate via C-allylation, followed by reduction of the two ethoxy-carbonyl functions. Exposure of 26 to benzaldehyde and a catalytic amount of camphorsulfonic acid (CSA) under dehydrating conditions accomplishes the simultaneous protection of both hydroxyl groups in the form of a benzylidene acetal (see intermediate 32, Scheme 4). Interestingly, when benzylidene acetal 32 is treated with lithium aluminum hydride and aluminum trichloride (1 4) in ether at 25 °C, a Lewis acid induced reduction takes place to give... 

The addition of sulphinyl chlorides to trimethylsilyl enol ether 138 affording a-ketosulphoxides 139 (equation 76) represents an extension of the reaction of sulphinyl chlorides with ketones. This reaction has attracted attention only recently. Sergeev and coworkers192 reported that treatment of sulphinyl chlorides with acyclic enol ethers afforded a-ketosulphoxides 139 in good to excellent yields. Meanwell and Johnson193 observed that in the case of cyclic enol ethers the corresponding sulphoxides were formed only in very low yields. They found, however, that the introduction of an equivalent amount of a Lewis acid into the reaction mixture markedly promotes the desired reaction, whereas the use of catalytic amounts of a Lewis acid led to a substantial reduction in the yield. This is most probably due to the formation of a complex, between the a-ketosulphoxide and the Lewis acid. 

The reaction of alkenylcarbene complexes and imines in the presence of a Lewis acid generates pyrroline derivatives as a result of a [3C+2S] cyclisation process [76]. This reaction has been extended to an asymmetric version by the use of chiral alkenylcarbene complexes derived from several chiral alcohols. However, the best results are found when (-)-8-phenylmenthol-derived complexes are used and catalytic amounts of Sn(OTf)2 are added to the reaction. In these conditions high levels of trans/cis selectivity are achieved and the hydrolysis of the major tram diastereoisomers allows the preparation of optically pure 2,5-disubstituted-3-pyrrolidinone derivatives (Scheme 29). 

Catalytic asymmetric aza-Diels-Alder reactions using a chiral lanthanide Lewis acid. Enantioselective synthesis of tetrahydroquinoline derivatives using a catalytic amount of a chiral source [98]... 

A variety of reagents can bring about these ring openings, including amines and anhydrides used in stoichiometric amounts, or Lewis acids such as SnCl4 and Lewis bases such as tertiary amines, used in catalytic amounts. 

P-Keto esters have been prepared in moderate to high yields by treatment of aldehydes with diethyl diazoacetate in the presence of a catalytic amount of a Lewis acid such as SnCL, BF3, or GeCL. The reaction was successful for both aliphatic and aromatic aldehydes, but the former react more rapidly than the latter, and the difference is great enough to allow selective reactivity. In a similar process, aldehydes react with certain carbanions stabilized by boron, in the presence of (F3CC0)20 or NCS, to give ketones. 

In a related reaction, the Danishefsky diene 1434 cyclizes with ethyl pyruvate 1435 in the presence of catalytic amounts of the asymmetric Lewis acid catalyst 1436, at -72 °C in THF, to give the Diels-Alder adduct 1437, in 85% yield and 91% ee, and the ring-opened product 1438, which cyclizes, however, with triflic acid to give 1437 [11] (Scheme 9.9). 

Whereas the Prins-type cyclizations reported in this and the preceeding chapter were performed using stoichiometric amounts of Fe salts as Lewis acids, a breakthrough in the field of catalysis was reported in 2009 when the first iron-catalyzed Prins- and aza-Prins cyclization was reported. The catalytic system, which is obtained by combining catalytic amounts of an iron salt with trimethylsilyl halides as a halide source, is widely applicable and promotes the construction of substituted six-membered oxa- and aza-cycles (Scheme 33) [44]. 

The thermal decompositions are catalyzed by Bronsted and Lewis acids [68]. In general, when M is electron poor and Lewis acidic, the thermal decompositions occur efficiently and at low temperatures (typically between 100 and 200 °C, but sometimes at lower temperature). The addition of a catalytic amount of a Lewis or Bronsted acid (i.e., AICI3 or HCl) has been observed to accelerate the ehmination of isobutylene and the formation of three-dimensional network structures [64,124-126]. Pioneering studies on pyrolyses of various metal alkoxides by Bradley and others have also shown that alkene eliminations represent a primary decomposition pathway [104]. 

By reacting aniline with 2,3-dihydrofuran or dihydropyran and a catalytic amount of Lewis acid such as indium chloride in water, various tetrahydroquinoline derivatives were obtained by Li via an in-situ hetero-Diels-Alder reaction.130 Alternatively, similar compounds... 

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