Unsaturated aldehydes with boranes

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Asymmetric Diels-AUer reactions The observation that simple acyloxy-boranes such as H2BOCOCH=CH2, prepared by reaction of BH3 with acrylic acid, can serve as Lewis acid catalysts for reactions of the a,P-unsaturated acids with cyclopentadiene (15, 2) has been extended to the preparation of chiral acyloxy-boranes derived from tartaric acid. The complex formulated as 3, prepared by reaction of BH3 with the monoacylated tartaric acid 2, catalyzes asymmetric Diels-Alder reactions of a,P-enals with cyclopentadiene with high enantioselectivity. The process is applicable to various dienes and aldehydes with enantioselectivities generally of 80-97 % ee. 

Lithium aminoborohydrides are obtained by the reaction of -BuLi with amine-boranes [FF2, FH5, NT2]. They can be generated in situ as THF solutions or as solids when formed in diethylether or hexane (n-BuLi must then be used in sub-stoichiometric amounts). They are stable under dry air and are slowly decomposed by water [NT2] or methanol so that workup of the reactions mixtures can be carried out with 3M HCl. They reduce alkyl halides (Section 2.1), epoxides (Section 2.3), aldehydes, and ketones (Section 3.2.1) (in the latter case with an interesting stereoselectivity [HFl]), and esters to primary alcohols (Section 3.2.5). a,(3-Unsaturated aldehydes, ketones, and esters are reduced to allyl alcohols (Section 3.2.9) [FF2, FS2]. Depending on the bulkiness of the amines associated with the reagent and to the substrate, tertiary amides give amines or alcohols (Section 3.2.8) [FFl, FF2]. Amines are also formed from imines (Section 3.3.1) [FB1 ] and from azides (Section 5.2) [AFl]. However, carboxylic acids remain untouched. 

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). 

Lithium trialkylalkynyl borates react in a stereoselective manner with benzo-dithiolithium tetrafluoroborate to give vinyl boranes. Hydrolysis of the vinyl boranes, the rate of which differs significantly for the E and Z-isomers gives protected unsaturated aldehydes (Scheme 25). 

Yamamoto developed a remarkable boron-derived catalyst for enantioselec-tive Diels-Alder reactions which is easily assembled from monoacylated tartaric acid and borane. Spectroscopic data provided evidence that supports the proposed catalyst structure 144 depicted in Equation 16 [79, 80]. Such chiral (acyloxy)borane (CAB) catalysts have been employed in numerous cyclo-additions with unsaturated aldehydes to afford the corresponding products, such as 145, with high selectivity (98% ee, endo exo > 99 1) [80]. 

The tartrate ester modified allylboronates, the diisopropyl 2-allyl-l,3,2-dioxaborolane-4,5-di-carboxylates, are attractive reagents for organic synthesis owing to their ease of preparation and stability to storage71. In the best cases these reagents are about as enantioselective as the allyl(diisopinocampheyl)boranes (82-88% ee with unhindered aliphatic aldehydes), but with hindered aliphatic, aromatic, a,/l-unsaturated and many a- and /5-alkoxy-substituted aldehydes the enantioselectivity falls to 55-75% ee71a-b... 

A modified version of the Brown-Negishi reaction using B-alkylcatechol-boranes was reported (Scheme 32). This novel method is based on a simple one-pot procedure involving the hydroboration of various substituted alkenes with catecholborane, followed by treatment with catalytic amount of oxygen/DMPU/water and a radical trap. Efficient radical additions to a,ft-unsaturated ketones and aldehydes have been reported. Primary alkyl radicals are efficiently generated by this procedure and the reaction has been applied to a 300 mmol scale synthesis of the y-side chain of (-)-perturasinic... 

With this spectrum of reagents, virtually all unsaturated hydrocarbons can be hydroborated. Moreover, many functional groups are tolerated in hydroboration with substituted borane derivatives, since double and triple C—C bonds react more rapidly than most other functionalities (see Table 1). Only aldehydes, ketones and carboxylic groups must be protected. Consequently, for the first time, functionalized organometall-ics are available. Chiral organoboranes of high optical purity can be prepared ... 

Borane, monoalkylboranes and dialkylboranes (such as thexylborane and 9-BBN) were shown to be powerful reducing agents for carbonyl derivatives (sec. 4.6.A). When the B—H unit is removed, as in trialkylboranes, addition to the alkene moiety of a,p-unsaturated ketones or aldehydes occurs. An alkyl group of the borane is transferred to the terminal position of the alkene moiety (1,4-addition) and boron is transferred to the oxygen, giving a boron enolate (sec. 9.4.D). Initial hydroboration of acrolein with tricyclopentylborane gave boron enolate 196.1 0... 

Because it is similar to RjSi as an electron-donating group, the CpMo(CO)2 fragment is cap le of stabilizing a P-carbocation center. By analogy with the allyl compounds of silane, borane, and tin, CpMo(CO)2(Tl -cyclohexadienyl) and its 6-substituted derivatives 106 a-c undergo BFj-promoted electrophilic addition to aldehydes and a,P-unsaturated enones by generating cyclohexadiene cationic precipitates 107, as shown in Scheme 22. 

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