Terminal allyl groups

Terminal Allylic Group in the Octadienediyl-Ni11 Complex. 184... 

The interconversion between octadienediyl-Ni11 stereoisomers may involve two different processes namely the syn-anti isomerization and also the enantioface conversion of one or both terminal allylic groups (Fig. 4). The isomerization of the allylic group is connected with two aspects firstly the interconversion of its syn and anti configuration and secondly the inversion of its enantioface.36 On the other hand, the process of enantioface conversion is not accompanied by alternation of the allylic configuration. 

The conversion processes of the terminal allylic groups of the octadienediyl-Ni11 complex show very similar characteristics for the two reaction channels. The influence of electronic and steric factors on the allylic isomerization will be scrutinized in Section 5.3 and the overall role played by allylic conversion will be elucidated in the context of the entire reaction course (see Sections 6.1 and 6.2). 

Among the several configurations of the crucial [Nin(octadienediyl)L] complex, all of which are in equilibrium, the p3, 1 1) species 2a and the bis(p3) species 4a are predicted to be prevalent. The odonor/71-acceptor ability of the ancillary ligand is shown to predominantly determine the position of the kinetically mobile 2a 4a equilibrium. The conversion of the terminal allylic groups via allylic isomerization and/or allylic enantioface conversion are indicated to be the most facile of all the elementary processes that involve the [NiII(octadienediyl)L] complex. Consequently, the several octadienediyl-Ni11 configurations and their stereoisomers are likely to be in a dynamic pre-established equilibrium, that can be assumed to be always present. 

Oxidative coupling via la —> 2a and the reductive elimination routes, that commence from 4a as the precursor, involve different stereoisomers along the most feasible pathway. Accordingly, the conversions of the terminal allylic groups of the [Nin(octadienediyl)L] complex represent indispensable elementary processes. 

Evidently, it is favorable for dihydridesiloxanes to attract terminal allyl groups rather than NH-group bonded to silicon atoms, surrounded with organic radicals under the conditions of polyhydrosilylation reactions. That leads to formation of macromolecules with linear structure (scheme 2) [6]. 

S-Bioallethrin (Fig. 5 ]9a]) is a S)mthetic pyrethroid insecticide with three asymmetric centers. The S designation refers to the l-K, 3-R, 4 -S isomer and is not a true designation of stereochemistry. Conversion of the terminal allylic group to —CHjCHjCHjOH, formation of hemisuccinate. 

Cyclopropanes are easily formed (Table 14, entries 14 and If)21, and can be thermally rearranged to cyclopentene derivatives. In these examples diastereoselectivity is independent of the olefin geometry since 7>c7-7 -interconversion involving a terminal allyl group is faster than nucleophilic attack. 

The various octadienediyl-Ni configurations 2-4 are likely to readily undergo mutual conversion for identical stereoisomers [16]. Syn-anti isomerization as well as enantioface conversion of the terminal allylic groups of the octadienediyl-Ni complex are the most hkely processes for interconverting different stereoisomeric forms (Fig. 4). Isomerization of the allylic group in-... 

Mo-Mo line. It is believed that symmetrically bridging allyl groups are as yet found only in these two compounds. The Mo-C distances, involving both bridging and terminal allyl groups, range from 2.22 to 2.53 A. The average C-C distance is 1.37 A. 

Some of these terminal allyl groups can undergo rearrangement (8) to propenyl groups in the basic medium. 

Scheme 8.3 One-pot synthesis of polyolefin dendrimers till G3 (3 = 243 terminal allyl groups) using the silane HSiMe2Cl...

Sun and coworkers (181) used a different approach to prepare block copolymers. A polyoxyethylene with a terminal allyl group was reacted with acrylic acid to produce a graft copolymer polyoxyethylene-g-poly (acrylic acid). 

In 2008, Stoltz and Enquist disclosed a novel approach to the total synthesis of the related cyathin diterpenoid (—)-cyanthiwigin F (119) [80]. They applied a simultaneous alkene RCM and cross metathesis (CM) protocol to achieve the required closure of the seven-membered ring as well as elaboration of the terminal allyl group (Fig. 31). This process was effectively and selectively conducted by treating polyene (120) with Grubbs-Hoveyda catalyst (81) and a vinyl boronate species to produce the desired bicycle (121) in 51% yield. 

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