Crotylation, with benzaldehyde

The reaction of benzaldehyde with unsymmetrical allylic bromides in the ionic liquid bmim also proceeds regioselectively to realize a carbon-carbon bond formation at the more substituted allylic carbon (Scheme 5). In the coupling of crotyl bromide with benzaldehyde, the product is a nearly 50 50 mixture of antilsyn diastereomers. Cinnamyl bromide predominantly gives the //-diastereomer. The regio- and diastereoselectivity is similar to that observed for these in aqueous media.104... 

The indium-mediated aqueous Barbier-type reaction of crotyl bromide with benzaldehyde shows no diastereo-selectivity. However, the use of preformed crotylindium sesquibromide in DMF affords. sy/z-l-phenyl-2-methyl-3-buten-l-ol (ca. 40% de) after aqueous acidic workup. At 22 °G in DMF prior to workup, a greater relative proportion of //-intermediate is decomposed as compared to its yy/z-diastereomer. The resultant kinetic diastereoselection upgrades the syn anti ratio to 99 1 with a concomitant drop in overall yield (Scheme 14).127... 

Chiral compounds of this class condense with aldehydes to give a reasonable degree of enantioselective synthesis of homoallylic alcohols. Reaction of (-)-(S)-(neomenthylcyclopentadienyl) Mo(NO)Cl(Ji-syn-crotyl) 47 with benzaldehyde affords (+)-( , /f)-2-methyl-l-phenyl-3-buten-l-ol 48 with >98% ee. (+) and (-) chiral... 

This chromium complex was tested in the addition of other aUyHc bromides to a series of aldehydes (Scheme 12.26). As examples, the crotylation of benzaldehyde afforded high diasteroeselectivities of 4.4 1 ratio favoring the a t/-product in 84% yields with 97% ee for both diaster-eomers. Crotylation of cyclohexanecarboxaldehyde gave an... 

Homoallylic alcohols (8, 111-112). The high rltreo-selectivity observed in the reaction of benzaldehyde with crotyl bromide (either irons or cis) is general for relatively unhindered aldehydes (equation I). High threo selectivity is still observed in the reaction with an a-methyl substituted aldehyde, but a-asymmetric induction (at C3) is rather low (2.2 1) with simple aldehydes (equation II).1... 

In addition, this silver-BINAP catalyst has been applied to the crotylation reaction between aldehydes and 2-butenylstannanes (Scheme 9.1).3 The addition of (E)-2-butenylstannane to benzaldehyde in the presence of 20 mol% AgOTf/BINAP gave the corresponding adduct with 94% ee with a high anti syn ratio. In contrast to other... 

Addition of cis- and trans-crotyl tributyltin to benzaldehyde could also be achieved with this catalyst system (Table 21) [37]. Interestingly, the anti product was predominant and the anttsyn ratio was unaffected by the cis.trans composition of the crotyl-stannane. These results are suggestive of an acyclic synclinal transition state for the reaction. 

Enantioenriched a-carbamoyloxy allylic stannanes can be prepared by lithiation of allylic carbamates in the presence of (-)-sparteine (Eq. 42) [62]. Ilie resulting lithiated sparteine complex reacts with BuaSnLi at the a-position to afford the substitution product. The crotyl derivative of 80% ee is thus prepared. This stannane undergoes thermal addition to benzaldehyde at 160 °C to afford the anti- S) adduct of 79% ee in 79% yield. 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

Rhodium catalyzes retro-allylation of homoallylic alcohols to generate nucleophilic allylic rhodium species. A combination of [RhCl(cod)]2, PMcj, and cesium carbonate catalyzes crotyl transfer from homoallylic alcohol to benzaldehyde in dioxane at 100 "C to afford the corresponding homoallylic alcohol (Scheme 5.37) [26]. Like allylic nickel, allylic rhodium experiences rapid and E-Z interconversions under the reaction conditions, which prevents stereoselectivity of the crotylation. Interestingly, a similar crotylation is followed by isomerization into saturated ketone with the aid of a bulkier catalyst [RhCl(cod)]2/PtBu3 at a higher temperature. 

Lactone synthesis through the reaction of 2-(alkoxycarbonyl)allyl metal species to carbonyl substrates holds a main position for the synthesis of the a-methylene derivatives. In 2(X)5, Hall et al. [30] obtained an interesting result in a TfOH-catalyzed reaction of a benzaldehyde derivative bearing electronically rich arene structure with (E)-crotyl boron reagent (E)-39 (Scheme 16). In this case, if the reaction proceeds via normal cyclic six-membered transition... 

Additions of methyl-substituted (Z)-crotyl reagent 51 to aldehydes give dramatically different stereochemical outcomes (Scheme 5.9) [60]. The minimization of the dominant A, 3 allylic interactions favors transition state 53 over the competing transition state 52, resulting in the formation of ( )-homo-allylic alcohols, such as 54, with superb selectivity (>99% ee). The lack of induction in the addition of 55 to benzaldehyde underscores the unique aspects of this class of reagents (Equation 6) [60]. It is worth noting that... 

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