Lewis acid derivatives

In a related study Porter et al. showed that a-bromo-y lactams 185 containing a pyridyl moiety can react with allyltrimethylsilane enantiose-lectively in the presence of chiral Lewis acids derived from zinc and 189 (Scheme 49) [142], In contrast to the above study, the ligand of choice for substrates 185 was found to be the bisoxazoline ligand 189. Excellent ee s were obtained in the presence of two equivalents of the chiral Lewis acid. Under substoichiometric amounts of the catalyst, lower selectivities were obtained. Different substituents on the pyridyl moiety were also examined although no predictable trend was observed. A trans octahedral model simi-... 

The asymmetric fluorination of enolates by means of chiral metal complexes has been reported with Selectfluor in the presence of a chiral Lewis acid derived from TADDOL (TiCl2/TADDOL), or with F-A-sulfonimide (NFSI) with palladium complexes and chiral phosphines. 

Lewis acid catalyzed 1,3-dipolar cycloadditions of olehns and nitrones are useful synthetic transformations that have the potential of dehning up to three contiguous chiral centers. Presumably, such reactions can also be catalyzed by chiral Lewis acids derived from metal-bis(oxazoline) complexes. 

It was also reported that diastereo- and enantioselective Mannich reactions of activated carbonyl compounds with a-imino esters were catalyzed by a chiral Lewis acid derived from Cu(OTf)2 and a bisoxazoline (BOX) ligand [31] [(Eq. (6)]. Catalytic enantioselective addition of nitro compounds to imines [32], and aza-Henry reactions of nitronates with imines [33] also proceeded under similar reaction conditions. 

Novel bidentate chiral Lewis acids derived from 1.8-naphthalenediylbis(dichloroborane) and modified amino acids as chiral auxiliary have been successfully utilized as effective catalysts for the asymmetric Diels-Alder reaction of a,[ -unsaturated aldehydes. The enantioselectivity is highly sensitive to the kind of chiral amino acids. Moderate enantioselectivity was obtained with the tryptophan-derived ligand for the endo adduct, but amino acids without aromatic groups... 

A chiral Lewis acid derived from Sn(OTf)2 and the proline derivative 17 has proven to catalyze the aldol reaction effectively. As Mukaiyama et al. [7J demonstrated, a high degree of enantio-selectivity was achieved (Scheme 3b). 

The titanium Lewis acid derived from diol (463) could be employed in catalytic amounts (0.1 mol equiv.) when molecular sieves were present (entries 2, 3). Under these conditions, cycloaddition of the crotonoyl dienophile was significantly more enantioselective compared with that of the acryloyl analog. On the other hand, both adducts (467a) as well as (467b) were obtained in 2% ee using the I wis acid (465) (0.1-0.2 mol equiv.), prepared in situ from bis-sulfonamide (464) and AlMes (entries 4, 5). 

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

In an intriguing example of size discrimination, a dendritic Lewis acid derived from 214 and Cu(OTf)2 can selectively activate the smaller dienophile 212 (Sch. 48) [91]. Diels-Alder reaction of 185 and an equimolar mixture of 212 and 213 provides the adduct 215 in higher yield. A generation-three dendrimer is required to obtain adequate size discrimination. 

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. 

It has been noted that although the linear gemnetry is consistently predicted for cationic Lewis acid carbonyl ctxnplexes in ab initio calculations, extrapolation of these results to neutral Lewis acid c< n-plexes may not be justified. Semiempirical MNDO calculations predicted the bent conformation as the lowest energy structure for neutral Lewis acidic derivatives of beryllium, boron and aluminum cmn-plexed with rranr-2,3-dimethylcyclopropanone, whereas linear structures were predicted for the cationic complexes of beryllium and aluminum (Table 1). ... 

A great majority of the catalytic aldol processes that have been developed over the last two decades involve Lewis acids derived from complexes of titanium, boron, tin, and, more recently, copper as well as silver. A recent, exciting area of rapid development for aldehyde addition reactions is represented by the catalytic aldol methods that utilize soft-metal and lanthanide coordination complexes which mediate addition reactions through metalloenolate intermediates. 

The catalytic asymmetric propargylation [108] and allenylation [109] of achiral aldehydes has been performed with high levels of enantioselection. The asymmetric propargylation promoted by the chiral Lewis acid derived from bind and Ti(0/-FT)4 are representative. Between 50 and 100 mol% of titanium is required for these reactions to go to completion (Scheme 10-70). The reaction of benzalde-hyde with allenyltributylstannane 170 and the chiral promoter produced the homo-propargylic alcohol 171 in >99% ee and 48% yield (7% of the undesired allenyl alcohol was also obtained). 

Table4.4 Asymmetric addition reaction ofTMSCN with aldehydes using Ti(ll)-Lewis acid derived from cinchonidine.

Masamune et al. examined the catalytic activity of several boron Lewis acids derived from BH3 THF and the p-toluenesulfonamides of simple a-amino acids towards the aldol reaction of benzaldehyde with TMS enolate 48 [121]. As a result, the borane catalysts derived from a,a-disubstituted glycine p-tolueriesulforiarriides were found to have high activity. The disubstitution would accelerate the second step (Step II) of the catalytic cycle (Scheme 10.43). On the basis of this observation, they developed chiral borane catalysts 47 c and 47 d, which enable highly enantioselective aldol reactions of KSA and thioketene silyl acetals (84—99% ee with 48). 

Keck and Krishnamurthy have shown that the Diels-Alder reaction of cy-clopentadiene and bromoacrolein is facilitated by a Lewis acid derived from titanium tetraisopropoxide and S-BINOL (59) (Scheme 47) [126]. The cycloaddition may be conducted with isoprene at slightly lower levels of enantioselectivi-ty methacrolein-cyclopentadiene Diels-Alder reactions are only moderately selective. 

Kamahori K,Tada S, Ito K, Itsuno S (1995) Tetrahedron Asymmetry 6 2547 Kamahori K, Ito K, Itsuno S (1996) J Org Chem 61 8321 Takasu M, Yamamoto H (1990) Synlett 194 Sartor D, Saffrich J, Helmchen G (1990) Synlett 197 Hawkins JM, Loren S (1991) J Am Chem Soc 113 7794 Hawkins JM, Loren S, Nambu M (1994) J Am Chem Soc 116 1657 A modified isopinocampheyldibromoborane catalyzes the reaction of cyclopentadiene with methyl acrylate in 48% ee Bir G, Kaufmann D (1990) J Organomet Chem 390 1 A Lewis acid derived from JY-tosyl tryptophan and l,8-naphthalenediylbis(dichlorobo-rane) is reported to catalyze the Diels-Adler reaction of methacrolein sind cyclopentadiene in 100% ee for the endo isomer (endo/exo = 37 63) ReiUy M, Oh T (1994) Tetrahedron Lett 35 7209... 

Chiral Lewis acids derived from tartaric acid have been used in the asymmetrical addition of dialkylzinc com-... 

One coordinate compounds studied include phosphaalkynes (15) as shown in Scheme 1. P-NMR spectroscopy showed that, in the presence of Lewis acids derived from group 13 elements, phosphaalkynes undergo spirocyclotrimerisation with incorporation of the Lewis acid to form the betaines (16). Signals of three different phosphorus environments were observed at 5 = —87.5 to —78.1, 257.4 to 262.2 and 412.3 to 422.4, the downfield shifts of the latter two resonances... 

Phenols have been condensed with alkenoylesters to give chromans by an oxa-Michael addition/electrophilic aromatic addition sequence with magnesium(II)- or copper(II)-bis-oxazoline complexes as chiral Lewis acid catalysts (Scheme 17b) [97]. This reaction may be initiated by an oxa-Michael reaction, followed by a hydroarylation of a carbonyl group. The authors suggest that the initial stereodetermining oxa-Michael addition is followed by a fast diastereoselective aromatic substimtion [97]. A nickel Lewis acid, derived from Ni(hfacac)2 (hfacac = 1,LL5,5,5-hexafluoro-3,5-dioxopentane enolate) and chiral Al-oxide ligands, catalyzes the enantioselective oxa-Michael cyclization of 2-tert-butyloxycarbonyl-2 -hydroxy-chalcones to 3-ferf-butoxycarbonyl flavanones, which can be decarboxylated to flavanons in a separate step (Scheme 17c) [98]. A Lewis acid activation of the unsaturated p-ketoester unit can be assumed. 

Syntheses (Scheme 1) of carborane-supported cyclic multidentate Lewis acid derivatives are based on the lithiation of icosahedral carborane l,2-C2BioH,2 (6) or its octaethyl derivative (7) and on... 

The metal-based Lewis acid derived from the camphor-derived Kgands such as (7.159) and La(OTf)3 is effective in the MBH reaction of aromatic aldehydes with a,P-unsaturated aldehydes mediated by l,4-diazabicyclo(2.2.2]octane (DABCO). The best ees (up to 95%) are obtained using sterically bulky acrylates such as a-naphthyl acrylate. More success has been obtained using Bronsted acidic organocatalysts. The partially reduced BINOL (7.160) has been used to effect enantioselective MBH reaction of aliphatic aldehydes such as (7.71) with 2-cyclohexen-l-one (7.161) mediated by triethylphosphine/ while bis(thio)ureas such as (7.163) provide up to 96% ee in the coupling of this ketone with cyclohex-anecarbaldehyde in the presence of DABCO. °... 

Although this reaction is still at an early stage of development, a few modifications have been exploited. These modifications include the extension of the Bellus-Claisen rearrangement to ring strain molecules (such as 2-vinyloxetane, 2-vinyloxirane, and 2-vinylaziridines ) and to tertiary allyl amines, the application of chiral Lewis acid derived from Mgl2 and bis(oxazolinyl)aryl ligands, and the implementation of electron-rich ketene equivalents (e.g., alkoxyketenes, aminoketenes) by the photolysis of chromium carbene complexes. 

Metal and Lewis acid-derived podand receptors... 

As previously mentioned, Lewis acid activation of a carbonyl to lower the energy of the LUMO can facilitate the Alder-Ene reaction. There are several examples using a variety of Lewis acids. Lewis acids derived from Al, B, In,... 

Weaker Lewis acids derived from AICI3, such as AlEtaCl and AlEts, require the presence of a co-initiator to initiate polymerization. - Similarly, BCI3 is an effective initiator for styrene and isobutylene polymerizations only in the... 

Systematic study of Lewis acids derived from aluminum in the de-O-benzylation of compounds bearing multiple alkyl ethers gave an insight into the mechanism. The proposed mechanism rationalizes also the regioselectivity observed in the de-O-alkylation reaction. 

Chiral Lewis acids derived from complexes between copper(I) chloride with phosphino sulfenyl ferrocenes are efficient catalysts for enantioselective aza-Diels-Alder reaction of A-sulfonyl imines with Danishefsky-type dienes (eq 47), and asymmetric conjugate addition of Grignard reagents to cyclic enones (eq 48).136... 

Other type of Lewis acid derived by a 1 2 mixture of Et2AlCl and chiral bifunctional ligand (69) having sulfoxide moiety as Lewis base site was found as an excellent catalyst for catalytic enantioselective Reissert reaction of nicotine amides (70) (Scheme 6.52) [67]. On the basis of ESI-MS studies, active catalyst species has... 

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