Dimethyl-2,2-but-2-ene

In the presence of alkenes, photolysis of alkyl (silyl)diazoacetates leads mainly to the formation of cyclopropanes as diastereomeric mixtures4,111,112. With (Z)- and ( )-but-2-ene, the cyclopropanation is not completely stereospecific with respect to the double bond configuration, but gives a small amount of the wrong isomer these results point to the participation of a triplet carbene in the cyclopropanation reaction. Allylic C,H insertion products are also formed their yield increases in the series 1,1-, 1,2-, tri- and tetrasubstituted C=C bond. With 2,3-dimethyl-but-2-ene, the allylic C,H insertion product is formed at the complete expense of the cyclopropane. 

The intramolecular 13C isotope effects on the ene reaction with 102 oxygen were determined for 2,3-dimethyl-l-buten-3-ol 43 prepared from 2,3-dimethyl-but-2-ene 89... 

In the same way, the l,4-dichloro-but-2-ene ozonide with the shorter retention time on silica gel yielded fractions with [a]40o = —2.66° and +4.2° the ozonide of l,4-dibromo-2,3-dimethyl-but-2-ene, which only could be made in one form with a melting point of 42 °C, yielded fractions with [ ]330 = +50.7° and —39.2°. Both, therefore, have the trans configuration. It is remarkable that the trans ozonide in this case is the only product, even when pure cis-l,4-dibromo-2,3-dimethyl-but-2-ene is ozonized. 

Addition versus rearrangement of chloro(methyl)-, chloro(ethyl)-, benzyl(chloro)-, chlo-ro(methoxymethyl)- and chloro(isopropyl)carbene generated by photolytic or thermal decomposition of 3-alkyl-3-chloro-3//-diazirines has also been investigated.81 However, it is known that chloro(isopropyl)- or ie/7-butylchlorocarbene does not undergo addition to 2,3-dimethyl-but-2-ene, which is used as a carbene trap. Photolysis or thermolysis of 3-arylmethyl-3-chloro-3//-diazirines in the presence of an alkene has also been studied extensively.84 Addition of the ambiphilic benzylchlorocarbene82 to an alkene to give 1-benzyl-1-chlorocyclopropanes is usually accompanied by its rearrangement to co-chlorostyrene (1,2-hydride shift, for details see ref 83). This isomerization was seriously impeded when the decomposition of 3-benzyl-3-chloro-3//-diazirines was performed in the presence of an alkene,85 with 2,3-dimethylbut-2-ene only traces of co-chlorostyrene were observed.81,86 The results of reactions of benzylchlorocar-benes with some alkenes are collected in Table 6.85... 

Physical Aspects.—A photoelectron spectroscopic study of perfluoro-2,3-dimethyl-but-2-ene, and cis- and /rans-perfluorobut-2-ene, where the vertical w ionization potentials are 12.61, 11.46, and 11.55 eV, respectively, indicates a planar carbon skeleton for these molecules. A careful gas-phase electron-diffraction study of the six fiuoroethylenes has produced the parameters shown in Scheme 1. Care is needed in this type of study, since the C C and C-F bond lengths obtained tend to be... 

Use the data in Table 7-1 to predict the energy difference between 2,3-dimethyl-but-l-ene and 2,3-dimethylbut-2-ene. Which of these double-bond isomers is more stable ... 

Rate earth X zeolites Ethylene, propylme, hex-l-ene. 2,3-dimethyl but-l-ene 18)... 

Preparation of (+)-6-ethyl-5-methyl-2-(4-sulfotolyl-l,3-dimethyl-but-l-ene)-yl-3,6-dihydro-2H-pyran ... 

To a stirred solution of methyl (+)-6-ethyl-5-methyl-2-(2-sulfonyl-benzene-3-methyl-hex-4-enoic acid)-3,6-dihydro-2H-pyran ester (130.0 mg, 0.31 mmol) in HMPA (2 ml) was added (Me)4N+ -OAc (370.0 mg, 2.78 mmol) and the mixture was heated at 96°-100°C for 17 h. The reaction mixture was cooled and diluted with EtOAc, washed with H20 several times, brine, dried over magnesium sulfate, and the solvent removed in vacuum. The residue was purified by chromatography over silica gel (hexane-EtOAc 9 1) to give 0.08 g (71%) of (+)-6-ethyl-5-methyl-2-(4-sulfotolyl-l,3-dimethyl-but-l-ene)-yl-3,6-dihydro-2H-pyran as a clear liquid. 

Both in situ infrared and multinuclear NMR under less severe conditions have been used to gain mechanistic insights. For the hydroformylation of 3,3-dimethyl but-l-ene, the formation and hydrogenolysis of the acylrhodium species Rh(C()R)(C())4( R=CH2CH2Bur) can be clearly seen by IR. NMR spectroscopy has also been very useful in the characterization of species that are very similar to the proposed catalytic intermediates. We have already seen (Section 2.3.3, Fig. 2.7) NMR evidence for equilibrium between a rhodium alkyl and the corresponding hydrido-alkene complex. There are many other similar examples. Conversion of 5.3 to 5.4 is therefore well precedented. In the absence of dihydrogen allowing CO and alkene to react with 5.1, CO adducts of species like 5.6 can be seen by NMR. Structures 5.11 and 5.12 are two examples where the alkenes used are 1-octene and styrene, respectively. 

One-bond CC couplings across a double bond have been determined by Wrackmeyer et for (Z)-l-chloro(dimethyl)silyl-l-(9-bora-bicylo[3.3.1]-non-9-yl)-3,3-dimethyl-but-l-ene and 2-[(Z)-1 -(3,3-dimethyl)butenylidene]-1,3-dihydro-1,1,4-trimethyl-l-sila-3-(9-bora-bicylo[3.3. l]non-9-yl)-4-aza-6a-aza-pentalene. 

Restricted Internal Rotation of Several Symmetric Tops. The tables of thermodynamic functions for an internal rotation of a single symmetric top may be used for several symmetric tops [with moments of inertia calculated from equation (20)] provided both potential energy and kinetic energy cross-terms between the tops can be neglected. Both assumptions have been generally made in calculations for molecules with several tops. Where there are reliable calorimetric data at one or more temperatures, the tables have been used to calculate appropriate potential barriers. Using this procedure thermodynamic contributions have been calculated for propane, 2-methylpropane, 2,2-dimethylpropane, cis-but-2-ene, rm a -but-2-ene, isobutene, o-xylene, > m-xylene, p-xylene, 1,2,3-trimethylbenzene, > 1,2,4-trimethylbenzene, dimethyl sulphide,2-chloro-2-methylpropane, and dimethyl-amine. In several cases thermodynamic contributions have been calculated using potential barriers estimated from those of related molecules. Examples of this procedure are found in calculations for 2-fluoro-2-methylpropane, 2-chloropropane, 2-bromopropane, 2-iodopro-pane, 2,2-dichloropropane, 2-bromo-2-methylpropane, 2-iodo-2-methylpropane, 1,3,5-trimethylbenzene, 1,2,3,5-tetramethylbenzene, 1,2,4,5-tetramethylbenzene, 1,2,3,4-tetramethylbenzene, pentamethyl-benzene, and hexamethylbenzene. ... 

Significant aliphatic sulfur compounds are methional, 3-methyl-but-2-ene-1-thiol, 3-mercapto-3-methylbutan-l-ol (8-124), its ester 3-mercapto-3-methylbutyl formate, methanethiol and dimethyltrisulfide. 3-Mercapto-3-methyl-l-ol also occurs in passion fruit and blackcurrant, and as a putative cat pheromone in cat urine, where it is formed as a degradation product of amino acid L-felinine (see Section 2.2.1.2.2). Of more than 70 known pyrazines, the most important compounds in roasted coffee are isopropylpyrazine, 2-isobutyl-3-methoxypyrazine, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 2,6-dimethyl-3-vinylpyrazine and 2-ethyl-6-methyl-3-vinylpyrazine. Pyridine and its alkyl derivatives and bicyclic pyridines have a negative impact on the quality of coffee aroma. Important aromatic... 

J as-Dimethyl. b (Z)-But-2-ene gave the m-dimethyl product ( )-but-2-cnc gave the /ram-dimethyl product. c (Z)-Hex-3-ene gave the cw-diethyl product (A )-hex-3-cnc gave the frans-diethyl product. d Product 2,2-dichloro-3- or -4-trimethylsilylcyclobutanonc. 

Experimentally, the kinetic data obtained from the pyrolysis of cyclobutane to ethene between 696 and 1007"C using a very low-pressure pyrolysis technique is consistent with a 1,4-diradical reaction route.82 A diradical process is also favored over a concerted pathway for the formation of nearly 1 1 mixtures of cis- and tranj-but-2-enes and [l,2-2H2]ethene from the thermal rupture of cb-l,2-dimethyl[cw,firn /-3,4-2H2]cyclobutane (5),83... 

Dramatic changeover is observed not only in the ene/HDA product ratio, but also in the absolute stereochemistry upon changing the central metal from Ti to Al. Thus, Jprgensen et al. reported the HDA-selective reaction of ethyl glyoxylate with 2,3-dimethyl-1,3-butadiene catalyzed by a BINOL-derived Al complex [25], where the HAD product was obtained with up to 89% periselectivity and high enantiopurity (Scheme 8C.9). The absolute configuration was opposite to that observed by using BINOL-Ti catalyst. 

Figure 3-13 compares the cis-trans isomers of but-2-ene with those of 1,2-dimethyl-cyclopentane. Make models of these compounds to convince yourself that cis- and trans-1,2-dimethylcyclopcntanc cannot interconvert by simple rotations about the bonds. 

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