Allylic boron-stabilized

Unlike aldehydes and ketones, allylic boron compounds are not ubiquitous, commercial organic substrates. There are several methods for the preparation of allylic boronates, however, and many of these have been developed in the past decade. This topic has been reviewed recently " so only the most common methods are emphasized in this section. As a result of the lesser stability of allylic boranes, methods to access these reagents are more limited and it is generally easier to prepare allylic boronates with a wide range of functional groups. 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

As a conseqnence of their superior stability, many types of allylic boronates can be isolated and purified. It should be noted that most pinacol allylic boronic esters and other bnlky esters are stable to hydrolysis and can be conveniently pnrified by chromatography on silica gel. A potential pitfall of all allylic boron componnds is their stereochemical integrity, and snbstituted allylic boranes are known to nndergo reversible borotropic rearrangements at temperatures above —45° (see Eq. 12, M = Eor this reason, allylic boranes are nor-... 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

Copper catalysed allylation of a boron stabilized secondary alkylzinc bromide preparation of 2-butyl-1-decen-4-ol8... 

Mesityllithium or lithium dicyclohexylamide react similarly with allyldimesitylborane (equation 19). Thus, boron-stabilized allyl units are available from either allyl- or vinyl-boranes. 

The boron-stabilized allyl carbanions thus formed react with electrophiles. 

These results support the intermediate allylic boron ate complex formed from stereochemically homogeneous allylmetal and il-OMe-9-BBN, with a plausible chair-like transition state Tj. This transition state is further stabilized by the adjacent four-membered ring formed by the second metal [2]. The alternative transition state Tj is destabilized by the 1,3-diaxial repulsion of the bulky cyclooctyl ring of 9-BBN (Fig. 6.1). erythro-threo Assignments are made... 

Yamamoto reported a convenient one-pot procedure for the synthesis of (Z)-2-alkenylsilanes and -tins (Scheme 24.3) [18]. The boron stabilized allyl carban-ions are prepared in situ by treatment of 1- or 2-alkenyl-9-BBN[19] with lithium 2,2,6,6-tetramethyl piperidide (LiTMP). The carbanion reacts with trialkylsilyl and -tin halide exclusively at the a position to give 1,1-borasilyl or 1,1-bora-tin derivatives, which are subsequently protonolyzed with H O. The results are summarized in Table 24.7 [18]. 

Low stability of the functionalized allyl boronates may limit their synthetic application. Alternative progress towards a workable Petasis borono-Mannich reaction offers a new solution to this problem in situ generation and use of allylboronates [18]. This one-pot route to stereo-defined a-amino acids is outlined in Scheme 8.6. 

Besides aUylsUanes or stannanes, allyl boron species have found widespread apphcations in synthesis. The Roush crotylation is a very well-estabhshed method with a broad substrate scope, and is therefore commonly used in organic synthesis [73]. The following example depicts a reversal in enantioselectivity induced by a cobalt complex present in the substrate. Roush et al. reported that the use of metal carbonyl complexes as substrate surrogates led to an improvement of enantioselectivity in the asymmetric crotylation of the respective aldehydes. These results were attributed to electronic effects exerted by the metal complexes that stabilize the transition state of the crotylation reaction (Scheme 3.47) [74]. 

In one of their notable examples, the hydroboration polymerization of low molecular weight allyl-telechelic polyisobutylene with tripylborane (trip = 2,4,6-triisopropylphenyl) was found to yield air-stable organoboron segmented block copolymers. These boron main-chain polymers (8) (Fig. 8), unlike the general ones, were stable to air. The stability was due to the steric hindrance of the bulky tripyl groups preventing oxygen attack of the borons.28... 

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