Carbon terminus

Vinylsilanes react readily with a range of electrophiles to give products of substitution (1). The overall stereochemistry of such substitution will depend on a number of factors, including the stereochemistry of addition and subsequent elimination when 1,2-adducts are discrete species. However, the regiochemistry of substitution is normally unambiguous, the -effect ensuring that carbonium-ion development on attack by the electrophile will occur at the carbon terminus remote, i.e. /3, to silicon ... 

The order of a sigmatropic rearrangement is expressed by two numbers set in brackets [ij]. These numbers can be determined by counting the atoms over which each end of the s bond has moved. Each of the original termini is given the number 1. Thus in the first example above, each terminus of the s bond has migrated from C-1 to C-3, so the order is [3,3]. In the second example, the carbon terminus has moved from C-1 to C-5, but the hydrogen terminus has not moved at all, so the order is [1,5]. 

A theoretical study of the reactivity of prototype ketene CH2=C=0 with several radicals including HsSi has been reported. Scheme 5.11 shows that three adducts are possible with the corresponding energy barriers for their formation calculated at B3LYP/6-31G level of theory [75]. However, all the levels of theory used in this study predicted that silyl radical prefers to add to the carbon terminus of ketene. 

The design of the peptide implies that interaction of the catalyst with its substrate relies heavily on hydrogen bonding. Initial studies indeed revealed that, in particular, N-acyl amino alcohols such as 25 and ent-25 were efficiently differentiated whereas both enantiomers of l-(l-naphthyl)ethanol were acetylated at identical rates [28]. Catalyst 23b, shown in Scheme 12.12, was the most efficient from a series of ten peptides. For best performance, proper matching of the sense of chirality of all three chiral amino acids is necessary, and the type of amino acid present at the carbon terminus enables further tuning (for example, L-Phe was found to be better than, e.g., L-Val, selectivity factor 21) [29]. 

Molecular orbital theory also predicts that a nucleophile of the sulfide type will bond at the carbon terminus of a conjugated ene carbonyl system that is, the nucleophile will bond with the electrophile in the Michael addition mode of reaction (20). Thus, the reaction of polysulfide dianion with an enone represented by a chalcone may proceed initially in such a manner as shown in Scheme 2, which reproduces one of the several pathways... 

Diazo ketones also possess an electrophilic diazo group, and hence are susceptible to diazo-coupling reactions with suitable soft nucleophiles. Examples are given in equations (11) and (12). Phospha-zines such as (19) are useful synthetic intermediates in their own right. The carbon terminus of the 1,3-dipole possesses nucleophilic properties and can participate in aldol-type reactions with the particularly electrophilic carbonyl groups in 1,2-di- and 1,2,3-tri-carbonyl compounds. Intramolecular condensations occur with greater ease (equation 13). Reaction of diazo ketones of the type summarized in equations (9)-(12) have been thoroughly reviewed. ... 

N-Methylation of 1-pyrrolidinocyclohexene is increased (110) fivefold by changing the electrophile from Mel to MeOS02p. general, the enamine alkylation is particularly amenable to chain lengthening at the carbon terminus by using soft acceptors such as a, -unsaturated esters, nitriles, and ketones (124). 

Enolates of carbonyl compounds are ambident anions. Although the negative charge resides predominantly at the oxygen, reactions of enolates with electrophiles can take place at the carbon terminus. C-Monoalkylation of enolates is usually accompanied by di- and polyalkylation owing to a rapid... 

Intramolecular nucleophilic attack at the allyl carbon terminus via this allyl nitroso cationic species is feasible. Examples are represented by compounds 34 and 36, ° whose side-chain carboxylate or hydroxyl termini undergo cyclization in basic conditions to give the cyclized products 35 and 37 in reasonable yields (> 50%). 

C-cycloadduct in which the carbon terminus of the dipole added to the p-carbon of the acceptor. When the process was catalysed by a combination of nickel perchlorate with chiral bisoxazoline ligand 9, the product was obtained in almost quantitative yield and with moderate enantioselectivity of 77% ee, as shown in Scheme 1.9. The authors showed that the enantioselectivity of... 

An intramolecular cycloaddition reaction involving a ketenimine with two different substituents on its sp carbon terminus 42 affords a mixture of mainly m-azeto[2,lb]-quinazoline 43 and the trans- isomer 44 in total yields of 29-84... 

When an electron withdrawing group is attached to the sp carbon terminus, 4-amino-3-trimethylsilylpyrazoles are obtained... 

The isomerization can be acid or base catalyzed. In the base-catalyzed reaction, the intermediate is the conjugated dienolate ion, which is reprotonated at the carbon terminus. 

Finally, an aza-Wittig rearrangement of acyclic enantio-enriched N,N-diallyllic a-amino alkylithium prepared via a tin-to-lithium exchange was reported, the process proceeding predominantly with inversion of configuration at the lithiumbearing carbon terminus [361]. 

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