Carbon-bonded substituents

Alkoxysilane hydrolysis—effects of substituents. A series of hydrolysis studies [4-9] have elucidated general trends. Under basic conditions, the hydrolysis of alkoxy groups usually takes place in a stepwise manner. Carbon-bonded substituents can have profound effects on the rate of hydrolysis. With the... 

The enthalpies of formation for phenols with carbon-bonded substituents appear in Table 3. 

TABLE 3. Enthalpies of formation of phenols with carbon-bonded substituents (kJ mol )... 

The compounds in this section are listed in Table 25. The ethyl-substituted derivatives have been arranged in the following way non-carbon-bonded substituents (designated X and Y) of one type (Nos. 2 to 11) or two types (Nos. 12 to 15), carbon-bonded groups on the C-1 atom and X groups on C-1 or C-2 (Nos. 16 to 25), and sp carbon atoms within the C2 chain and/or cyclic carbon-bonded groups (Nos. 26 to 54). 

Several further C-nucleosides have been obtained by elaboration of short carbon-bonded substituents on furanosyl rings, Starting materials and... 

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

The term allylic refers to a C=C—C unit The singly bonded carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon Conversely doubly bonded carbons are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents... 

The three carbons of allene he in a straight line with relatively short carbon-carbon bond distances of 131 pm The central carbon because it bears only two substituents is sp hybridized The terminal carbons of allene are sp hybridized... 

For a carbon-carbon bond located along a polymer backbone, the preceding molecular representation must be modified to Fig. 1.8c. The chain segments on either side of the bond of interest are substituents for which the amount of steric hindrance follows a slightly different pattern than for the unsubstituted ethane. Using the same convention for [Pg.58]

The polarity of covalent bonds between carbon and substituents is the basis of important structure-reactivity relationships in organic chemistry. The effects of polar bonds are generally considered to be transmitted in two ways. Successive polarization through bonds is called the inductive fect. It is expected that such an effect would diminish as the number of intervening bonds increases. 

The Heck reaction is considered to be the best method for carbon-carbon bond formation by substitution of an olefinic proton. In general, yields are good to very good. Sterically demanding substituents, however, may reduce the reactivity of the alkene. Polar solvents, such as methanol, acetonitrile, N,N-dimethylformamide or hexamethylphosphoric triamide, are often used. Reaction temperatures range from 50 to 160 °C. There are various other important palladium-catalyzed reactions known where organopalladium complexes are employed however, these reactions must not be confused with the Heck reaction. 

The same principles just developed for butane apply to pentane, hexane, and all higher alkanes. The most favorable conformation for any alkane has the carbon-carbon bonds in staggered arrangements, with large substituents arranged anti to one another. A generalized alkane structure is shown in Figure 3.10. 

Figure 3.10 The most stable alkane conformation is the one in which all substituents are staggered and the carbon-carbon bonds are arranged anti, as shown in this model of decane.

Newman projection (Section 3.6) A means of indicating stereochemical relationships between substituent groups on neighboring carbons. The carbon-carbon bond is viewed end-on, and the carbons are indicated by a circle. Bonds radiating from the center of Ihe circle are attached to the front carbon, and bonds radiating from the edge of the circle are attached to the rear carbon. 

An interesting free radical carbon-carbon bond formation with concomitant elimination of a /5-thio substituent was achieved during the course of Boger s impressive synthesis of CC-1065.26-27 In the event, treatment of aryl bromide 70 (see Scheme 13) with tri-n-... 

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