2.3.3- Trimethyl-l-butene

Trimethyl-l-butene gives only (CH3)3CC=CH2 1-Octene gives a mixture of... 

The asymmetric nickel-catalyzed hydroalumination of prochiral terminal alkenes using adducts of BujAl and chiral amines was reported in 1981 [74], Among the different amines investigated, (-)-N,N-dimethylmenthylamine (DMMA) gave the best enantioselectivities. For example, reaction of 2,3,3-trimethyl-l-butene (39) at room temperature with 0.33 equiv. of the DMMA/iBu3Al adduct in the presence of 0.6 mol% of Ni(mesal)2 gave, after oxidation of the intermediate organoaluminum compounds, 2,3,3-trimethyl-l-butanol 40 in 76% yield and 27% ee (Scheme 2-19). 

Subtle ligand changes have pronounced effects in hydroboration of 1,1-disubstituted alkenes. Addition of HBcat across the C=C bond in 2,3,3-trimethyl-l-butene is catalyzed by [Rh(COD)Cl]2/DIOP at —5°C affording 2-t-butyl-l-propanol in 69% ee upon oxidation. In the case of 2-phenylpropene, [Rh(C2H4)2Cl]2 combines with either DIOP or BINAP to provide higher enantiomeric excesses than with [Rh(COD)Cl]2.46... 

Both resonance forms are equivalent, and so 2,3,3-trimethyl-l-butene gives a single bromide on treatment with /V-bromosuccinimidc (NBS). 

The McBride synthesis has been applied to the preparation of chiral phosphetane oxides by the reaction of optically active dichlorophosphines with 2,3,3-trimethyl-l-butene. Thus, myrtanyl-, bornyl-, and isopinocamphenyldichloro-phosphines afforded the corresponding phosphetane oxides having chirality localized on both the phosphorus substituent and the phosphorus center. In all cases, epimeric mixtures are obtained (Equation 22) <1997JOC297>. 

Similar unsatisfactory results, usually with even lower inductions are obtained with other terminal aliphatic alkenes such as 1-pentene, 2-methy]-l -butene, 3-methyl-l -butene, 2,3-dimethyl-l-butene, 3,3-dimethyl-l-butene, 2,3,3-trimethyl-l-butene, 2-ethyl-l-hexene, I-hex-ene and 1-octene (Table 1). 

An important result is the hydroboration of 2,3,3-trimethyl-l-butene with catecholborane in the presence of [Rh(cod)Cl]2 2 Diop leading to 2,3-trimethyl-l-butanol with 69% ee112. Other asymmetric hydroborating agents do not show a comparable cnanlioseleclivity in reactions with simple 2-methyl-1-alkenes. 

Evaluate 2,3,3-trimethyl-l-butene as a candidate for free-radical bromination. How many allylic bromides would you expect to result from its treatment with A/-bromosuccinimide ... 

Figure 5. Correlation of rate of hydration of terminal alkenes with

Addition of X2 in nucleophilic solvents presents us with another opportunity. We can measure the regioselectivity of the reaction. For example, in the bromina-tion of 2,3,3-trimethyl-l-butene in carbon tetrachloride, there is no way to determine the regiochemistry of the bromide attack of the bromonium ion. Bromination in water allows us to see where the hydroxyl group goes as it opens the bromonium ion. There are two possibilities (Fig. 10.17). The hydroxyl group can attach to the more substituted or to the less substituted carbon of the original double bond. 

In a concerted process, an olefin can add cis or trans to MA. These are represented as syn-syn and anti-anti concerted processes and are symmetry allowed.We have seen that the predominant product obtained is a trans-olefin however, thermodynamic stabilities of the products cannot be ruled out as a contributing factor. To eliminate this stability bias, Friedrich et al have investigated the reaction of 2,3,3-trimethyl-l-butene 68 and its deuter-ated analogue. From the NMR of the product, it was concluded that starting with deuterated olefin, H2 and H3 (i.e., those originating from MA) were cis. Since the limit of detectability of the other isomer is 30%, it may be concluded that the ene reaction is at least 70% c/5-stereospecific. 

---