Aldehydes boron trifluoride complexes

The influence of Lewis acids on the diastereoselectivity of the cycloaddition of /f-alkoxyalde-hydes has also been studied35. Magnesium bromide, highly effective for a-alkoxyaldehydes, fails in the case of the cycloaddition of aldehyde 10 to diene 2 and the reaction does not exhibit any selectivity, probably due to a change of mechanism to Mukaiyama s aldol type. One reason may be the change of solvent from tetrahydrofuran to a mixture of benzene and diethyl ether. The additions of aldehyde 10 to other dienes are more selective but diastereoselectivity is still much lower than for the a-alkoxy aldehydes. Boron trifluoride-diethyl etherate complex also leads to a mixture of four possible products. Excellent selectivity is achieved for the titanium(IV) chloride catalyzed addition of aldehyde 10a to diene 2b, 11c is obtained as the only product. 

The initial reaction is a Prins reaction, catalysed by the boron trifluoride complex. In order to achieve maximum overlap of the -orbitals of the olefin and aldehyde groups, the aldehyde must approach the olefin from below, as will easily be seen using molecular models. This means that the resultant alcohol function is located on the downward side of the molecule as shown in Figure S21. A 1,2-carbon shift followed by a transannular bond formation with concomitant loss of a proton, provides the skeletal rearrangement to the product. It may not be too obvious in the figure, but an experiment with molecular models will soon... 

For the monoprotected a-amino aldehydes, the best results in yield and stereoselectivity were obtained under kinetic control conditions which gave the expected sw-com pounds. The addition of tin(lV) chloride did not result in increased syn selectivity, and the use of boron trifluoride diethyl ether complex did not provide the ann -isomer as the major product. 

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

Effective 1,4-asymmetric induction has been observed in reactions between 2-(alkoxyethyl)-2-propenylsilanes and aldehydes. The relative configuration of the product depends on the Lewis acid used. Titanium(IV) chloride, in the presence of diethyl ether, gave 1,4-ijn-products with excellent stereoselectivity with boron trifluoride-diethyl ether complex, the amt-isomer was the major product, but the stereoselectivity was less83. 

The stereoselectivity of Lewis acid promoted reactions between 2-butenylstannanes and aldehydes has been widely studied, and several very useful procedures for stereoselective synthesis have been developed. In particular syn-products are formed stereoselectively in reactions between trialkyl- and triaryl(2-butenyl)stannanes, and aldehydes induced by boron trifluoride-diethyl ether complex, irrespective of the stannane geometry66. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

Boron trifluoride-diethyl ether complex induced reactions of both (E)- and (Z)-tributyI(3-methoxy-2-propenyl)stannane with aldehydes give. vj-w-products with useful slereoselectivi-... 

Alkoxyallylstannanes can be generated in situ by stannylation of allyl ethers or by 1,3-isomerization of isomers, and trapped by boron trifluoride-diethyl ether complex induced addition to aldehydes to give syn-diol derivatives 13,120. 3-Alkylthioallylstannanes can similarly be generated and trapped84. 

The stoichiometric equivalents of halofluorides have been recently applied to transform alkylene dithioacetals into gcm-difluorides.70-71 Dithioacetals such as 1,3-dithiolanes and 1,3-dithianes arc readily obtained from the corresponding carbonyl compounds by the reaction with ethane-1,2-dithiol or propane-1,3-dithiol in the presence of the complexes boron trifluoride-bis(acetic acid) or boron trifluoride-diethyl ether. Using a two-step procedure, a range of aldehydes and ketones can be converted into gem-difluorides under mild conditions. 

A selection of A//p gp values has already been given in Table 2-4 in Section 2.2.6. This new Lewis basicity scale is more comprehensive and seems to be more reliable than the donor number scale. Analogously, a Lewis basicity scale for 88 carbonyl compounds (esters, carbonates, aldehydes, ketones, amides, ureas, carbamates) has been derived from their standard molar enthalpies of complexation with gaseous boron trifluoride in dichloromethane solution [143]. The corresponding Aff Q gp values range from 33 kJ mol for di-t-butyl ketone to 135 kJ mol for 3-diethylamino-5,5-dimethyl-cyclohexen-2-one. 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Perfluoroalkyllithium reagents react with electrophiles such as aldehydes, ketones, silicon and tin halides, alkyl halides, carbonates, esters, boron trifluoride-imine complexes, and sulfur dioxide. ... 

The aldol reactions of these complexes were first observed by Corey in the presence of Lewis acid such as boron trifluoride for ketones and tin chlorides for aldehydes (Scheme 3.8). 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

Cobalt-complexed propargylic aldehydes can be alkylated using both cyclic and acyclic trimethylsilyl enol-ethers and -esters in the presence of a Lewis acid. Boron trifluoride etherate is the most frequently employed Lewis acid. 

Diethyl phosphorocyanidate adds to a,/J-unsaturated aldehydes or ketones in the presence of lithium cyanide in a 1,2-fashion28. Boron trifluoride-diethyl ether complex catalyzed rearrangement of these allylic phosphates shows high E selectivity (>85 15) for the adducts derived from aldehydes and Z selectivity (>90 10) for ketone adducts. The selectivity of the rearrangement can be explained by assuming a chairlike transition state, in which the sterically more demanding x-substituent occupies the quasi-equatorial position. The steric requirement decreases in the order of R1 > CN > H. Thus, the cyano substituent occupies the quasi-equatorial position in the aldehyde-derived adduct (R1 = H), but the quasi-axial position in the ketone-derived adduct (R1 = CH3, C6H5). 

Mcthoxyoxazaphosphorinane 20 reacts with aldehydes in a tetrahydrofuran solution at — 78 C in the presence of lithium iodide and boron trifluoride-diethyl ether complex, followed by treatment with aqueous sodium hydroxide, to give diastereomeric a-hydroxy oxazaphospho-rinanes 21 in good yields but with low stereoselectivity [d.r. (21 a/21 b) from 52 48 to 70 30]65. 

Recently, Sisko and Weinreb have developed a new, in situ procedure for effecting A -sulfonyl imine Diels-Alder reactions. The method consists of treating an aldehyde with an A -sulfinyl sulfon-amide/boron trifluoride etherate in the presence of a diene (Scheme 3). Under these conditions the IV-sul-finyl sulfonamide reacts with the aldehyde to produce an /V-sulfonyl imine. This reaction, developed by Kresze, has been used previously for preparing imines from simple, non-enolizable aldehydes (c/. 1,2), but had not been applied to more complex aldehydes. As can be seen from the scheme, the cycload-... 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

Similarly, allylboranes 7, easily prepared by lithiation of allyl(dipenylamine) 5 followed by reaction with (-)-fi-methoxydiisopinocampheylborane (6) and subsequent addition of boron trifluoride-diethyl ether complex, react with aldehydes to give, after alkaline hydrogen peroxide workup, enantiomerically pure (> 95% ee) cyclopropylamine derivatives 8 as single diaste-reomers (diastereoselectivity > 95 5). However, yields of 8 are low (25-28% for alkyl aldehydes, 10-15% for aryl aldehydes) and vicinal anti-j8-diphenylamino alcohols 9 (> 95% ee, 23-48% yield) are always the main products. ... 

Reduction of 2-hydroxycyclobutanone by lithium aluminum hydride gave a 1 1 mixture of cis- and trans-cyclobutane-1,2-diols (80%). Slow distillation of the mixture of these diols in the presence of a catalytic amount of boron trifluoride-diethyl ether complex heated to 230°C in a metal bath gave cyclopropanecarbaldehyde (1) in 65-80% yield, providing an easy route to this aldehyde. Heating the diols in benzene in the presence of p-toluenesulfonic acid was less effective moreover, formation of the cyclopropanecarbaldehyde acetal subsequently occurred under these conditions. ... 

With various Lewis acids [aluminum trichloride, zinc(ll) chloride, tin(IV) chloride], boron trifluoride-diethyl ether complex and acetic acid, 1,2-epoxy-1-methylcyclobutane gave, on heating in chloroform at 50 C for 15 hours, in 87% yield a mixture of 1-methylcyclopropane-carbaldehyde (70%) and 2-methylcyclobutanone (30%), while with strong protie acids, i.e. sulfuric and hydrochloric acid, ring contraction occurred at 20 °C and the aldehyde was obtained, exclusively, in 65% yield. 

Even unsymmetrically substituted 1,3-diketones (338) are transformed into 1,3-dithiolanes (339) and (340) with boron trifluoride diethyl ether complex as a catalyst, as long as the reaction temperature does not exceed 15°C (Scheme 61) <88T2283>. 2-Oxo-aldehydes (341) react with ethane-... 

In a similar manner trans- and c -cyclobutane-l,2-diols afforded cyclopropyl aldehydes and ketones 7 and 8 on treatment with 4-toluenesulfonic acid in benzene or boron trifluoride-diethyl ether complex in dichloromethane.3840 If R1 R2, a mixture of cyclopropyl aldehydes or ketones was obtained. 

A transannular 6-endo reaction takes place in the boron trifluoride-diethyl ether complex induced cyclization of a bicyclic aldehyde. After the Markovnikov-orientated ring-closure step, a tertiary cation is generated, which is stabilized by //-proton loss and tetracycle 18, bearing a cyclopropyl unit, is formed in 67% yield24. In this case an ene reaction is impossible. 

Aldehydes obtained by oxidation of the C-6 position of carbohydrates are highly diastereose-lective dienophiles. Addition of diene 10b to the galactose derived aldehyde 9 in the presence of boron trifluoride - diethyl ether complex leads to a single adduct 11, while the addition to the ribose derived aldehyde 12 produces adduct 13 with a d.r. [(25,35 )/(21 ,31 )] 94 69. Eu(fod)3 catalyzed addition of 12 to ( )-l-methoxy-3-trimethylsilyloxy-l,3-butadiene (14) produces adduct 1543. 

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