Dimethyl-butanol

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

Evidence for the intermediate carhocations in the acid-catalvzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3-dimethyl-2-butene from 3,3-dimethyl-2-butanol. ... 

Dimethyl-2-butanol [464-07-3] C6Hi40 MW = 102.18 Table 1. Experimental and recommended values with uncertainties. ... 

Early studies of the asymmetric reduction of prochiral ketones by chiral aluminum alkoxides have been reviewed by Morrison and Mosher (1). Doering and Young (123) reported the reduction of methyl cyclohexyl ketone with chiral 3-methyl-2-butanol in the presence of a catalytic amount of aluminum alkoxide to give the (S)-( + )-carbinol in a 22% optical yield. Jackman and co-workers (124) similarly reduced methyl n-hexyl ketone with chiral 3,3-dimethyl-2-butanol to the (S)-( - )-carbinol in a 6% optical yield. Other attempts resulted in similar low optical yields or gave only racemic products. Since the reductions were carried out under equilibrium conditions, racemization could have accounted for the low optical yields. 

From 3,3-dimethyl-2-butanol, the major product of rearrangement is 2,3-dimethyl-1-butene. The distribution of the primary dehydration products is far from equilibrium. The maximum ratio of 2,3-dimethyl-1-butene to 2,3-dimethyl-2-butene obtained from 2,3-dimethyl-2-butanol is about 10. This is higher than that to be expected if a proton is removed from the l,l,2-trimethyl-2-propyl carbonium ion in a statistical manner. The maximum ratio of the two olefins obtained from 2,3-dimethyl-2-butanol is also about 10. Hence it can be argued that the high yield of 2,3-dimethyl-1-butene from 3,3-dimethyl-2-butanol does not necessarily rule out a classical carbonium ion mechanism. It is very unlikely, however, that the same intermediate is involved from both alcohols. If such were the case the product of dehydration of 2,3-dimethyl-2-butanol would contain appreciable amounts of 3,3-dimethyl-l-butene. 

The products from the dehydration of 3,3-dimethyl-2-butanol can be explained by anchimeric assistance of the methyl group and the removal of the proton from the y-carbon atom ... 

The dehydration of the two alcohols over alumina catalyst in the presence of piperidine was studied by Pillai and Pines [84). The experimental results which are given in Table X indicate that, although carbonium ion mechanism can interpret the products obtained from the tertiary alcohols, another mechanistic path has to prevail in order to account for the formation of the various dehydration products from 3,3-dimethyl-2-pentanol. The mechanism, as proposed above for the dehydration of 3,3-dimethyl-2-butanol, would also explain the hydrocarbons formed from the dehydration of 3,3-dimethyl-2-pentanol. 

H-NMR spectra15c of esters of 3,3-dimethyl-2-butanol with three different chiral selector acids are depicted in Figure 2. The two diastereomers can be clearly distinguished in each case. Note the different order of chirality of MTPA esters compared to the other esters. 

Figure 2. NMR Spectra (60 MHz)15c of 3,3-dimethyl-2-butanol esters of four different nonracemic enantiomeric mixtures of carboxylic acids. Excess of the (R,R)- or (5,5 )-diasLereomers mandelate 22% atrolactate 18% MTPA 7.5% 0-methylmandelate 12.8%.

Chemical shift differences of O-methylmandelates of four different methylcarbinols have been shown15a to be rather small, varying most in 3,3-dimethyl-2-butanol (4 Hz at 110 °C 13 Hz at — 90 °C). Pirkle and Simmons36 have studied the effect of temperature on ATEA derivatives and found that the configuration about the rotationally hindered amide bond is Z and is quite stable to temperature. 

Problem 6.55 Dehydration of 3,3-dimethyl-2-butanol, (CH JjCCHOHCH, yields two alkenes, neither of which is (CHj),CCH==CH2. What are their structures <... 

Although the alcoholysis of Cr(OBu )4 with several primary and secondary alcohols leads to oxidation by Cr to the aldehyde or ketone,1316 alcoholysis by the secondary alcohol 3,3-dimethyl-2-butanol affords a Cr alkoxide (equation 84)1318 which is sensitive to oxygen and moisture, but otherwise stable. Presumably the bulky f-butyl groups prevent the molecule from achieving the conformation required for hydrogen transfer in the oxidation step. 

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