Diethylaluminum

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Triethylaluminum Preparation. Triethyl aluminum [97-93-8], C H Al, can be prepared by a two-step or a one-step process. In the former, aluminum [7429-90-5], Al, powder is added to recycled triethylaluminum and the slurry reacts first with hydrogen [1333-74-0], to produce diethylaluminum hydride [871-27-2], which in the second step reacts with ethylene [74-85-1], to produce triethylaluminum. In the one-step process,... 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

The eailiest Ziegler-Natta catalysts were combinations of titanium tetrachloride (TiCl4) and diethylaluminum chloride [(CH3CH2)2A1C1], but these have given way to more effective zirconium-based metallocenes, the simplest of which is bis(cyclopentadi-enyl)ziiconium dichloride (Section 14.14). 

Addition of (R,S)-9 to the aromatic benzaldehyde proceeded with higher enantiosclcctivity than the addition of the diastereomeric reagent (S,S)-9. The reverse is true for additions to aliphatic aldehydes. Thus, the highest enantioselectivity of 92% ee was observed in the addition of (R,R)- 9 to cyclohexanccarboxaldehyde. The low chemical yields of most addition reactions can be improved by addition of the Lewis acid diethylaluminum ethoxide. The presence of the Lewis acid solely enhanced the chemical yield without changing the enantioselectivity of the addition reactions. 

Z)-2-Butenyldiethylaluminum has been generated at — 78°C by the reaction of (Z)-2-butenylpotassium and diethylaluminum chloride, but its reactions with aldehydes have not been systematically investigated1. The reaction of (Z)-2-butenyldiethylaluminum and chiral aldehyde 1, that provided 2 with 3 1 selectivity, was performed as one step in Still s monensin synthesis. 

Alkoxy-substituted allylaluminum reagents diethyl[(Z)-3-methoxy-2-propenyl]- and -[(Z)-3-(l-methoxy-l-methylethyl)-2-propenyl]aluminum have been prepared by treatment of the corresponding alkoxyallyllithiums with diethylaluminum chloride in tetrahydrofuran at — 78 =C4. These reagents provide the syn-diastereomer with 9-11 1 selectivity in reactions with aldehydes at — 78 °C. The reaction of diethyir(Z)-3-methoxy-2-propenyl]a]uminum and acetophenone provided the iy -diastereomer with 4 1 selectivity. 

A solution of 1.5 mol equiv of butyllithium in hexane is added to 1.5 mol equiv of a 1 M solution of hexabutylditin in THF at 0 C under nitrogen, and the mixture is stirred for 20 min. The solution is cooled to — 78 °C and a solution of 1.5 mol equiv of diethylaluminum chloride in toluene is added. After stirring for 1 h at — 78 °C, a solution of 0.05 mol equiv of [tetrakis(triphenyl)phosphine]palladium(0) in THF is added followed by a solution of the allyl acetate in THF. The mixture is warmed to r.t., and stirred until the allyl acetate has reacted (TLC). The solution is cooled to 0°C, and an excess of aq ammonia slowly added. After an aqueous workup, the products arc isolated and purified by flash chromatography on silica gel using 1 % triethylamine in the solvent to avoid acid-induced loss of stannane. 

It must be noted that the cyclic model fails to account for the role of the additional alkyllithium and diethylaluminum that are required in precise amounts to achieve high selectivity. A simple model that recognizes a possible role for the additional reagents suggests the intermediacy of an extended, noncyclic transition state G with the aldchydic oxygen coordinated to some undefined Lewis acidic species derived from the additional components of the reaction mixture26,44. Aggregates composed of enolate, alkyllithium and dialkylaluminum species are also possible. 

Conducting the aldol reaction at temperatures below —78 "C increases the diastereoselectivity, but at the cost of reduced yields45. Transmetalation of the lithium enolate 2 a by treatment with diethylaluminum chloride generated an enolate species that provided high yields of aldol products, however, the diastereoselectivity was as low as that of the lithium species45. Pre treatment of the lithium enolate 2a with tin(II) chloride, zinc(II) chloride, or boron trifluoridc suppressed the aldol reaction and the starting iron-acyl complex was recovered. 

Reaction of the lithium enolate 2 with prochiral aldehydes at low temperature proceeds with little selectivity, producing all four possible diastereomers 3, 4, 5, and 6 in similar amounts50. Transmetalation of the lithium enolate by treatment with three equivalents of diethylaluminum chloride or with one equivalent of copper cyanide generates the corresponding cthylaluminum and copper enolates which react at — 100°C with prochiral aldehydes to produce selectively diastereomers 1 and 2, respectively50. The reactivity of tin enolates of iron- propanoyl complexes has not been described. 

Extended acyclic transition states, such as G are also possible since, after the transmetalation, two additional equivalents of diethylaluminum remain that may serve as a Lewis acid26,44. 

In contrast, transmetalation of the lithium enolate at —40 C by treatment with one equivalent of copper cyanide generated a species 10b (M = Cu ) that reacted with acetaldehyde to selectively provide a 25 75 mixture of diastereomers 11 and 12 (R = CH3) which are separable by chromatography on alumina. Other diastereomers were not observed. Similar transmetalation of 10a (M = Li0) with excess diethylaluminum chloride, followed by reaction with acetaldehyde, produced a mixture of the same two diastereomers, but with a reversed ratio (80 20). Similar results were obtained upon aldol additions to other aldehydes (see the following table)49. 

Transmetalation of 19 by treatment with two equivalents of diethylaluminum chloride generates the aluminum enolate species 23. The latter reacts with acetaldehyde to produce the stable aluminum aldolates 24 which do not undergo the Peterson elimination23. A protic quench then provides the a-silylated aldol adducts of tentative structures (2 R)-25 and (2 V)-25 with little diastereoselectivity. Other diastereomers are not observed. 

According to these results the yields and E. F. decrease with the increasing number of Si-Cl bonds in the functional initiators. It is possible that Si-Cl bonds may slowly react with the diethylaluminum chloride coinitiator and thus reduce the expected E.F. as follows40 ... 

The addition of HCN to aldehydes or ketones produces cyanohydrins. This is an equilibrium reaction. For aldehydes and aliphatic ketones the equilibrium lies to the right therefore the reaction is quite feasible, except with sterically hindered ketones such as diisopropyl ketone. However, ketones ArCOR give poor yields, and the reaction cannot be carried out with ArCOAr since the equilibrium lies too far to the left. With aromatic aldehydes the benzoin condensation (16-54) competes. With oc,p-unsaturated aldehydes and ketones, 1,4 addition competes (15-33). Ketones of low reactivity, such as ArCOR, can be converted to cyanohydrins by treatment with diethylaluminum cyanide (Et2AlCN see OS VI, 307) or, indirectly, with cyanotrimethylsilane (MesSiCN) in the presence of a Lewis acid or base, followed by hydrolysis of the resulting O-trimethylsilyl cyanohydrin (52). The use of chiral additives in this latter reaction leads to cyanohydrins with good asymmetric... 

Cyanide addition has also been done under Lewis acid catalysis. Triethylaluminum-hydrogen cyanide and diethylaluminum cyanide are useful reagents for conjugate... 

Diethylaluminum cyanide mediates conjugate addition of cyanide to a, (3-unsaturated oxazolines. With a chiral oxazoline, 30-50% diastereomeric excess can be achieved. Hydrolysis gives partially resolved a-substituted succinic acids. The rather low enantioselectivity presumably reflects the small size of the cyanide ion. 

Lewis acids such as zinc chloride, boron trifluoride, tin tetrachloride, aluminum chloride, methylaluminum dichloride, and diethylaluminum chloride catalyze Diels-Alder reactions.22 The catalytic effect is the result of coordination of the Lewis acid with the dienophile. The complexed dienophile is more electrophilic and more reactive toward electron-rich dienes. The mechanism of the addition is believed to be concerted and enhanced regio- and stereoselectivity is often observed.23... 

Scheme 7.5 gives some examples of the Reformatsky reaction. Zinc enolates prepared from a-haloketones can be used as nucleophiles in mixed aldol condensations (see Section 2.1.3). Entry 7 is an example. This type of reaction can be conducted in the presence of the Lewis acid diethylaluminum chloride, in which case addition occurs at -20° C.171... 

The experimental isotope effects have been measured for the reaction of 2-methylbutene with formaldehyde with diethylaluminum chloride as the catalyst,27 and are consistent with a stepwise mechanism or a concerted mechanism with a large degree of bond formation at the TS. B3LYP/6-31G computations using H+ as the Lewis acid favored a stepwise mechanism. 

Diethylaluminum cyanide can also be used for preparation of (3-hydroxynitriles. 

Similarly, diethylaluminum azide gives (3-azido alcohols. The epoxide of 1-methylcyclohexene gives the tertiary azide, indicating that the regiochemistry is controlled by bond cleavage, but with diaxial stereoselectivity. 

Among other reagents that effect epoxide ring opening are diethylaluminum 2,2,6,6-tetramethylpiperidide and magnesium (V-cyclohexyl-A-O -propy amide. 

However, when (+)-methylneophylphenyltin deuteride, (+)-(56) ([ot] s + 10.7) is kept in the dark mixed with five equivalents of diethylaluminum hydride for ten hours at room temperature in benzene, optically inactive (72) is formed 44). (In the absence of (Et2AlH)2 less than 3 % of (12) is racemized under these conditions). The four-center transition state is therefore very unlikely. 

On the other hand, many reactions are known where in a first intermolecular step a functionality is introduced which than can undergo an intramolecular reaction. A nice example is the reaction of dienone 0-34 with methyl acrylate in the presence of diethylaluminum chloride to give the bridged compound 0-35 (Scheme 0-11). The first step is an intermolecular Michael addition, which is followed by an intramolecular Michael addition. This domino process is the key step of the total synthesis of valeriananoid A, as described by Hagiwara and coworkers [21]. 

Twofold Michael additions have been utilized by the groups of Spitzner [2] and Hagiwara [3] to construct substituted bicyclo[2.2.2]octane frameworks. In Hagiwara s approach towards valeriananoid A (2-6) [4], treatment of trimethylsily-enol ether 2-2, prepared from the corresponding oxophorone 2-1, and methyl acrylate (2-3) with diethylaluminum chloride at room temperature (r.t.) afforded the bicyclic compound 2-4 (Scheme 2.2). Its subsequent acetalization allowed the selective protection of the less-hindered ketone moiety to provide 2-5, which could be further transformed into valeriananoid A (2-6). 

Materials. 5-Methyl-1,4-hexadiene was obtained by the codimerization of isoprene and ethylene with a catalyst (18) consisting of iron octanoate, triethylaluminum and 2,2 -bi-pyridyl. The product mixture which contained principally 5-methyl-1,4-hexadiene and 4-methy1-1,4-hexadiene was fractionated through a Podbielniack column to yield high purity (>99%) 5-methylxhexadiene, b.p. 92.80C,njj 1.4250 (Lit. (19) b.p. 88-89°C, np 1.4249). 1-Hexene (99.9% purity), 1-decene (99.6% purity), 4-methyl-1-hexene (99.5% purity) and 5-methyl-l-hexene (99.7% purity) were obtained from Chemical Samples Co. 6-TiCl3 AA (Stauffer Chemical Co.j contains 0.33 mole AICI3 per mole of TiClj). Diethylaluminum Chloride was obtained from Texas Alkyls (1.5 M in hexane). 

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