Tert-carbon

Regioselectivity in hydrogenolysis of internal allylic systems depends on the bulkiness of substituents. The alkene 441 was obtained regioselectively from tert-carbonate 440. The hydride attacked the congested tert-carbon, but not i ec-carbon [169]. 

Concomitant rise in the viscosity of the reaction mixture was also noticed. NMR spectroscopy manifested the appearance of a new peak at 8 69.83 (Me3C-OSi) representative of a silane substituted tertiary carbon center of tert-butanol beside the peak at 8 68.02 (Me3C-OH). After 34 h, the intensity of the silane substituted tert-carbon peak reached optimum level, while the unsubstituted carbon peak disappeared completely. Si NMR spectra also corroborated complete transformation of Si-H... 

The properties of bioisosteric compounds prompted Tacke et al. to prepare sila-fexofenadine (49) and to study the bioisosterism of this and some related Hi-receptor antagonists [55, 56]. Replacement with a silicon atom of the tert-carbon atom bearing the OH group in fexofenadine was attempted, and the synthesis of sila-fexofenadine was achieved according to Schemes 10.11-10.13. 

With reference to Table I an excellent agreement is seen between the spectral data for the polymers and for the model compounds (the signals for a cis isomer have been found to be significantly different). The mode of ring-opening is just the opposite of that found in case of the polymerization of 7 where the tert. carbon is subjected to the nucleophilic attack. The tertiary position in case of 9 and 11 are severely shielded and for that reason the attack occurs only on the methylene carbon. 

Benzoylamino-2-methyl-l-propanol allowed to react at —30 to —40° with p-toluenesulfonyl chloride in pyridine, and the product isolated after ca. 15 min. -> 4,4-dimethyl-2-phenyl-2-oxazoline. Y 84%.—The amido group may be attached to prim., sec., or tert. carbon and the alcohol may be prim, or sec. Yields vary widely depending on the structure of the startg. m. F. e. s. R. N. Boyd and R. C. Rittner, Am. Soc. 82, 2032 (1960). 

Figure 9.2. Carbon Is photoelectron spectrum Is core-hole-state spectra for the 2-norbornyl cation of tert-butyl cation and Clark s simulated spectra for the classical and nonclassical ions.

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Note 1. The lithiation of monoalky1al 1 enes is not completely regiospecific. The ratio of a- to ylithiated allene varies from about 80 20 for methyl-allene to 93 7 for hexylallene. tert.-Butylallene, however, is metallated exclusively on the terminal carbon atom. 

If a bromomethyl- or vinyl-substituted cyclopropane carbon atom bears a hydroxy group, the homoallyiic rearrangement leads preferentially to cyclobutanone derivatives (J. Sa-laun, 1974). Addition of amines to cydopropanone (N. J. Turro, 1966) yields S-lactams after successive treatment with tert-butyl hypochlorite and silver(I) salts (H.H. Wasserman, 1975). For intramolecular cyclopropane formation see section 1.16. 

In peptide syntheses, where partial racemization of the chiral a-carbon centers is a serious problem, the application of 1-hydroxy-1 H-benzotriazole ( HBT") and DCC has been very successful in increasing yields and decreasing racemization (W. Kdnig, 1970 G.C. Windridge, 1971 H.R. Bosshard, 1973), l-(Acyloxy)-lif-benzotriazoles or l-acyl-17f-benzo-triazole 3-oxides are formed as reactive intermediates. If carboxylic or phosphoric esters are to be formed from the acids and alcohols using DCC, 4-(pyrrolidin-l -yl)pyridine ( PPY A. Hassner, 1978 K.M. Patel, 1979) and HBT are efficient catalysts even with tert-alkyl, choles-teryl, aryl, and other unreactive alcohols as well as with highly bulky or labile acids. 

Those derived from isobutane are the 2 methylpropyl (isobutyl) group and the 1 1 dimethylethyl (tert butyl) group Isobutyl is a primary alkyl group because its poten tial point of attachment is to a primary carbon tert Butyl is a tertiary alkyl group because Its potential point of attachment is to a tertiary carbon... 

The transition state is closer m energy to the carbocation (tert butyl cation) so Its structure more closely resembles the carbocation than it resembles tert butyloxonium ion The transition state has considerable carbocation character meaning that a significant degree of positive charge has developed at carbon... 

The properties of tert butyl cation can be understood by focusing on its structure which IS shown m Figure 4 9 With only six valence electrons which are distributed among three coplanar ct bonds the positively charged carbon is sp hybridized The unhybridized 2p orbital that remains on the positively charged carbon contains no elec Irons Its axis is perpendicular to the plane of the bonds connecting that carbon to the three methyl groups... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

Together these two products contain all eight carbons of the starting alkene The two carbonyl carbons correspond to those that were doubly bonded m the original alkene One of the doubly bonded carbons therefore bears two methyl substituents the other bears a hydrogen and a tert butyl group The alkene is identified as 2 4 4 trimethyl 2 pentene (CH3)2C=CHC(CH3)3 as shown m Figure 6 15... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

Step 1 Protonation of the carbon-carbon double bond to form tert butyl cation... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

Step 1 Once generated by the reaction of tert butyl chloride and aluminum chloride tert butyl cation attacks the TT electrons of benzene and a carbon-carbon bond is formed... 

Here the electrophile is tert butyl cation formed by a hydride migration that accompa nies ionization of the carbon-chlorine bond... 

Carbon-hydrogen stretching vibrations with frequencies above 3000 cm are also found m arenes such as tert butylbenzene as shown m Figure 13 33 This spectrum also contains two intense bands at 760 and 700 cm which are characteristic of monosub stituted benzene rings Other substitution patterns some of which are listed m Table 13 4 give different combinations of peaks... 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the for mation of imines and enamines Carbocation (Section 4 8) Positive ion in which the charge re sides on carbon An example is tert butyl cation (CH3)3C Carbocations are unstable species that though they cannot normally be isolated are believed to be intermediates in certain reactions... 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

J5)i-tert-55ky. peroxides ate thermally stable organic peroxides, having 10-h HLTs of 110—135°C for acycHc peroxides and 10-h HLTs approaching 200°C for five- to six-membeted cycHc peroxides, eg, 129°C for d4-tert-huty peroxide (in decane) (22), 200°C for 3,3,5,5-tettamethyl-l,2-dioxolane [22431-90-9] (in benzene) (70), and 197°C for 3,3,6,6-tettamethyl-l,2-dioxane (in carbon tetrachloride) (71). 

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