Selection equations

Equation 39 can often be simplified by adopting the concept of a mass transfer unit. As explained in the film theory discussion eadier, the purpose of selecting equation 27 as a rate equation is that is independent of concentration. This is also tme for the Gj /k aP term in equation 39. In many practical instances, this expression is fairly independent of both pressure and Gj as increases through the tower, increases also, nearly compensating for the variations in Gj. Thus this term is often effectively constant and can be removed from the integral ... 

Furanones are a class of chiral dienophiles very reactive in thermal cycloadditions. For example, (5R)-5-(/-menthyloxy)-2-(5H)-furanone (28) underwent Diels Alder reaction with cyclopentadiene (21) with complete re-face-selectivity (Equation 2.10), affording a cycloadduct which was used as a key intermediate in the synthesis of dehydro aspidospermidine [27]. 

Dienes 581 and 583 undergo smooth Diels-Alder cycloaddition with a wide range of dienophiles affording the corresponding products. Their reaction with PTAD gives the respective products 582 and 584 with complete Jt-facial selectivity (Equations 82 and 83) <2005CEJ5136>. 

The heteroatom-assisted Diels-Alder reaction has emerged as an extremely powerful method for the preparation of complex heterocycles. In several cases, such reactions with TADs are described. For example, the reaction of vinyl pyrrolidone 594 with MTAD provides a 7 2 mixture of diastereomers 595 and 596 in 95% combined yield, showing only low face selectivity (Equation 88) <2005JOC5221>. 

Accordingly, chiral 7r-allylpalladium complex 204 was found to catalyze enantioselective allylation of imines with good to high selectivity (Equation (98)).4S7c 457e 457g 4S71... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

The kinetic resolution by etherification has also been conducted through the cyclization of epoxy aliphatic alcohols.274 In these reactions catalyzed by monomeric complex 51, the ring closure of acyclic substrates occurred with exclusive / -selectivity (Equation (74)), whereas m -openings were observed in the desymmetrization of... 

Olefination of aldehydes with gem-dichromium reagents.1 Reduction of 1,1-diiodoethane with CrCl2 in THF provides a 1,1-dichromioethane reagent (a) that reacts with aldehydes to furnish products of ethylidenation in 80-99% yield with high (E)-selectivity (equation I). 

Reactions with other gem-diiodoalkanes under these conditions proceed in low yield however, addition of DMF (and ultrasonic irradiation) provides a very general method for alkylidenation of aldehydes with good (E)-selectivity (equation II). 

Axial addition to cyclohexanones.1 Addition of carbanions to cyclic ketones generally favors equatorial products. This preference may result from nonbonded interactions, since Trost et al. now find that the addition of LiCH2CN to cyclohexanones is axial selective (equation I). The preference for axial addition is even higher in the case of cyclohexenones ( 20 1). The axial selectivity of LiCH2CN is... 

Ohfune and coworkers78 used Diels-Alder reactions between 2-trimethylsilyloxy-l,3-butadiene (63) and acrylate esters 64 to synthesize constrained L-glutamates which they intended to use for the determination of the conformational requirements of glutamate receptors. The reactions between 63 and acrylate esters 64a and 64b did not proceed. Changing the ethyl and methyl ester moieties into more electron-deficient ester moieties, however, led to formation of Diels-Alder adducts, the yields being moderate to good. In nearly all cases, the cycloadducts were obtained as single diastereomers, which is indicative of a complete facial selectivity (equation 22, Table 1). Other dienes, e.g. cyclopentadiene and isoprene, also showed a markedly enhanced reactivity toward acrylate 64g in comparison with acrylate 64a. 

Nitrosocarbonyl-D-bomane-10,2-sultam 233 adds to cyclopentadiene to yield 234 with complete facial selectivity (equation 123)114. 

A selective hydrogenation of conjugated dienes was carried out with a Pd complex which was preactivated with oxygen. Besides the conversion of dienes with good selectivity (98%), diene esters, ketones and nitro compounds were also hydrogenated with fairly good selectivities (equation 9)29. 

The iron carbonyl complex [Fe(CO)5] in basic media hydrogenated steroidal dienes selectively (equations 10-12)30. 

PAIRS ALTERNATING BETWEEN EQUATION NUMBER AND VARIABLE NUMBER. THE ORIGINAL ASSIGNMENT OF EQUATION AND VARIABLE NUMBERS ARE ARBITRARY. ENTER IN COLUMNS 1-3 AN EQUATION NUMBER. ENTER IN COLUMNS 5-7 ONE OF THE VARIABLE NUMBERS IN THE SELECTED EQUATION. IN COLUMNS 9-11 ENTER AN EQUATION NUMBER (THE SAME EQUATION OR ANOTHER) AND IN COLUMNS 13-15 ENTER THE NUMBER OF ONE OF THE VARIABLES IN THE SPECIFIED EQUATION. 

It is noteworthy that the rearrangement of crotyl propargyl ether 86 with f-BuLi/f, )-27 at —95 °C carried out in pentane provided [2,3]-shift product 87 in high enantiopurity (89% ee, 47% yield) with high (>95%) anf/-selectivity (equation 48). 

Oxidative kinetic resolution of secondary alcohols mediated with a catalytic amount of optically active binaphthyl-type iV-oxyl has been performed with high selectivity". Also, it has mediated oxidative asymmetric desymmetrization of primary alcohols with good selectivity (equation 25)". ... 

Hydroboration and hydrosilylation reactions ofalkynes, when they do not proceed spontaneously, can be catalyzed by numerous transition metals [17]. Metal vinyli-dene-mediated processes uniquely provide (Z)-alkenes via trans-addition . In 2000, Miyaura and coworkers discovered that both Rh(I)- and Ir(I)-complexes supported by bulky electron-rich phosphine ligands catalyze the hydroboration of terf-butylacety-lene to give alkenylboronate 41 with >95 5 Z/E selectivity (Equation 9.5) [18]. 

Sulfmyl hydrazones, such as 68, reacting as 1-azabuta-l,3-dienes, undergo cycloaddition reactions with A -methyl-maleimide (NMM), to form tetrahydropyrrolo[3,4-. ]pyridine derivatives in low yield. The reaction occurs to form enantiomerically pure adduct 69 with complete endo- and facial selectivities (Equation 14) <200382241 >. 

A method to form nucleosides from chlorofuranoses was shown to be highly /3-selective (Equation 5) <2000TL8207>. For example, the reaction of 6-(benzyloxy)purine with 41 provided nucleoside 42 with a >99 1 ratio of [3- to a-anomers. Product distribution was found to be independent of the anomeric ratio of the chlorofuranose starting materials. 

The enolate generated by reaction of lactone 88 with lithium diisopropylamide (LDA) is quenched with an excess of methyl iodide to give methyl lactone 89 in excellent yield. As expected, the electrophilic attack is stereoselective for the less sterically hindered convex face of the lactone enolate, giving the product with the desired 7iJ-stereochemistry with greater than 95 5 selectivity (Equation 22) <1997TL3817>. 

Baldwin has utilized an intramolecular nitrene insertion reaction in an approach to hydrolytically stable analogues of penicillin <2001T4999, 2003T8233>. Although the reaction proceeded in only moderate yields, the product is formed with excellent facial selectivity (Equation 43). 

The carbonylation of the 2-aminostyrene 187 in dichloromethane under a carbon monoxide and hydrogen atmosphere produces the lactam in the presence of a palladium acetate/tricyclohexylphosphine catalyst in excellent yield and selectivity (Equation 123) <1996JA4264>. 

By choosing an appropriate titanium complex, a /ra r-isomer of 1,2-cyclohexanols can be prepared selectively. Because intramolecular pinacol coupling of hexanedials with Sml2 usually produces t-isomers of cyclohexane-1,2-diols, the titanium-mediated reaction complements the samarium-mediated cyclization (Equation (17)). In addition, when a /-butyl group fixes the conformation of the substrate, one of the diastereomers is produced selectively (Equation (18)). ... 

In 2002, Grubbs and co-workers reported the first CM reactions of allyl phosphines.In an initial reaction, subjecting allyl diphenylphosphine to catalyst 5 (5 mol%) failed to produce any of the desired cross-product. However, by protecting the phosphine as its borane complex, CM reactions could be achieved in good yield with high E-selectivity (Equation (5)). Notably, catalyst 5 failed to dimerize borane-protected vinyl diphenylphosphine. This result was attributed to substrate trapping of the catalyst as an unreactive Fischer carbene, a situation analogous to that observed in the CM reactions of alkyl vinyl ethers. 

The synthesis of a-methylene-7-lactone 317 through carbonylation of but-3-yn-l-ol 316 catalyzed by Pd(ll)-PPh2(2-Py) has been carried out in l-butyl-3-methylimidazolium tetrafluoroborate 315 as reaction medium in high yield with excellent product selectivity (Equation (30))4 Although the ionic liquid containing the catalytic species was recovered, a significant decrease in yield occurred with the recycled catalyst, which appears to be attributed to the decomposition of the catalyst during the isolation procedure ... 

C without a catalyst, yielding a,/3-enones in excellent yields and with complete ( )-selectivity (equation 121) . 

High yields and diastereoselectivities have also been observed for the addition of Grignard reagents to imines like 221 derived from phenylglycinol (222), which are existing in equilibria with 1,3-oxazolidines. Also, the imine derived from methoxyacetone affords amino-ethers with excellent diastereoselectivities. The addition of a Lewis acid (MgBr2) has a strong effect on both the yield and the selectivity (equation 151) . ... 

The diastereoselective cyclopropanation of acyclic allylic alcohols is a very useful synthetic transformation. It was recognized early on by Pereyre and coworkers that the cyclopropanation of Z-substituted chiral allylic alcohol was highly syn selective (equation 58) °. It was later shown that most zinc carbenoids react with these substrates with high syn stereocontrol. 

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