Terminal substitution

This reaction proceeds through a chain mechanism. Free-radical additions to 1-butene, as in the case of HBr, RSH, and H2S to other olefins (19—21), can be expected to yield terminally substituted derivatives. Some polymerization reactions are also free-radical reactions. 

The success of the above alkylation and acylations, without obtaining ring-opening products , extends the usefulness of this method particularly when the anion 323 is being used in a regio- and stereo-specific manner . Thus, the combination of direct alkylation and thermal extrusion of sulfur dioxide provides an ideal route for the preparation of terminally substituted conjugated dienes. 

It is interesting to note that the reactivity of the excited states of (25), (26), (27), and (28) in Table 8.4 increases in this order as stabilizing terminal substitution is increased. Zimmerman suggests that vinyl-vinyl bridging (the start of bond formation between 2 and 4) controls the reaction rate. 

Based on a known synthesis of spiro compounds with six-membered rings [63] we succeeded in the stepwise assembly of terminal substituted mono-[64] and dispiranes [65]. Hereby the cyclisation of the dibromide 35 with TOSMIC (Iter. 1) is the keystep of the reaction sequence as the initial ketone functional group (36) is recovered. Four subsequent reactions led to the spirodibromide 40 (Iter. 2). Final spirocyclisation afforded the dispirane 41, representing a precursor for new calamitic liquid crystals (Fig. 16) [66]. 

Fig. 16. Stepwise synthesis of terminal substituted dispiranes as precursors for new liquid crystals... 

Formation of cis-l,2-DVCB and ds,dv-COD commences through the formation of a a-bond between the terminal substituted carbons, C3, C6, and the terminal unsubstituted carbons, C1, C8, of the two r 3-allylic groups along 4a —> 9a and 4a —> 10a, respectively (Fig. 8). The transition states TS[4a-9a] and TS[4a 10a] occur at a distance of 1.9 and 2.1 A for the newly formed C-C bond and decay into 9a and 10a, respectively, where the cyclodimers are each coordinated to Ni° by two olelinic double bonds. The several stereochemical pathways are connected with activation barriers that differ significantly. Moderate barriers have to be overcome for 4a —> 9a... 

Based on the data collected in this section, one must conclude that the addition of radicals to dienes is certainly rapid enough to compete against the typical chain-breaking processes and that especially the addition of electrophilic radicals to polyenes appears to bear significant potential. Terminally substituted polyenes are likely to be unsuitable for radical addition reactions due to their lower addition rates and to undesirable side reactions. 

The inter- and intramolecular catalytic reductive couplings of alkynes and aldehydes recently have experienced rapid growth and are the topic of several recent reviews.5 h-8k 107 With respect to early transition metal catalysts, there exists a single example of the catalytic reductive cyclization of an acetylenic aldehyde, which involves the titanocene-catalyzed conversion of 77a to ethylidene cyclopentane 77b mediated by (EtO)3SiH.80 This process is restricted to terminally substituted alkyne partners (Scheme 53). 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

The origin of this preference is illustrated by Eq. 13.3. When the allene is terminally substituted, as it is in 6, it is axially chiral. Consequently, there are two reaction pathways that the allenyl vinyl ketone (10 in Eq. 13.3) can follow. The major reaction pathway for 10 involves counterclockwise conrotation, leading to 11, whereas the... 

The planar geometry of the allyl group and the partial double-bond character of both C—C bonds permit terminally substituted jr-allyl... 

SPOCC resin is based on the cross-linking of long-chain Poly(ethylene glycol) (PEG)terminally substituted with oxetane by cationic ring-opening polymerization. 

The addition of O2 to acyclic dienes proved to be strongly dependent on terminal substitution and the substituents at other positions of the conjugated system, and, furthermore, it must be accompanied by photoisomerization of ( ,Z)-dienes to ( , )-dienes because singlet oxygen adds exclusively to ( , )-dienes to yield endoperoxides (cf Tables 6(a), 6(b), and 7, and references therein). 

Kinetic product, ortho products dominate for additions to terminally-substituted dienes para products dominate for additions to internally-substituted dienes... 

To generate the catalytically active species, the diene ligand of 18 was removed by hydrogenation with 1 atm H2 to afford the bis-aquo species (PMc3)2Rh(H2O)2 (23). In the absence of 1, addition of small allyl alcohols quickly yielded the isomerized product. However, the reaction did not tolerate terminal substitution, and isomerization of crotyl alcohol was not observed (Table 7.4). Significantly, when both crotyl alcohol and allyl alcohol were added to 23, neither substrate underwent isomerization, suggesting that crotyl alcohol inhibits the reaction. 

Internal alkenes and cycloalkenes react much more slowly. If double-bond migration, however, can lead to an isomeric terminal olefin, slow transformation to the terminally substituted alkylsilane is possible.401,423 126 All three isomeric... 

Radical addition reactions provide synthetic access to 1-substituted and 1,3-disubstituted bicyclo[l. 1. l]pentanes. They have been reviewed recently6, but a large number of new examples has accumulated in the literature since. Under suitable conditions, these processes produce significant yields of terminally substituted oligomers of bicyclo[l.l.l]pentane, [ ]staffanes (Table 13). The course of the reaction depends on the... 

Cyclization of P-allenylamine systems to six-membered ring products has been observed recently. Cyclization of terminally substituted p-allenylamides with silver tetrafluoroborate produced lactam products (equation 137).267 The cyclization shown in equation (138) was used to generate the indolizidine ring system of allopumiliotoxin alkaloids.2676... 

Not surprisingly, a completely staggered conformation is maintained in the symmetrical, linear, terminally substituted diiodidooc-... 

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