Borane, derivatives allylic

Borylation. Directed by a silylated heteroatom the borylation opens a new trail for o-functionalization of arenes. Allylic boranes derived from aUcenes by the same protocol can be used to synthesize homoallylic alcohols and alkenylarenes. ... 

Syntheses with boranes and acetylene derivs. Allyl-type rearrangement a-Alkoxystyrenes from alkoxyacetylenes... 

The tartrate-derived allyl and crotyl boronates 25 developed by Roush [33, 42-48] represent a convenient and elegant alternative to the terpene-derived boranes described above, as a consequence of their ease of handling and configurational stability (Equation 3). Representative examples are summarized in Table 5.2 [42, 45, 47]. These reagents have also enjoyed widespread applications in diastereoselective, reagent-controlled additions to chiral aldehydes, displaying superb levels of diasteroinduction [43, 44, 48], An explanation for the observed stereoselectivity has been proffered by Roush [32] and supported by ab initio calculations [62] minimization of lone pair/lone pair interactions as well as an attractive interaction between the ester lone pairs and the polarized aldehyde are believed to result in a preference for transition state structure 26. 

Allylic derivatives are particularly important in the case of boranes, silanes, and stannanes. Allylic boranes effect nucleophilic addition to carbonyl groups via a cyclic TS that involves the Lewis acid character of the borane. 1,3-Allylic transposition occurs through the cyclic TS. 

The allylation reaction has been extended to enantiomerically pure allylic boranes and borinates. For example, the 3-methyl-2-butenyl derivative of (Ipc)2BH reacts with aldehydes to give carbinols of greater than 90% e.e. in most cases.39... 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

However, the classical version of the ABAC, when triallyl-, trimethallyl-, or tricrotylborane are involved in the reaction with RR1CHC=CH (e.g., propargylic ethers), can be applied only for the synthesis of 1-boraadamantane derivatives 35, 3,5-dimethyl- 36 and 4,6-dimethyl-l-boraadamantanes 37 (Scheme 7). Hence, the stmctures of the final cage compounds are rigidly restricted by the stmctures of the starting allylic boranes. 

This procedure using allylic and methallylic boranes, Grignard reagents, and methylacetylene (Scheme 9) presents a unique route to design the 3-borabicyclo[3.3.1]non-6-ene derivatives 38 and 39 differentiated only by methyl group location relative to the double bond <2005MI1, 2005MI3> and inaccessible by any other chemistry. 

In addition to the predominant allyl and crotyl reagents, a large nnmber of allylic borane 1 and boronate derivatives 2 (Eq. 1) with varions snbstitnents (R -R" ) have been reported. Interested readers can refer to the comprehensive Tabnlar Snrvey at the end of this monograph, which covers the literatnre np to the end of 2005. Several reviews on allylic boron componnds and other allylmetal reagents and their additions to carbonyl compounds and imines have been written prior to this one," " and these sonrces may be consnlted if a more in-depth historical perspective is desired. 

The immediate products of additions between carbonyl substrates and allylic boranes 1 or boronate derivatives 2 are borinate or borate esters, respectively. To cleave the covalent B-O bond in these intermediates (structme 6, Scheme 1) and to obtain the desired free alcohol, a hydrolytic or oxidative work-up is required. This issue is discussed in detail in the section Work-Up Conditions . In the interest of simplifying chemical equations, specific work-up conditions are not inclnded in most of the examples highlighted in this chapter. 

The boron-oxygen mesomeric effect described in the previous section explains the lower reactivity of allylic boronates towards carbonyl compounds compared to that of allylic boranes. The use of Lewis acids, however, allows boronate derivatives, including hindered ones, to react at temperatures comparable to the analogous boranes. As described above (see section Mechanism and Stereochemistry ), the most reactive allylic boronates are those with the most electrophilic boron centers.The nucleophilicity of the y-position of an allylic boron reagent (the position that forms the new C-C bond with the aldehyde) is also important to the reactivity of the reagent. For example, allylic boronates with... 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

In scientific and patent literature on diene polymerization the following Nd(III)-salts are most often cited as catalyst precursors halides, carboxylates, alcoholates, phosphates, phosphonates, allyl compounds, cyclopentadienyl derivatives, amides, boranes and acetylacetonates. The catalyst systems based on these Nd-sources are reviewed in the following subsections. 

Optically active TV-sulfonylamino alcohols derived from D-camphor or norephedrine were found to be efficient chiral ligands for the enantioselective allylboration of iV-silylimines (Equation (170)) 646-648 B-Allyl(diisopinocampheyl)borane allylated iV-diisobutylaluminum imines with 87% ee (Equation (171)).649,650... 

The possibility of such a cyclic mechanism along with the enhanced Lewis acidity of allylic boranes accounts for their ability to add easily to carbonyl derivatives whereas normal trialkylboranes do not. 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

Reaction of acetylenic derivatives of the elements of group 14 with triorganoboranes gives products of 1,1-organoboration [eq. (I)].1 At the same time, allylic boranes react with triple bonds by 1,2-addition mechanism through a six-membered transition state [eq. (2)].2 Therefore two different reaction pathways may be expected for interaction of A113B with Si-, Ge- and Sn-containing acetylenes. 

Thus ailylboration of carbonyl compounds with B-allylic derivatives of 9-BBN, followed by transesterification with ethanolamine, provides a simple, convenient method for the synthesis of homoallylic alcohols It has been reported that enantiomerically enriched allylic boranes provide homoallylic alcohols with high degree of enantio- and diastereoselectivity. The scheme is illustrated in Eqs. 48 and 49... 

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