Grignard reagents butenyl

Chelation control does not operate in the addition of 2-butenyl Grignard reagents to a-oxyalkanals, since with (racemic) 2-benzyloxy-, 2-(benzyloxymethoxy)- and 2-(t< rt-butyldi-methylsilyoxy)propanal similar ratios of isomers are formed28. Several cations were investigated, but the best choices, e.g., allylboronates or allylsilanes, were not included in this study. 

Racemic l-methyl-2-butenylboronates (E)- and (Z)-3 may be prepared selectively via reactions of the l-methyl-2-butenyl Grignard reagent with the appropriate borate ester. Use of triisopropyl borate provides a 96 4 mixture of (E)-3l(Z)-3 on a 0.36 mol scale15. Use of a bulkier borylating agent, such as 2-isopropyloxy-4,4,5,5-tetramethyl-l,3,2-dioxaborolane, reverses the selectivity, enabling a 91 9 mixture of (Z)-3/( )-3 to be obtained on a 0.5 mol scale. The diastereomeric purity of this mixture may be enhanced to 95 5 by treatment with 0.15 equivalents of benzaldehyde, since ( )-l-mcthyl-2-butenylboronatc ( )-3 is more reactive than (Z)-3. Repetition of this process provides (Z)-3 that is 98% isomerically pure. 

Sturmer via the reaction of the chiral borate ester (45, 5S)-4,5-dicyclohexyl-2-isopropyloxy-1,3,2-dioxaborolane, and racemic Grignard reagent (l-methyl-2-butenyl)magnesium chloride16. A 97 3 mixture of (S)-4 and its tf-diastereomer was obtained in 89% yield. 

The parent compound, 69, has been synthesized and characterised <2003ZFA1475>. 4-Chloro-hepta-l,6-diene was reacted with Mg. No Grignard rearrangement was noticed but instead the Grignard reagent was converted into l-allyl-3-butenylphosphonous dichloride by reaction with PC13. Reduction with LiAlH. produced l-allyl-3-butenyl-phosphane. Radical-initiated cyclization led to the product, l-phosphabicyclo[3.3.0]octane. Four derivatives were similarly prepared and characterized (70-73). Compound 74 was similarly prepared via a radical reaction < 1997PS(123)141 >. 

Crotylzinc bromide can also add twice to alkynyl Grignard reagents and the resulting bis-adducts exclusively incorporate two 2-butenyl groups (equation 141)169,170. 

Trimethyl silane does not react under these conditions. However, under similar conditions, heptamethylcyclotetrasiloxane reacts with isoprene by 1,2-addition (78). Thiophene reacts with isoprene in the presence of phosphoric acid to give mainly 2-(3-methyl-2-butenyl)thiophene and some higher boiling compounds (79). Reactions of isoprene with Grignard reagents have been described (80,81). 

A similar mechanism can be written for the formation of 1-butene from the erotyl form of the butenyl Grignard reagent and hydroxylated materials. 

Similarly, for reactions of 2-(3-butenyl)phenyl bromide and iodide. D-model equation (7.31) predicts 78% cyclized Grignard reagent and 22% uncyclized. Much less eyeli/ation is observed. 

The fairly recent literature has included a review of the preparation and reactions of allyl and butenyl Grignard reagents (5), a review of the reactions of allylic organometallic compounds (II), and a review of organomagnesium rearrangements, in which allylic derivatives were discussed in some detail (29). Since more recent publications do not appear to include new information that would significantly modify earlier conclusions, the previous discussions should suffice. 

A possible method seemed to be to make use of the Thorpe-Ingold, or gem-dimethyl, effect which, in one statement 49), is that the accumulation of substituents along the chain of a bifunctional system appears to selectively stabilize the corresponding cyclic forms. Maercker and co-workers (6S) have recently used this effect to shift the equilibrium between a 3-butenyl Grignard reagent and a cyclopropyl methyl Grignard reagent in favor of the cyclopropylmethyl compound. 

---