Tributylstannane aldehydes

A related convenient and mild method for the preparation of aliphatic and aromatic aldehydes in high yield from carboxylic acids is the reductive cleavage of l-acyl-3-methylimidazole-2-thiones by tributylstannane. 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

Chemical reduction of aromatic aldehydes to alcohols was accomplished with lithium aluminum hydride [5i], alane [770], lithium borohydride [750], sodium borohydride [757], sodium trimethoxyborohydride [99], tetrabutylam-monium borohydride [777], tetrabutylammonium cyanoborohydride [757], B-3-pinanyl-9-borabicyclo[3.3.1]nonane [709], tributylstannane [756], diphenylstan-nane [114], sodium dithionite [262], isopropyl alcohol [755], formaldehyde (crossed Cannizzaro reaction) [i7i] and others. 

Chiral allenylmetal compounds provide convenient access to enantioenriched homopropargylic alcohols through Se2 additions to aldehydes. The syn adducts can be obtained through addition of allenyl tributylstannanes in the presence of stoichiometric boron trifluoride etherate (BF3-OEt2). The use of allenylmetal halides derivatives of Sn, Zn, and In lead to the anti diastereomers. The former additions proceed through an acyclic transition slate whereas the latter are thought to involve a cyclic transition state, thus accounting for the difference in diastereoselectivity. 

Comparison of reaction rates and selectivities for BF3-promoted additions of crotyl tributylstannanes to aldehydes revealed that the trans crotyl isomer reacts faster and is more yn-selective than the cis isomer (Table 8) [18]. It is proposed that the synclinal transition state arrangement for the frans-crotylstannane is stabilized by a favorable interaction between the LUMO of the carbonyl oxygen and the allylic tin sigma HOMO. The analogous transition state for addition of the dx-crotylstannane is destabilized by unfavorable steric interactions (Fig. 3). 

Table 8. Diastereoselection for BF3-promoted additions of trans- and c -crotyl tributylstannanes to aldehydes.

A parallel trend is observed for MgBr2-promoted additions of cis- and trans-crotyl tributylstannanes to a-benzyloxy aldehydes but the effect is much smaller (Table 9) [18], In such reactions the orientation of the allylic stannane and the chelated aldehyde is governed by steric effects in which the vinylic y-hydrogen orients over the five-membered chelate (Fig. 4). Support for this picture is provided by competition experiments in which y3,)8-dimethylallyl tributyltin was found to be markedly slower than the crotyl or allyl derivatives in additions to a-benzyloxypropanal. The observed rate decrease was attributed to the disfavored relationship of a vinylic methyl substituent with the chelate ring resulting in unfavorable steric interactions. 

Table 16. Additions of crolyl tributylstannanes to aldehydes catalyzed by a chiral acyloxyborane (CAB). ArC02...

When the aldehyde and alkoxystannane are enantioenriched the issue of matching and mismatching must be addressed. An early examination of this issue involved S)-2-benzyloxypropanal and the enantiomeric y-OMOM tributylstannanes derived from crotonaldehyde (Eq. 44) [64], Two sets of experiments were performed. In the first BF3 OEt2 was used as the Lewis acid promoter. Matching was observed with the R)-stannane and the anti, syn- E) adduct was formed as the major component of a separable 92 8 mixture. The mismatched (5)-stannane gave a 67 33 mixture of syn, syn-(E) and anti, anti- E) under these conditions. 

Additions of the transient y-OMOM crotyl indium chloride reagents to a-oxyge-nated aldehydes are strongly reagent-controlled. Thus the (R) and (S) reagents add to protected threose and erythrose aldehydes with high diastereoselectivity (Eq. 59) [75]. These additions are complementary to those previously effected with the y-BusSn counterparts (Eq. 45). It is thus possible to prepare precursors to the eight diastereomeric hexoses and their enantiomers from threose- and erythrose-derived aldehydes and their enantiomers plus the a-OMOM crotyl tributylstannane enantiomers. 

Additions of allenic tributylstannanes to aldehydes, like those of their allylic counterparts, require Lewis-acid promoters [83]. The favored promoter is BF3-OEt2 (Table 50). Promotion can also be effected by MgBr2, although less effectively. The use of other common Lewis acids, for example TiCU or AICI3, is complicated by competing exchange reactions. This type will be covered in a later section. 

Table SO. Additions of an allenyl tributylstannane to aldehydes promoted by BF3 OEt2 or MgBf2.

The tributyltin enolates 74 are readily prepared from the corresponding enol acetates and tributyltin methoxide in the absence of solvent [34]. The tin enolates thus obtained occur in the 0-Sn form and/or the C-Sn form, and both species can be used for the aldol reaction of this system. Although the tin enolates themselves have adequate reactivity toward aldehydes [34c], in the presence of the BINAP silver(I) catalyst the reaction proceeds much faster even at -20 °C. Optimum conditions entail the use of THF as solvent and the results employing these conditions in the catalytic enan-tioselective aldol reaction of a variety of tributyltin enolates with typical aromatic, a,/3-unsaturated, and aliphatic aldehydes are summarized in Table 2. TTie characteristic features are (i) All reactions proceed to furnish the corresponding aldol adducts 75 in moderate to high yield in the presence of 10 mol % (i )-BINAP AgOTf complex at -20 °C for 8 h, and no dehydrated aldol adduct is observed (ii) with an a,j3-unsaturated aldehyde, the 1,2-addition reaction takes place exclusively (entry 3) (iii) a bulky alkyl substituent of tin enolate increases the enantioselectivity of the aldol reaction. For instance, the highest ee (95 % ee) is obtained when the tin enolate prepared from pinacolone 77 or rert-butyl ethyl ketone 79 is added to aldehydes (entries 2, 7, and 8) (iv) addition of the cyclohexanone-derived enol tributylstannane 78 (( )-... 

Alkylations. The effect of subjoined Lewis acid (e.g., trimethyl borate) on the catalytic ally lation of aldehydes with allylstannanes promoted by a BINOL-Ti complex has been examined. With allenyltributylstannane the products are homopropargyl alcohols.. Allylation in a Sn(II)-mediated Barbier reaction exhibits much lower ee, although jllenylation with (terminally substituted) propargyltributylstannanes shows good results. 4-Trimethylsilylbut-2-ynyl)tributylstannane undergoes destannylative addition to... 

Carbonylative Carbon-Carbon Bond Formation. A general, mild (50 °C), and high yielding conversion of halides and triflates into aldehydes via Pd(Ph3P)4-catalyzed carbonylation (1-3 atm CO) in the presence of Tributylstannane has been described (eq 21). The range of usable substrates is extensive and includes Arl, benzyl and allyl halides, and vinyl iodides and triflates. The reaction has been extended to include ArBr by carrying out the carbonylation at 80 °C under pressure (50 atm CO), using poly(methylhydrosiloxane) (PMHS) instead of tin hydride. ... 

A number of amine-tethered tributylstannanes 94, termed SnAP (Sn amino protocol) reagents, have been developed, and when reacted with various aldehydes 95 formed a diverse set of N-unprotected and saturated l,4-ox(di)azepines 96 in a mild, scalable, one-pot process with excellent functional group tolerance (14NC310). 

For example, treatment with p-toluenesulfonic acid in methanol affords dimethoxyacetals which can be further hydrolyzed to aldehydes (eq 2). Aldehydes are also available in one step by oxidation of the sulfide to the sulfoxide with m-chloroperbenzoic acid and hydrolytic workup (eq 2). Vinyl ethers are produced via a thermal elimination of benzenesulfenic acid (eq 3). Alternately, the anion formed from the addition of methoxy(phenylthio)methyllithium to aldehydes can be treated with carbon disulfide and iodomethane to form a xanthate which when treated with tributylstannane effects a radical reductive elimination to form (Z)- and ( )-enol ethers (eq 4). ... 

The synthesis of the TT 36, possessing a 3-mercaptoprop-l-yl tmit at C-2 was achieved through the reaction of aldehyde 6 with lithium-2-trimethylsilyl-l,3-dithian-2-ide (34) and tributylstannane successively (Scheme 7) [26]. 

The first report on the conceptually new asymmetric catalysis described that both a stoichiometric amount of SiCU and a catalytic amount of chiral phosphoramide (107) promote highly enantioselective allylation and propargylation of aromatic aldehydes with allyl- and allenyl-tributylstannane, respectively [41], The allylation does not proceed without (107). In the proposed mechanism, SiCU, a weak achiral Lewis acid, accepts the Lewis base (107) to form a strong chiral Lewis acid by polarization of the Si-Cl bonds. The active Lewis acid promotes the asymmetric reaction to give trichlorosilylated adducts with regeneration of (107) (Scheme 9.74). 

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