Boranes, reaction with ketenes

TABLE 8B1.12. Asymmetric Aldol Reaction of Ketene Silyl Acetal 32 with Aldehydes Catalyzed by Borane Complex 33 (Scheme 8B1.20)... 

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

Kiyooka et al. have reported that stoichiometric use of chiral oxazaborolidines (e.g. (S)-47), derived from sulfonamides of a-amino acids and borane, is highly effective in enantioselective aldol reactions of ketene TMS acetals such as 48 and 49 (Scheme 10.39) [117]. The use of TMS enolate 49 achieves highly enantioselective synthesis of dithiolane aldols, which can be readily converted into acetate aldols without epimerization. The chiral borane 47-promoted aldol reaction proceeds with high levels of reagent-control (Scheme 10.40) [118] - the absolute configuration of a newly formed stereogenic center depends on that of the promoter used and not that of the substrate. 

The reaction of borane with keten forms a single addition product BC2H5O (bimolecular rate constant of formation =4 x 10 1 mol s at low partial pressures of reactants). Kinetic and mass-spectrometric data are consistent with the formulation (64). 

Paterson et al. [98] in their attempt used a similar disconnection for rhizopodin as described by Menche (fragments 144 and 149) (Scheme 2.151). However, unlike, Menche, they used silyl ketene acetal 16 in an asynunetric VMAR for the addition to ( )-iodoacrolein (142) to obtain dioxinone 143 in 94% ee. Methanolysis removed the aceto-nide, and the subsequent Narasaka reduction [99] provided the syn-diol 144 in 80% yield and a 10 1 selectivity for the desired isomer. The synthesis of segment 149 started with aldehyde 145, which was ultimately derived from Roche ester. Carbon chain extension was achieved through a chelation-controlled Mukaiyama aldol reaction with silyl ketene acetal 146, which installed the new chiral center with excellent stereocontrol (20 1 dr). For the installation of the third secondary alcohol, six-membered lactone 148 was obtained by treatment with K COj in methanol. Subsequent borane reduction provided stereospecifically the desired alcohol, which was then further transformed to the desired acid (149). 

Yamamoto and coworkers have also shown that tris(pentafluorophenyl)borane is a highly active catalyst for analogous imino-aldol reactions. Here, both the N-trialkylsilyl imines (72) (Equation 46) [44] and the N-benzyl imines (73) (Equation 47) [45] are applicable as electrophiles with ketene silyl acetals (74), and the reaction gives remarkably high yields of the N-benzyl imino-aldol adducts. 

Another clever approach by Masamune et al. is an asymmetric aldol reaction of the ketene silyl acetal 32. This reaction is effectively promoted by 20 mol % of chiral borane complexes 33, prepared from BH3 -THF and a,a-disubstituted glycine tosylamide 34, undo-reaction conditions in which aldehydes are slowly added to the reaction mixture (Scheme 8B1.20, Table 8B1.12). The catalysts and the reaction conditions have been designed according to the proposed catalytic cycle shown in Scheme 8B1.21. Thus, the use of a geminally disubstituted catalyst accelerates the ring closure of intermediate 35 as expected from the Thorpe-Ingold effect, and the slow addition of the aldehyde reduces the accumulation of 35, which might catalyze the aldol reaction with low enantioselectivity. Extremely... 

The use of CAB as a chiral reagent seems to be more effective for this reaction, which proceeds faster and with higher yields and enantiomeric excess. Kiyooka et al. first described the use of various chiral oxaborolidines, derived from sulfonamides of a-amino acids and borane, in the course of the selective aldol reaction between silyl ketene acetals and aldehydes (Eq. 47) [43a]. Stereoselectivity and yields were relatively high. 

Chiral boranes have been recommended as Lewis acids catalysts by Reetz [689], Yamamoto [787, 788], Kiyooka [795, 1302], Masamune and their coworicers [796, 797], These groups used, respectively, boranes 2.61, 3.9 (R = H, R = /-Pr), 3.10 (R = i-Pr or tert-Bu, R = H) and derivatives of 3.12 and 3.13. These boranes are very efficient catalysts in asymmetric additions of symmetrically substituted ketene silylacetals 6.113 to aldehydes (Figure 6.94). Similar reactions can also be conducted with enoxysilanes derived from methylketones or from tert-Bu thiolacetate [787, 794, 796], Oxazaborolidine 3.10 derived from tryptophan 3.11 is also a very potent catalyst [794],... 

With the exception of 3.9, these borane catalysts give lower selectivities with enoxysilanes of propionic esters or ethylketones (< 80%) [796, 1302], Using 3.9 as a catalyst, high diastereo- and enantioselectivities are observed in the reactions of the E-ketene silylacetal of phenyl propionate with a,P-unsaturated aldehydes [788] and in the reaction of the enoxysilane of diethylketone with PhCHO [787] (Figure 6.95). All these results are interpreted by acyclic transition state models in which steric repulsions are minimized (Figure 6.95). 

Other reports deal with a pyrrolidine-catalysed homo-aldol condensation of aliphatic aldehydes (further accelerated by benzoic acid), a diastereoselective aldol-type addition of chiral boron azaenolates to ketones,the use of TMS chloride as a catalyst for TiCU-mediated aldol and Claisen condensations, a boron-mediated double aldol reaction of carboxylic esters, gas-phase condensation of acetone and formaldehyde to give methyl vinyl ketone, and ab initio calculations on the borane-catalysed reaction between formaldehyde and silyl ketene acetal [H2C=C(OH)OSiH3]. ... 

Successful examples of the Mukayama aldol reaction include the use of the thermally stable tris(pentafluorophenyl)borane Lewis acid. Yamamoto and coworkers reported that 2 mol% of (C6Es)3B smoothly catalyzes the Mukaiyama-aldol reaction of various silyl enol ethers or ketene silyl acetals with aldehydes (Equation 45). ... 

---