Allyltri-n-butylstannane

Reactions of the a-methyl- -benzyloxy aldehyde 97 with allyltri-n-butylstannane 98 are summarized in Table 11-7 [83]. While little stereocontrol is observed in the BF3 OEt2-promoted allylation reaction, the chelate-controlled reaction catalyzed by either TiC or SnCl4 is much more selective, favoring formation of the Cram-chelate adduct 100 with up to 98 2 selectivity. The chelate transition state 101, where C-C bond formation occurs anti to the aldehyde a-methyl group, rationalizes the observed stereoselective formation of 100. Although the BF3-OEt2-cata-... 

The diastereoselectivity of these reactions is consistent with product formation occurring through transition state 137, where the reactive conformation of the aldehyde in the transition state (corresponding to the normal Felkin-Anh model) minimizes steric interactions with the allylstannane as well as the 1,3-dipole interactions of the aldehyde and the /(-alkoxy group. The allylation reaction of the 2,3-syn aldehyde 138, however, with allyltri-n-butylstannanes 98, generates the anti-Felkin adducts 139 preferentially (Eq. (11.9)) [93], The stereochemistry of these reactions is consistent with product formation occurring preferentially through transition state 140, in which 1,3-dipole interactions of the aldehyde and the P-... 

The synthesis of 145 (Scheme 11-4) began with the allylation adduct 121, obtained through a chelate-controlled addition of crotyltri- -butylstannane to the a-benzyloxy aldehyde 55 (see Eq. (11.4)). Adduct 121 was converted in five steps to aldehyde 146, which subsequently underwent a highly diastereoselective chelate-controlled allylation reaction with allyltri-n-butylstannane 98. The stereochemistry of the resulting adduct, 147, is consistent with formation through a chelate transition state analogous to 101 (Table 11-7). 

Either antipode of bromoborane 332 can be prepared in a six-step sequence from benzil (330, Scheme 11-19) [127, 254, 258J. Reaction of benzil with cyclohexanone in the presence of ammonium acetate and acetic acid generates a cyclic bis-imine which is subsequently reduced with lithium in ammonia. The resulting racemic fra .s-imidazolidine is subsequently hydrolyzed to the diamine 331. Resolution of 331 is accomplished by crystallization with either antipode of tartaric acid. The enantiomerically enriched stein ligand 331 is then sulfonylated and condensed with boron tribromide, giving the chiral bromoborane 332. Transmetalla-tion of allyltri-n-butylstannane with bromoborane (R,/ )-332 then affords the allyl-boron reagent (R,/ )-198. 

The BINOL/BINAP Lewis acid complexes and the CAB catalyst are complementary in the following respects in general, the BINOL/BINAP-Lewis acid complexes provide excellent enantiocontrol in the reactions of aldehydes with allyltri-n-butylstannane, but poor diastereocontrol (syn anti) in the reactions of aldehydes with crotyltri- -butylstannane. In contrast, when the CAB catalyst is used to promote the reaction of aldehydes and crotylsilane or crotylstannane reagents, excellent levels of diastereo- and enantioselectivity are achieved, while in the corresponding reactions with allyltri-n-butylstannane poor levels of enantioselectivity are realized. 

Keck and Tagliavani reported within months of each other the asymmetric ally-lation reactions with allyltri-n-butylstannane and various aldehydes with BINOL-Ti(IV) catalysts 451 and 452, respectively [289, 2901. Although the two catalysts give similar yields and enantioselectivities with a range of aldehydes, the diiso-propoxide catalyst 451 has been used more extensively. Keck and co-workers have shown that a variety of aldehydes react with allyl and methallyltri-n-butyl-stannane in modest to excellent yield and with good to excellent enantioselection using (/ )-451 as the catalyst (Table 11-25) [289, 296, 2971. 

Allyltri-n-butylstannane (EC-2) similarly terminates the copper-catalyzed polymerization of MA to give allyl-functionalized polymers via elimination of the stannyl group accompanying the bromine originated from the dormant polymer terminal.346 Allyl a -end PMMA was obtained also by the copper-catalyzed reaction between allyl bromide (EC-3) and the isolated bromine-capped PMMA, although the functionalization was 57%.226 Another allyl derivative (EC-4) similarly leads to methacrylate-based macromonomers quantitatively in the presence of Cu(0).347... 

Scheme 5.2.10 Felkin-Ahn addition of allyltri-n-butylstannane to the 2- R)-methyl diastereomer 41...

Some improvements have been described for the enantioselective addition of allyltri-n-butylstannane to simple aldehydes using a catalyst prepared from BINOL and Zr(0-t Bu)4 in toluene.122 Reactions proceed... 

Allyltri-n-butylstannane reacts with aromatic and branched a-alkylimines in the presence of TiCU and BFvEt20 (entries 8-11, Table 2) yields using TiCU tend to be higher. Keck and Enholm have demonstrated that the Lewis acid participates by activating the imine, not by exchanging with the metal of the organometallic reagent. TTiis is confirmed by the fact that no homoallylamines are formed when the... 

Analogous reactions of allylstannanes (131) with primary amines and aqueous formaldehyde yield bishomoallylamines (132 equation 28).57 Reactions are conducted at ambient temperature in a 1 1 mixture of methanol and chloroform using 37% aqueous formaldehyde (2.1 equiv.), 2.0 equiv. of the allyltri-n-butylstannane and 1.1 equiv. of the amine trifluoroacetate. The failure to observe 4-hydroxy-... 

Vinyl-ftmctionalized poly(methacrylate) has also been prepared by the reaction of allyltri-n-butylstannane. The reaction of organic halides with allyltri-n-butylstannane was first discovered by Keck and Migita. " ° Methyl acrylate was polymerized in bulk by ATRP to 93% conversion, upon which benzene and allyltri-n-butylstannane (3 molar equivalents) were added to the reaction mixture and allowed to react for 8 h. After purification, the presence of the vinyl end group was confirmed by NMR and ESI. The degree of functionalization was not reported. 

A mixture of (i )-l,r-binaphthalene-2,2 -diol (22.5 mg, 0.078 mmol) and IM titanium isopropoxide (39 pL, 0.039 mmol in dichloromethane) in dichloromethane was stirred at room temperature for 1 h. Benzaldehyde (41.2 mg, 0.394 mmol) was added to the red-brown solution, the contents were cooled to 0 °C and allyltri-n-butylstannane (144 mg, 0.435 mmol) was added. After 3 h at 0 °C, saturated NaHCOa (0.5 mL) was added and the contents stirred for Ih, dried (Na2S04) and filtered. The crude material was purified by chromatography over silica gel eluting with 19 1 (v/v) hexanes/acetone followed by 17 3 (v/v) hexanes/acetone to give (i )-(-t)-phenyl-3-buten-l-ol as a clear oil (54.9 mg, 94% yield, 95% ee). 

Abstract The SnCl4 promoted reactions of three (3-alkoxy aldehydes with allyltri-n-butylstannane have been investigated in detail, primarily via H Sn NMR spectroscopy. Using this technique, all processes involved in a rather complex manifold of reactions can be observed at low temperature. These three substrates span the range of possible behavior upon complexation with SnCl4 at 1 1 stoichiometry 2-methyl-3-benzyloxypropanal is a "strong... 

In Fig. 5, we show the reaction of that same aldehyde with allyltri-n-butylstannane, in the presence of SnCl4 as... 

Lewis acidr as monitored by Sn NMR spectroscopy. The protocol employed here is a "normal one, in which the aldehyde is precomplexed with SnCl4 and the NMR spectrum is recorded. We observe the presence of a chelate with SnCl4 which we have previously characterized by and NMR spectroscopy, at -573 ppm and a very small amount of what is in fact a 2 1 complex at -558 ppm. Thus we are using a very small excess of aldehyde to insure the absence of SnCl4 as can be discerned from the spectrum. Allyltri-n butylstannane is then added by a technique which we will discuss later, and one obtains the spectrum above after eight minutes of exposure of allyltri-n-butylstannane to... 

Since we have now defined the reactivity of the chelate formed from 2-methyl-3-benzyloxypropanal and the 2 1 complex formed between that same substrate and SnCl4, we are now in a position to address the Curtin-Hammet problem that was alluded to in the introduction of this paper that is, which is the more reactive substrate, the chelate or the 2 1 complex. Shown in Fig. 9 are the results of a competition experiment where, by appropriate choice of stoichiometry, a solution containing both chelate and 2 1 complex was prepared and then allowed to react with allyltri-H butylstannane. This particular experiment was done in such a way that allyltri-n-butylstannane was mixed in in small portions and the progress of the reaction was followed, and the acquisition times were such that allyl-tri-n butylstannane is never evident in the spectra. In the bottom trace we can see a mixture of chelate and 2 1... 

One sees free SnCl4 at -155 ppm, a chelate at -585 ppm, and a 2 1 complex at -566 ppm. Upon warming that sample to -60 one can see that neither chelation nor simple 2 1 complexation is particularly viable with this substrate because the spectrum essentially disappears at -40 one sees no spectrum at all, which is indicative of exchange and concommitant line broadening. At 0, in the limit of fast exchange, one sees one broad peak displaced towards the position for free SnCl4. It is not hard to imagine then what should happen upon the admixture of allyltri-n-butylstannane to the solution. Shown in Fig. 12 is the reaction of allyltri-H butylstannane with the mixture of... 

In summary, then, we have seen three substrates. One, 2-methyl 3-benzyloxypropanal, is a strong chelator, and forms tight well-defined rigid chelates with SnCl4. The addition reaction of allyltri-n-butylstannane with this chelate proceeds without transmetalation. The isomeric 3-benzyloxybutanal compound is in fact a weak chelator. At low temperatures the chelate is in equilibrium with a 2 1... 

We then investigated the addition reaction of allyltri-n-butylstannane with this substrate. Shown in Fig. 18 is again the 2 1 complex in the absence of free... 

SnCl4. In the trace immediately above, allyltri-n-butyl-stannane has been mixed in, and one observes allyltri-H butylstannane at -14 ppm and the 2 1 complex at -578 ppm. After 50 minutes at -80 addition products are being formed as evidenced by the production of tri-n-butyltinchloride, allyltri-n-butylstannane is still present, and the 2 1 complex has been consumed. We should point out however, that the 2 1 complex will of course be consumed when only half of the aldehyde has been converted to addition products. After 2 1/2 h at -70, consumption of the allyltri-n-butylstannane is complete, and quenching allows for the isolation of the expected addition products. We have also investigated the reactivity of this same substrate with both allyltrichlorostannane and diallyldichlorostannane. 

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